EP0357133B1 - Protection device for neutron tubes - Google Patents

Description

The invention relates to a device for protecting a neutron tube comprising an ion source whose anode is brought to a positive potential with respect to the cathode by means of a source power supply and whose accelerated ion beam strikes a target disposed on an electrical insulating support and brought to a negative potential supplied by a HV power supply, said protection device consisting of elements for electrical limitation of the tube current and / or of the target voltage.

In their current application, neutron tubes operate under conditions compatible with the possibilities of heat dissipation, in particular of the target and its support.

• le fonctionnement en mode continu nécessite un dimensionnement en conséquence des isolements électriques et thermiques,
• le fonctionnement en mode pulsé et à récurrence rapide conduit à des structures d'extraction permettant des courants d'ions relativement élevés.

Their definitions are also provided for operating modes with broad dynamics:

• continuous mode operation requires sizing as a result of electrical and thermal insulation,
• operation in pulsed mode and rapid recurrence leads to extraction structures allowing relatively high ion currents.

The ion sources themselves, often of the Penning type, are provided with fairly large extraction orifices in order to allow a high extraction yield to be obtained at a low operating pressure. Furthermore, they can work in an arc type discharge regime for pressure values still compatible with operation of the tube and the very high voltages applicable on these tubes can make it possible to extract large ion currents for short times.

• augmentation du courant tube au moyen d'un accroissement du courant de décharge de la source d'ions en régime d'arc avec une tension anode plus élevée,
• augmentation possible de la tension cible d'un facteur voisin de 1,5.

All of these considerations show that, in particular for tubes of small dimensions, it is possible to use a neutron tube far beyond its nominal use for, for example, use with intense pulsed neutron flux. This can be achieved by:

• increase of the tube current by means of an increase in the discharge current of the ion source in arc mode with a higher anode voltage,
• possible increase in the target voltage by a factor close to 1.5.

The electrical limitations usually mounted on the supply circuits of the anode of the ion source and the target for the purpose of protecting the supply and the tube can be eliminated and replaced by new elements adapted to a new use.

Such limiter assembly is known in particular for the publication: PHYSICA IV ^No. 11, December 1937, pages 1190-1199, Article: "A neutron tube without pumping" FM Penning and JHA MOUBIS.

The object of the invention is to provide the tube manufacturer with a means of prevention against such changes.

To this end, the invention is remarkable in that said limitations are made unalterable by including said elements inside the neutron tube so that any attempt to modify the electrical parameters fixing the nominal operating conditions of the tube requires the opening of said tube.

Said tube current limiting elements include a resistor connected between the positive terminal of the source supply and the anode of the ion source, as well as a voltage limiter at a value slightly greater than the specified value, connected between said positive terminal and the negative terminal of the source power supply connected to ground.

The resistor and the voltage limiter can be placed inside a sealed box supplied by sealed passages or in a sealed glass bulb. They can be produced in screen-printed technology or made up of mixed assemblies of screen-printed elements and discrete components compatible with the quality of vacuum necessary for the neutron tube.

• résistance sérigraphiée disposée en hélice sur la face externe d'un cylindre isolant électrique servant de support à la cible ;
• fil résistif isolé électriquement (technologie haute température) et bobiné spire à spire ou en galette ;
• éléments résistifs haute tension en série disposés à l'intérieur d'une enveloppe d'alumine (ou tout autre isolant électrique compatible avec la technologie tube et les contraintes THT) pouvant être soit en communication avec le tube soit sous atmosphère de gaz selon la technologie desdits éléments résistifs (tenue en température, dégazage), le mode de liaison et d'assemblage (champ électrique au nivieau des résistances et des fils de raccordement) et le niveau maximum de chute de tension accepté dans la résistance par le constructeur.

The target voltage is limited by a resistor connected between the negative terminal of the HV supply and the target and a voltage limiter connected between said negative terminal and the positive terminal of the HV supply connected to ground. Said resistance can be produced in one of the following forms:

• screen-printed resistor arranged helically on the external face of an electrical insulating cylinder serving as support for the target;
• resistive wire electrically insulated (high temperature technology) and wound from whorl to wafer;
• high voltage resistive elements in series arranged inside an alumina envelope (or any other electrical insulator compatible with tube technology and THT constraints) which can either be in communication with the tube or in a gas atmosphere according to technology of said resistive elements (temperature resistance, degassing), the connection and assembly mode (electric field at the level of the resistors and the connection wires) and the maximum level of voltage drop accepted in the resistance by the manufacturer.

