EP0813223B1 - Magnetic field generation means and ECR ion source using the same - Google Patents

Description

Technical field and prior art

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

In general, the process implemented works to produce radioactive ions with a such source is to bomb a thick target with a beam of high energy heavy ions. The heavy ion beam stops in the target and produces elements by fragmentation thereof, or of the projectile. The target is heated to a temperature very high, around 2000 ° C, in order to reduce the output time of the elements created. These latter then broadcast to an ECR source in which they are ionized, in order to be accelerated by a cyclotron.

In some structures, the target is distant from plasma from an ECR source by a few tens of centimeters. Transporting items produced by the impact of the ion beam on the target is provided by a tube connecting it to the ECR source. Elements broadcast from target to source through this tube.

This transfer between the target and the source poses problem for the condensable elements as well as for those with a very short lifespan (e.g. 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 includes the combination, in a microwave cavity, of a field high frequency electromagnetic and field magnetic. Magnitude of magnetic field is chosen so that the cyclotron frequency associated electronics, equal to the frequency of the electromagnetic field: this condition allows strong ionization of neutral atoms, since the emitted electrons are strongly accelerated due to electronic cyclotron resonance.

Magnets 2, 4 are provided, arranged in symmetrically about an axis AA 'which crosses a zone 6, or plasma confinement zone. A high frequency injection line 8, coaxial, east aligned along axis AA '. Ions are extracted by an opening 10, also with axial symmetry AA ′, for example using extraction electrodes arranged near 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 cash produced must be transported from this target towards containment zone 6. In addition, in this type of device, the environment (magnet or coil) has a limited lifespan, when exposed to a flow of charged neutrons or particles.

The device illustrated in FIG. 2 schematically represents a target-source assembly, called "Nanomafira; this assembly 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 line 22 d high frequency, radial or coaxial injection. Means 24 are also provided 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 located close to the plasma confinement zone 18. This 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 beam axis 26 , and the apex of which can be approximately located in the target 20, undergo, within a few hours of operation, effects of rapid demagnetization, 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 2 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 2 ₂ = 150 °.

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

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

Statement of the invention

There is therefore the problem of finding a magnetic structure, in particular for ECR source, with proximity of which we can have a target, and whose magnetic elements are not exposed to demagnetization effects. Furthermore, such structure must allow easy access to the volume confined.

• 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 emboítés les uns dans les autres,
• un ensemble de moyens magnétiques à structure multipolaire permettant d'obtenir un champ magnétique radial 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 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 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, near which a target can be arranged: thus, problems related to transport of elements via a tube are avoided. In addition, such a structure makes it possible to direct a primary beam of particles, towards a target, without the magnetic elements being exposed to neutrons induced by the primary beam. Finally, this structure makes it possible to minimize the sets to change after long-term operation: all the magnetic elements being distributed coaxially, and nested one inside the other, a translation of one of the elements in relation to the others is possible, which allows to clear this element for have access to it.

The structure according to the invention is revealed 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, we obtain a compact system, offering a wide opening to the elements produced from of a target placed near the device according to the invention.

One (or more) insulating element (s), coaxial (coaxial) to N magnetic systems and the set of magnetic means with structure multipolar, 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. As part of the present invention, the insulator (or insulators) is (are) on the contrary very simple form, since it (s) has (s) an axial symmetry.

The set of N magnetic systems for forming an axial magnetic field can be reduced to 2 (N = 2) magnetic systems, the most exterior defining a mean axial field, while the internal system makes it possible to establish locally at minus a gradient of this mean axial field. Such device provides a B field structure at minimum.

In particular, the interior system can have a single magnetic system, allowing establish an axial field gradient at one end of the outermost system.

Alternatively, the interior system can have two magnetic subsystems, allowing establish an axial field gradient for each of the two ends of the outermost system.

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

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

The invention also relates to a method of production of radioactive ions using a ECR source as described above.

