EP2311061B1 - Electron cyclotron resonance ion generator - Google Patents

Description

The present invention relates to an electron cyclotron resonance ion generating device.

In a known manner, electron cyclotron resonance sources, referred to as ECR sources, are commonly used to produce single-charged or multicharged ions (that is to say atoms to which one or more electrons have been torn off).

The principle of these ECR sources is to couple, inside a vacuum chamber fed with atoms (these atoms can come from a gas or a metal), a high-frequency wave with a magnetic field B, in order to obtain the conditions under which a cyclotron resonance is likely to appear and to ionize the atoms present, thus generating a plasma. The residual pressure prevailing in the vacuum chamber is of the order of 10 ^-6 to 10 ^-1 Pa.

Generally, the chamber containing the plasma has a symmetry of revolution with respect to a longitudinal axis. The magnetic field is produced by means external to the vacuum chamber. These means may consist of a set of coils with an electric current or a set of permanent magnets. The coils used, if they consist of superconducting materials, must be cooled to a specified temperature by a suitable cryogenic system.

dans laquelle e représente la charge de l'électron, m sa masse et f la fréquence de l'onde électromagnétique.The cyclotron resonance is obtained thanks to the combined action of the high frequency wave injected into the enclosure, and a magnetic field having a so-called "minimum B" structure. In particular, the magnetic field has a module Br which satisfies the condition (1) of electronic cyclotron resonance: $$\mathrm{Br}=\mathrm{f}\mathrm{.2}\mathrm{\pi m}/\mathrm{e}$$

in which e represents the charge of the electron, m its mass and f the frequency of the electromagnetic wave.

An ion extraction system, located on the side of the chamber opposite to that of the injection of the high frequency, or disposed laterally with respect to the axis of the source facing the plasma is also provided.

In this type of source, the quantity of ions that can be produced results from the competition between two processes: on the one hand the formation of ions by electronic impact on neutral atoms constituting the gaseous medium to be ionized, and on the other hand, the losses of these same ions by recombination with the neutral or charged particles present in the plasma volume or by diffusion of the neutral atoms to the walls of the chamber.

It is planned to confine in the chamber the ions formed as well as the electrons used for their ionization. This is achieved by superimposing on the magnetic field of axial symmetry a magnetic field of radial symmetry. This radial magnetic field is obtained using a multipolar structure generally consisting of permanent magnets. A positive field gradient is created in all directions (on the axis and towards the wall of the chamber) and is decelerating. The plasma electrons are trapped in a magnetic potential well axially and radially. This magnetic mirror configuration is obviously not perfect (creepage) and this is used to extract the charged particles that will form the beam at the output of the plasma electrode.

The superposition of the radial magnetic field and the axial magnetic field leads to the formation of closed magnetic field equimodule surfaces having no contact with the walls of the enclosure. The total magnetic field is adjusted so that there is at least one completely closed magnetic surface on which the electronic cyclotron resonance condition (1) is satisfied.

The patent application FR 2668642 A discloses a highly charged ion source with polarizable probe and electron cyclotron resonance, said source having improved compactness.

The patent EP946961 filed by the applicant describes an ECR source using a magnetic field symmetry of revolution. This source comprises magnetic means whose vector sum of the fields created by these magnetic means makes it possible to define at least one closed line of minima of the module B of the vector sum, within one or more internal volume (s) ( s) to the cavity and delimited by surfaces of equimodule Bf of the magnetic field closed in space. The closed module surface B _f encompasses an interior volume where the magnetic field may, in particular, have a very low minimum B, unlike what occurred in previously known ECR sources.

The electron density of the ECR source plasmas is between 10 ⁹ and 10 ¹² electrons per cm ³ . The neutral particles are injected into the volume of the vacuum chamber containing the plasma. If they are not ionized during their first journey within the plasma, they stick on the walls of the room. Their bonding time depends on the chemical species to which they belong. This time can be very important for particles whose physico-chemical properties allow a reaction with the walls. Their probability of ionization therefore depends directly on the ionization capacity of the plasma.

Ionization efficiencies close to 100% can be observed for non-reactive gases with the walls. Indeed, the successive rebounds of the particles on the walls multiply the number of paths of the particles in the plasma and allow their ionization if they are not ionized at the passage of the zones of the strongest electron excitation of the plasma (around the zones of resonance).

On the other hand, it is not the same for the condensable elements (Pb, Ge for example) of Mendeleev's periodic table. The latter, if they are not ionized at the first passage in the plasma, are stuck to the walls as soon as they reach them and can only come off if the temperature of the wall is sufficient for the element in question. As a result, conventional ECR ion sources with cold walls lead to low total ionization efficiencies as atoms that are not ionized at their first pass through the plasma condense on the walls of the chamber and are lost for beam production. Thus, in view of the electron impact ionization cross sections, the ionization efficiencies for the condensable elements are from a few to a thousand, for a wave frequency of 2.45 GHz up to 20% for a wave frequency of 15 Ghz.