• figure 1, le schéma de principe des alimentations du tube neutronique munies des éléments de limitation de courant et de tension,
• figures 2a, 2b et 2c, quelques modes de réalisation des éléments de limitation du courant tube,
• figures 3 et 4 respectivement, un premier et un second exemple de disposition des éléments de limitation de courant dans le tube neutronique,
• figure 5, un exemple de disposition des éléments de limitation de la tension cible autour du support cible,
• figure 6, un autre exemple de disposition des éléments de limitation de la tension cible à l'intérieur d'une enveloppe électriquement isolante.

The invention will be better understood with the aid of the following description of some embodiments of devices given by way of non-limiting example, said description being accompanied by drawings which represent:

• FIG. 1, the block diagram of the neutron tube power supplies provided with current and voltage limiting elements,
• FIGS. 2a, 2b and 2c, some embodiments of the elements for limiting the tube current,
• FIGS. 3 and 4 respectively, a first and a second example of arrangement of the current limiting elements in the neutron tube,
• FIG. 5, an example of arrangement of the elements for limiting the target voltage around the target support,
• FIG. 6, another example of arrangement of the elements for limiting the target voltage inside an electrically insulating envelope.

The elements which correspond in these figures are indicated by the same reference numbers.

The neutron tube shown in Figure 1 includes an ion source obtained from a deuterium-tritium mixture contained in the reservoir 1 and in which there is an anode 2, a cathode 3 and a permanent magnet 4 which establishes a field magnetic axial. The ion beam from this source is accelerated by the acceleration electrode 5 and hits the target 6. The envelope of the tube consists of a conductive part 7 grounded and an electrically insulating part 8 surrounding the target support 9 secured to the acceleration electrode.

The description above relates to a Penning type tube, but it can be easily extended to other types of tubes (electrostatic confinement, "multi-cusp", etc.).

The ion source is supplied by the DC voltage generator 10 of value V _aa , the negative pole of which is connected to the ground of the tube and the positive pole of which is connected to the anode 2 through the resistor 11 of value R _a .

• un régime basse pression pour lequel le courant de décharge I_d varie en fonction de la pression P_DT à l'intérieur du tube et de la tension anode-cathode V_ak suivant la relation :
  
  avec γ et ν voisins de 1.Cette relation montre que l'augmentation du courant de décharge est obtenue par une augmentation de la pression P_DT avec une limite supérieure correspondant au régime d'arc ou par augmentation de la tension anode-cathode V_ak ; dans ce dernier cas, il est nécessaire d'augmenter le champ magnétique B de façon à garder un rapport V_ak/B constant.
• un régime fort courant correspondant au régime d'arc ; dans ce cas, le courant de décharge est pratiquement limité par le circuit d'alimentation externe et la tension aux bornes de la structure anode-cathode V_ak
  
  est presque constante. On a donc :
  
  Le courant I_d est alors donné par :
  

   Dans les deux régimes, le courant I_d est limité par la tension d'alimentation V_aa et par la résistance R_a.Schematically, the discharge current in the neutron tube can be subdivided into two regimes:

• a low pressure regime for which the discharge current I _d varies as a function of the pressure P _DT inside the tube and of the anode-cathode voltage V _ak according to the relationship:
  
  with γ and ν neighbors of 1. This relationship shows that the increase in the discharge current is obtained by an increase in the pressure P _DT with an upper limit corresponding to the arc regime or by increase in the anode-cathode voltage V _ak ; in the latter case, it is necessary to increase the magnetic field B so as to keep a constant V _ak / B ratio.
• a very current regime corresponding to the arc regime; in this case, the discharge current is practically limited by the external supply circuit and the voltage across the anode-cathode structure V _ak
  
  is almost constant. So we have :
  
  The current I _d is then given by:
  

In both regimes, the current I _d is limited by the supply voltage V _aa and by the resistance R _a .

