EP2220658B1 - System for creating a magnetic field via a superconducting magnet - Google Patents

Abstract

The present invention relates to a system (100) for creating a magnetic field via a superconducting magnet (102) intended to produce said magnetic field. The system (100) according to the invention comprises a first branch including the superconducting magnet (102) formed by a coil inductance (L′) in series with a residual resistance (R′2), a second branch comprising a protection resistance (R′3) and a third branch comprising a power source (103). Furthermore, the system comprises a fourth branch formed by a resistance (R′1) mounted in series with a current-limiting superconducting device (106) switching from a low-resistance state to a high-resistance state when the current passing therethrough exceeds a breaking current, said first, second, third and fourth branches being mounted in parallel.

Description

The present invention relates to a system for creating a magnetic field via a superconducting magnet for producing said magnetic field. A superconducting magnet is formed of a superconducting winding (for example a Niobium-Titanium composite) maintained at a temperature such that the superconducting state of the material constituting the winding is ensured (for example at 4.2 K in a liquid helium bath at atmospheric pressure for a Niobium-Titanium composite subjected to a field typically less than 10 T). The zero electrical resistance thus achieved makes it possible to create very high magnetic field intensities within the confines of the transport capacities of the superconducting materials. The invention finds a particularly interesting application in the field of nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI).

Applications in the field of NMR and MRI require intense magnetic fields (up to several tens of teslas) and stable over time.

A known configuration consists in using a superconducting magnet short-circuited: this mode of operation, called persistent mode, is achieved by disconnecting the power supply from the coil and the presence of a superconducting switch forming a closed circuit with the coil. A superconducting switch formed by a superconducting composite coupled to a heating element (hereinafter referred to indifferently as the heater) is a thermal switch which has a zero resistance when the heater associated with it is turned off, the switch is then said " closed "and a large resistance in front of the other resistors of the circuit when the heater is energized, the switch is then said" open ". The resistance of the switch is that of the resistive matrix of the superconducting composite above a so-called critical temperature, and is almost zero below this temperature. The Equivalent electrical circuit thus formed is composed of the inductance of the magnet, typically several hundred henrys, the resistance of the magnet and the resistance of the short circuit formed by the superconducting switch.

This solution presents some difficulties, however.

Indeed, for magnetic field drift over time to be low, typically less than 0.05 ppm / h, it is necessary that the resistances of the circuit be extremely low, typically less than 1 nΩ for a 100 H magnet. .

However, for reasons related to the magnet technology (use of superconductor at high critical temperature), or accidental causes (fault on a junction inside the winding of the magnet), the residual resistance of the magnet may be greater than the value that makes system operation possible in persistent mode.

• Une première branche comportant un électroaimant 2 supraconducteur modélisé par une inductance L de bobinage en série avec une résistance R₂ représentant la résistance résiduelle de l'aimant empêchant le fonctionnement en mode persistant,
• une deuxième branche formée par un interrupteur supraconducteur thermique S₁ en série avec une résistance R₁.
• une troisième branche formée par une source d'alimentation en courant 3.

A known solution to this problem is described in the document US6624732 and consists in compensating the time drift of the magnet. The figure 1 illustrates the electrical circuit 1 allowing the implementation of this compensation. The electric circuit 1 comprises:

• A first branch comprising a superconducting electromagnet 2 modeled by an inductance L of winding in series with a resistor R ₂ representing the residual resistance of the magnet preventing operation in persistent mode,
• a second branch formed by a superconducting thermal switch S ₁ in series with a resistor R ₁ .
• a third branch formed by a power supply source 3.

The three branches are connected in parallel.

The value of the resistance R ₁ is in a ratio of 10 to 1000 times the value of the resistance R ₂ .

• un mode de charge de l'aimant : lorsque l'injection de courant par la source 3 dans la bobine L de l'aimant commence, l'interrupteur supraconducteur S₁ est ouvert ;
• un mode de fonctionnement normal (ou mode nominal) : lorsque le courant dans la bobine de l'aimant a atteint sa valeur nominale (courant stabilisé), l'interrupteur supraconducteur S₁ est fermé ; après fermeture de cet interrupteur S₁, au lieu de déconnecter la source d'alimentation 3, on la laisse connectée à la bobine L de l'aimant pour compenser les pertes; le courant est injecté dans l'interrupteur S₁, jusqu'à la stabilisation du champ produit par l'aimant à la valeur souhaitée.

Circuit 1 operates in the following two modes of operation:

• a charging mode of the magnet: when the injection of current by the source 3 into the coil L of the magnet begins, the superconducting switch S ₁ is open;
• a normal mode of operation (or nominal mode): when the current in the coil of the magnet has reached its nominal value (stabilized current), the superconducting switch S ₁ is closed; after closing this switch S ₁ , instead of disconnecting the power source 3, it is left connected to the coil L of the magnet to compensate for the losses; the current is injected into the switch S ₁ , until the stabilization of the field produced by the magnet to the desired value.

To limit the time drift due to the current supply source 3, the first resistive branch formed by R ₁ is added so that all the pulsations of the power supply will pass in this branch, and the current in the coil will be perfectly continued. If I denote by I _op the nominal operating current of the magnet and ΔI the current flowing in the resistor R ₁ , the stabilization is obtained by the relation: $${\mathrm{<figure-callout\; id="2"\; label="R"\; filenames="imgf0001.png,imgf0005.png"\; state="\{\{state\}\}">R</figure-callout>}}_2\mathrm{.}{\mathrm{I}}_{\mathrm{o}\mathrm{p}}={\mathrm{<figure-callout\; id="1"\; label="R"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">R</figure-callout>}}_1\mathrm{.}\mathrm{\Delta }\mathrm{I}\mathrm{.}$$

However, the implementation of the circuit as described in the document US6624732 also poses some difficulties.

Thus, the magnet can locally lose its superconducting properties and transit in a dissipative mode (We speak of "Quench" of the magnet). Such a transition implies that the latter is protected on itself (ie that the resistance developed in the magnet during the transition is sufficient to discharge the current in the magnet at such a speed that the heating of the conductor remains limited) . However, for technological reasons such as the very high energy stored in the magnet (typically above 100 MJ) it is sometimes difficult to apply this type of protection. The Joule effect that is generated could then lead to an abnormal heating of the magnet and thus to a permanent deterioration of its superconducting properties.

One solution to this problem is to add an additional branch to the terminals of the magnet and the power supply consisting of a protection resistor: such a circuit 10 is illustrated in FIG. figure 2 .

• Une première branche comportant un électroaimant 2 modélisé par une inductance L de bobinage en série avec une résistance R₂ représentant la résistance résiduelle de l'aimant empêchant le fonctionnement en mode persistant,
• une deuxième branche formée par un interrupteur supraconducteur thermique S₁ en série avec une résistance R₁,
• une troisième branche formée par une source d'alimentation en courant 3 montée en série avec un organe de coupure S₂ (et éventuellement un organe de coupure de redondance S₃),
• une quatrième branche formée par une résistance R₃, dite résistance de protection, qui peut se situer aussi bien dans l'enceinte du cryostat de l'aimant qu'à l'extérieur à température ambiante.

The electrical circuit 10 comprises (the elements common to the circuit 1 of the figure 1 have the same references):

• A first branch comprising an electromagnet 2 modeled by a winding inductance L in series with a resistor R ₂ representing the residual resistance of the magnet preventing operation in persistent mode,
• a second branch formed by a superconducting thermal switch S ₁ in series with a resistor R ₁ ,
• a third branch formed by a current supply source 3 connected in series with a cut-off device S ₂ (and possibly a redundancy cut-off device S ₃ ),
• a fourth branch formed by a resistor R ₃ , said protection resistor, which can be located both inside the cryostat chamber of the magnet and outside at room temperature.

In case of quench, the cutoff member S ₂ is open (and possibly S ₃ ) so that the coil L discharges into the resistor R ₃ whose value is optimized to obtain a fast discharge without degradation of the magnet. The decay rate of the current is then determined by the value of the protection resistor.

As mentioned above, the superconducting switch S ₁ in series with R ₁ must be closed, ie with low impedance in comparison with the other resistors of the circuit, in normal operation with stabilized current.

On the other hand, this same switch S ₁ must be open (ie with high impedance in comparison with the other resistances of the circuit (R ₁ , R ₂ , R ₃ )) during the charge / discharge of the coil L and during the protection of the magnet (fast discharge of the coil L in R ₃ ).

Indeed, for the stabilization process to work, it is necessary to use a resistor R ₁ having a value much lower than the protection resistor R ₃ and a value greater than that of R ₂ (included in a ratio from 10 to 1000 times the value of the resistance R ₂ as we already mentioned above). To limit heat losses, the resistance R ₁ must be such that the Joule power dissipated in the stabilization regime remains low, typically less than a few milliwatts.

In case of fast discharge (protection mode), the current of the magnet must flow in R ₃ , because it is the only resistance dimensioned to receive a high energy and it is it which controls the speed of decay of the current to protect the magnet: switch S ₁ must therefore be open.