Note that it is the same for the production of radioactive ions whose effectiveness will be very dependent on the life of these elements.

For non-reactive gases with the wall, the ionization efficiency is obviously higher than for the condensable elements; however, in parallel, the total transformation time of the neutral particles increases, this time being related to both the different rebounds and the take-off time of the particles.

In this context, the object of the present invention is to provide an electronic cyclotron resonance ion generating device which makes it possible to increase the direct ionization capacity before any bounce on the walls of the vacuum chamber.

• une chambre étanche sous vide destinée à contenir un plasma, ladite chambre étant à symétrie axiale suivant un axe longitudinal,
• des moyens pour engendrer un champ magnétique dans ladite chambre, ledit champ magnétique possédant une symétrie de révolution par rapport au dit axe longitudinal,
• des moyens de propagation d'une onde haute-fréquence à l'intérieur de ladite chambre,

ledit dispositif étant caractérisé en ce que ladite chambre comporte :

• un premier étage d'ionisation situé au niveau d'une extrémité de ladite chambre, ledit premier étage comportant une zone d'ionisation dans laquelle sont générés des ions, ledit champ magnétique étant sensiblement parallèle au dit axe longitudinal dans ladite zone d'ionisation,
• un deuxième étage de confinement magnétique desdits ions générés dans ladite zone d'ionisation, ledit deuxième étage utilisant une première onde haute-fréquence se propageant dans ladite chambre issue desdits moyens de propagation d'une onde haute-fréquence,

ledit champ magnétique étant sensiblement parallèle au dit axe longitudinal entre ladite zone d'ionisation et ledit deuxième étage de confinement de sorte que les ions générés dans ladite zone d'ionisation migrent vers ledit deuxième étage de confinement et que lesdits premier et deuxième étages comportent un même plasma continu.To this end, the invention proposes an electron cyclotron resonance ion generating device comprising:

• a vacuum sealed chamber for containing a plasma, said chamber being axially symmetrical along a longitudinal axis,
• means for generating a magnetic field in said chamber, said magnetic field having a symmetry of revolution with respect to said longitudinal axis,
• means for propagating a high-frequency wave inside said chamber,

said device being characterized in that said chamber comprises:

• a first ionization stage located at an end of said chamber, said first stage comprising an ionisation zone in which ions are generated, said magnetic field being substantially parallel to said longitudinal axis in said ionization zone,
• a second magnetic confinement stage of said ions generated in said ionization zone, said second stage using a first high-frequency wave propagating in said chamber resulting from said means of propagation of a high-frequency wave,

said magnetic field being substantially parallel to said longitudinal axis between said ionization zone and said second confinement stage so that the ions generated in said ionization zone migrate to said second confinement stage and said first and second stages comprise a same continuous plasma.

Magnetic field having a symmetry of revolution with respect to the longitudinal axis is understood to mean a magnetic field whose radial and axial components are symmetrical irrespective of the points situated on a circle about said axis.

Thanks to the invention, the transformation time of the neutral particles into ions is reduced while ensuring a high efficiency of ionization. The device according to the invention has a magnetic field with symmetry of revolution defining the volume of a plasma contained in a chamber comprising two distinct zones or stages. The ions are essentially created in the first zone while the second zone ensures the confinement of the ions according to the principle of the electron cyclotron resonance source. Between the two zones, the directions of the vectors of the magnetic field are parallel to the axis common to the two stages, namely the longitudinal axis of the chamber: there is thus between these two zones a purely axial magnetic field (no radial component magnetic field). The two zones have no magnetic break and define a volume containing a single plasma, that is to say a single set composed of ions, electrons, atoms and molecules, globally electrically neutral (ie with as many positive charges as negative charges). The fact of using coaxial magnetic field vectors between the two stages implies implicitly that the magnetic field is symmetrical of revolution and requires the migration of ions from the first zone to the second zone.

In the first stage, the ionization efficiency of a particle depends on the means used to achieve this ionization. The ionized particles migrate to the second ECR stage in which they are confined or even multicharged; it should be noted in this respect that the second stage can maintain or increase the state of charge of the ions coming from the first stage.

The ions confined by the second stage can then be used in the form of a single or multicharged particle beam. The beam thus produced will have the characteristics given by a RCE type source with symmetry of revolution as described in the patent. EP946961 of the plaintiff.

As regards the ionisation of particles of short duration (radioactive atoms, unstable molecules, etc.), the device according to the invention makes it possible to increase the probability of ionizing them before they have changed state by reducing the time required for the transformation process.

