EP0199625B1 - Electron cyclotron resonance negative ion source - Google Patents

Electron cyclotron resonance negative ion source Download PDF

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Publication number
EP0199625B1
EP0199625B1 EP86400726A EP86400726A EP0199625B1 EP 0199625 B1 EP0199625 B1 EP 0199625B1 EP 86400726 A EP86400726 A EP 86400726A EP 86400726 A EP86400726 A EP 86400726A EP 0199625 B1 EP0199625 B1 EP 0199625B1
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Prior art keywords
enclosure
negative ion
ion source
ions
extraction
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German (de)
French (fr)
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EP0199625A1 (en
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Göran Hellblom
Claude Jacquot
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Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
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Commissariat a lEnergie Atomique CEA
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    • 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
    • G—PHYSICS
    • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
    • G21K—TECHNIQUES FOR HANDLING PARTICLES OR IONISING RADIATION NOT OTHERWISE PROVIDED FOR; IRRADIATION DEVICES; GAMMA RAY OR X-RAY MICROSCOPES
    • G21K1/00—Arrangements for handling particles or ionising radiation, e.g. focusing or moderating
    • G21K1/14—Arrangements for handling particles or ionising radiation, e.g. focusing or moderating using charge exchange devices, e.g. for neutralising or changing the sign of the electrical charges of beams
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J27/00—Ion beam tubes
    • H01J27/02—Ion sources; Ion guns
    • H01J27/028—Negative ion sources

