EP0813223A1 - Magnetic field generation means and ECR ion source using the same - Google Patents

Abstract

The plasma confinement field has N axially symmetric assemblies (A1 ..... AN). When N=2, the outer assembly defines a mean axial field and the inner one establishes at least locally a gradient of this axial field. The inner one is divided into two sub-assemblies to establish gradients at each end of the outer assembly. The ratio of the length of the device measured parallel to the common axis of symmetry (MM') to the length of the multipolar assembly (32) is less than 1.5. In a cyclotron, the multipolar structure defines a plasma confinement volume with the target (42) at one end (S1, S2) of the volume, lying opposite a primary beam injector (44).

Description

Technical field and prior art

The invention relates to the field of magnetic devices for creating a magnetic field, in particular for application to an ECR source (source with Electronic Cyclotronic Resonance). Such sources are used to produce ions, for example radioactive ions.

Generally, the process used to produce radioactive ions with such a source consists in bombarding a thick target with a beam of high energy heavy ions. The beam of heavy ions stops in the target and produces elements by fragmentation of this one, or of the projectile. The target is heated to a very high temperature, around 2000 ° C, in order to decrease the exit times of the created elements. The latter then diffuse towards an ECR source in which they are ionized, in order to be accelerated by a cyclotron.

In some structures, the target is distant from the plasma of an ECR source by a few tens of centimeters. The transport of the elements produced by the impact of the ion beam on the target is ensured by a tube connecting the latter to the ECR source. The elements diffuse from the target to the source via this tube.

This transfer between the target and the source poses a problem for the condensable elements as well as for those whose lifetime is very short (for example for those whose period is of the order of a few milliseconds).

FIG. 1 represents a known ECR source, also called ECR4.

This type of device comprises the combination, in a microwave cavity, of a high frequency electromagnetic field and a magnetic field. The amplitude of the magnetic field is chosen so that the associated electronic cyclotron frequency is equal to the frequency of the electromagnetic field: this condition allows a strong ionization of neutral atoms, since the emitted electrons are strongly accelerated due to electronic cyclotron resonance.

Magnets 2, 4 are provided, arranged symmetrically with respect to an axis AA 'which crosses a zone 6, or plasma confinement zone. A high frequency injection line 8, coaxial, is aligned along the axis AA '. The ions are extracted through an opening 10, also with axial symmetry AA ′, for example using extraction electrodes placed near the orifice 10.

We see that, in this type of device, the available space is occupied on the one hand by the magnets 2, 4 and on the other hand by the coaxial injection line 8. The target must therefore be placed at a distance, and the produced species must be transported from this target to containment zone 6. In addition, in this type of device, the environment (magnet or coil) has a limited lifespan, when exposed to a flux of neutrons or of charged particles.

The device illustrated in FIG. 2 schematically represents a target-source assembly, called "Nanomafira; this set is described in the Communication by P. Sortais et al." Developments of compact permanent magnets ECRIS ", 12th International Workshop on ECR Ion Sources, April 25-27, 1995, Riken, Japan. A set of magnets 12, 14, 16 is placed around a plasma confinement zone 18. A target 20 is placed, at the end of a high frequency injection line 22, radial or coaxial. designed to be able to direct a primary beam 26 towards the target 20: thus, the place of production of the species (interaction zone between the beam 26 and the target 20) is close to the plasma confinement zone 18. type of device makes it possible to efficiently produce condensable elements, in particular radioactive elements. However, the magnetized zones arranged in a cone of 150 ° around the axis of the beam 26, and the apex of which can be approximately located in the target 20, are subjected to, in some s hours of operation, rapid demagnetization effects, due to the energy neutrons emitted by the interaction of the primary beam with the target. The areas of the cone closest to the axis of the beam 26 are the most affected by these effects (in FIG. 2, this is the area included in the opening cone 2θ ₁ = 60 °). As one moves away from the axis of the beam 26, the demagnetization effects diminish, and they are no longer felt outside the opening cone 2θ ₂ = 150 °.