Moreover, the parallelism between said magnetic field and the longitudinal axis is determined by the Larmor radius of the ion of interest (radius of gyration of the ion around the field lines). Thus, the radius of gyration increases with the mass of the ions of interest. Since, according to the invention, the ionized particles in the ionization zone must migrate towards the confinement zone, the requirement of parallelism of the magnetic field with the axis will depend on the Larmor radius of this ion. It is easy to understand that for an ion of interest of low mass, it will be possible to tolerate a greater parallelism error between the magnetic field and the longitudinal axis than for an ion of greater mass interest with a high Larmor radius and which could, for a magnetic field having too much angle with the longitudinal axis, deviate from the planned direction and not migrate towards the second stage. In general, it can be said that the maximum angle θ between the magnetic field vectors located in the ionization zone and between this ionization zone and the entry of the confinement zone must remain less than 30 °. Note that it is possible to decrease the Larmor radius of the ion of interest by having a high magnetic field module in the ionization zone.

• Ledit premier étage d'ionisation est une source d'ions à résonance cyclotronique électronique ;
• Le dispositif selon l'invention comporte un guide d'ondes pour l'injection d'une deuxième onde haute-fréquence dans ladite zone d'ionisation ;
• Le dispositif selon l'invention comporte un système d'injection des éléments à ioniser agencé à proximité de la zone de résonance formant ladite zone d'ionisation de ladite source d'ions à résonance cyclotronique, ledit système restant hors de ladite zone de résonance ;
• Ledit système d'injection est un four injectant de la vapeur d'éléments condensable à ioniser dans ladite zone de résonance ;
• Ledit premier étage d'ionisation est choisi parmi les sources suivantes :
  • o source à décharge,
  • o source à ionisation de surface,
  • o source à thermo-ionisation,
  • o source à Laser,
  • o source ionisation de champ,
  • o source à échange de charge ;
• Le dispositif selon l'invention comporte des moyens pour augmenter localement le module du champ magnétique dans ladite zone d'ionisation ;
• lesdits moyens pour augmenter localement le module du champ magnétique sont formés par un anneau en fer doux ;
• lesdits moyens pour engendrer un champ magnétique dans ladite chambre comportent des aimants permanents dont l'axe de révolution se confond sensiblement avec ledit axe longitudinal ;
• lesdits moyens pour engendrer un champ magnétique dans ladite chambre comportent au moins une bobine, parcourue par un courant d'intensité déterminée, ladite bobine étant réalisée avec un matériau supraconducteur ou un matériau conventionnel ;
• Le dispositif selon l'invention comporte une zone d'extraction desdits ions localisée à l'extrémité opposée à celle dans laquelle se situe ledit premier étage d'ionisation, ledit champ magnétique étant sensiblement parallèle au dit axe longitudinal dans ladite zone d'extraction ;
• lesdits moyens pour engendrer un champ magnétique dans ladite chambre permettent de définir dans ledit deuxième étage de confinement magnétique au moins une ligne fermée de minima dudit champ magnétique, à l'intérieur d'un ou plusieurs volume(s) intérieur(s) à ladite chambre et délimité(s) par des surfaces d'équimodule du champ magnétique fermées dans l'espace.

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

• Said first ionization stage is an electron cyclotron resonance ion source;
• The device according to the invention comprises a waveguide for injecting a second high-frequency wave into said ionization zone;
• The device according to the invention comprises an injection system of the elements to be ionized arranged close to the resonance zone forming said ionization zone of said source of cyclotron resonance ions, said system remaining outside said resonance zone;
• Said injection system is an oven injecting condensable element vapor to be ionized in said resonance zone;
• Said first ionization stage is chosen from the following sources:
  • o discharge source,
  • o surface ionization source,
  • o thermionic ionization source,
  • o Laser source,
  • o field ionization source,
  • o source of charge exchange;
• The device according to the invention comprises means for locally increasing the modulus of the magnetic field in said ionization zone;
• said means for locally increasing the modulus of the magnetic field are formed by a soft iron ring;
• said means for generating a magnetic field in said chamber comprise permanent magnets whose axis of revolution substantially coincides with said longitudinal axis;
• said means for generating a magnetic field in said chamber comprises at least one coil, traversed by a current of determined intensity, said coil being made with a superconductive material or a conventional material;
• The device according to the invention comprises an extraction zone of said ions located at the end opposite to that in which said first ionization stage is located, said magnetic field being substantially parallel to said longitudinal axis in said extraction zone;
• said means for generating a magnetic field in said chamber makes it possible to define in said second magnetic confinement stage at least one closed minimum line of said magnetic field, within one or more volume (s) inside said chamber and delimited by equimodule surfaces of the magnetic field closed in space.