Definitions

  • the present invention relates to a source of negative ions with electronic cyclotron resonance. It is advantageously applied in the production of high intensity H- ion beams (greater than 1 A) or of its D- or T- isotopes, these beams being mainly used for the production of beams of energetic neutral atoms ( intensity of several tens of amps and energy from 200 to 500 KeV) used in particular as efficient means of heating thermonuclear plasmas produced in fusion devices with magnetic confinement. Furthermore, high intensity H-, D- or T- ion beams can also be used in nuclear physics and in particular in accelerators of the Van de Graaf-Tandem type or in the medical field using accelerators of the type variable energy cyclotron.
  • volume ionization One of the techniques currently known for producing beams of negative ions and in particular of H-, D- and T- ions is volume ionization. This technique is based on the formation, from a gas or a vapor contained in a closed enclosure, of a plasma constituted mainly in the case of hydrogen of H-, H + ions and electrons .
  • This technique consists first of all of creating molecules of hydrogen, deuterium or tritium depending on the starting gas used, vibrational excited by hot or energetic electrons, that is to say whose kinetic energy is higher at 20 eV, according to the following reaction scheme (1), in the case of hydrogen:
  • H-, D- or T- ions are formed by the following dissociative attachment reaction (2), in the case of hydrogen:
  • the intermediate compound is unstable.
  • the effective cross-sections of attachment are high for so-called cold electrons having a kinetic energy at most equal to 1 eV.
  • This dissociative attachment phenomenon has in particular been described in an article by M. BA-CAL et al., Phys. Rev. Letters, 42, 1538 (1979).
  • the difficulty in such a technique for producing negative ions is to create in the closed enclosure of the ion source a population of energetic or hot electrons and a population of cold electrons, spatially separated so that the hot electrons do not destroy the negative ions formed by a collision in the case of hydrogen, of the type:
  • the destruction of the negative ions formed by reaction with the hot electrons of the plasma is relatively large, which is detrimental to the production of a beam of intense negative ions.
  • the number of negative ions constituting the plasma created in the enclosure represents only 10% of the number of positive ions.
  • the object of the present invention is precisely a source of negative ions making it possible to remedy the various drawbacks above.
  • it makes it possible to produce an intense negative ion beam, in particular of H-, D- or T- ions, using as physical phenomena the dissociative attachment technique as well as electronic cyclotron resonance.
  • This resonance phenomenon is generally used to produce multi-charged positive ions.
  • the European patent application No. 0,127,523 filed in the name of applicant describes a positive ion source operating on the principle of electron cyclotron resonance.
  • This condition of electronic cyclotron resonance makes it possible to create energetic or hot electrons, of kinetic energy greater than 20 eV, in a direction perpendicular to the magnetic field.
  • These hot electrons by collision on the molecules of gas or vapor contained in the source, generate other electrons which will also be accelerated by cyclotronic resonance.
  • the hot plasma of electrons thus formed makes it possible, according to the reaction mechanism (1), to excite the molecules of the gas or of the vapor.
  • the electrons formed by the interaction of the electromagnetic wave and gas or vapor molecules, have a lower energy, for example at most equal to 1 eV. These cold electrons interact with the unexcited neutral molecules of gas or vapor, creating positive ions and other cold electrons, thus forming a cold plasma of electrons. Given the profile of the amplitude of the magnetic field, this cold electron plasma is mainly located in the extraction zone of negative ions. According to the reaction mechanism (2), this cold plasma of electrons formed allows the formation of negative ions.
  • the source of negative ions according to the invention allows the formation of a hot electron plasma and a cold electron plasma well separated spatially, thus making it possible to form negative ions, and in particular H- ions, D- or T-, by dissociative attachment and by electronic cyclotron resonance, while avoiding the destruction of negative ions formed by collisions with energetic electrons, according to the reaction mechanism (3).
  • the negative ions thus formed and extracted from the plasma can advantageously be accelerated using appropriate means located downstream of the extraction means.
  • This final acceleration of the ions can for example be obtained by using an electrode, pierced with one or more openings to allow the passage of the ions, brought to a positive potential relative to that of the extraction means.
  • means for reducing the amplitude of the magnetic field at the level of the ion extraction means can be provided.
  • This local cancellation of the amplitude of the magnetic field can advantageously be carried out using as means of extraction of the negative ions an electrode or plate made of a ferromagnetic substance, pierced with slots or holes to allow the passage of the negative ions formed.
  • This cancellation of the amplitude of the magnetic field at the level of the extraction of the ions traps the electrons which have not reacted with the gas or vapor molecules, thus making it possible to avoid their acceleration between the means of extraction and of acceleration, and therefore their output from the source.
  • the means for injecting the electromagnetic field comprise a waveguide whose end, mounted on the enclosure, is equipped with a window in a dielectric material.
  • the source of negative ions comprises a containment vacuum enclosure 2 constituting a resonant cavity which can be excited by a microwave electromagnetic field.
  • This enclosure 2 has an axis of symmetry Z, which in the case of a cylindrical enclosure represents the axis of revolution.
  • the electromagnetic wave produced by a source 4 such as a klystron is introduced into the resonant cavity 2 by means of a waveguide 6, of circular or rectangular section, comprising at its end mounted on the enclosure a window 8 made of a dielectric material such as A1 2 0 3 .
  • This wave can be continuous or pulsed and have a frequency between 1 and 100 GHz.
  • a pipe 10 makes it possible to introduce a gas or a vapor of a material inside the cavity 2 intended to form a plasma in said cavity.
  • this introduction of gas is carried out near the introduction of the electromagnetic wave.
  • the enclosure 2 can for example be filled with hydrogen, deuterium or tritium at a pressure of 1 to 10 mtorr (0.134 to 1.34 Pa).
  • Means not shown, such as a diffusion or cryogenic pump, mounted on the cavity 2 allow the maintenance of a high vacuum inside the cavity.
  • the cavity 2 is brought to an electrostatic potential -V with respect to the mass.
  • it is surrounded by two coils 12 and 14, the coil 12 being supplied in counter-field, making it possible to create a magnetic field of axial symmetry.
  • the axis of symmetry of this magnetic field can be merged with the axis of symmetry Z of the cavity 2.
  • the arrows 16 represent the field lines of the magnetic field. This magnetic field can be either continuous or pulsed.
  • the source of negative ions according to the invention further comprises means making it possible to extract the ions formed.
  • These means consist for example of a conductive plate 18 brought to a positive potential with respect to the enclosure 2, for example to a potential -V + AV. They are mounted on one end of the enclosure and isolated from them by means of an insulating ring 19.
  • These means 18 are equipped with at least one hole or a slot 20 allowing the passage of ions negative. This extrac orifice tion 20 is for example located on the axis of symmetry Z of the microwave cavity.
  • V and of V can be between -1500 V and -2000 V and A V between 5 and 20 volts.
  • the extraction electrode 18, negative ions can be followed by another electrode 22 brought to a positive potential with respect to the extraction electrode 18, and for example to the ground potential, in order to accelerate negative ions formed to their final value.
  • This electrode 22 is of course equipped with at least one opening 24, located in particular on the axis of symmetry Z of the cavity, allowing the exit of the negative ions formed, outside the source.
  • the positions of the extraction 18 and acceleration 20 electrodes are advantageously adjustable along the Z axis.
  • the electromagnetic waveguide 6 and the extraction 18 and acceleration 22 electrodes of the ion source are arranged at the two opposite ends of the resonant cavity 2; the axis of symmetry of the waveguide 6 and those of the openings 20 and 24, reciprocally made in the electrodes 18 and 22, coincide with the axis of symmetry Z of the cavity.
  • the coils 12 and 14 surrounding the cavity 2 make it possible, as shown in FIG. 2, to create a magnetic field of axial symmetry in the enclosure whose amplitude B increases from the window 8 of the injector of the electromagnetic wave at the extraction electrode 18.
  • This magnetic field has, at a point Z R taken on the axis of symmetry of the cavity 2, and approximately at the center of said cavity, an amplitude B R satisfying the condition of electronic cyclotronic resonance (4), thus allowing the formation of energetic electrons used for the vibrational excitation of the molecules of the gas contained in enclosure 2.
  • this magnetic field has a maximum amplitude B M just upstream of l extraction electrode 18, the position of which is marked by the dimension Z e .
  • the electrons acquire a strong kinetic energy, perpendicular to the magnetic field.
  • these electrons undergo a mirror effect and are subjected to a force.
  • ⁇ being the magnetic moment of the electron. They are therefore accelerated towards window 8 of the electromagnetic injector; the direction of movement of these electrons is illustrated by the arrow F.
  • the energetic electrons entrain, by electrostatic or ambipolar effect, the positive ions such as H + , D + or T + formed during the ionization of the hydrogen gas, deuterium or tritium contained in the enclosure 2.
  • the positive ions such as H + , D + or T + formed during the ionization of the hydrogen gas, deuterium or tritium contained in the enclosure 2.
  • This more positive potential is responsible for the self-acceleration of the H- ions, represented by the arrow F ', these ions being created in the ion extraction zone, that is to say near and upstream of the electrode 18.
  • Negative ions and, for example, H-, D- or T- ions are created preferentially in the region of extraction of the ions, because the molecules of gas vibratively excited, equation (1), are insensitive to the magnetic field; they can therefore diffuse isotropically.
  • the amplitude of the magnetic field can advantageously cancel out at the level of the extraction electrode 18, that is to say at the Ze dimension, in order to carry out a trapping of the electrons of the plasma. thus making it possible to avoid their acceleration between the extraction electrode 18 and the electrode 22.
  • This cancellation of the magnetic field can for example be obtained by using an extraction electrode 18 made of a ferromagnetic substance.
  • the source of negative ions according to the invention allowed the production of a beam of H- ions having an energy of 2 KeV per nucleon and an intensity of 10 mA using an average microwave power of 1 kW, a cyclotronic frequency. 10 GHz electronics and a magnetic field whose amplitude increases from 0.2 to 0.45 T.
  • the ion source had a cylindrical cavity 10 cm in diameter and 15 cm in length; it was brought to a negative potential of -2000 volts and the extraction electrode 18 to a potential of 2 volts higher than that of the cavity, ie -1998 V.
  • the pressure of the hydrogen gas contained in the enclosure was 0 , 2 Pa.
  • the magnetic field with axial symmetry can be produced by ferrites instead of the use of two coils supplied in counter-field and surrounding the microwave cavity.
  • the cavity may have a shape other than a cylindrical shape, for example a parallelepiped shape.
  • the source according to the invention can produce other types of negative ions and in particular oxygen, sodium, lithium and iodine ions.