In addition, in all of these structures, access to the confined volume (volume 6 in FIG. 1 or volume 18 in FIG. 2) is extremely difficult. When elements are to be changed, after long-term operation, disassembly of the assembly is difficult.

Finally, any modification of existing structures is very delicate, because the ECR sources are intended to be coupled to generators of ion beams or particles, and possibly to means of studying the ions produced: the environment of these structures is therefore very restrictive.

Statement of the invention

The problem therefore arises of finding a magnetic structure, in particular for an ECR source, close to which a target can be placed, and the magnetic elements of which are not exposed to demagnetization effects. In addition, such a structure must allow easy access to the confined volume.

comportant :

• un ensemble de N(N≥2) systèmes magnétiques, à symétrie axiale, pour former un champ magnétique axial (B_a), ces N systèmes étant coaxiaux et emboîtés les uns dans les autres,
• un ensemble de moyens magnétiques à structure multipolaire permettant d'obtenir un champ magnétique radial $\overrightarrow{\text{B}}$
  
  _rad, cet ensemble de moyens magnétiques étant disposé à l'intérieur des N systèmes magnétiques et étant coaxial à ceux-ci.

To this end, the invention relates to a device for generating a magnetic field $\overrightarrow{\text{B}}$

comprising:

• a set of N (N≥2) magnetic systems, with axial symmetry, to form an axial magnetic field (B _a ), these N systems being coaxial and nested one inside the other,
• a set of magnetic means with a multipolar structure making it possible to obtain a radial magnetic field $\overrightarrow{\text{B}}$
  
  _rad , this set of magnetic means being arranged inside the N magnetic systems and being coaxial with them.

The coaxial structure chosen makes it possible to obtain a compact device, close to which a target can be placed: thus, the problems associated with transport of elements via a tube are avoided. In addition, such a structure makes it possible to direct a primary beam of particles, in the direction of a target, without the magnetic elements being exposed to the neutrons induced by the primary beam. Finally, this structure makes it possible to minimize the assemblies to be changed after long-term operation: all of the magnetic elements being distributed coaxially, and nested one inside the other, a translation of one of the elements relative to the others is possible, which allows to clear this element to have access to it.

The structure according to the invention proves all the more advantageous, in an ECR source, that the spatial and space constraints are extremely critical in this type of source.

Preferably, the system is dimensioned so that the ratio L ₁ / L, where L is the length of the set of magnetic elements with multipolar structure and where L1 is the length of the device, measured parallel to the axis of common symmetry, is less than 1.5.

Thus, a compact system is obtained, offering a large opening to the elements produced from a target arranged near the device according to the invention.

One (or more) insulating element (s), coaxial (coaxial) to the N magnetic systems and to the set of magnetic means with multipolar structure, can also be provided in the device: all the insulating elements of the devices known according to the prior art have a geometry which depends on the geometry of the source, and this results insulators of fairly complex shape. In the context of the present invention, the insulator (or insulators) is (are) on the contrary very simple in shape, since it (s) has (s) an axial symmetry.

The set of N magnetic systems to form an axial magnetic field can be reduced to 2 (N = 2) magnetic systems, the outermost system defining an average axial field, while the internal system makes it possible to locally establish at least one gradient of this mean axial field. Such a device makes it possible to obtain a minimum B field structure.

In particular, the interior system can comprise a single magnetic system, making it possible to establish an axial field gradient, at one end of the outermost system.

According to a variant, the interior system can comprise two magnetic subsystems, making it possible to establish an axial field gradient at each of the two ends of the outermost system.

The invention also relates to an ECR source comprising a device for generating a magnetic field, as described above, the interior or magnetic volume with a multipolar structure defining a confinement enclosure for plasma, and means for placing a target, of preferably at one end of the multipole structure.

Such a device also comprises means for injecting a primary beam in the direction of a target, allowing the injection of the primary beam along the axis common to the N magnetic systems and to the magnetic means with multipolar structure.

The invention also relates to a method for producing radioactive ions using an ECR source as described above.

Such a process allows the production of ions, including condensable or unstable elements: in fact, the ECR source then does not require any tube to transport the elements.