• Première difficulté : l'interrupteur supraconducteur S₁ a maintenant un rôle prépondérant dans la sécurisation du fonctionnement de l'aimant car en l'absence d'ouverture, le courant dans l'aimant ne décroît pas à la vitesse attendue. Or ce retard peut causer un échauffement anormal du bobinage conduisant à une détérioration irréversible de ses propriétés supraconductrices,
• Deuxième difficulté : en cas de décharge de l'aimant, la tension aux bornes de l'interrupteur supraconducteur S₁ peut être de plusieurs centaines voire milliers de volts. L'énergie déposée lors de la décharge dans l'interrupteur S₁ est donc très grande. Afin d'éviter une détérioration de l'interrupteur S₁, la masse de ce dernier doit donc être telle que son échauffement reste limité à typiquement moins de 100 K. L'interrupteur supraconducteur S₁ présente donc une grande difficulté de réalisation par rapport aux interrupteurs de l'état de l'art (qui supportent 1000 A sous quelques volts) et est nécessairement volumineux et lourd.
• Troisième difficulté : lors des opérations de charge et décharge en courant de l'aimant, l'interrupteur S₁ doit être maintenu dans l'état résistif à l'aide de sa chaufferette électrique (usuellement quelques centaines de milliwatt). Or, du fait de la taille imposée par la forte tension de décharge, la puissance nécessaire pour le maintenir ouvert est de quelques watts, ce qui est très sollicitant pour les systèmes cryogéniques assurant la régulation de la température de l'aimant.

The implementation of the configuration 10 of the figure 2 therefore presents several difficulties:

• First difficulty: the superconducting switch S ₁ now has a preponderant role in securing the operation of the magnet because in the absence of opening, the current in the magnet does not decrease at the expected speed. But this delay can cause an abnormal heating of the winding leading to an irreversible deterioration of its superconducting properties,
• Second difficulty: in case of discharge of the magnet, the voltage across the superconducting switch S ₁ can be several hundred or even thousands of volts. The energy deposited during the discharge in the switch S ₁ is very large. In order to avoid a deterioration of the switch S ₁ , the mass of the latter must therefore be such that its heating remains limited to typically less than 100 K. The superconducting switch S ₁ therefore presents a great difficulty of realization with respect to State-of-the-art switches (which support 1000 A at a few volts) and are necessarily bulky and heavy.
• Third difficulty: during current charging and discharging operations of the magnet, the switch S ₁ must be maintained in the resistive state using its electric heater (usually a few hundred milliwatt). However, because of the size imposed by the high discharge voltage, the power required to keep it open is a few watts, which is very demanding for cryogenic systems providing the regulation of the temperature of the magnet.

A known solution is to replace the superconducting thermal switch with a mechanical switch. A configuration of this type is described in the document US2007 / 0024404 . This solution provides a technological answer in principle to the second and third difficulties mentioned above but leaves in suspense the first difficulty related to the fact that the reliability of the protection of the magnet is dependent on the reliability of the switch and its circuit control.

In this context, the present invention aims to provide a system for creating a magnetic field to overcome the three difficulties mentioned above while ensuring an effective charge of the coil, a very small drift of the magnetic field in the time and fast discharge without degradation of the magnet in case of quenching.

• une première branche comportant un aimant supraconducteur destiné à produire ledit champ magnétique, ledit aimant étant modélisé par une inductance de bobinage en série avec une résistance résiduelle ;
• une deuxième branche comportant une résistance, dite résistance de protection,
• une troisième branche comportant une source d'alimentation ;

ledit système étant caractérisé en ce qu'il comporte une quatrième branche formée par une résistance montée en série avec un dispositif supraconducteur limiteur de courant basculant d'un état à résistance basse vers un état à résistance haute lorsque le courant le traversant dépasse un courant de déclenchement, ledit dispositif supraconducteur ayant une inductance au moins 10⁵ fois inférieure à celle de la bobine, et lesdites première, deuxième, troisième et quatrième branches étant montées en parallèle,
ledit système présentant au moins trois modes de fonctionnement :

• un premier mode de fonctionnement, dit mode de charge ou de décharge de l'aimant, dans lequel :
  • o ladite source d'alimentation est reliée au dit aimant de façon à augmenter ou diminuer le courant dans l'aimant,
  • o ledit limiteur de courant est dans son état à résistance haute ;
• un deuxième mode de fonctionnement, dit mode normal de fonctionnement, dans lequel :
  • o ladite source d'alimentation est reliée au dit aimant,
  • o ledit limiteur est dans son état à résistance basse ;
• un troisième mode de fonctionnement, dit mode de décharge rapide de l'aimant dans ladite résistance de protection, dans lequel :
  • o ladite source d'alimentation est déconnectée dudit aimant,
  • o ledit limiteur est dans son état à haute résistance ;

l'activation de l'état dudit limiteur dans lesdits trois modes de fonctionnement se faisant de manière passive sans recours à une commande externe.To this end, the invention proposes a system for creating a magnetic field including:

• a first branch comprising a superconducting magnet for producing said magnetic field, said magnet being modeled by a series winding inductance with a residual resistance;
• a second branch comprising a resistor, called a protection resistor,
• a third branch having a power source;

said system being characterized in that it comprises a fourth branch formed by a resistor connected in series with a tilting current limiting superconductor device from a low resistance state to a high resistance state when the current flowing through it exceeds a current of tripping, said superconducting device having an inductance at least ⁵ times smaller than that of the coil, and said first, second, third and fourth branches being connected in parallel,
said system having at least three modes of operation:

• a first mode of operation, said mode of charge or discharge of the magnet, wherein:
  • said source of power is connected to said magnet so as to increase or decrease the current in the magnet,
  • said current limiter is in its high resistance state;
• a second mode of operation, called normal operating mode, in which:
  • said source of power is connected to said magnet,
  • o said limiter is in its low resistance state;
• a third mode of operation, said fast discharge mode of the magnet in said protection resistor, wherein:
  • said source of power is disconnected from said magnet,
  • o said limiter is in its high-resistance state;

activation of the state of said limiter in said three modes of operation being done passively without recourse to an external command.

The limiter must have the lowest possible inductance, firstly to ensure the stabilizing function as described in the patent US6624732 and on the other hand to minimize the transition time between the "closed" state and the "open" state. With the experimental devices used, it is of the order of some microhenry.

The superconducting material is selected such that its critical temperature is greater than the temperature of the medium in which it is placed.

During the rapid discharge phase of the magnet in the protection resistor, the temperature of the superconducting wire forming said limiter passes through a maximum value called T _max. This value must be such that the limiter is not damaged if the superconducting wire constituting it reaches, locally or in totality, the value T _max . This value T _max must be at least lower than the temperature at which the superconducting properties of the superconducting wire chosen are not degraded, for example around 300 ° C. for NbTi. In the choice of this value, it is sometimes necessary to take into account the effect of the mechanical deformation related to the expansion of the materials. In order to overcome this effect, T _{max is} sometimes chosen to be less than 100 K because below this value, most materials no longer deform under the effect of a temperature variation.

A superconducting limiter is understood to mean a device based on the transition of superconductors between a non-dissipative state (almost zero resistance) and a dissipative state (non-zero resistance). This transition of the superconductors is characterized in particular by the presence of a critical current beyond which the device switches into the dissipative state. The limiter according to the invention differs from the limiters intended for electrical distribution networks where the current limiting requirements only last for a few hundred milliseconds. In contrast, in the context of the invention, the operation in limitation must be able to last several minutes or even several hours. Thermal exchanges that were neglected in this type of application are of great importance here.

Indeed, these temporal conditions have a direct influence on the exchanges between the superconductor and the cooler (cryogenic fluid or cold point). These exchanges are almost negligible in the case of a network limiter (practically adiabatic regime) while the exchanges are of great importance in the invention and allow to optimize the dimensioning of the limiter. It should also be noted that the limiters used in the distribution networks limit the current to a peak value; conversely, the role of the limiter according to the invention is to lower (not clipping) the current when it reaches a critical value.

Thanks to the invention, the superconducting switch controlled by a heating system according to the state of the art is advantageously replaced by a superconductive limiter requiring no external control to switch to resistive mode when charging or discharging the coil or its fast discharge. Such a configuration has a considerable advantage in terms of operational safety insofar as the efficiency of the fast discharge in case of quench is no longer conditioned by the opening of the switch controlled by its external control; the limiter according to the invention intrinsically allows to switch from its on state to its resistive state during the three operating modes that are the charging or discharging of the coil, the normal operating mode and the rapid discharge of the coil in the resistor protection when detecting a quench of the magnet.

• le limiteur ne perturbe pas la charge ou la décharge de l'aimant car il réagit intrinsèquement sans action extérieure ; les pertes dans le limiteur dans ces régimes peuvent être maintenues à un niveau faible par un dimensionnement adapté du limiteur,
• le limiteur en limitant naturellement et automatiquement le courant lors d'une décharge rapide de l'aimant ne modifie pas la protection de l'aimant,
• le fonctionnement du limiteur est automatique, il ne nécessite ni circuit de détection ni de donneur d'ordre.

The advantages of a current limiter over a controlled superconducting switch are therefore the following:

• the limiter does not disturb the charge or the discharge of the magnet because it reacts intrinsically without external action; the losses in the limiter in these regimes can be kept at a low level by a suitable dimensioning of the limiter,
• the limiter by naturally and automatically limiting the current during a fast discharge of the magnet does not modify the protection of the magnet,
• the operation of the limiter is automatic, it does not require a detection circuit or a client.