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  • Engineering & Computer Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Combustion & Propulsion (AREA)
  • Physics & Mathematics (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • General Engineering & Computer Science (AREA)
  • High Energy & Nuclear Physics (AREA)
  • Electron Sources, Ion Sources (AREA)
  • Particle Accelerators (AREA)

Description

La présente invention a pour objet une source d'ions négatifs à résonance cyclotronique électronique. Elle s'applique avantageusement dans la production de faisceaux d'ions H- de forte intensité (supérieure à 1 A) ou de ses isotopes D- ou T-, ces faisceaux étant principalement utilisés pour la production de faisceaux d'atomes neutres énergétiques (intensité de plusieurs dizaines d'ampères et énergie de 200 à 500 KeV) employés notamment comme moyens efficaces de chauffage des plasmas thermonucléaires produits dans les dispositifs de fusion à confinement magnétique. Par ailleurs, les faisceaux d'ions H-, D- ou T- de forte intensité peuvent aussi être utilisés en physique nucléaire et en particulier, dans des accélérateurs du type Van de Graaf-Tandem ou dans le domaine médical utilisant des accélérateurs du type cyclotron à énergie variable.The present invention relates to a source of negative ions with electronic cyclotron resonance. It is advantageously applied in the production of high intensity H- ion beams (greater than 1 A) or of its D- or T- isotopes, these beams being mainly used for the production of beams of energetic neutral atoms ( intensity of several tens of amps and energy from 200 to 500 KeV) used in particular as efficient means of heating thermonuclear plasmas produced in fusion devices with magnetic confinement. Furthermore, high intensity H-, D- or T- ion beams can also be used in nuclear physics and in particular in accelerators of the Van de Graaf-Tandem type or in the medical field using accelerators of the type variable energy cyclotron.

L'une des techniques actuellement connue pour produire des faisceaux d'ions négatifs et en particulier des ions H-, D- et T- est l'ionisation en volume. Cette technique est basée sur la formation, à partir d'un gaz ou d'une vapeur contenu dans une enceinte fermée, d'un plasma constitué principalement dans le cas de l'hydrogène d'ions H-, H+ et d'électrons.One of the techniques currently known for producing beams of negative ions and in particular of H-, D- and T- ions is volume ionization. This technique is based on the formation, from a gas or a vapor contained in a closed enclosure, of a plasma constituted mainly in the case of hydrogen of H-, H + ions and electrons .

Cette technique consiste tout d'abord à créer des molécules d'hydrogène, de deutérium ou de tritium selon le gaz de départ utilisé, excitées vibrationnellement par des électrons chauds ou énergétiques, c'est-à-dire dont l'énergie cinétique est supérieure à 20 eV, selon le schéma réactionnel (1) suivant, dans le cas de l'hydrogène:

Figure imgb0001
This technique consists first of all of creating molecules of hydrogen, deuterium or tritium depending on the starting gas used, vibrational excited by hot or energetic electrons, that is to say whose kinetic energy is higher at 20 eV, according to the following reaction scheme (1), in the case of hydrogen:
Figure imgb0001

Ensuite, à partir de ces molécules excitées (H2 *) on forme des ions H-, D- ou T- par la réaction d'attachement dissociatif (2) suivante, dans le cas de l'hydrogène:

Figure imgb0002
Then, from these excited molecules (H 2 * ), H-, D- or T- ions are formed by the following dissociative attachment reaction (2), in the case of hydrogen:
Figure imgb0002

Dans ce schéma réactionnel, le composé intermédiaire est instable. Les sections efficaces d'attachement sont élevées pour des électrons, dits froids, présentant une énergie cinétique au plus égale à 1 eV. Ce phénomène d'attachement dissociatif a notamment été décrit dans un article de M. BA-CAL et al., Phys. Rev. Letters, 42, 1538 (1979).In this reaction scheme, the intermediate compound is unstable. The effective cross-sections of attachment are high for so-called cold electrons having a kinetic energy at most equal to 1 eV. This dissociative attachment phenomenon has in particular been described in an article by M. BA-CAL et al., Phys. Rev. Letters, 42, 1538 (1979).