Brief description of the figures

• les figures 1 et 2, déjà décrites, représentent des sources ECR selon l'art antérieur,
• la figure 3 représente schématiquement une structure d'aimants pour un système magnétique selon l'invention,
• la figure 4 représente schématiquement un système d'aimants, avec deux sous-systèmes pour former un champ axial, conformément à l'invention,
• les figures 5A et 5B représentent respectivement un premier mode de réalisation d'un dispositif avec deux sous-systèmes pour le champ axial, et les variations, le long de l'axe, des champs magnétiques obtenus,
• les figures 6A et 6B représentent respectivement un autre mode de réalisation d'un dispositif avec deux sous-systèmes pour former le champ axial, et les valeurs des champs magnétiques le long de l'axe,
• les figures 7A et 7B représentent un autre mode de réalisation d'un dispositif avec deux sous-systèmes pour former le champ axial, et les champs magnétiques résultant le long de l'axe,
• les figures 8A et 8B représentent un autre mode de réalisation d'un dispositif avec deux sous-systèmes pour former le champ axial, et les variations des champs magnétiques le long de l'axe,
• la figure 9 représente un prototype de source réalisé avec une structure magnétique selon l'invention,
• la figure 10 représente la variation des champs magnétiques, le long de l'axe, dans le cas d'un prototype de source réalisé avec une structure magnétique selon l'invention.

In any case, the characteristics and advantages of the invention will appear better in the light of the description which follows. This description relates to the exemplary embodiments, given by way of explanation and without limitation, with reference to the appended drawings in which:

• FIGS. 1 and 2, already described, represent ECR sources according to the prior art,
• FIG. 3 schematically represents a structure of magnets for a magnetic system according to the invention,
• FIG. 4 schematically represents a system of magnets, with two subsystems for forming an axial field, in accordance with the invention,
• FIGS. 5A and 5B respectively represent a first embodiment of a device with two subsystems for the axial field, and the variations, along the axis, of the magnetic fields obtained,
• FIGS. 6A and 6B respectively represent another embodiment of a device with two subsystems for forming the axial field, and the values of the magnetic fields along the axis,
• FIGS. 7A and 7B represent another embodiment of a device with two subsystems for forming the axial field, and the magnetic fields resulting along the axis,
• FIGS. 8A and 8B represent another embodiment of a device with two subsystems for forming the axial field, and the variations of the magnetic fields along the axis,
• FIG. 9 represents a prototype source produced with a magnetic structure according to the invention,
• FIG. 10 represents the variation of the magnetic fields, along the axis, in the case of a source prototype produced with a magnetic structure according to the invention.

Detailed description of embodiments of the invention

FIG. 3 schematically represents a device according to the invention, comprising a multipolar system 32, making it possible to generate a magnetic field with radial symmetry, around an axis MM 'of a confinement zone 34. The multipolar system 32 can be for example of the type described in the document by R. Geller entitled "Micromafios, multicharged ion source based on the cyclotron resonance of the electrons", published in "Revue de Physique Applée", vol. 15, n ° 5, MAY 1980, page 995-1005. It can also be a multipole structure of the type described in European patent application (CEA) EP-138 642.

An axial component is superimposed on the radial component of the magnetic field. This axial component is obtained using a set A ₁ , ... A _N of coaxial and nested magnetic systems. Each system A _i has an axial symmetry around the axis MM ', which is therefore common to all of the N magnetic systems making it possible to obtain the axial component of the magnetic field, but also to the multipolar system 32.

The resulting axial magnetic field B _a is the sum of the magnetic fields obtained with each of the elements A _i .

Preferably, all of the magnetic systems are configured so as to produce a magnetic field B having a "minimum" structure. In other words, the magnetic field is then formed by the superposition of the multipolar radial component, which has a minimum amplitude in the central part of the cavity, and of an axial magnetic field with symmetry of revolution, having a gradient following axis MM ′, the resulting magnetic field is adjusted so that there exists in the cavity at least one ply 35 completely closed, and having no contact with the walls of the cavity, ply on which the condition of electronic cyclotron resonance is satisfied, so as to obtain an ionization of the gas passing through it.