• ledit limiteur est formé par un fil supraconducteur comportant une pluralité de filaments élémentaires supraconducteurs intégrés dans une matrice résistive ;
• le fil supraconducteur peut aussi être constitué du dépôt ou de plusieurs dépôts d'un matériau supraconducteur sur un substrat résistif (par exemple un matériau supraconducteur fait à partir de céramiques tel qu'YBaCuO par exemple) ;
• la résistivité de ladite matrice résistive est supérieure à 10^-7 Ω.m ;
• ladite matrice résistive est réalisée en CuNi ;
• lesdits filaments élémentaires sont réalisés en NbTi ou dans un matériau dit « haut Tc » tel que le MgB₂ ;
• ladite résistance montée en série avec ledit limiteur présente une valeur 10 à 1000 fois supérieure à celle de la résistance résiduelle de l'aimant ;
• le fil supraconducteur formant ledit limiteur est choisi de sorte de son courant critique soit supérieur à (R'₂/R'₁)I_op où R'₂ désigne la valeur de ladite résistance résiduelle dudit aimant, R'₁ désigne ladite deuxième résistance montée en série avec ledit limiteur et I_op désigne le courant circulant dans ladite première branche lors dudit mode normal de fonctionnement ;
• la longueur du fil supraconducteur formant ledit limiteur est déterminée de sorte que la température dudit fil supraconducteur reste toujours inférieure ou égale à une valeur maximale de température prédéterminée T_max ;
• ladite longueur dudit fil supraconducteur est inférieure à une longueur I déterminée par $$l=U_0\sqrt{\frac{\mathrm{<figure-callout\; id="2"\; label="\tau "\; filenames="imgf0001.png,imgf0005.png"\; state="\{\{state\}\}">\tau </figure-callout>}}{2{\int }_{T_{\mathit{H}\mathit{e}}}^{T_{\max }}{C}_p\left(T\right)\rho \left(T\right)dT}}$$
  
  
  où S, C_p et ρ sont respectivement la section, la chaleur spécifique volumique et la résistivité dudit fil avec ses brins supraconducteurs et sa matrice, T_He désigne la température initiale du bain cryogénique dudit limiteur, U₀ désigne la tension initiale aux bornes dudit aimant avant ladite décharge rapide dans ladite résistance de protection et τ désigne une constante de temps donnée par le rapport L'/R'₃, L' représentant ladite inductance de bobinage et R'₃ représentant ladite résistance de protection ;
• ledit limiteur est formé par un fil supraconducteur entouré par une couche isolante dont l'épaisseur est déterminée de sorte que la puissance déposée dans le bain cryogénique dudit limiteur soit inférieure à une valeur prédéterminée ;
• ledit limiteur et ledit aimant sont localisés dans des bains cryogéniques séparés ;
• ledit limiteur est formé par un bobinage en deux couches, les deux couches étant bobinées en sens inverse et étant mises soit en parallèle soit en série, ceci dans le but d'obtenir un limiteur avec la plus faible inductance possible;
• le système selon l'invention comporte des moyens de commande pour faire basculer ledit limiteur de son état à résistance basse vers son état à résistance haute ;
• lesdits moyens de commande sont formés par un élément chauffant ;
• lesdits moyens de commande comportent des moyens pour générer un signal de courant alternatif circulant dans ledit limiteur de sorte que ledit limiteur bascule de son état à résistance basse vers son état à résistance haute, notamment sous l'effet de l'élévation de la température entraînée par la circulation dudit courant alternatif ;
• lesdits moyens pour générer un signal de courant alternatif comportent des moyens transformateurs de tension recevant en entrée la tension du réseau électrique et fournissant en sortie une tension abaissée à la même fréquence que la tension du réseau électrique ;
• la fréquence f dudit signal de courant alternatif est choisie suffisamment élevée pour que le courant alternatif soit bloqué par l'inductance de la bobine ;
• lesdits moyens de commande comportent des moyens pour générer un courant supérieur au dit courant de déclenchement permettant de faire basculer ledit limiteur ;
• lesdits moyens pour générer un courant supérieur au dit courant de déclenchement permettant de faire basculer ledit limiteur sont formés par des moyens générant une impulsion de courant d'intensité et de durée suffisante pour faire basculer ledit limiteur ;
• lesdits des moyens pour générer un courant supérieur au dit courant de déclenchement permettant de faire basculer ledit limiteur sont intégrés à ladite source d'alimentation.

The system according to the invention may also have one or more of the following characteristics, considered individually or in any technically possible combination:

• said limiter is formed by a superconducting wire having a plurality of superconducting elementary filaments embedded in a resistive matrix;
• the superconducting wire may also consist of the deposit or several deposits of a superconductive material on a resistive substrate (for example a superconductive material made from ceramics such as YBaCuO for example);
• the resistivity of said resistive matrix is greater than 10 ^-7 Ω.m;
• said resistive matrix is made of CuNi;
• said elementary filaments are made of NbTi or a so-called "high Tc" material such as MgB ₂ ;
• said resistor connected in series with said limiter has a value 10 to 1000 times higher than that of the residual resistance of the magnet;
• the superconducting wire forming said limiter is chosen so that its critical current is greater than (R ' ₂ / R' ₁ ) I _op where R ' ₂ denotes the value of said residual resistance of said magnet, R' ₁ denotes said second resistance mounted in series with said limiter and I _op designates the current flowing in said first branch during said normal mode of operation;
• the length of the superconducting wire forming said limiter is determined so that the temperature of said superconducting wire always remains less than or equal to a predetermined maximum temperature value T _max ;
• said length of said superconducting wire is less than a length I determined by $$l=U_0\sqrt{\frac{\mathrm{<figure-callout\; id="2"\; label="\tau "\; filenames="imgf0001.png,imgf0005.png"\; state="\{\{state\}\}">\tau </figure-callout>}}{2{\int }_{T_{\mathit{H}\mathit{e}}}^{T_{\max }}{\mathrm{VS}}_p\left(T\right)\rho \left(T\right)dT}}$$
  
  
  where S, C _p and ρ are respectively the cross section, the volume specific heat and the resistivity of said wire with its superconducting wires and its matrix, T _He denotes the initial temperature of the cryogenic bath of said limiter, U ₀ denotes the initial voltage across said magnet before said rapid discharge in said protection resistor and τ denotes a time constant given by the ratio L '/ R' ₃ , L 'representing said winding inductance and R' ₃ representing said protection resistor;
• said limiter is formed by a superconducting wire surrounded by an insulating layer whose thickness is determined so that the power deposited in the cryogenic bath of said limiter is less than a predetermined value;
• said limiter and said magnet are located in separate cryogenic baths;
• said limiter is formed by a two-layer winding, the two layers being wound in opposite directions and being put in parallel or in series, this in order to obtain a limiter with the lowest possible inductance;
• the system according to the invention comprises control means for switching said limiter from its low resistance state to its high resistance state;
• said control means is formed by a heating element;
• said control means comprise means for generating an alternating current signal flowing in said limiter so that said limiter switches from its low resistance state to its high resistance state, in particular under the effect of the elevation of the driven temperature by the circulation of said alternating current;
• said means for generating an AC signal comprises voltage transformer means receiving as input the voltage of the electrical network and outputting a voltage lowered at the same frequency as the voltage of the electrical network;
• the frequency f of said alternating current signal is chosen high enough so that the alternating current is blocked by the inductance of the coil;
• said control means comprises means for generating a current greater than said tripping current for tilting said limiter;
• said means for generating a current greater than said tripping current for flipping said limiter is formed by means generating a current pulse of intensity and duration sufficient to switch said limiter;
• said means for generating a current greater than said tripping current for tilting said limiter are integrated with said power source.

• génération d'une rampe de courant avec une consigne fixée à la nouvelle valeur de courant à atteindre dans l'aimant ;
• génération d'une impulsion de courant dont la durée et l'intensité sont telles que ledit limiteur bascule dans son état à haute résistance.

The present invention also relates to a method of adjusting the current in a magnet included in a system according to the invention comprising the following steps considered in any order:

• generation of a current ramp with a set point set to the new current value to be reached in the magnet;
• generating a current pulse whose duration and intensity are such that said limiter switches to its high-resistance state.

Advantageously, the method according to the invention comprises a step of generating a current slot that follows the step of generating said current pulse, the value of the current of this slot being equal to the sum of the current flowing in said protection resistor and the current flowing in said limiter when it is in its high resistance state.

• la figure 1 est une représentation schématique d'un premier circuit selon l'art antérieur;
• la figure 2 est une représentation schématique d'un second circuit selon l'art antérieur;
• la figure 3 est une représentation schématique d'un système selon l'invention ;
• la figure 4 est une représentation schématique d'un système selon l'invention incorporant des moyens de commande pour faire basculer le limiteur de son état à résistance basse vers son état à résistance haute selon un premier mode de réalisation ;
• la figure 5 est une représentation schématique d'un système selon l'invention incorporant des moyens de commande pour faire basculer le limiteur de son état à résistance basse vers son état à résistance haute selon un deuxième mode de réalisation ;
• la figure 6 représente respectivement l'évolution du courant de l'alimentation, le courant dans l'aimant et le courant dans le limiteur en fonction du temps en utilisant un système tel que représenté en figure 5.