The devices which are the subject of the invention will therefore make it possible to act on these two parameters and will include the resistor 11 of value R _a compatible with the maximum instantaneous flux provided by the manufacturer and a voltage limiter 12 (spark gap, varistor, gas diode, ...) of impedance Z _a , connected to the terminals of the supply 10, limiting the operating voltage to a value slightly higher than the supply voltage desired by the manufacturer and guaranteeing good reliability of the tube.

Given the rapid evolution of the neutron emission as a function of the target voltage, a system of the same principle as that used on the anode is possible; it comprises the DC voltage generator 13 of value V _ac whose positive pole is grounded and whose negative pole is connected to the acceleration electrode through the resistor 14 of value R _c . This limitation of the target current I _c can be supplemented by a voltage limitation at by means of a limiter 15 (spark gap or varistor) of impedance Z _c , connected to the terminals of the current generator 13.

• limitation du courant tube par limitation du courant de la source d'ions,
• limitation de la tension cible par limitation de la tension d'alimentation de la cible.

The limiting devices shown diagrammatically in FIG. 1 can be used simultaneously or separately; they are of two types:

• limitation of the tube current by limitation of the current of the ion source,
• limitation of the target voltage by limitation of the target supply voltage.

The corresponding structures depend on the structure of the tube and the descriptions given (types of components, examples of location) are only non-limiting illustrations. A point common to all the solutions adopted is their compatibility with the stresses in particular thermal resulting from the technology for producing the tubes.

avec 0,3<k<1 et 3<β<4 pour 80 kV<V_c<150 kV. Ceci est valable pour I_TU exprimé en ampères, V_c en kilovolts, Q_n en neutrons/seconde et pour des pressions du mélange de gaz hydrogéné inférieures ou égales à 7,5.10⁻⁵ Pa (10⁻² torr).Before detailing some examples of limitation devices of the two types, it is recalled that the instantaneous neutron emission Q _n is globally related to the electrical parameters current tube I _TU and target voltage V _c by a relation of the type

with 0.3 <k <1 and 3 <β <4 for 80 kV <V _c <150 kV. This is valid for I _TU expressed in amperes, V _c in kilovolts, Q _n in neutrons / second and for pressures of the mixture of hydrogenated gases less than or equal to 7.5.10⁻⁵ Pa (10⁻² torr).

The tube current limiting elements can be mounted so as to form a single limiting assembly 16. FIGS. 2a, 2b and 2c show some examples of assemblies.

In FIG. 2a, the resistor 11 and the voltage limiter 12 are arranged in a sealed box BO supplied by sealed passages PAS1 and PAS2.

In FIG. 2b, the resistor 11 and the voltage limiter 12 are placed in a sealed tube TU.

In FIG. 2c, the resistor 11 and the breakdown voltage limiter 12 are produced in screen-printed technology and must be compatible with the quality of vacuum essential for the neutron tube. The resistor 11 consists of a screen-printed strip B between the supply pads P₁ and P₂; the spark gap 12 is formed by the groove located between the pad P₁ and the pad P₃ or between the bar B and the pad. The assembly is placed on the PL insulating plate (alumina, glass) and is compatible with tube technology (high temperature and low degassing rate).

Figures 3 and 4 show two examples of arrangement of the anode current limiter assembly inside the neutron tube itself according to the invention.

In FIG. 3, the anode 2 is held mechanically by the electrically insulating wall 17 on the ion source side. The sealed housing 16 containing the limiting elements 11 and 12 is provided with electrically isolated outputs 18 and 19 serving respectively for its voltage supply and for its connection to the anode; this connection is protected by an insulating sleeve 20.

In FIG. 4, the anode is mounted directly on the sealed casing 16 by means of the electrically insulating support 21 through which the connection of the limiting resistor 11 is made to the anode 2; the spark gap (or varistor) 12 is connected to the voltage supply through the insulated passage 22. The box 16 is connected to the support of the reservoir 23 and to the frame of the tube 7 by a "three-lip" weld 24.

The limitation assembly 16 can be produced using different types of components, by direct mounting of discrete components (resistance, spark gap) assembled in parallel in the base of the tube; the technology of these components must be compatible with ultra-vacuum. Mixed assemblies of screen-printed elements and discrete components can also be assembled according to their compatibility with ultra-vacuum.

Given the rapid evolution of the neutron emission as a function of the target voltage, a system of the same principle as that used on the anode is implemented by means of a high voltage resistor which can be produced in different forms.