FIG. 4 presents a device according to the invention, comprising a multipole structure 32 intended to generate the radial component of the field, and a set of two systems A ₁ , A ₂ making it possible to generate the axial component of the magnetic field in the manner described above. Closed surfaces, of magnetic equimodules 36, 38 are obtained inside the confinement zone 34. For the creation of ions, using an ECR source, a field electromagnetic HF is injected into this area 34 by means not shown in Figures 3 and 4. The internal layer 36 corresponds to the resonant layer (the electronic cyclotron frequency is equal to the frequency of the electromagnetic field); the external ply 38 corresponds to a closed surface of magnetic equimodule having no contact with the walls of the cavity 34.

As can be seen in FIG. 3, each of the systems A _i , contributing to the axial component of the magnetic field, can be composed of neither magnetic subsystems A _i1 , ... A _in . These magnetic subsystems are not necessarily juxtaposed: this is the case of the A2 system (A21 and A22 subsystems) of Figure 3.

Those skilled in the art will be able to configure each level of the magnetic system A _i , so as to obtain a desired, predetermined configuration of axial field.

The systems described above, in conjunction with Figures 3 and 4 have many advantages.

First of all, the arrangement of the various elements, coaxial and mechanically nested one inside the other, allows, in the event of one of them failing, to perform a simple translation of said element (movement symbolized by the arrow 40 for the system A _n-1 of FIG. 3) to repair this element or replace it with another element.

In addition, access to the confined volume 34 is free: the zones or surfaces S ₁ , S ₂ defined by the ends of the multipolar structure 32, as well as their vicinity, are available to be able to have in the immediate vicinity of the plasma, or the plasma confinement zone, an assembly consisting of a target 42, with its heating and cooling systems, its diagnostic means (for example thermocouple), its mechanical holding, dismantling and intervention devices, its reflectors, etc. This arrangement is therefore much more flexible to use than the devices of the prior art, which often required an installation of the target at a distance from the confinement zone 34 such that means of transport or transit of the species produced had to be planned. On the contrary, in the device according to the invention, the fact of being able to install a target near one of the ends S ₁ , S ₂ of the multipolar structure makes it possible to efficiently produce radioactive or condensable species, the lifetimes of which are very weak (of the order of a millisecond).

Finally, this structure is compatible with a positioning of a high energy incident beam 44 along the axis MM ', directed towards the target 42. No magnetic element is found in the field of a cone 45 of angle at vertex θ ₃ = 150 °, and whose vertex is approximately on the target 42: consequently no magnetic element risks being demagnetized due to the influence of the flux of neutrons or charged particles 44, of high energy.

As illustrated in FIG. 4, such a device can be dimensioned so that the internal diameter φ of the multipolar system 32 and the total length L of the magnetic device are in a ratio φ / L of between 0.1 and 0.8 . Such a geometry allows, when a target 42 is positioned near an end S ₂ of the multipolar system 32, to immediately expose the elements produced to from the target to a plasma seen under a large width, for a determined volume of plasma. The production of ionized species is improved.

Preferably, the different assemblies constituting the magnetic structure according to the invention are presented as coaxial cylinders, nested one inside the other; this has the effect of making them independent mechanically, but also electrically.

It is often necessary to carry out a high voltage isolation of the source. The structure of magnetic devices according to the invention is compatible with the introduction of an insulating element also having an axial symmetry with respect to the axis MM '. As illustrated in FIG. 4, this isolation element can be introduced at several levels, for example in the zone 46 outside the magnetic system A ₁ or in the zone 48 between the system A ₁ and the system A2 or in the zone 50 included between the A2 system and the multipolar system 32. In general, isolation can be done at different diameters: between the multipoles and the axial system, or between the components of the axial system; it is also possible to make insulations on several diameters, which allows higher voltages to be maintained. The insulating elements are generally made of PVC or Bakelite.