Other features and advantages of the invention will emerge clearly from the description which is given below, as an indication and in no way limiting, with reference to the appended figures, among which:

• the figure 1 is a schematic representation of a first circuit according to the prior art;
• the figure 2 is a schematic representation of a second circuit according to the prior art;
• the figure 3 is a schematic representation of a system according to the invention;
• the figure 4 is a schematic representation of a system according to the invention incorporating control means for switching the limiter from its low resistance state to its high resistance state according to a first embodiment;
• the figure 5 is a schematic representation of a system according to the invention incorporating control means for switching the limiter from its low resistance state to its high resistance state according to a second embodiment;
• the figure 6 represents respectively the evolution of the current of the power supply, the current in the magnet and the current in the limiter as a function of time using a system as represented in FIG. figure 5 .

• Les figures 1 et 2 ont déjà été décrites en référence à l'état de la technique.
• La figure 3 est une représentation schématique d'un système 100 de création d'un champ magnétique selon l'invention.

In all the figures, the common elements bear the same reference numbers.

• The figures 1 and 2 have already been described with reference to the state of the art.
• The figure 3 is a schematic representation of a system 100 for creating a magnetic field according to the invention.

• Une première branche comportant un électroaimant supraconducteur 102 modélisé par une inductance L' de bobinage en série avec une résistance R'₂ représentant la résistance résiduelle de l'aimant empêchant le fonctionnement en mode persistant,
• une deuxième branche formée par une résistance de protection R'₃ localisée dans l'enceinte du cryostat de l'aimant ou à l'extérieur de l'enceinte à température ambiante
• une troisième branche formée par une source d'alimentation en courant 103 montée en série avec un organe de coupure 104 (et éventuellement un organe de coupure de redondance 105),
• une quatrième branche formée par un limiteur de courant supraconducteur 106 en série avec une résistance R'₁.

The system 100 comprises:

• A first branch comprising a superconductive electromagnet 102 modeled by an inductance L 'of winding in series with a resistance R ' ₂ representing the residual resistance of the magnet preventing operation in persistent mode,
• a second branch formed by a protection resistor R ' ₃ located in the chamber of the cryostat of the magnet or outside the chamber at room temperature
• a third branch formed by a current supply source 103 connected in series with a cut-off member 104 (and possibly a redundancy cut-off member 105),
• a fourth branch formed by a superconducting current limiter 106 in series with a resistor R ' ₁ .

The superconducting limiter 106 is composed of a superconducting wire formed by a plurality of superconducting elementary filaments integrated in a resistive matrix, the superconducting wire may also consist of the deposition of a superconductive material on a resistive substrate; we will return in the following description on the choice of material for the realization of the resistive matrix.

The limiter 106 is characterized by two currents: the tripping current of the limitation I _o and the recovery current I _r .

The trip current represents the current beyond which the limiter develops a significant resistance that limits the current. This current is close to the critical current I _c characteristic of the superconducting material and is defined by the current for which the driver develops a given electric field (10 μV / m or 100 μV / m).

The recovery current is the thermal balance current of the driver with his environment. This current is reached after a long enough time (of the order of a few seconds) and is not a conventional parameter of a limiter. It is defined by the characteristics of the conductor, in particular its resistance per unit length, and the cooling conditions (thickness of insulation surrounding the limiter and thermal conductivity of the limiter).

• La charge (ou décharge) lente de l'aimant 102 constitue un premier mode de fonctionnement qui peut être très long (plusieurs heures). Pendant ce mode de fonctionnement, les organes de coupure 104 et 105 sont fermés. Au début de ce fonctionnement, le limiteur 106 transite vers son état haute résistance et le courant s'établit rapidement à son courant de récupération I_r. La puissance dissipée dans le limiteur est égale à |V_o|I_r où V_o est la tension de charge ou de décharge. Le dimensionnement du limiteur, en particulier son isolation thermique permet d'adapter son courant de récupération et d'ajuster la puissance dissipée dans les phases de montée et de descente du courant. A titre d'exemple, en considérant un aimant présentant une inductance de L'=300 H sous une tension de V_o=10V et si on souhaite atteindre un courant nominal I_N=1500A, on a en faisant l'approximation que la montée en courant dans l'aimant est linéaire: $$V_0=L\frac{d{I}_N}{dt}\Rightarrow \mathrm{\Delta }t=\frac{{\mathit{LxI}}_N}{V_0}=12,5\mathrm{heures}\mathrm{.}$$
  
  
  Durant cette période, il est important que le limiteur 106 n'échange pas trop d'énergie avec le bain d'hélium du cryostat dans lequel il se trouve.
• Une deuxième phase de fonctionnement est formée par le mode nominal ou mode normal de fonctionnement. Dans ce cas, les organes de coupure 104 et 105 sont fermés et ce mode de fonctionnement correspond au régime établi de courant dans l'aimant 102. La source d'alimentation 103, reste connectée à l'aimant 102. Le courant ΔI qui traverse le limiteur 106 est une faible fraction du courant opérationnel I_op traversant la bobine L'. Ce courant ΔI est fonction du rapport R'₂/R'₁. En première approximation, on a ΔI= (R'₂/R'₁)I_op. Bien entendu, ΔI doit être inférieur au courant de déclenchement I_o du limiteur 106 pour que le limiteur 106 présente une résistance basse.
• Le troisième mode de fonctionnement concerne la décharge rapide de l'aimant, en cas de transition du type quench. Cette phase assure la protection de l'aimant lorsqu'on vide l'aimant de son courant dans la résistance de protection R'₃. Dans ce cas, l'un au moins des organes de coupure 104 ou 105 est ouvert. Ce mode est caractérisé par une tension élevée (plusieurs centaines à milliers de Volts) aux bornes de l'aimant pour décharger rapidement le courant et ainsi limiter la montée en température du conducteur supraconducteur qui se trouve à l'état normal. L'aimant se décharge alors dans la résistance de protection R'₃. Durant cette phase, le limiteur 106 développe automatiquement et naturellement une résistance élevée et limite le courant dans la quatrième branche comportant la résistance R'₁ à une valeur bien inférieure au courant qui circule dans la résistance de protection R'₃. Cette phase est sensible car la protection de l'aimant en dépend. Le limiteur 106 présente une caractéristique très sûre de ce point de vue puisque le pire défaut pour le limiteur est sa destruction qui conduit à une résistance équivalente infinie et donc à une protection de l'aimant. Même si le temps de décharge est ici beaucoup plus court (de l'ordre de quelques minutes) que le temps de charge mentionné plus haut en référence au premier mode de fonctionnement, la tension appliquée aux bornes du limiteur 106 est beaucoup haute et entraîne une température du limiteur 106 beaucoup plus élevée que dans le mode de charge.

Three phases of operation of the limiter can be distinguished for the application concerned:

• The slow charging (or discharging) of the magnet 102 constitutes a first mode of operation that can be very long (several hours). During this operating mode, the breaking members 104 and 105 are closed. At the beginning of this operation, the limiter 106 transits to its high resistance state and the current is rapidly established at its recovery current I _r . The power dissipated in the limiter is equal to | V _o | I _r where V _o is the charging or discharging voltage. The dimensioning of the limiter, in particular its thermal insulation, makes it possible to adapt its recovery current and to adjust the power dissipated in the rise and fall phases of the current. By way of example, considering a magnet having an inductance of L '= 300 H under a voltage of V _o = 10V and if it is desired to achieve a nominal current I _N = 1500A, it is approximated that the rise when running in the magnet is linear: $$V_0=\mathrm{The}\frac{d{I}_{\mathrm{NOT}}}{dt}\Rightarrow \mathrm{\Delta }t=\frac{{\mathit{LXI}}_{\mathrm{NOT}}}{V_0}=12,5\mathrm{hours}\mathrm{.}$$
  
  
  During this period, it is important that the limiter 106 does not exchange too much energy with the helium bath of the cryostat in which it is located.
• A second phase of operation is formed by the nominal mode or normal mode of operation. In this case, the cut-off members 104 and 105 are closed and this mode of operation corresponds to the established current regime in the magnet 102. The power source 103 remains connected to the magnet 102. The current ΔI which passes through the limiter 106 is a small fraction of the operational current I _op through the coil L '. This current ΔI is a function of the ratio R ' ₂ / R' ₁ . As a first approximation, we have ΔI = (R ' ₂ / R' ₁ ) I _op . Of course, ΔI must be lower than the tripping current I _o of the limiter 106 so that the limiter 106 has a low resistance.
• The third mode of operation concerns the fast discharge of the magnet, in case of transition of the quench type. This phase ensures the protection of the magnet when the magnet is emptied of its current into the protection resistor R ' ₃ . In this case, at least one of the organs cutoff 104 or 105 is open. This mode is characterized by a high voltage (several hundreds to thousands of volts) at the terminals of the magnet to quickly discharge the current and thus limit the rise in temperature of the superconducting conductor which is in the normal state. The magnet then discharges into the protection resistor R ' ₃ . During this phase, the limiter 106 automatically and naturally develops a high resistance and limits the current in the fourth branch comprising the resistor R ' ₁ to a much lower value than the current flowing in the protection resistor R' ₃ . This phase is sensitive because the protection of the magnet depends on it. The limiter 106 has a very reliable characteristic from this point of view since the worst fault for the limiter is its destruction which leads to an infinite equivalent resistance and therefore to a protection of the magnet. Even if the discharge time here is much shorter (of the order of a few minutes) than the charging time mentioned above with reference to the first mode of operation, the voltage applied across the limiter 106 is very high and leads to limiter temperature 106 much higher than in the charging mode.