Such a process allows the production of ions, including comprised of condensable or unstable elements: indeed, the ECR source 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

Figure 3 shows schematically a device according to the invention, comprising a system multipolar 32, allowing to generate a field magnetic with radial symmetry, around an axis MM ' of a containment zone 34. The multipolar system 32 may for example be of the type described in R. Geller's document entitled "Micromafios, source multi-charged ion based on resonance cyclotronic of electrons ", published in" Revue de applied physics ", vol. 15, n ° 5, MAY 1980, page 995-1005. It can also be a multipolar structure of the type described in the 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 magnetic systems and nested one inside the other. 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 systems magnetic is configured to achieve a magnetic field B having a structure "to minimum ". In other words, the magnetic field is then constituted by the superposition of the component radial multipolar, which has an amplitude minimal in the central part of the cavity, and a axial magnetic field with symmetry of revolution, having a gradient along the axis MM ', the field resulting magnetic is set so that it exists in the cavity at least one ply 35 completely closed, and having no contact with walls of the cavity, sheet 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, an HF electromagnetic field is injected into this zone. 34 by means not shown in FIGS. 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 (A ₂₁ and A ₂₂ 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 different elements, coaxial and mechanically nested one inside the other, makes it possible, 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 constituted by 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 provided. 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  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  of the multipolar system 32 and the total length L of the magnetic device are in a ratio  / 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 from the target to a plasma seen under a large width, for a volume of plasma determined. The production of ionized species is improved.

Preferably, the different sets constituting the magnetic structure according to the invention appear as coaxial cylinders, nested into each other; this has the effect of mechanically independent 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 A ₂ or in the zone 50 comprised between the system A ₂ 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 makes it possible to withstand higher voltages. 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 interior system A ₂ comprises a single magnetic subsystem, making it possible to establish an axial field gradient at one end of the exterior 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 from 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 exposed in connection with Figures 3 to 8A are, in the case of a ECR source, combined with means for injecting a HF radiation in containment zone 32, at means for placing a target, means for inject a primary beam towards the position of a target and means for extracting ions from a plasma formed in the containment 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 multipole element 32, an external system A ₁ of cylindrical coils allowing to establish an average axial field in the zone 34 of confinement of 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 of Figure 9 further comprises means 54 allowing the cooling of the target and its environment, a passage 56 for the connection of a thermocouple, itself located nearby of the target, and a current arrival 58 for the target heating.

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 position the target 42 near one of the ends of the multipolar system 32 allows the bomb with an incident ion beam at high energy 44 flowing through the extraction system everything maintaining the structures, sensitive to neutrons, producing the magnetic field at higher angles 90 ° to the axis MM '. Of course, the device according to the invention, for producing a field magnetic, does not prohibit, if necessary, 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 alone (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)

1. Means for generating a magnetic field B comprising:

   an assembly of N, (N≥2), magnetic systems (A₁, ...,A_N), having axial symmetry, for forming an axial magnetic field (B_a), these N systems being coaxial and nested one within another,

   an assembly of magnetic means (32) of multipolar structure allowing the production of a radial magnetic field (B_rad), this assembly of magnetic means being arranged within the N magnetic systems and being coaxial with them.
2. Means according to Claim 1, the field B having a minimum structure.
3. Means according to one of Claims 1 or 2, the ratio L₁/L, where L is the length of the assembly of magnetic means of multipolar structure (32) and where L₁ is the length of the means measured parallel to the common axis of symmetry (MM'), being less than 1.5.
4. Means according to one of Claims 1 to 3, including in addition one or more insulators (60) coaxial with the N magnetic systems (A₁, ... A_N) and with the assembly of magnetic means (32) of multipolar structure.
5. Means for generating a magnetic field according to one of Claims 1 to 4, comprising 2, (N=2), magnetic systems having axial symmetry (A₁, A₂), for forming an axial magnetic field, the outermost system (A₁) defining a mean axial field, the inner system (A₂) allowing the establishment locally of at least one gradient of this axial field.
6. Means according to Claim 5, the inner system (A₂) comprising a single magnetic system allowing the establishment of an axial field gradient, at one end of the outermost system (A₁).
7. Means according to Claim 5, the inner system (A₂) comprising two magnetic sub-systems (A₂₁, A₂₂) allowing the establishment of an axial field gradient at each of the two ends of the outermost system (A₁).
8. ECR source comprising means according to one of Claims 1 to 7, the interior volume of the magnetic means of multipolar structure defining a confining cavity for plasma (34) and means for receiving a target (42).
9. ECR source according to Claim 8, the means for receiving the target allowing the positioning of the latter at one of the ends (S₁, S₂) of the multipolar structure (32).
10. ECR source according to Claim 8, means being provided for injecting a primary beam (44) in the direction of the position of a target (42), along the axis common to the N magnetic systems (A₁, ..., A_N) and to the magnetic means (32) of multipolar structure.
11. Method of producing ions using an ECR source according to one of Claims 8 to 10.
12. Method according to Claim 11, the ions produced being radioactive ions.

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