La difficulté dans une telle technique de production d'ions négatifs est de créer dans l'enceinte fermée de la source d'ions une population d'électrons énergétiques ou chauds et une population d'électrons froids, séparées spatialement de façon que les électrons chauds ne détruisent pas les ions négatifs formés par une collision dans le cas de l'hydrogène, du type:

Figure imgb0003
The difficulty in such a technique for producing negative ions is to create in the closed enclosure of the ion source a population of energetic or hot electrons and a population of cold electrons, spatially separated so that the hot electrons do not destroy the negative ions formed by a collision in the case of hydrogen, of the type:
Figure imgb0003

Or, dans les sources d'ions négatifs connues, fonctionnant sur le principe ci-dessus, la destruction des ions négatifs formés par réaction avec les électrons chauds du plasma est relativement importante, ce qui est néfaste à la production d'un faisceau d'ions négatifs intense. En général, le nombre d'ions négatifs constituant le plasma créé dans l'enceinte ne représente que 10% du nombre d'ions positifs.However, in the known sources of negative ions, operating on the above principle, the destruction of the negative ions formed by reaction with the hot electrons of the plasma is relatively large, which is detrimental to the production of a beam of intense negative ions. In general, the number of negative ions constituting the plasma created in the enclosure represents only 10% of the number of positive ions.

Par ailleurs, dans les sources d'ions négatifs, créés à partir d'un plasma, il existe une autre difficulté liée à la technique d'extraction des ions négatifs par effet électrostatique ou ambipolaire. En effet, l'extraction ou la décharge par effet électrostatique de particules (ions positifs, électrons, etc...) dans une source de particules quelconque est toujours réalisée au moyen d'électrodes d'extraction portées à un potentiel positif par rapport aux parois de l'enceinte formée; ceci est dû à la grande mobilité des électrons du plasma. Or, si pour l'extraction d'ions positifs, ce potentiel positif aide à l'extraction, dans le cas d'extraction d'ions négatifs, ce potentiel empêche les ions négatifs de sortie et les confine électrostatiquement dans l'enceinte. Ceci est encore néfaste à la production d'un faisceau d'ions négatifs intense.In addition, in the sources of negative ions, created from a plasma, there is another difficulty linked to the technique of extraction of negative ions by electrostatic or ambipolar effect. Indeed, the extraction or the discharge by electrostatic effect of particles (positive ions, electrons, etc ...) in any source of particles is always carried out by means of extraction electrodes brought to a positive potential compared to walls of the enclosure formed; this is due to the high mobility of plasma electrons. However, if for the extraction of positive ions, this positive potential helps the extraction, in the case of extraction of negative ions, this potential prevents the negative ions from leaving and confines them electrostatically in the enclosure. This is still detrimental to the production of an intense negative ion beam.

La présente invention a justement pour objet une source d'ions négatifs permettant de remédier aux différents inconvénients ci-dessus. Elle permet notamment de produire un faisceau d'ions négatifs intense notamment d'ions H-, D- ou T-, en utilisant comme phénomènes physiques la technique d'attachement dissociatif ainsi que la résonance cyclotronique électronique. Ce phénomène de résonance est généralement utilisé pour produire des ions positifs multichargés. La demande de brevet européenne n° 0 127 523 déposée au nom du demandeur décrit une source d'ions positifs fonctionnant sur le principe de la résonance cyclotronique électronique.The object of the present invention is precisely a source of negative ions making it possible to remedy the various drawbacks above. In particular, it makes it possible to produce an intense negative ion beam, in particular of H-, D- or T- ions, using as physical phenomena the dissociative attachment technique as well as electronic cyclotron resonance. This resonance phenomenon is generally used to produce multi-charged positive ions. The European patent application No. 0,127,523 filed in the name of applicant describes a positive ion source operating on the principle of electron cyclotron resonance.

De façon plus précise, l'invention a pour objet une source d'ions négatifs comportant une enceinte fermée contenant un gaz ou une vapeur d'un matériau destiné à former un plasma, se caractérisant en ce qu'elle comprend:

  • - des moyens d'injection dans l'enceinte d'un champ électromagnétique de haute fréquence formant par ionisation du gaz ou de la vapeur des électrons,
  • - des moyens pour créer dans l'enceinte un champ magnétique de symétrie axiale dont l'amplitude croît le long de l'axe de symétrie, cette amplitude, maximale à proximité et en amont de la zone d'extraction des ions négatifs, présentant dans la région centrale de l'enceinte une valeur pour laquelle la condition de résonance cyclotronique électronique est satisfaite, et
  • - des moyens d'extraction des ions négatifs formés, portés à un potentiel positif par rapport à l'enceinte.
More specifically, the subject of the invention is a source of negative ions comprising a closed enclosure containing a gas or a vapor of a material intended to form a plasma, characterized in that it comprises:
  • means for injecting into the enclosure a high frequency electromagnetic field forming, by ionization of the gas or the vapor of the electrons,
  • - Means for creating in the enclosure a magnetic field of axial symmetry whose amplitude increases along the axis of symmetry, this amplitude, maximum near and upstream of the extraction zone of negative ions, having in the central region of the enclosure a value for which the condition of electronic cyclotron resonance is satisfied, and
  • means for extracting the negative ions formed, brought to a positive potential with respect to the enclosure.