• Etape 1 : on définit la valeur de la résistance R'₁ en fonction de la valeur de la résistance résiduelle R'₂ de l'aimant dans un rapport de 10 à 1000.
• Etape 2 : de façon à ne pas faire transiter le limiteur 106 vers une impédance haute lors du mode de fonctionnement normal, on choisit un fil supraconducteur présentant un courant critique I_c supérieur à (R'₂/R'₁)I_op.
• Etape 3 : comme nous l'avons déjà mentionné plus haut, la température maximale T_max vue par le limiteur 106 se produit pendant la phase de décharge rapide de l'aimant 102 dans la résistance de protection R'₃. Le dimensionnement du limiteur 106 implique le choix de cette température maximale admissible, T_max, sur le limiteur 106 en cas de décharge de l'aimant 102.
• Etape 4 : Il est important que le limiteur 106 n'échange pas trop d'énergie avec le bain d'hélium, notamment pendant les opérations de chargement et déchargement de l'aimant 102 avec l'alimentation 103. Dès lors, le dimensionnement du limiteur 106 implique également le choix de la puissance maximale admissible sur le bain cryogénique, W_max, lors des opérations de chargement et déchargement de l'aimant.
• Etape 5 : Cette étape vise à calculer la longueur de fil strictement nécessaire pour maintenir le fil à une température inférieure à la température T_max fixée à l'étape 3 (lors de la décharge rapide de l'aimant 102). La tension aux bornes de l'aimant 102, U(t), et donc du limiteur 106, est fournie par la relation suivante : $U\left(t\right)=U_0{e}^{-\frac{t}{\tau }}$
  
  où τ est une constante de temps caractéristique de décharge donnée par le rapport L'/R'₃. Dans une hypothèse adiabatique où on néglige tout transfert de chaleur entre le limiteur 106 et le bain d'hélium, la chaleur produite par effet joule est absorbée par le fil lui-même. Par ailleurs, en supposant que la vitesse du front résistif qui fait transiter le fil supraconducteur est infinie, on obtient la relation suivante : $$\frac{{\left(U_0{e}^{\frac{-t}{\tau }}\right)}^2}{\frac{l}{S}\rho \left(T\right)}={\mathit{SlC}}_p\left(T\right)\frac{\mathit{dT}}{\mathit{dt}}$$
  
  
  où I, S, C_p et ρ sont respectivement la longueur, la section, la chaleur spécifique volumique et la résistivité du fil avec ses brins supraconducteurs et sa matrice. Ainsi, si on néglige de façon très conservative l'échange thermique entre le bain d'hélium et le limiteur 106, la longueur maximale de fil qu'il est avantageux de donner au limiteur est donnée par la formule suivante (obtenue en intégrant la relation précédente) : $$l=U_0\sqrt{\frac{\mathrm{<figure-callout\; id="2"\; label="\tau "\; filenames="imgf0001.png,imgf0005.png"\; state="\{\{state\}\}">\tau </figure-callout>}}{2{\int }_{T_{\mathit{H}\mathit{e}}}^{T_{\max }}{C}_p\left(T\right)\rho \left(T\right)dT}}$$
  
  
  Il convient bien entendu de noter que le calcul précédent donne une valeur maximale de la longueur de fil (liée à l'hypothèse adiabatique) ; une longueur de fil plus faible permet donc également de répondre aux exigences en température. Une longueur plus importante est également possible du point de vue technique, mais peu intéressante du point de vue économique.
• Etape 6 : cette étape vise à déterminer l'isolation thermique nécessaire sur le limiteur 106 pour limiter la puissance déposée sur le bain. Cette isolation est caractérisée par le flux thermique par unité de longueur de fil, w_isolation, entre le bain d'hélium et le limiteur 106 une fois le régime stationnaire établi. Lors des charges et décharges de l'aimant 102, la tension aux bornes du limiteur est constante et imposée par l'alimentation 103, U_Alim. L'équilibre thermique entre le bain et le limiteur s'écrit donc $$\frac{{\mathit{U}}_{\mathit{A}\lim }^{\mathrm{2}}}{{\mathit{R}}_{\lim }{\mathit{l}}_{\mathit{trans}}}\mathit{=}{\mathit{w}}_{\mathit{isolation}}{\mathit{l}}_{\mathit{trans}}\mathit{=}{\mathit{W}}_{\max }$$
  
  
  où R_lim est la résistance par unité de longueur de fil et I_trans est la longueur de fil transité dans le limiteur une fois l'équilibre thermique atteint. Pour une tension d'alimentation fixée, la longueur de fil transité est donc imposée par l'isolation. La nature et l'épaisseur de l'isolation peuvent donc être ajustées de sorte que la puissance déposée sur le bain soit inférieure à W_max.

The three modes of operation described above make it possible to define a method of dimensioning the limiter 106 comprising the following steps:

• Step 1: the value of the resistance R ' ₁ is defined as a function of the value of the residual resistance R' ₂ of the magnet in a ratio of 10 to 1000.
• Step 2: so as not to pass the limiter 106 to a high impedance in the normal operating mode, we choose a superconducting wire having a critical current I _c greater than (R ' ₂ / R' ₁ ) I _op .
• Step 3: As already mentioned above, the maximum temperature T _max seen by the limiter 106 occurs during the rapid discharge phase of the magnet 102 in the protection resistor R ' ₃ . The dimensioning of the limiter 106 implies the choice of this maximum permissible temperature, T _max , on the limiter 106 in the event of discharge of the magnet 102.
• Step 4: It is important that the limiter 106 does not exchange too much energy with the helium bath, especially during the loading and unloading operations of the magnet 102 with the feed 103. Therefore, the dimensioning of the limiter 106 also implies the choice of the maximum permissible power on the cryogenic bath, W _max , during the loading and unloading of the magnet.
• Step 5: This step aims to calculate the length of wire strictly necessary to maintain the wire at a temperature below the temperature T _max set in step 3 (during the rapid discharge of the magnet 102). The voltage at the terminals of the magnet 102, U (t), and therefore of the limiter 106, is provided by the following relation: $U\left(t\right)=U_0{e}^{-\frac{t}{\tau }}$
  
  where τ is a discharge characteristic characteristic given by the ratio L '/ R' ₃ . In an adiabatic hypothesis in which all transfer of heat between the limiter 106 and the helium bath is neglected, the heat produced by joule effect is absorbed by the wire itself. Moreover, assuming that the speed of the resistive edge that passes the superconducting wire is infinite, we obtain the following relation: $$\frac{{\left(U_0{e}^{\frac{-t}{\tau }}\right)}^2}{\frac{l}{S}\rho \left(T\right)}={\mathit{SLC}}_p\left(T\right)\frac{\mathit{dT}}{\mathit{dt}}$$
  
  
  where I, S, C _p and ρ are respectively the length, the section, the volume specific heat and the resistivity of the wire with its superconducting strands and its matrix. Thus, if the thermal exchange between the helium bath and the limiter 106 is very conservatively neglected, the maximum length of wire which it is advantageous to give to the limiter is given by the following formula (obtained by integrating the relationship previous) : $$l=U_0\sqrt{\frac{\mathrm{<figure-callout\; id="2"\; label="\tau "\; filenames="imgf0001.png,imgf0005.png"\; state="\{\{state\}\}">\tau </figure-callout>}}{2{\int }_{T_{\mathit{H}\mathit{e}}}^{T_{\max }}{\mathrm{VS}}_p\left(T\right)\rho \left(T\right)dT}}$$
  
  
  It should of course be noted that the above calculation gives a maximum value of the length of yarn (related to the adiabatic hypothesis); a shorter wire length therefore also meets the requirements in temperature. A longer length is also possible from the technical point of view, but unattractive from an economic point of view.
• Step 6: This step aims to determine the necessary thermal insulation on the limiter 106 to limit the power deposited on the bath. This insulation is characterized by the thermal flux per unit length of wire, w _insulation , between the helium bath and the limiter 106 once the steady state established. During the charges and discharges of the magnet 102, the voltage across the limiter is constant and imposed by the supply 103, U _Alim . The thermal equilibrium between the bath and the limiter is therefore written $$\frac{{\mathit{U}}_{\mathit{AT}\lim }^{\mathrm{2}}}{{\mathit{R}}_{\lim }{\mathit{l}}_{\mathit{trans}}}\mathit{=}{\mathit{w}}_{\mathit{insulation}}{\mathit{l}}_{\mathit{trans}}\mathit{=}{\mathit{W}}_{\max }$$
  
  
  where R _lim is the resistance per unit length of wire and I _trans is the length of wire passed through the limiter once thermal equilibrium is reached. For a fixed supply voltage, the transited wire length is therefore imposed by the insulation. The nature and thickness of the insulation can therefore be adjusted so that the power deposited on the bath is less than W _max .

This relationship also shows that it is the voltage supplied by the power supply that keeps the limiter open, I _trans non-zero, during charging and discharging operations.