• la figure 1 est une représentation schématique simplifiée du dispositif selon un premier mode de réalisation de l'invention incluant une carte d'intensités du module, d'équimodules et de vecteurs du champ électromagnétique régnant dans le dispositif selon l'invention;
• la figure 2 est une vue en trois dimensions de la configuration mécanique du dispositif de la figure 1 ;
• la figure 3 donne deux spectres d'ions multichargés respectivement avec et sans le fonctionnement du premier étage du dispositif de la figure 1 ;
• la figure 4 donne trois spectres d'ions multichargés respectivement à trois puissances de chauffage différentes du micro-four utilisé pour l'injection des particules neutres dans le premier étage du dispositif de la figure 1 ;
• la figure 5 est une représentation schématique simplifiée du dispositif selon un second mode de réalisation de l'invention incluant une carte d'intensités du module, d'équimodules et de vecteurs du champ électromagnétique régnant dans le dispositif selon l'invention ;
• la figure 6 est une représentation schématique simplifiée du dispositif selon un troisième mode de réalisation de l'invention incluant un dispositif générateur d'ions par thermo-ionisation, une carte d'intensités du module, d'équimodules et de vecteurs du champ électromagnétique régnant dans le dispositif selon l'invention ;
• la figure 7 est une représentation schématique simplifiée du dispositif selon un quatrième mode de réalisation de l'invention incluant un générateur d'ions par excitation laser, une carte d'intensités du module, d'équimodules et de vecteurs du champ électromagnétique régnant dans le dispositif selon l'invention.

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 simplified schematic representation of the device according to a first embodiment of the invention including a module intensities map, equimodules and vectors of the electromagnetic field prevailing in the device according to the invention;
• the figure 2 is a three-dimensional view of the mechanical configuration of the device of the figure 1 ;
• the figure 3 gives two multicharged ion spectra respectively with and without the operation of the first stage of the device of the figure 1 ;
• the figure 4 gives three spectra of multicharged ions respectively at three different heating powers of the micro-furnace used for the injection of the neutral particles in the first stage of the device of the figure 1 ;
• the figure 5 is a simplified schematic representation of the device according to a second embodiment of the invention including a module intensities map, equimodules and vectors of the electromagnetic field prevailing in the device according to the invention;
• the figure 6 is a simplified schematic representation of the device according to a third embodiment of the invention including a device for ion generator by thermo-ionization, a module intensities map, equimodules and vectors of the electromagnetic field prevailing in the device according to the invention;
• the figure 7 is a simplified schematic representation of the device according to a fourth embodiment of the invention including a generator of ions by laser excitation, a module intensities map, equimodules and vectors of the electromagnetic field prevailing in the device according to the invention.

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

The figure 1 is a simplified schematic representation of a device 1 according to a first embodiment of the invention. It will be noted that certain mechanical elements represented on the figure 2 are not represented on the schematic diagram of the figure 1 for a better understanding of this figure. The figure 2 is a three-dimensional view of the mechanical configuration of the device of the figure 1 (for a better understanding of the device 1, the figure 2 represents a section along a vertical plane passing through the longitudinal axis of the device 1).

• une chambre 2 étanche sous vide ayant un axe de symétrie longitudinale AA';
• trois aimants permanents 3, 4 et 5 sensiblement identiques ayant une forme d'anneau et disposés les uns à coté des autres de façon à ce que leur axe de révolution se confonde sensiblement avec l'axe longitudinal AA' de la chambre 2, l'aimant 3 se trouvant à une première extrémité du dispositif 1, l'aimant 5 à l'extrémité opposée et l'aimant 4 étant localisé entre l'aimant 3 et l'aimant 5 ;
• un élément conique en fer doux 6 agencé de sorte que son extrémité de section rétrécie se trouve à l'intérieur de l'aimant 4 intermédiaire.

The device 1 comprises:

• a vacuum sealed chamber 2 having a longitudinal axis of symmetry AA ';
• three substantially identical permanent magnets 3, 4 and 5 having a ring shape and arranged next to each other so that their axis of revolution is substantially confused with the longitudinal axis AA 'of the chamber 2, the magnet 3 being at a first end of the device 1, the magnet 5 at the opposite end and the magnet 4 being located between the magnet 3 and the magnet 5;
• a soft iron conical element 6 arranged so that its narrowed section end is inside the intermediate magnet 4.

The permanent magnets 3, 4 and 5 may be monoblock magnets or magnets composed of several sectors mounted with a magnetization in the same direction.

The figure 1 also includes a map of the intensity of the modules, equimodules and vectors of the electromagnetic field prevailing in the device 1 according to the invention.

Thus, the intensity of the modulus of the magnetic field is represented by dashed lines: the modulus prevailing in the chamber 2 is all the more intense as the dotted lines are dense.