In FIG. 5, a resistor 14 is arranged on the outside of the electrically insulating cylinder 9 serving to support the target 6; the end at the base of the cylinder is connected to the negative pole of the high-voltage power supply and the end at the top of the cylinder is connected to the acceleration electrode 5.

This resistor 14 can consist for example of screen-printed elements deposited on the outside of the cylinder 9 in the form of a helix or resistive wire electrically insulated according to high temperature technology and wound from turn to turn or into wafer.

The acceleration electrode 5 is held by the electrically insulating support 26.

In FIG. 6, the resistor 14 is formed of resistors in series and deposited inside an alumina envelope 27 serving as a support for the acceleration electrode 5 connected to the upper end of the resistor, the target 6 being supported by the electrically insulating wall of the tube 8.

This current limitation by means of the resistor 14 can be supplemented by a voltage limitation by means of a spark gap (or a varistor) 28 disposed inside the alumina envelope and connected between the mass and the "three-lip" weld 29 connected to the negative pole of the target HV supply.

The interior of the alumina casing 27 can be placed either in communication with the tube, or in a gas atmosphere depending on whether the orifice 30 is found to be open or closed. The solution will depend on the compatibility of the varistors and resistors to the ultra-vacuum and the technology of the resistors. (temperature resistance, degassing). One could for example place the target voltage limiting elements under controlled gas of sulfur hexafluoride type under the pressure of one to a few bars.

Claims (10)

1. A protection device for a neutron tube, comprising a ion source whose anode (2) is brought to a positive potential relative to the cathode (3) by means of a source supply (10) and whose accelerated ion beam strikes a target (6) disposed on an electrically insulating support (9) and brought to a negative potential supplied by a high-voltage supply, said protection device being composed of electric elements for limiting the tube current (11, 12) and/or the target voltage (15), characterized in that these limiters are made unalterable by enclosing said elements inside the neutron tube, so that any attempt to modify the electrical parameters determining the nominal operating conditions of the tube necessitates the opening of said tube.
2. A device as claimed in Claim 1, characterized in that said tube current limiter elements comprise a resistor (11) connected between the positive terminal of the source supply (10) and the anode (2) of the ion source, as well as a limiter (12) for limiting the voltage to a value slightly higher than the specified value, connected between said positive terminal and the negative terminal of the grounded source supply.
3. A device as claimed in Claim 2, characterized in that said tube current limiter elements are placed inside a leaktight casing (16) fed through leaktight passages (PAS1, PAS2).
4. A device as claimed in Claim 2, characterized in that said tube current limiter elements (11, 12) are placed in a sealed glass envelope (TU).
5. A device as claimed in Claim 2, characterized in that said tube current limiter elements (11, 12) are produced by screen printing technology compatible with the quality of the vacuum required for the neutron tube, said resistor being composed of a screen printed bar (B) and said limiter being composed of a groove separating said bar (B) from another screen printed element, the whole arrangement being disposed on an insulating plate (PL) (alumina, glass, etc.).
6. A device as claimed in Claim 2, characterized in that said tube current limiter elements (11, 12) are composed of mixed arrangements of screen printed elements and discrete components, depending on their compatibility with ultrahigh vacuum.
7. A device as claimed in Claim 1, characterized in that said target voltage is limited by a resistor (14) connected between the negative terminal of the high-voltage supply (13) and the target (16), and by a voltage limiter (15) connected between said negative terminal and the positive terminal of the grounded high-voltage supply.
8. A device as claimed in Claim 7, characterized in that said resistor (14) is a screen printed resistor disposed helically on the outer face of an electrically insulating cylinder serving as support for the target.
9. A device as claimed in Claim 7, characterized in that said resistor is composed of electrically insulated resistive wire (high temperature technology) wound turn-to-turn or in the form of a pane coil.
10. A device as claimed in Claim 7, characterized in that said resistor (14) is composed of high-voltage resistors connected in series and disposed inside an alumina envelope (27) which may either be in communication with the tube or in a gas atmosphere, depending on the technology of said resistors (temperature characteristics, degassing), the method of connection and assembly (electric field at the level of the resistors and connection wires), and the maximum level of voltage drop accepted in the resistor by the manufacturer.

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