L'utilisation d'un champ électromagnétique de haute fréquence ou hyperfréquence permet d'ioniser les molécules de gaz ou de la vapeur contenues dans l'enceinte par transfert d'énergie. Les électrons ainsi formés sont soumis à l'action du champ magnétique de symétrie axiale et, grâce au mécanisme d'absorption cyclotronique, se trouvent fortement accélérés dans la région centrale de l'enceinte où le champ magnétique présente une amplitude BR définie par l'équation (4): BR = 2n-fm/e, dans laquelle e représente la charge de l'électron, m sa masse et f la fréquence du champ électromagnétique.The use of a high frequency or microwave electromagnetic field makes it possible to ionize the gas or vapor molecules contained in the enclosure by energy transfer. The electrons thus formed are subjected to the action of the magnetic field of axial symmetry and, thanks to the cyclotronic absorption mechanism, are strongly accelerated in the central region of the enclosure where the magnetic field has an amplitude B R defined by equation (4): B R = 2n-fm / e, in which e represents the charge of the electron, m its mass and f the frequency of the electromagnetic field.

Cette condition de résonance cyclotronique électronique permet de créer des électrons énergétiques ou chauds, d'énergie cinétique supérieure à 20 eV, dans une direction perpendiculaire au champ magnétique. Ces électrons chauds, par collision sur les molécules du gaz ou de la vapeur contenues dans la source, engendrent d'autres électrons qui seront aussi accélérés par résonance cyclotronique. Le plasma chaud d'électrons ainsi formé permet, selon le mécanisme réactionnel (1), d'exciter les molécules du gaz ou de la vapeur.This condition of electronic cyclotron resonance makes it possible to create energetic or hot electrons, of kinetic energy greater than 20 eV, in a direction perpendicular to the magnetic field. These hot electrons, by collision on the molecules of gas or vapor contained in the source, generate other electrons which will also be accelerated by cyclotronic resonance. The hot plasma of electrons thus formed makes it possible, according to the reaction mechanism (1), to excite the molecules of the gas or of the vapor.

En dehors de la zone de résonance, les électrons, formés par interaction de l'onde électromagnétique et des molécules de gaz ou de vapeur, présentent une énergie plus faible, par exemple au plus égale à 1 eV. Ces électrons froids interagissent avec les molécules neutres non excitées de gaz ou de vapeur créant ainsi des ions positifs et d'autres électrons froids formant ainsi un plasma froid d'électrons. Compte tenu du profil de l'amplitude du champ magnétique, ce plasma d'électrons froids est principalement localisé dans la zone d'extraction des ions négatifs. Ce plasma froid d'électrons formé permet, selon le mécanisme réactionnel (2) la formation des ions négatifs.Outside the resonance zone, the electrons, formed by the interaction of the electromagnetic wave and gas or vapor molecules, have a lower energy, for example at most equal to 1 eV. These cold electrons interact with the unexcited neutral molecules of gas or vapor, creating positive ions and other cold electrons, thus forming a cold plasma of electrons. Given the profile of the amplitude of the magnetic field, this cold electron plasma is mainly located in the extraction zone of negative ions. According to the reaction mechanism (2), this cold plasma of electrons formed allows the formation of negative ions.

La source d'ions négatifs selon l'invention permet la formation d'un plasma d'électrons chaud et d'un plasma d'électrons froid bien séparés spatialement, permettant ainsi de former des ions négatifs, et en particulier des ions H-, D- ou T-, par attachement dissociatif et par résonance cyclotronique électronique, tout en évitant la destruction des ions négatifs formés par collisions avec les électrons énergétiques, selon le mécanisme réactionnel (3).The source of negative ions according to the invention allows the formation of a hot electron plasma and a cold electron plasma well separated spatially, thus making it possible to form negative ions, and in particular H- ions, D- or T-, by dissociative attachment and by electronic cyclotron resonance, while avoiding the destruction of negative ions formed by collisions with energetic electrons, according to the reaction mechanism (3).

Les ions négatifs ainsi formés et extraits du plasma pourront avantageusement être accélérés en utilisant des moyens appropriés situés en aval des moyens d'extraction. Cette accélération finale des ions peut par exemple être obtenue en utilisant une électrode, percée d'une ou plusieurs ouvertures pour permettre le passage des ions, portée à un potentiel positif par rapport à celui des moyens d'extraction.The negative ions thus formed and extracted from the plasma can advantageously be accelerated using appropriate means located downstream of the extraction means. This final acceleration of the ions can for example be obtained by using an electrode, pierced with one or more openings to allow the passage of the ions, brought to a positive potential relative to that of the extraction means.

Selon un mode préféré de réalisation de la source d'ions selon l'invention, des moyens pour diminuer l'amplitude du champ magnétique au niveau des moyens d'extraction des ions peuvent être prévus. Cette annulation locale de l'amplitude du champ magnétique peut avantageusement être réalisée en utilisant comme moyens d'extraction des ions négatifs une électrode ou plaque réalisée en une substance ferromagnétique, percée de fentes ou de trous pour permettre le passage des ions négatifs formés.According to a preferred embodiment of the ion source according to the invention, means for reducing the amplitude of the magnetic field at the level of the ion extraction means can be provided. This local cancellation of the amplitude of the magnetic field can advantageously be carried out using as means of extraction of the negative ions an electrode or plate made of a ferromagnetic substance, pierced with slots or holes to allow the passage of the negative ions formed.