Similarly, several surfaces of equimodules are represented on the figure 1 by continuous lines.

Finally, the magnetic field vectors are represented by arrows.

• un premier étage d'ionisation 7 situé au niveau d'une extrémité de la chambre 2, le premier étage 7 comportant une zone d'ionisation 10 ;
• un deuxième étage de confinement magnétique 8 des ions générés par le premier étage 7 ;
• une zone de migration 12 des ions du premier étage 7 vers le deuxième étage 8 ;
• une zone d'extraction 9 des ions, cette zone pouvant être latérale située dans la zone 8.

The device 1 comprises:

• a first ionization stage 7 situated at one end of the chamber 2, the first stage 7 comprising an ionization zone 10;
• a second magnetic confinement stage 8 of the ions generated by the first stage 7;
• a migration zone 12 of the ions of the first stage 7 to the second stage 8;
• an ion extraction zone 9, this zone being able to be lateral situated in zone 8.

The ionization zone 10 is here an ECR zone (it should be noted that the injection systems of the ions and of the high frequency wave are not represented on the figure 1 ). This ECR zone 10 is here typically a high density zone with a resonance zone operating at 15 GHz (value given purely for guidance for a waveguide for conveying a frequency wave between 8 GHz and 18 GHz). It will be noted that this zone only ensures the ionization of the injected neutral particles and not the confinement of these same ionized particles. This resonance frequency at 15 GHz implies the presence of a magnetic field with a modulus of about 5,300 G to ensure the resonance phenomenon that will allow effective ionization of the neutral particles (obtaining single-charged and multicharged ions). The configuration of the magnetic field of the first stage is ensured by the magnets 3 and 4 as well as by the soft iron conical element 6. The soft iron conical element makes it possible to locally increase the value of the magnetic field module so to obtain the resonance magnetic field at the ionization zone 10. As shown in FIG. figure 2 , the high frequency wave at 15 GHz is transmitted via a waveguide 13 so that the high frequency wave at 15 GHz is injected at the resonance zone 10.

The device 1 also comprises a tube 14 in which a micro-furnace (not shown) is inserted: this micro-furnace makes it possible, by heating a compound to be ionized, to a vapor pressure sufficient to produce condensable elements of the periodic table of Mendeleyev. (Pb for example). The micro-oven is also substantially placed along the longitudinal axis AA 'and must be very close to the resonance zone 10 without however entering this zone. Typically, the micro-oven can be placed 2 mm recessed (see location illustrated by the reference 15) of the end of the waveguide 13: this oven is for example charged with ²⁰⁸ Pb. Remember that the ionization of a condensable element is a fundamental criterion for qualifying the device according to the invention since the non-ionized condensable elements at first pass through the known devices are glued to the walls as soon as they reach them and can only come off if the temperature of the wall is sufficient for the element under consideration.

The ions produced by the first stage 7 at the level of the ionization zone 10 are taken up by the magnetic field substantially parallel to the longitudinal axis AA '(ie the radial component of the magnetic field is substantially zero) both in the ionization zone 10 and then between the ionization zone 10 and the inlet of the second confinement stage so that the ions generated in said ionization zone migrate spontaneously by winding around the field lines to said second stage of containment 8 (note that all ions, mono and multicharged, is supported and migrates to the second stage 8). It will also be noted that the fact of imposing a substantially collinear magnetic field on the axis AA 'implies in fact having a magnetic field with symmetry of revolution. As already mentioned above, the parallelism between the magnetic field and the longitudinal axis AA 'is determined by the Larmor radius of the ion of interest. Thus, the radius of Larmor increases with the mass of the ions of interest (the radius of gyration of the Ar is thus lower than the radius of gyration of the Pb, heavier than the Ar). Since, according to the invention, the ionized particles in the ionization zone 10 must migrate towards the second confinement stage 8, the requirement of parallelism of the magnetic field with the axis will depend on the Larmor radius of this ion . It is easy to understand that for an ion of interest of low mass, it will be possible to tolerate a greater parallelism error between the magnetic field and the longitudinal axis than for an ion of greater mass interest with a high Larmor radius and which could, for a magnetic field having too great an angle with the longitudinal axis AA ', deviate from the intended direction and not migrate towards the second stage 8. In general, it can be said that the maximum angle θ between the magnetic field vectors located in the ionization zone and between this ionization zone and the entry of the confinement zone, must remain below 30 °. It will be noted that it is possible to reduce the Larmor radius of the ion of interest by having a high magnetic field module in the ionization zone 10: the soft iron cone 6 makes it possible to concentrate the magnetic field in this region. zoned.

The two permanent magnets 4 and 5 serve to generate the magnetic field with symmetry of revolution.