FIG. 5A represents a device according to the invention, comprising a multipole element 32 as described above, and two systems A ₁ , A ₂ for producing the axial field. The outermost system, A ₁ , makes it possible to establish an average axial field. The system inside A ₂ has a single magnetic subsystem, allowing an axial field gradient to be established at one end of the outside system A ₁ . FIG. 5B represents the evolution, along the axis MM 'of the radial component obtained by A ₁ (mean field: curve I ₅ ), of the gradient induced by the subsystem A ₂ (modulation, curve II ₅ ), and of the resulting axial field (curve III ₅ ). The resulting axial field has a mirror ratio B _f / B _min greater than 1.1: this criterion clearly establishes that a minimum field structure is obtained.

FIG. 6A represents another structure of a device according to the invention, with multipolar system 32 around an axis MM ', and two systems A ₁ , A ₂ for producing the axial field. The system A2 is broken down into two subsystems A ₂₁ and A ₂₂ , each of these subsystems making it possible to establish, at one end of the external system A ₁ , an axial field gradient. The polarities of these elements are represented in FIG. 6A by arrows. The curves I ₆ , II ₆ , III ₆ represent, in FIG. 6B, the respective evolution of the mean field, of the modulation fields, and of the resulting total field. Here again, the mirror ratio is greater than or equal to 1.1, this results in a minimum structure.

FIG. 7A represents another structure with two levels of magnetic systems A ₁ and A ₂ for producing the axial component of the magnetic field. The difference between the device of this figure and that of Figure 6A lies in the polarity of the subsystem A ₂₂ . FIG. 7B shows the evolution, along the axis MM ', of the mean axial field (obtained using the system A ₁ : curve I ₇ ), of the axial modulation field (obtained by the combined action of the subsystems A ₂₁ , A ₂₂ : curve II ₇ ; curve II ' ₇ represents the axial field resulting from the action of the system A ₂ ), and the total resulting axial field (curve III ₇ ). Here again, a minimum field structure is obtained (the criterion being the obtaining of a mirror ratio B _f / B _min greater than or equal to 1.1).

A fourth embodiment of a device, with two levels of magnetic systems for obtaining the axial component of the field, is shown in FIG. 8A. Compared to the structures of FIGS. 6A and 7A, the difference again lies in the magnetization of the subsystem A ₂₂ . Along the axis MM ′, the mean axial field obtained by the system A ₁ evolves as illustrated by the curve I ₈ in FIG. 8B. The curves II ₈ represent the evolution of the axial modulation field, the curve II ' ₈ representing the evolution of the component of the axial field resulting from the system A ₂ . Curve III8 represents the axial evolution of the total resulting axial field. Again, we can clearly see that a minimum structure has been obtained (B _f / B _min greater than or equal to 1.1).

For the four embodiments illustrated in FIGS. 5A to 8A, the system A ₁ can be constituted by a coil, the modulation systems (A ₂ , A ₂₂ , A ₂₁ ) being constituted by permanent magnets or coils.

The various embodiments set out in connection with FIGS. 3 to 8A are, in the case of a ECR source, combined with means for injecting HF radiation into the confinement zone 32, with means for placing a target, with means for injecting a primary beam towards the position of a target and with means for extracting ions from a plasma formed in the confinement zone.

FIG. 9 represents an example of an ECR source using a magnetic structure according to the invention. In this figure, we recognize the presence of the multipolar element 32, of an external system A ₁ of cylindrical coils making it possible to establish an average axial field in the zone 34 for confining a plasma 35. A second level of means magnetic A ₂ (in fact, composed of two permanent cylindrical magnets A ₂₁ and A ₂₂ ) allows to establish the gradients necessary for the modulation of the axial field, inside the confinement zone 34. A target 42, and its heating means, are arranged near one end of the multipolar system 32. Means 52 allow lateral injection of high frequency radiation into the confinement zone 34. B _r represents the value of the radial component of the magnetic field at inside the zone 34, and it is preferably arranged so that the frequency f of the electromagnetic field HF injected into this zone satisfies the condition of electronic cyclotron resonance: B _r = f.2πm / e ( where m is the mass of the electron and e its charge). Such a condition allows a strong ionization of the atoms in the confinement zone 34: in fact, the emitted electrons are then strongly accelerated due to the electronic cyclotron resonance.