Cette annulation de l'amplitude du champ magnétique au niveau de l'extraction des ions réalise un piégeage des électrons n'ayant pas réagi avec les molécules de gaz ou de vapeur, permettant ainsi d'éviter leur accélération entre les moyens d'extraction et d'accélération, et donc leur sortie de la source.This cancellation of the amplitude of the magnetic field at the level of the extraction of the ions traps the electrons which have not reacted with the gas or vapor molecules, thus making it possible to avoid their acceleration between the means of extraction and of acceleration, and therefore their output from the source.

Selon un autre mode préféré de réalisation de la source d'ions de l'invention, les moyens d'injection du champ électromagnétique comportent un guide d'onde dont l'extrémité, montée sur l'enceinte, est équipée d'une fenêtre en un matériau diélectrique.According to another preferred embodiment of the ion source of the invention, the means for injecting the electromagnetic field comprise a waveguide whose end, mounted on the enclosure, is equipped with a window in a dielectric material.

D'autres caractéristiques et avantages de l'invention ressortiront mieux de la description qui va suivre, donnée à titre illustratif et non limitatif, en référence aux figures annexées, dans lesquelles:

  • - la figure 1 représente, schématiquement, en coupe longitudinale une source d'ions négatifs conformément à l'invention,
  • - la figure 2 représente une courbe donnant l'amplitude B du champ magnétique régnant dans la source de la figure 1 en fonction de la distance Z prise sur l'axe de révolution de la source, et
  • - la figure 3 représente une courbe donnant les variations du potentiel électrique U à l'intérieur de la source de la figure 1 en fonction de la distance Z.
Other characteristics and advantages of the invention will emerge more clearly from the description which follows, given by way of illustration and not limitation, with reference to the appended figures, in which:
  • FIG. 1 schematically represents, in longitudinal section, a source of negative ions in accordance with the invention,
  • FIG. 2 represents a curve giving the amplitude B of the magnetic field prevailing in the source of FIG. 1 as a function of the distance Z taken on the axis of revolution of the source, and
  • FIG. 3 represents a curve giving the variations of the electrical potential U inside the source of FIG. 1 as a function of the distance Z.

En se référant à la figure 1, la source d'ions négatifs selon l'invention comprend une enceinte à vide de confinement 2 constituant une cavité résonnante pouvant être excitée par un champ électromagnétique hyperfréquence. Cette enceinte 2 présente un axe de symétrie Z, qui dans le cas d'une enceinte cylindrique représente l'axe de révolution. L'onde électromagnétique produite par une source 4 telle qu'un klystron est introduite dans la cavité résonnante 2 au moyen d'un guide d'onde 6, à section circulaire ou rectangulaire, comportant à son extrémité montée sur l'enceinte une fenêtre 8 réalisée en un matériau diélectrique tel A1203. Cette onde peut être continue ou pulsée et présenter une fréquence comprise entre 1 et 100 GHz.Referring to Figure 1, the source of negative ions according to the invention comprises a containment vacuum enclosure 2 constituting a resonant cavity which can be excited by a microwave electromagnetic field. This enclosure 2 has an axis of symmetry Z, which in the case of a cylindrical enclosure represents the axis of revolution. The electromagnetic wave produced by a source 4 such as a klystron is introduced into the resonant cavity 2 by means of a waveguide 6, of circular or rectangular section, comprising at its end mounted on the enclosure a window 8 made of a dielectric material such as A1 2 0 3 . This wave can be continuous or pulsed and have a frequency between 1 and 100 GHz.

Une conduite 10 permet d'introduire un gaz ou une vapeur d'un matériau à l'intérieur de la cavité 2 destiné à former un plasma dans ladite cavité. Avantageusement, cette introduction de gaz est réalisée à proximité de l'introduction de l'onde électromagnétique. L'enceinte 2 peut par exemple être remplie d'hydrogène, de deutérium ou de tritium à une pression de 1 à 10 mtorr (0,134 à 1,34 Pa).A pipe 10 makes it possible to introduce a gas or a vapor of a material inside the cavity 2 intended to form a plasma in said cavity. Advantageously, this introduction of gas is carried out near the introduction of the electromagnetic wave. The enclosure 2 can for example be filled with hydrogen, deuterium or tritium at a pressure of 1 to 10 mtorr (0.134 to 1.34 Pa).

Des moyens non représentés, tels qu'une pompe à diffusion ou cryogénique, montés sur la cavité 2 permettent le maintien d'un vide poussé à l'intérieur de la cavité.Means not shown, such as a diffusion or cryogenic pump, mounted on the cavity 2 allow the maintenance of a high vacuum inside the cavity.

La cavité 2 est portée à un potentiel électrostatique -V par rapport à la masse. En outre, elle est entourée par deux bobines 12 et 14, la bobine 12 étant alimentée en contre-champ, permettant de créer un champ magnétique de symétrie axiale. En particulier, l'axe de symétrie de ce champ magnétique peut être confondu avec l'axe de symétrie Z de la cavité 2. Les flèches 16 représentent les lignes de champ du champ magnétique. Ce champ magnétique peut être soit continu, soit pulsé.The cavity 2 is brought to an electrostatic potential -V with respect to the mass. In addition, it is surrounded by two coils 12 and 14, the coil 12 being supplied in counter-field, making it possible to create a magnetic field of axial symmetry. In particular, the axis of symmetry of this magnetic field can be merged with the axis of symmetry Z of the cavity 2. The arrows 16 represent the field lines of the magnetic field. This magnetic field can be either continuous or pulsed.