Placing the current limiter 106 in the liquid helium causes the limiter to consist, for example, of a superconducting wire formed by a plurality of niobium-titanium elementary filaments (NbTi) whose transition temperature is equal to 9. , 5K if it is subjected to a zero magnetic induction and whose diameter is preferably less than 120 microns integrated in a resistive matrix. The resistive matrix is preferably highly resistive so as to reduce the length of wire (as mentioned above, the maximum wire length is inversely proportional to the resistivity of the wire and its matrix): a highly resistive matrix allows therefore reduce the size of the limiter. The matrix may for example be made of cupronickel (CuNi). Choosing a highly resistive matrix also makes it possible to accelerate the superconducting transition and to have a high resistance after transition. Indeed, the resistivity of cupronickel is very high (about 0.4 × 10 ^-6 Ω.m) in comparing a copper matrix (10 ^-10 Ω.m to 4.2 K) for example, the limitation will be improved.

It is also possible to place the limiter at a higher temperature and use in this case a superconducting material of the high Tc (higher critical temperature) type such as magnesium diboride (MgB ₂ ) or a ceramic superconductor, such as YBaCuO for example.

The presence of the current limiter 106 near the magnet 102 which aims to be as stable as possible requires the limiter to have the lowest inductance possible so that the current variations flow well in the limiter branch. and not in the magnet. In addition, the lower the inductor of the limiter 106 will be the faster the current limitation. The length of the wire must therefore be arranged in such a way that the inductance of the limiter 106 is as small as possible in order to have a reduced response time, not to induce overvoltages and to ensure good stabilization. One solution is to use a winding in two layers, the two layers being wound in the opposite direction (two coils of the same length nested one inside the other and separated by an insulator to prevent dielectric breakdown between the two coils).

In a first configuration, the two layers are paralleled at each end: this configuration is interesting because it distributes the voltage over a large distance (the distance between the two ends) and avoids dielectric breakdown.

According to a second configuration, the two layers are put in series.

We will apply in the following the sizing process described above to a superconducting magnet producing a field of 7 teslas formed of a superconducting winding of Niobium-Titanium (integrated in a copper matrix) bathed in liquid helium at atmospheric pressure (ie at a temperature of 4.2K). The following numerical values (data at the temperature of 4.2 K) used are given in Table 1 below: Table 1 Nominal current of the magnet I _op 400 A Residual resistance of the magnet R ' ₂ 10μΩ Inductance of the magnet The 0.68 H Resistance of protection R ' ₃ 0,5Ω

• Etape 1 : Choix de la résistance de stabilisation R'₁ à 1 mΩ pour assurer un rapport R1/R2 de 100.
• Etape 2 : Choix d'un fil supraconducteur de diamètre 0,2 mm non isolé composé de filaments supraconducteurs en NbTi de 30 µm de diamètre dans une matrice en CuNi avec 30% de Ni en poids. Le rapport de la section de Cu sur la section de NbTi est de 1.2 ce qui permet d'assurer un courant critique supérieur à (R'₂/R'₁)I_op, soit 4 A.
• Etape 3 : Choix de la température maximale admissible T_max à 100 K.
• Etape 4 : Choix de la puissance maximale admissible sur le bain cryogénique W_max à 1 W.
• Etape 5 : En appliquant la relation 1, on trouve une longueur maximale de fil nécessaire d'environ 250 m. Comme nous l'avons déjà précisé, cette valeur est fortement majorée ; ainsi, des essais démontrent qu'une longueur 50 m est suffisante.
• Etape 6 : Le limiteur est isolé du bain hélium par exemple avec une résine isolante (d'époxy par exemple) présentant une épaisseur de 1 mm. Si cela est nécessaire, l'épaisseur de la couche isolante peut être augmentée afin de diminuer la puissance dissipée à une valeur inférieure à la valeur seuil W_max souhaitée en régime stationnaire avec le limiteur dans son état haute impédance.

By rolling out the six steps mentioned above:

• Step 1: Choice of the stabilizing resistor R ' ₁ to 1 mΩ to ensure a ratio R1 / R2 of 100.
• Step 2: Choice of an uninsulated 0.2 mm diameter superconducting wire composed of NbTi superconducting filaments of 30 μm diameter in a CuNi matrix with 30% Ni by weight. The ratio of the Cu section to the NbTi section is 1.2, which makes it possible to ensure a critical current greater than (R ' ₂ / R' ₁ ) I _op , ie 4 A.
• Step 3: Choice of the maximum permissible temperature T _max at 100 K.
• Step 4: Choice of the maximum permissible power on the cryogenic bath W _max at 1 W.
• Step 5: Applying the relation 1, we find a maximum length of wire required of about 250 m. As we have already stated, this value is strongly increased; thus, tests demonstrate that a length of 50 m is sufficient.
• Step 6: The limiter is isolated from the helium bath for example with an insulating resin (epoxy for example) having a thickness of 1 mm. If necessary, the thickness of the insulating layer may be increased in order to reduce the power dissipated to a value lower than the desired threshold value W _max in steady state with the limiter in its high impedance state.

It should be noted that the invention applies both to a configuration in which the magnet 102 and the limiter 106 are in the same cryogenic bath as to a configuration in which the magnet 102 and the limiter 106 are in baths. separated; in the latter case, one possible application consists in using two helium baths, one containing superfluid helium at a temperature of between 1.7 and 2.2 K (of the order of 1.8 K) for the purpose of the magnet 102 and the other containing 4.2 K liquid helium, the two baths being interconnected by a channel of reduced section according to the principle of "Bain Claudet". Such a configuration allows easier access to the limiter 106 separated from the magnet 102.

In the case of certain applications in MRI or NMR, it may sometimes be necessary to open the limiter in a controlled manner, for example to adjust the current flowing in the magnet. With a limiter without opening control, this adjustment could cause a problem because it would be necessary to increase the current in the limiter to the limit limiting current which is then injected into the magnet, then adjust the value current once the limiter is open. For magnets sized very close to their critical values, or whose protection is sensitive to rapid changes in the current, such a constraint can be prohibitive.

A first solution is to add a heater to temporarily put the limiter in "open" mode, without degrading the safety related to the intrinsic operation of the limiter.

Advantageously, a second solution consists in injecting, via the current leads of the magnet (into the coil of the magnet and the protection branches situated in parallel between the breaking members 104 and 105), a sinusoidal or impulsive alternating current which superimposes itself on the operating current. The frequency of this current is chosen high enough that the alternating current is blocked by the inductance L 'of the coil, so that the latter does not receive a thermal energy capable of passing it out of the superconducting state. The frequency may for example be chosen so that more than 99.9% of this alternating current passes through the limiter. The transition of the limiter from its low impedance state to its high impedance state is obtained either by the temperature rise caused by the circulation of the alternating current (elevation created by the losses induced by the alternating current) or because the rms value of the current alternative exceeds the value of the tripping current of the limiter. In practice, a frequency equal to or greater than 50 Hz is sufficient for known applications. This alternating current can be generated by specific internal circuits designed for this purpose, or externally to the supply by a secondary power supply located from preferably in parallel with the main power supply. However, it is not contrary to the invention to provide this secondary supply by a device placed in series with the main power supply. An example of a system 200 for creating a magnetic field according to the invention incorporating a control device 201 generating such a signal is illustrated in FIG. figure 4 .

The system 200 is identical to the system 100 of the figure 3 with the difference that it comprises the control device 201 forming means for the tilting of the limiter 106 from its low resistance state to its high resistance state by enabling the generation of a sinusoidal current signal able to switch the limiter 203 and it does not include a second redundancy cutoff member 105.

• un transformateur abaisseur de tension TBT (Très Basse Tension) 205 dont le courant de court circuit Icc est supérieur au courant nécessaire pour faire transiter le limiteur 106 et la tension de sortie suffisante pour maintenir le limiteur 106 dans l'état résistif compte tenu des résistances des lignes (Icc = 38 A avec Ucc = 0,80 V),
• un autotransformateur variable 204 permettant d'ajuster la tension du réseau (230 V) pour obtenir ces deux valeurs de courant de court-circuit et de tension de sortie suffisante pour maintenir le limiteur 106 dans son état résistif,
• un interrupteur 203 permettant de connecter le dispositif de commande 201 au circuit de l'aimant pendant la phase opératoire.
• un interrupteur 202 permettant de connecter le dispositif de commande 201 au réseau électrique 230V/50Hz (ou 115V/60Hz) pour sa mise en service.

The control device 201 comprises:

• a voltage-reducing transformer TBT (Very Low Voltage) 205 whose short-circuit current Isc is greater than the current required to pass the limiter 106 and the output voltage sufficient to maintain the limiter 106 in the resistive state given the resistors lines (Icc = 38 A with Ucc = 0.80 V),
• a variable autotransformer 204 for adjusting the mains voltage (230 V) to obtain these two values of short circuit current and sufficient output voltage to maintain the limiter 106 in its resistive state,
• a switch 203 for connecting the controller 201 to the circuit of the magnet during the operating phase.
• a switch 202 for connecting the control device 201 to the 230V / 50Hz mains (or 115V / 60Hz) for commissioning.

We will illustrate the operation of the control device 201 in the case of a superconducting magnet of inductance L '= 0.68 H giving a nominal magnetic field of 7 T for a current of 400 A. A resistance R' ₂ of 10 μΩ (resistance simulating the resistive connections of a superconducting magnet) is connected in series with the coil L '.

With the switch 203 closed, the control device 201 is put into operation by the closing of the switch 202 (connection to the 230V / 50Hz network).

In a first phase, the autotransformer 204 is set to the voltage of 230 V.