The device in FIG. 9 further comprises means 54 allowing the target and its environment to be cooled, a passage 56 for the connection of a thermocouple, itself located near the target, and a current inlet 58 for heating the target.

The structure of FIG. 9 also includes an insulating cylinder 60, placed between the two systems A ₁ and A ₂ for the production of the axial component of the magnetic field.

The target 42 is bombarded by an incident ion beam 44 whose direction is aligned on the common axis of the magnetic device A ₁ -A ₂ . The plasma ions are extracted through the same opening, using extraction electrodes 62.

As already explained above, the fact of positioning the target 42 near one of the ends of the multipolar system 32 makes it possible to bombard it with a high-energy incident ion beam 44 which passes through the extraction system, while now the structures, sensitive to neutrons, producing the magnetic field, at angles greater than 90 ° relative to the axis MM '. Of course, the device according to the invention, for producing a magnetic field, does not prevent, if necessary, from placing a target far from the source.

The evolution, along the axis MM ', of the various components of the magnetic field obtained with the device illustrated in FIG. 9, is represented in FIG. 10. The curve I represents the evolution of the component due to the system A ₁ only (this system consists of a coil supplied at 700 amps), curve II represents the evolution of the axial component due to the system A ₂ alone (system consisting of magnets A ₂₁ and A ₂₂ ). Curve III represents the axial field resulting from the contribution of each of the systems A ₁ and A ₂ . This curve III shows again, that a minimum field structure is well obtained.

Claims (12)

Device for generating a magnetic field $\overrightarrow{\text{B}}$

comprising: - a set of N (N≥2) magnetic systems (A ₁ , ..., A _N ), with axial symmetry, to form an axial magnetic field (B _a ), these N systems being coaxial and nested one inside the other other, - a set of magnetic means (32) with a multipolar structure making it possible to obtain a radial magnetic field $\overrightarrow{\text{B}}$

_rad , this set of magnetic means being arranged inside the N magnetic systems and being coaxial with them. Device according to claim 1, the field $\overrightarrow{\text{B}}$

having a minimum structure. Device according to either of Claims 1 and 2, the ratio L1 / L, where L is the length of the set of magnetic means with multipolar structure (32) and where L ₁ is the length of the device, measured parallel to the common axis of symmetry (MM '), being less than 1.5. Device according to one of claims 1 to 3, further comprising one or more insulators (60) coaxial with the N magnetic systems (A ₁ , ... A _N ) and with the set of magnetic means (32) with multipolar structure . Device for generating a magnetic field, according to one of claims 1 to 4, comprising 2 (N = 2) magnetic systems with axial symmetry (A ₁ , A ₂ ), for forming an axial magnetic field, the outermost system ( A ₁ ) defining a mean axial field, the interior system (A ₂ ) allowing to locally establish at least one gradient of this axial field. Device according to claim 5, the internal system (A ₂ ) comprising a single magnetic system making it possible to establish an axial field gradient, at one end of the outermost system (A ₁ ). Device according to claim 5, the internal system (A ₂ ) comprising two magnetic subsystems (A ₂₁ , A ₂₂ ) making it possible to establish an axial field gradient at each of the two ends of the outermost system (A ₁ ). ECR source comprising a device according to one of claims 1 to 7, the volume inside the magnetic means with a multipolar structure defining a confinement enclosure for plasma and means for placing a target. ECR source according to claim 8, the means for placing a target making it possible to position the latter at one of the ends (S ₁ , S ₂ ) of the multipolar structure (32). ECR source according to claim 8, means being provided for injecting a primary beam (44) towards the position of a target (42), along the axis common to the N magnetic systems (A ₁ , ..., A _N ) and magnetic means (32) with a multipole structure. Method for producing ions using an ECR source according to one of claims 8 to 10. The method of claim 11, the ions produced being radioactive ions.

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