La source d'ions négatifs selon l'invention comprend de plus des moyens permettant d'extraire les ions formés. Ces moyens sont constitués par exemple d'une plaque conductrice 18 portée à un potentiel positif par rapport à l'enceinte 2, par exemple à un potentiel -V + AV. Ils sont montés sur l'une des extrémités de l'enceinte et isolés de celles-ci au moyen d'une bague isolante 19. Ces moyens 18 sont équipés d'au moins un trou ou d'une fente 20 permettant le passage des ions négatifs. Cet orifice d'extraction 20 est par exemple situé sur l'axe de symétrie Z de la cavité hyperfréquence.The source of negative ions according to the invention further comprises means making it possible to extract the ions formed. These means consist for example of a conductive plate 18 brought to a positive potential with respect to the enclosure 2, for example to a potential -V + AV. They are mounted on one end of the enclosure and isolated from them by means of an insulating ring 19. These means 18 are equipped with at least one hole or a slot 20 allowing the passage of ions negative. This extrac orifice tion 20 is for example located on the axis of symmetry Z of the microwave cavity.

Selon le gaz ou la vapeur utilisé, on choisira telle ou telle valeur de V et de àV. Par exemple pour de l'hydrogène ou ses isotopes V peut être compris entre -1500 V et -2000 V et AV compris entre 5 et 20 volts.Depending on the gas or vapor used, this or that value of V and of V will be chosen. For example, for hydrogen or its isotopes V can be between -1500 V and -2000 V and A V between 5 and 20 volts.

Selon l'invention, l'électrode d'extraction 18, des ions négatifs peut être suivie d'une autre électrode 22 portée à un potentiel positif par rapport à l'électrode d'extraction 18, et par exemple au potentiel de la masse, afin d'accélérer des ions négatifs formés à leur valeur finale. Cette électrode 22 est bien entendu équipée d'au moins une ouverture 24, située notamment sur l'axe de symétrie Z de la cavité, permettant la sortie des ions négatifs formés, hors de la source.According to the invention, the extraction electrode 18, negative ions can be followed by another electrode 22 brought to a positive potential with respect to the extraction electrode 18, and for example to the ground potential, in order to accelerate negative ions formed to their final value. This electrode 22 is of course equipped with at least one opening 24, located in particular on the axis of symmetry Z of the cavity, allowing the exit of the negative ions formed, outside the source.

Les positions des électrodes d'extraction 18 et d'accélération 20 sont avantageusement réglables le long de l'axe Z.The positions of the extraction 18 and acceleration 20 electrodes are advantageously adjustable along the Z axis.

Comme représenté sur la figure 1, le guide d'onde électromagnétique 6 et les électrodes d'extraction 18 et d'accélération 22 de la source d'ions sont disposées aux deux extrémités opposées de la cavité résonnante 2; l'axe de symétrie du guide d'onde 6 et ceux des ouvertures 20 et 24, pratiquées réciproquement dans les électrodes 18 et 22, sont confondus avec l'axe de symétrie Z de la cavité.As shown in FIG. 1, the electromagnetic waveguide 6 and the extraction 18 and acceleration 22 electrodes of the ion source are arranged at the two opposite ends of the resonant cavity 2; the axis of symmetry of the waveguide 6 and those of the openings 20 and 24, reciprocally made in the electrodes 18 and 22, coincide with the axis of symmetry Z of the cavity.

Les bobines 12 et 14 entourant la cavité 2 permettent, comme représenté sur la figure 2, de créer un champ magnétique de symétrie axiale dans l'enceinte dont l'amplitude B croît de la fenêtre 8 de l'injecteur de l'onde électromagnétique à l'électrode d'extraction 18. Ce champ magnétique présente, en un point ZR pris sur l'axe de symétrie de la cavité 2, et approximativement au centre de ladite cavité, une amplitude BR satisfaisant à la condition de résonance cyclotronique électronique (4), permettant ainsi la formation d'électrons énergétiques e- servant à l'excitation vibrationnelle des molécules du gaz contenues dans l'enceinte 2. Par ailleurs, ce champ magnétique présente un maximum d'amplitude BM juste en amont de l'électrode d'extraction 18 dont la position est repérée par la cote Ze.The coils 12 and 14 surrounding the cavity 2 make it possible, as shown in FIG. 2, to create a magnetic field of axial symmetry in the enclosure whose amplitude B increases from the window 8 of the injector of the electromagnetic wave at the extraction electrode 18. This magnetic field has, at a point Z R taken on the axis of symmetry of the cavity 2, and approximately at the center of said cavity, an amplitude B R satisfying the condition of electronic cyclotronic resonance (4), thus allowing the formation of energetic electrons used for the vibrational excitation of the molecules of the gas contained in enclosure 2. Furthermore, this magnetic field has a maximum amplitude B M just upstream of l extraction electrode 18, the position of which is marked by the dimension Z e .

