Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J27/00—Ion beam tubes
  • H01J27/02—Ion sources; Ion guns
  • H01J27/16—Ion sources; Ion guns using high-frequency excitation, e.g. microwave excitation
  • H01J27/18—Ion sources; Ion guns using high-frequency excitation, e.g. microwave excitation with an applied axial magnetic field
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05H—PLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
  • H05H3/00—Production or acceleration of neutral particle beams, e.g. molecular or atomic beams
  • H05H3/02—Molecular or atomic beam generation

Definitions

• the present invention relates to an electron cyclotron resonance ion generating device.
• ECR sources 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 in the vacuum chamber is of the order of 10- 6 to 10- 1 Pa.
• 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 certain temperature by a suitable cryogenic system.
• 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.
• the magnetic field has in particular a Br module which satisfies the condition (1) of electronic cyclotron resonance:
• Br f.2 ⁇ m / e (1)
• 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.
• the quantity of ions that can be produced is the result of 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 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 enclosure.
• 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.
• Patent EP946961 filed by the Applicant describes an ECR source using a magnetic field with 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.
• the ionization efficiencies for the condensable elements are a few per thousand, for a 2.45 GHz frequency wave up to 20% for a 15 Ghz frequency wave. . Note that it is the same for the production of radioactive ions whose effectiveness will be very dependent on the life of these elements.
• 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.
• 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.
• an electron cyclotron resonance ion generating device comprising:
• a vacuum sealed chamber intended to contain a plasma, said chamber being axially symmetrical along a longitudinal axis,
• said chamber comprises:
• a first ionization stage situated at one end of said chamber, said first stage comprising an ionization 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 issuing 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.
• 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).
• 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.
• 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 rotationally symmetrical RCE-type source as described in patent EP946961 of the applicant.
• 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.
• 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).
• 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.
• 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 near the resonance zone forming said ionization zone of said cyclotronic resonance ion source, said system remaining outside said zone. resonance;
• 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 ionisation field source, o source with 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 merges with said longitudinal axis;
• said means for generating a magnetic field in said chamber comprise 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, inside one or more internal volume (s); ) to said chamber and bounded by equimodular surfaces of the magnetic field closed in space.
• FIG. 1 is a simplified schematic representation of the device according to a first embodiment of the invention including an intensity map of the module, equimodules and vectors of the electromagnetic field prevailing in the device according to the invention;
• FIG. 2 is a three-dimensional view of the mechanical configuration of the device of FIG. 1;
• FIG. 3 gives two multi-charged ion spectra respectively with and without the operation of the first stage of the device of FIG. 1;
• FIG. 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 into the first stage of the device of FIG. 1;
• FIG. 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;
• FIG. 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;
• FIG. 7 is a simplified schematic representation of the device according to a fourth embodiment of the invention including a laser excitation ion generator, a module intensity map, equimodules and vectors of the electromagnetic field prevailing in the device according to the invention.
• Figure 1 is a simplified schematic representation of a device 1 according to a first embodiment of the invention. Note that some mechanical elements shown in Figure 2 are not shown in the block diagram of Figure 1 for a better understanding of this figure.
• Figure 2 is a three-dimensional view of the mechanical configuration of the device of Figure 1 (for a better understanding of the device 1, Figure 2 shows a section along a vertical plane passing through the longitudinal axis of the device 1).
• the device 1 comprises: a vacuum-tight chamber 2 having a longitudinal axis of symmetry AA ';
• 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.
• FIG. 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.
• 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.
• the device 1 comprises:
• first stage 7 situated at one end of the chamber 2, the first stage 7 comprising an ionization zone 10;
• the ionization zone 10 is here an ECR zone (it will be noted that the injection systems of the ions and of the high frequency wave are not shown in FIG. 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 which will allow the efficient 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.
• 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 level of the resonance zone 10.
• the device 1 also comprises a tube 14 in which a micro-oven (not shown) is inserted: this micro-oven allows, by heating a compound to be ionized, up to a pressure of sufficient vapor to produce condensable elements of the periodic table of Mendeleyev (Pb for example).
• the micro-furnace is also substantially placed along the longitudinal axis AA 'and must be very close to the resonance zone 10 without however entering this zone.
• 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.
• the ionization of a The 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 not come off unless 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 towards said second zone containment stage 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.
• the parallelism between the magnetic field and the longitudinal axis AA ' is determined by the Larmor radius of the ion of interest.
• the radius of Larmor increases with the mass of the ions of interest (the radius of gyration of I Ar is thus smaller than the radius of gyration of Pb, heavier than I 1 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 .
• 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 minima
• Reference 16 in FIG. 1 designates a surface of equimodule I Bf
• the maximum operating frequency of the second stage 8 is defined by the closed area 16 of maximum field module
• the RCE confinement stage typically operates with a corresponding 2.45 GHz frequency wave. to the closed line 1 1 shown in 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 then are extracted in the so-called extraction zone 9.
• 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 objectives sought, to 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.
• first and second stages 7 and 8 comprise one and the same continuous plasma.
• 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.
• AIN- if, in the case of the ionization of 208 Pb, a carrier gas, for example He, can be used.
• the waveguide system 13 and the injection system of the neutral elements 14 are connected in a perfectly sealed manner to the chamber 2 by means of appropriate joints not shown.
• the injection of the neutral elements into the ionization zone has been more particularly described in the case of the use of a microfur for condensable elements; obviously, the invention is also applicable to other known sources of production of neutral elements (gas bottle for example).
• Figure 4 shows the evolution of the intensities of 208 Pb with the variation of the power of the micro-furnace.
• a test (not shown) of operation of the first stage alone (without operating the confinement stage RCE) shows a very low production of ions.
• FIG. 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.
• the device 100 comprises:
• a vacuum tight chamber 102 having a longitudinal axis of symmetry AA ';
• Two permanent magnets 104 and 105 substantially identical 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;
• FIG. 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.
• 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.
• several surfaces of equimodules are represented in FIG. 5 by continuous lines.
• the device 100 comprises:
• a first ionization stage 107 situated at one end of the chamber 102, the first stage 7 comprising an ionization zone 1 10;
• the ionization zone 110 is here a higher frequency RCE zone than the ECR zone of FIG. 1 made by means of the coil 101.
• This RCE zone 1 10 here is typically a high density zone with a resonance zone operating at 29 GHz. As for FIG. 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 1 10.
• the device 100 of FIG. 5 is identical to the device 1 of FIG. 1 and operates in a similar manner.
• the various embodiments described so far (FIGS. 1 and 2 and FIG. 5) all included a first ECR stage. 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.
• the first ionization stage can also be chosen from the following sources: gas discharge source,
• FIGS. 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, equimodules and vectors intensity map. the electromagetic 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 retained the same references for the elements common to the device 1 of FIG. 1.
• the device 200 of FIG. 6 differs from the device 1 of FIG. 1 only in that the ionization source 201 is an ionization source. surface, the ionization stage 207 of the device 200 is not a device RCE.
• 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.
• the device 300 of FIG. 7 differs from the device 1 of FIG. 1 only in that the ionization source 301 is a laser excitation and ionization source (one of the principles of which is that of a light beam Focused laser that heats a target punctually, thermal expansion locally creates a shock wave that expels a "quill" plasma very hot and dense, another principle is a resonant laser ionization source to remove a peripheral electron), l Ionization stage 307 of the device 300 is therefore not an ECR device.
• 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.

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