The second stage 8 therefore forms a magnetic confinement zone RCE: the magnets 4 and 5 are chosen so that the vector sum of the magnetic fields created at each point of the second stage 8 leads to obtaining a closed line profile of minimal | B | . The reference 16 on the figure 1 designates a surface of equimodule | Bf | (Maximum modulus of the magnetic field in the second stage 8) while the reference 17 designates ellipsoids of revolution defined by lower values of magnetic field. Inside these ellipsoids 17, closed lines of minima are defined. The maximum operating frequency of the second stage 8 is defined by the closed area 16 of maximum field module | Bf |. Such a configuration is described in the patent EP946961 filed by the plaintiff. By way of illustration, the confinement stage RCE typically operates with a corresponding frequency wave of 2.45 GHz at the closed line 11 shown on the figure 1 (corresponding to a magnetic field module approximately equal to 870 G). The high frequency wave at 2.45 GHz is injected via a not shown waveguide inserted in the tubing 18. The ions coming from the ionization zone 10 belonging to the first stage 7 remain confined in the confinement zone 8 and are then extracted. in the so-called extraction zone 9. It will be noted that the confinement zone RCE 8 not only makes it possible to ensure the confinement function of the charged ions during their passage in the ionization zone 10 but also, according to the desired objectives, maintain or increase the state of charge of the ions from the first stage. The second stage can also allow the creation of monocharged ions (in particular in the case of the recombination of certain atoms within the confinement zone 8). The ion extraction zone 9 is located at the end opposite to that in which the first ionization stage 7 is located, the magnetic field being substantially parallel to the longitudinal axis AA 'in this extraction zone 9: as soon as an electron leaves the confinement zone 8 (it preferentially leaves this zone in the extraction zone 9 in which the magnetic field is coaxial with the longitudinal axis of symmetry AA '), there is an ion which will follow the electron and leave the containment zone so as to respect the neutrality of the plasma.

It will be noted that the first and second stages 7 and 8 comprise one and the same continuous plasma.

It will also be noted that it is possible to use a single waveguide for injecting the two high frequency waves (for example a first frequency for the first ionization stage 7 equal to 18 GHz and a second frequency for the second stage confinement equal to 8 GHz transmitted by the same waveguide).

It is also possible to use a carrier gas (injected via a capillary not shown in chamber 2) which makes it possible to increase the electronic population. This carrier gas is preferably a gas whose atoms are of lower mass than those for obtaining the ions of interest. So, in the case of the ionization of ²⁰⁸ Pb, it is possible to use a carrier gas, for example He.

Of course, the waveguide system 13 and the injection system of the neutral elements 14 are connected perfectly tightly to the chamber 2 by means of appropriate joints not shown.

Moreover, the injection of the neutral elements into the ionization zone has been more particularly described in the case of the use of a micro-furnace for condensable elements; obviously, the invention is also applicable to other known sources of production of neutral elements (gas bottle for example).

With a device as represented in Figures 1 and 2 , two spectra of different ions given on the figure 3 depending on whether the first ionization stage does not work (curve in full bold line) or works (non-bold solid line curve). These spectra give the intensity, expressed in microamperes, of the ion current I leaving the device as a function of the current in the analysis magnet, expressed in amperes; this current of analysis gives the ratio Q / A where Q is the charge of the ion and has its mass. The spectra were obtained in the case of ionization of ²⁰⁸ Pb with a micro-oven power equal to 3.75W, a first-stage frequency equal to 9.347 GHz and a second-stage frequency equal to 2.45 GHz. The figure 3 clearly shows a gain in the ionization efficiency according to whether or not the first stage is in operation. Thus, a gain (ratio of ion currents between the spectrum with operation of the first stage and the non-functioning spectrum of the first stage) is observed equal to 3.1 for the ²⁰⁸ Pb ³⁺ ion and 2.7 for the ²⁰⁸ Pb ion. ²⁺ . There is also a gain in efficiency for ²⁰⁸ Pb ⁴⁺ and ²⁰⁸ Pb ¹⁺ ions.

The figure 4 shows the evolution of intensities of ²⁰⁸ Pb with the variation of the power of the micro-furnace. The higher the power of the micro-oven, the greater the ionization is important. Overall, we note a direct gain in total intensity of ²⁰⁸ Pb (in particles) ranging from a gain of 1.4 (for a micro-oven power of 3.36 W) to 2.2 (for a micro-oven power of 5.37W) .

Note also that a test (not shown) of operation of the first stage alone (without operating the confinement stage RCE) shows a very low production of ions.