Since 2πfL 'is much greater than the resistor of the limiter 106, the current flows only in the mesh of the limiter 106 until the transition thereof. Note that in the example cited here, the limiter 106 transits because the short circuit current Icc (corresponding to the rms value of the sinusoidal current supplied by the TBT transformer 205) is greater than the tripping current required to pass the limiter 106. However, it is also possible to control the opening of the limiter 106 if a voltage supplied by the TBT transformer 205 is chosen such that the current flowing in the limiter does not cause the trip current to be exceeded, but a rise in temperature going to beyond the critical temperature used to switch the limiter 106: such a solution requires working at higher operating frequencies.

In a second phase, the limiter 106 being resistive, the current passing through it is low (a few tens of mA) and the voltage necessary to maintain the transient limiter 106 is thus a few volts (about 1 V at the output of the autotransformer 205) . This voltage will circulate a current of about 2 A in the discharge resistor R ' ₃ and a very low alternating current in the mesh of the coil inversely proportional to its inductance L'. This alternating current does not modify the main DC current in the coil.

In a third phase, it is possible to increase (or decrease) the main current in the coil by modifying the current supplied by the power supply 103. During this phase, the switch 203 is either closed to keep the limiter 106 open or open (in in this case, the current which keeps the limiter 106 open is supplied by the power supply 103 for the time necessary for the modification of the current). The open switch 203 makes it possible to make current adjustments without being disturbed by the alternating signals.

In a fourth phase, as soon as the necessary adjustments have been made, the limiter 106 becomes again superconducting following the opening of the switch 203. In fact, without external energy input, the limiter 106 finds the temperature of the cryogenic bath typically after a few seconds. The return time in the closed state depends above all on the level of thermal insulation between the limiter and the cryogenic bath.

It should be noted that the example above relates to a sinusoidal signal but that other types of alternative signals (square - triangular - pulsed, ...) can also be used.

It is also possible to directly use the main power supply 103 to generate a current pulse of a few milliseconds at a current value greater than the tripping current of the limiter 106 sufficient to make the latter pass therethrough.

The figure 5 illustrates the implementation of such a command on a circuit 300 substantially identical to the circuit 100 of the figure 3 (With the difference that it does not have a switch 105).

Circuit 300 presented in figure 5 is composed of a superconducting magnet of 0.68 H inductance giving a nominal magnetic field of 7 T for a current I ₂ of 400 A. The resistor R ' ₂ simulating the resistive connections of the superconducting magnet and connected in series with the coil L 'has a value of 10 μΩ. A current regulated power supply 103 (1000A - 10V) is connected to the load by closing the switch 104.

As we have already explained with reference to the figure 1 to protect the magnet, the resistor R ' ₃ (of a value here equal to 0.5 Ω) is connected in parallel with the branch of the magnet. In the case of the circuit 300, the resistor R ' ₃ is inside the cryostat C. During a serious problem, the switch 104 is open causing the fast discharge of the energy of the magnet in the resistor protection R ' ₃ . The limiter 106 and the stabilizing resistor R ' ₁ (here equal to 1 mΩ) are connected in parallel to the magnet. The current I ₁ in this branch must be such that R ' ₂ .I ₂ = R' ₁ .I ₁ in steady state, where I ₂ denotes the current flowing in the inductance L '.

The power supply 103 comprises means for generating a current pulse for a sufficient duration (here> 5 ms) and amplitude I _p (here> 40 A) greater than the tripping current making it possible to switch the limiter 106 from its low resistance state to its high resistance state. One solution for generating this pulse is to intervene in the control loop of the power supply 103. It is also possible to use an auxiliary power supply for generating this pulse.

The power supply 103 regulated current, generates a current ramp (with a di / dt here chosen between 2 and 10 A / s). A minimum ramp value is imposed so that the voltage U _c at the terminals of the magnet is sufficient to maintain the limiter 106 in its resistive mode.

où R'₁O ≈ 10 Ω désigne la résistance du limiteur supraconducteur 106 dans son état haute résistance.In steady state we can write the following relation: $${\mathrm{U}}_{\mathrm{vs}}\mathrm{=}{\mathrm{<figure-callout\; id="2"\; label="L\text{'}dl"\; filenames="imgf0001.png,imgf0005.png"\; state="\{\{state\}\}">L\text{'}dl</figure-callout>}}_{\mathrm{2}}\mathrm{/}\mathrm{dt}\mathrm{+}{\mathrm{R\text{'}}}_{\mathrm{2}}{\mathrm{<figure-callout\; id="2"\; label="l"\; filenames="imgf0001.png,imgf0005.png"\; state="\{\{state\}\}">l</figure-callout>}}_{\mathrm{2}}\mathrm{=}{\mathrm{R\text{'}}}_{\mathrm{3}}{\mathrm{<figure-callout\; id="3"\; label="l"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">l</figure-callout>}}_{\mathrm{3}}\mathrm{=}\left({\mathrm{R\text{'}}}_{\mathrm{1}}\mathrm{+}{\mathrm{R\text{'}}}_{\mathrm{1}}\mathrm{O}\right){\mathrm{<figure-callout\; id="1"\; label="l"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">l</figure-callout>}}_{\mathrm{1}}$$

where R ' ₁ O ≈ 10 Ω designates the resistor of the superconducting limiter 106 in its high resistance state.

$${\mathrm{l}}_{\mathrm{3}}\mathrm{=}\mathrm{1}\mathrm{,}\mathrm{36}\mathrm{/}\mathrm{0}\mathrm{,}\mathrm{5}\mathrm{=}\mathrm{2}\mathrm{,}\mathrm{72}\mathrm{A}$$

$${\mathrm{l}}_1\mathrm{=}\mathrm{1}\mathrm{,}\mathrm{36}\mathrm{/}10\mathrm{=}0\mathrm{,}136\mathrm{A}\mathrm{.}$$

Thus, for a value of di / dt of 2 A / s, the following values are obtained: $${\mathrm{U}}_{\mathrm{vs}}\mathrm{\approx }\mathrm{0}\mathrm{,}\mathrm{68}\mathrm{x}\mathrm{2}\mathrm{=}\mathrm{1}\mathrm{,}\mathrm{36}\mathrm{<figure-callout\; id="3"\; label="V\; l"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">V\; </figure-callout>}$$

$${\mathrm{l}}_{}$$$${\mathrm{}}_{\mathrm{3}}\mathrm{=}\mathrm{1}\mathrm{,}\mathrm{36}\mathrm{/}\mathrm{0}\mathrm{,}\mathrm{5}\mathrm{=}\mathrm{2}\mathrm{,}\mathrm{72}\mathrm{AT}$$

$${\mathrm{<figure-callout\; id="1"\; label="l"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">l</figure-callout>}}_1\mathrm{=}\mathrm{1}\mathrm{,}\mathrm{36}\mathrm{/}10\mathrm{=}0\mathrm{,}136\mathrm{AT}\mathrm{.}$$

• d'une part des phases transitoires de durées d'autant plus importantes que l'inductance L' de l'aimant est élevée et,
• d'autre part le risque que la tension U_c soit trop faible pendant les premières secondes pour maintenir le limiteur 106 en mode résistif et ce particulièrement pour des aimants de grande inductance.

It should be noted that, just after the switching of the limiter 106 in its resistive state, the current will essentially switch in the resistor R ' ₃ . As a result, the rise in current I ₂ in the magnet is established with a ramp of time constant close to L '/ R' ₃ (ie there is a certain delay before the current I ₂ in the magnet catches the current ramp delivered by the power supply 103). Similarly, at the end of the ramp, the current ends up in the magnet with the same time constant. Such a behavior of the current, I ₂ can cause two disadvantages:

• on the one hand, transient phases of durations which are all the more important because the inductance L 'of the magnet is high and,
• on the other hand the risk that the voltage U _c is too low during the first seconds to maintain the limiter 106 in resistive mode and particularly for magnets of high inductance.

An effective solution to overcome these drawbacks is to generate a current slot Ic through the power supply 103 immediately after the Ip pulse of switching limiter 106, the value of Ic being chosen such that Ic = I ₃ + I ₁ . This current slot will have the same duration as the ramp up (ie corresponding to the adjustment time of the magnet).

• réaliser le chargement de l'aimant avec une tension constante aux bornes de l'aimant avec une alimentation régulée en tension et une valeur de di/dt inférieure à 10 A/s. Cette solution nécessite une alimentation spécifique pour le chargement et n'est pas particulièrement adaptée à des petits ajustements en courant.
• monter une diode en opposition en série avec R'₃ ce qui permet d'annuler le courant I₃. Dans ce cas, le circuit électrique n'est plus symétrique et ne fonctionne pour des descentes en courant.

Two other solutions can also be used:

• perform the charging of the magnet with a constant voltage across the magnet with a regulated voltage supply and a value of di / dt less than 10 A / s. This solution requires a specific power supply for charging and is not particularly suitable for small adjustments in current.
• to mount a diode in opposition in series with R ' ₃ which makes it possible to cancel the current I ₃ . In this case, the electric circuit is no longer symmetrical and does not work for current descents.

• on commence par annuler le courant de stabilisation I₁ en passant la consigne de l'alimentation 103 à 400 A ;
• après quelques secondes (typiquement 2 s), on fixe une nouvelle consigne de courant de 410 A ;
• on génère impulsion de 40 A pendant quelques millisecondes (typiquement 10 ms) pour rendre le limiteur 106 résistif ;
• on génère immédiatement après l'impulsion un créneau en courant à une valeur de courant Ic telle que Ic = I₃ + I₁ = 2,72 + 0,14 = 2,86 A
• dès que le courant de l'alimentation 103 atteint 410 A (typiquement après 5 s), on arrête le créneau de courant (Ic = 0). Le courant dans l'aimant est alors de 410 A et les courants I₁ et I₃ sont quasiment nuls (< 10 mA). Le limiteur 106 se refroidit et redevient supraconducteur en quelques secondes, recouvrant ainsi son état basse résistance.
• après quelques secondes, on injecte un courant de stabilisation I₁ égal à 4,1 A dans la résistance R '₁ choisi de sorte que R '₁I₁ = R '₂I₂, l'injection se faisant par une nouvelle consigne donnée à l'alimentation à 414,1 A.

We will describe in the following the steps allowing the passage (ie adjustment) of a current of 400 A to a current of 410 A in the magnet, the current ramp always being 2 A / s:

• the stabilizing current I _{1 is first} canceled by passing the setpoint of the supply 103 to 400 A;
• after a few seconds (typically 2 s), a new current set point of 410 A is set;
• a pulse of 40 A is generated for a few milliseconds (typically 10 ms) to make the limiter 106 resistive;
• immediately after the pulse is generated a current slot at a current value Ic such that Ic = I ₃ + I ₁ = 2.72 + 0.14 = 2.86 A
• as soon as the current of the supply 103 reaches 410 A (typically after 5 s), the current slot (Ic = 0) is stopped. The current in the magnet is then 410 A and currents I ₁ and I ₃ are almost zero (<10 mA). The limiter 106 cools and becomes superconductive again in a few seconds, thus covering its low resistance state.
• after a few seconds, a stabilization current I ₁ equal to 4.1 A is injected into the resistor R ' ₁ selected so that R' ₁ I ₁ = R ' ₂ I ₂ , the injection being done by a new instruction given at the feed at 414.1 A.

1. 1. à t=2s (valeur purement illustrative correspondant au départ de la rampe), on fixe une consigne de chargement de 30 A à l'alimentation : on observe donc le démarrage d'une rampe de courant pour la courbe d'alimentation.
2. 2. Le limiteur étant passant, il a l'impédance la plus faible du circuit ; le courant s'écoule donc dans sa branche et la courbe du limiteur suit la rampe de courant de l'alimentation.
3. 3. On envoie alors une impulsion de courant (35 A) sur l'alimentation qui dépasse le courant de déclenchement du limiteur. On observe que l'impulsion est également vue par le limiteur.
4. 4. Le limiteur passe en mode résistif et le courant bascule essentiellement dans la résistance de protection R'₃. La montée en courant dans l'aimant s'établit avec une rampe de constante de temps proche de L'/R'₃. Cette phase transitoire peut être évitée en utilisant un créneau de courant tel qu'évoqué plus haut. Le courant dans l'aimant rattrape ensuite la rampe de courant délivrée par l'alimentation.
5. 5. Durant toute la poursuite de la rampe de courant, le limiteur reste en mode résistif car une tension est maintenue à ses bornes et le chargement de l'aimant se poursuit donc normalement.
6. 6. Une fois arrivé à la consigne de courant souhaitée dans l'aimant, le limiteur redevient passant (non représenté sur la figure 6).

As an illustration, a rise from 0 to 30 A (the principle would be identical from 400 to 410 A) was performed experimentally by applying the steps outlined above (without the slot generation step). This climb is illustrated on the figure 6 which represents the evolution as a function of time respectively of the current of the power supply 103, the current in the magnet and the current in the limiter 106. The current and time scales are the same for the three curves. We can distinguish the following stages:

1. 1. at t = 2s (purely illustrative value corresponding to the start of the ramp), a charging setpoint of 30 A is set at the power supply: the start of a current ramp for the supply curve is thus observed.
2. 2. The limiter being passing, it has the weakest impedance of the circuit; the current flows in its branch and the limiter curve follows the current ramp of the power supply.
3. 3. A current pulse (35 A) is then sent to the supply which exceeds the trip current of the limiter. It is observed that the pulse is also seen by the limiter.
4. 4. The limiter goes into resistive mode and the current switches essentially into the protection resistor R ' ₃ . The rise in current in the magnet is established with a ramp of time constant close to L '/ R' ₃ . This transient phase can be avoided by using a current slot as discussed above. The current in the magnet then catches up with the current ramp delivered by the power supply.
5. 5. During the entire current ramp, the limiter remains in resistive mode because a voltage is maintained at its terminals and the charging of the magnet therefore continues normally.
6. 6. Once arrived at the desired current setpoint in the magnet, the limiter turns on again (not shown on the figure 6 ).

The noise observed on the current measurement in the magnet is related to the measurement noise due to the very low resistance value (R ₂ = 10μΩ) used for the measurement of this current.

Note that in the example given, the delay is important (about 3 seconds) between the beginning of the ramp and the pulse; this delay is only intended to illustrate the operating principle but can be reduced to zero.

Claims (16)

1. A system (100) for creating a magnetic field including:

   - a first branch comprising a superconducting magnet (102) intended to produce said magnetic field, said magnet being formed by a coil inductance (L') in series with a residual resistance (R'2);

   - a second branch comprising a resistance (R'3), said protection resistance,

   - a third branch comprising a power supply (103);

   said system being characterized in that it comprises a fourth branch formed by a resistance (R'1) mounted in series with a current-limiting superconducting device (106) switching from a low-resistance state to a high-resistance state when the current passing therethrough exceeds a breaking current, said superconducting device (106) having an inductance at least 10⁵ times less than that of the coil (L'), and said first, second, third and fourth branches being mounted in parallel,
   said system (100) presenting at least three modes of operation:

   - a first mode of operation, known as charge mode or discharge mode of the magnet, in which:

   o said power source (103) is connected to said magnet (102) so as to increase or reduce the current in the magnet, ○ said current limiter (106) is in its high-resistance state;

   - a second mode of operation, known as normal mode of operation, in which:

   o said power source (103) is connected to said magnet (102),

   o said limiter (106) is in its low-resistance state; - a third mode of operation, known as the rapid discharge mode of the magnet in said protection resistance (R'3), in which:

   o said power source (103) is disconnected from said magnet (102),

   o said limiter (106) is in its high-resistance state; Activation of the state of said limiter (106) in said three operation modes is done in a passive manner without resorting to an external command.
2. The system (100) according to the previous claim characterized in that said resistance (R'₁) mounted in series with said limiter (106) presents a value 10 to 1000 times greater than that of the residual resistance of the magnet (R'2).
3. The system (100) according to one of the previous claims characterized in that the superconductor wire forming said limiter (106) is chosen such that its critical current is greater than (R'2/R'1)Iop where R'2 designates the value of said residual resistance of said magnet, R'₁ designates said resistance mounted in series with said limiter (106) and Iop designates the current circulating in said first branch during said normal operation mode.
4. The system (100) according to one of the previous claims characterized in that the length of the superconductor wire forming said limiter (106) is determined such that the temperature of said superconductor wire always remains less than or equal to a predetermined maximum temperature value T_max.
5. The system according to one of the previous claims characterized in that said limiter is formed by a superconductor wire surrounded by an insulating layer whose thickness is determined such that the power deposited in the cryogenic bath of said limiter is less than a predetermined value.
6. The system according to one of the previous claims characterized in that said limiter is formed by a coil in two layers, the two layers being wound in opposing directions and being placed either in parallel or in series.
7. The system (200) according to one of the previous claims characterized in that the system comprises control means (201) to cause said limiter (106) to switch from its low-resistance state to its high-resistance state.
8. The system according to the previous claim characterized in that said control means are formed by a heating element.
9. The system (200) according to claim 7 characterized in that said control means (201) comprise means (202, 203, 204, 205) for generating a signal of alternating current circulating in said limiter (106) such that said limiter (106) switches from its low-resistance state to its high-resistance state.
10. The system (200) according to claim 9 characterized in that said means (202, 203, 204, 205) to generate an alternating current signal comprise voltage transformer means (204, 205) receiving in input the voltage from the electrical network and providing in output a lowered voltage at the same frequency as the voltage of the electrical network.
11. The system (200) according to one of claims 9 or 10 characterized in that the frequency f of said alternating current signal is chosen sufficiently high so that said alternating current is blocked by the coil inductance (L').
12. The system according to claim 7 characterized in that said control means comprise means for generating a current greater than said breaking current enabling said limiter to be caused to switch.
13. The system according to claim 12 characterized in that said means for generating a current greater than said breaking current enabling said limiter to be caused to switch are formed by means generating a current pulse of a sufficient intensity and duration to cause said limiter to switch.
14. The system according to one of claims 12 or 13 characterized in that said means for generating a current greater than said breaking current enabling said limiter to be caused to switch are integrated into said power source.
15. A method of adjusting the current in a magnet included in a system according to one of claims 13 or 14 comprising the following steps, considered in any order:

    - generation of a current ramp with a setting set to the new current value to be reached in the magnet;

    - generation of a current pulse in which the duration and intensity are such that said limiter switches in its high-resistance state.
16. The method according to the previous claim characterized in that the method comprises a step of generating a current slot that follows the step of generating said current pulse, the value of the current in this slot being equal to the sum of the current circulating in said protection resistance and of the current circulating in said limiter when it is in its high-resistance state.

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