The magnetic field in the example shown in Figures 1 and 2 is created by a permanent magnet system. However, to obtain magnetic fields corresponding to higher ECR operating frequencies, the use of a coil system is also possible. So, the figure 5 illustrates a simplified schematic representation of a device 100 according to a second embodiment of the invention including a module intensities map, equimodules and vectors of the electromagnetic field prevailing in the device according to the invention

• une chambre 102 étanche sous vide ayant un axe de symétrie longitudinale AA';
• deux aimants permanents 104 et 105 sensiblement identiques ayant une forme d'anneau et disposés les uns à coté des autres de façon à ce que leur axe de révolution se confonde sensiblement avec l'axe longitudinal AA' de la chambre 102 ;
• une bobine 101 parcourue par un courant d'intensité déterminée et dont l'axe de révolution se confond sensiblement avec l'axe longitudinal AA' de la chambre 102
• un élément conique en fer doux 106 agencé de sorte que son extrémité de section rétrécie se trouve à l'intérieur de la bobine 101.

The device 100 comprises:

• a vacuum sealed chamber 102 having a longitudinal axis of symmetry AA ';
• two substantially identical permanent magnets 104 and 105 having a ring shape and arranged next to each other so that their axis of revolution is substantially confused with the longitudinal axis AA 'of the chamber 102;
• a coil 101 traversed by a current of determined intensity and whose axis of revolution substantially coincides with the longitudinal axis AA 'of the chamber 102
• a soft iron cone member 106 arranged so that its narrowed end section is within the coil 101.

As for the figure 1 , the figure 5 also includes a map of the intensity of the modules, equimodules and vectors of the electromagnetic field prevailing in the device 100 according to the invention.

Thus, the intensity of the modulus of the magnetic field is represented by dotted lines: the modulus prevailing in the chamber 102 is all the more intense as the dotted lines are dense.

Similarly, several surfaces of equimodules are represented on the figure 5 by continuous lines.

Finally, the magnetic field vectors are represented by arrows.

• un premier étage d'ionisation 107 située au niveau d'une extrémité de la chambre 102, le premier étage 7 comportant une zone d'ionisation 110 ;
• un deuxième étage de confinement magnétique 108 des ions générés par le premier étage 7 ;
• une zone de migration 112 des ions du premier étage 107 vers le deuxième étage 108 ;
• une zone d'extraction 109 des ions.

The device 100 comprises:

• a first ionization stage 107 located at one end of the chamber 102, the first stage 7 having an ionization zone 110;
• a second magnetic confinement stage 108 of the ions generated by the first stage 7;
• a migration zone 112 of the ions of the first stage 107 to the second stage 108;
• an extraction zone 109 of the ions.

The ionization zone 110 here is an ECR zone with a higher frequency than the ECR zone of the figure 1 This RCE zone 110 is here typically a high density zone with a resonance zone operating at 29 GHz. As for the figure 1 this zone only ensures the ionization of the injected neutral particles and not the confinement of these same ionized particles. This resonant frequency at 29 GHz implies the presence of a very high magnetic field to ensure the resonance phenomenon that will allow effective ionization of neutral particles (obtaining single-charged and multicharged ions). The soft iron cone element 106 makes it possible to locally increase the value of the magnetic field module so as to obtain the resonance magnetic field at the ionization zone 110.

With the only difference of the coil 101 which makes it possible to use a higher frequency area RCE for the ionization stage, the device 100 of the figure 5 is identical to device 1 of the figure 1 and works in a similar way.

The different embodiments described so far ( Figures 1 and 2 and figure 5 ) all included an ECR first floor. It is important to note, however, that the device according to the invention can operate with other types of ion sources, the only condition being that the ions are produced in an area where the magnetic field is coaxial with the axis of symmetry. of the chamber so that the ions created spontaneously migrate to the second confinement stage.

• source à décharge,
• source à ionisation de surface,
• source à thermo-ionisation,
• source à Laser,
• source ionisation de champ,
• source à échange de charge.

Thus, in place of a first ECR stage, the first ionization stage can also be chosen from the following sources:

• discharge source,
• surface ionization source,
• thermionic ion source,
• source at Laser,
• ionization field source,
• source with charge exchange.

By way of illustration, Figures 6 and 7 illustrate a simplified schematic representation of device 200 and 300 respectively according to a third and a fourth embodiment of the invention including a module intensities map, equimodules and vectors of the electromagnetic field prevailing in the device according to the invention .

The devices 200 and 300 are identical to the device 1 of the figure with the difference that the first ionization stage is not an RCE stage. We have kept the same references for the common elements with the device 1 of the figure 1 .

The device 200 of the figure 6 differs from device 1 of the figure 1 only in that the ionization source 201 is a surface ionization source, the ionization stage 207 of the device 200 therefore not being an ECR device. The end of the source 201 is in the zone 10 forming the ionization zone of the device 200 in which the magnetic field is coaxial with the longitudinal axis AA 'of the chamber 2 of the device 200. It can be seen that the the permanent magnet 3 and the soft iron cone 6 have been preserved so as to obtain a concentration of the field module Magnetic at the ionization zone 10: this field concentration makes it possible to have ions with weaker Larmor rays and is particularly useful for heavy particles.

The device 300 of the figure 7 differs from device 1 of the figure 1 only in that the ionization source 301 is a source of laser excitation and ionization (one of the principles of which is that of a focused laser light beam which heats a target on a local scale; thermal expansion locally creates a shock wave which expels a "feather" plasma very hot and dense, another principle is a laser resonant ionization source for removing a peripheral electron), the ionization stage 307 of the device 300 is not a device RCE. The end of the source 301 is in the zone 10 forming the ionization zone of the device 300 in which the magnetic field is coaxial with the longitudinal axis AA 'of the chamber 2 of the device 300. Again, it can be seen that that the permanent magnet 3 and the soft iron cone 6 have been preserved so as to obtain a concentration of the magnetic field module at the level of the ionization zone 10.

Claims (13)

1. An electron cyclotron resonance ion generator device (1, 100, 200, 300) including:

   - a vacuum sealed chamber (2, 102) for containing a plasma, said chamber (2, 102) being of axial symmetry along a longitudinal axis (AA'),

   - means (3, 4, 5, 6, 101, 104, 105, 106) for generating a magnetic field in said chamber (2, 102), said magnetic field having a revolution symmetry relative to said longitudinal axis (AA'),

   - means for propagating a high frequency wave inside said chamber (2, 102),

   said device (1, 100, 200, 300) being characterised in that said chamber (2, 102) includes:

   - a first ionisation stage (7, 107, 207, 307) located at one end of said chamber (2, 102), said first stage including an ionisation zone (10, 110) in which ions are generated, said magnetic field being substantially parallel to said longitudinal axis (AA') in said ionisation zone (10, 110),

   - a second stage (8, 108) for magnetically confining said ions generated in said ionisation zone (10, 110), said second stage (8, 108) using a first high frequency wave propagating in said chamber (2, 102) from said means for propagating a high frequency wave,

   said magnetic field being substantially parallel to said longitudinal axis (AA') between said ionisation zone (10, 110) and said second confinement stage (8, 108) such that the ions generated in said ionisation zone (10, 110) migrate to said second confinement stage (8, 108) and in that said first and second stages (7, 107, 207, 307, 8, 108) include a same continuous plasma.
2. The device (1, 100) according to claim 1, characterised in that the first ionisation stage (7, 107) is an electron cyclotron resonance ion source.
3. The device (1) according to claim 2, characterised in that it includes a waveguide (13) for injecting a second high frequency wave into said ionisation zone (10).
4. The device (1) according to one of claims 2 or 3, characterised in that it includes a system for injecting elements to be ionised arranged in the proximity of the resonance zone forming said ionisation zone (10) of said cyclotron resonance ion source, said system remaining off said resonance zone.
5. The device (1) according to claim 4, characterised in that said injecting system is an oven injecting vapour of condensable elements to be ionised in said resonance zone.
6. The device (1) according to claim 5, characterised in that said oven is arranged such that it is substantially parallel to said longitudinal axis.
7. The device (200, 300) according to claim 1, characterised in that the first ionisation stage is chosen from the following sources:

   - discharge source,

   - surface ionisation source (207),

   - thermo-ionisation source,

   - laser source (307),

   - field ionisation source,

   - charge exchange source.
8. The device (1, 100, 200, 300) according to one of the preceding claims, characterised in that it includes means (6, 106) for locally increasing the module of the magnetic field in said ionisation zone.
9. The device (1, 100, 200, 300) according to the preceding claim, characterised in that said means (6, 106) for locally increasing the module of the magnetic field are formed by a soft iron ring.
10. The device (1) according to one of the preceding claims, characterised in that said means for generating a magnetic field in said chamber include permanent magnets (3, 4, 5) the revolution axis of which is substantially the same as said longitudinal axis (AA').
11. The device (100) according to one of the preceding claims, characterised in that said means for generating a magnetic field in said chamber include at least one coil (101) through which a current having a determined intensity passes.
12. The device (1, 100, 200, 300) according to one of the preceding claims, characterised in that it includes a zone (9, 109) for extracting said ions located at the opposite end to that in which said first ionisation stage is located, said magnetic field being substantially parallel to said longitudinal axis (AA') in said extracting zone (9, 109).
13. The device according to one of the preceding claims, characterised in that said means for generating a magnetic field in said chamber make it possible to define in said second magnetic confining stage at least one closed line of minima of said magnetic field, inside one or more volume(s) internal to said chamber and delimited by magnetic field-equimodular surfaces closed in space.

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