Etant donné le fort couplage entre l'onde électromagnétique et les électrons créés par ionisation au point ZR, les électrons acquièrent une forte énergie cinétique, perpendiculaire au champ magnétique. Dans le champ magnétique d'amplitude croissante vers l'électrode 18, ces électrons subissent un effet miroir et sont soumis à une force

Figure imgb0004
µ étant le moment magnétique de l'électron. Ils sont donc accélérés vers la fenêtre 8 de l'injecteur électromagnétique; le sens de déplacement de ces électrons est illustré par la flèche F.Given the strong coupling between the electromagnetic wave and the electrons created by ionization at point Z R , the electrons acquire a strong kinetic energy, perpendicular to the magnetic field. In the magnetic field of increasing amplitude towards the electrode 18, these electrons undergo a mirror effect and are subjected to a force.
Figure imgb0004
 µ being the magnetic moment of the electron. They are therefore accelerated towards window 8 of the electromagnetic injector; the direction of movement of these electrons is illustrated by the arrow F.

Dans leur entraînement axial, les électrons énergétiques entraînent, par effet électrostatique ou ambipolaire, les ions positifs tels que H+, D+ ou T+ formés lors de l'ionisation du gaz hydrogène, deutérium ou tritium contenu dans l'enceinte 2. Il en résulte, comme représenté sur la figure 3, un potentiel dit de plasma plus positif vers l'électrode d'extraction 18 (cote Ze) que dans le centre de la cavité (cote ZR). Ce potentiel plus positif est responsable de l'autoaccélération des ions H-, représentée par la flèche F', ces ions étant créés dans la zone d'extraction des ions, c'est-à-dire à proximité et en amont de l'électrode 18.In their axial drive, the energetic electrons entrain, by electrostatic or ambipolar effect, the positive ions such as H + , D + or T + formed during the ionization of the hydrogen gas, deuterium or tritium contained in the enclosure 2. It As a result, as shown in FIG. 3, a so-called more positive plasma potential towards the extraction electrode 18 (dimension Z e ) than in the center of the cavity (dimension Z R ). This more positive potential is responsible for the self-acceleration of the H- ions, represented by the arrow F ', these ions being created in the ion extraction zone, that is to say near and upstream of the electrode 18.

Les ions négatifs et par exemple les ions H-, D-ou T- sont créés préférentiellement dans la région d'extraction des ions, du fait que les molécules de gaz excitées vibrationnellement, équation (1), sont insensibles au champ magnétique; elles peuvent donc diffuser de façon isotrope.Negative ions and, for example, H-, D- or T- ions are created preferentially in the region of extraction of the ions, because the molecules of gas vibratively excited, equation (1), are insensitive to the magnetic field; they can therefore diffuse isotropically.

Etant donné la polarité très légèrement positive +AV de l'électrode d'extraction 18 par rapport à la cavité hyperfréquence 2, l'extraction du plasma des ions négatifs formés, par exemple H- pour l'hydrogène, sera facilitée.Given the very slightly positive polarity + AV of the extraction electrode 18 with respect to the microwave cavity 2, the extraction of the plasma from the negative ions formed, for example H- for hydrogen, will be facilitated.

Comme représenté sur la figure 2, l'amplitude du champ magnétique peut avantageusement s'annuler au niveau de l'électrode d'extraction 18, c'est-à-dire à la cote Ze, afin de réaliser un piégeage des électrons du plasma permettant ainsi d'éviter leur accélération entre l'électrode d'extraction 18 et l'électrode 22. Cette annulation du champ magnétique peut par exemple être obtenue en utilisant une électrode d'extraction 18 réalisée en une substance ferromagnétique.As shown in FIG. 2, the amplitude of the magnetic field can advantageously cancel out at the level of the extraction electrode 18, that is to say at the Ze dimension, in order to carry out a trapping of the electrons of the plasma. thus making it possible to avoid their acceleration between the extraction electrode 18 and the electrode 22. This cancellation of the magnetic field can for example be obtained by using an extraction electrode 18 made of a ferromagnetic substance.

La source d'ions négatifs selon l'invention a permis la production d'un faisceau d'ions H- ayant une énergie de 2 KeV par nucléon et une intensité de 10 mA en utilisant une puissance hyperfréquence moyenne de 1 kW, une fréquence cyclotronique électronique de 10 GHz et un champ magnétique dont l'amplitude croît de 0,2 à 0,45 T. La source d'ions présentait une cavité cylindrique de 10 cm de diamètre et de 15 cm de longueur; elle était portée à un potentiel négatif de -2000 volts et l'électrode d'extraction 18 à un potentiel de 2 volts supérieur à celui de la cavité, soit -1998 V. La pression du gaz hydrogène contenu dans l'enceinte était de 0,2 Pa.The source of negative ions according to the invention allowed the production of a beam of H- ions having an energy of 2 KeV per nucleon and an intensity of 10 mA using an average microwave power of 1 kW, a cyclotronic frequency. 10 GHz electronics and a magnetic field whose amplitude increases from 0.2 to 0.45 T. The ion source had a cylindrical cavity 10 cm in diameter and 15 cm in length; it was brought to a negative potential of -2000 volts and the extraction electrode 18 to a potential of 2 volts higher than that of the cavity, ie -1998 V. The pressure of the hydrogen gas contained in the enclosure was 0 , 2 Pa.

La description donnée précédemment n'a bien entendu été donnée qu'à titre illustratif, toute modification, sans pour autant sortir du cadre de l'invention, pouvant être envisagée.The description given above has of course only been given by way of illustration, any modification, without departing from the scope of the invention, which can be envisaged.

En particulier, il est possible d'utiliser des moyens distincts pour extraire les ions négatifs et pour annuler l'amplitude du champ magnétique au niveau de ces moyens d'extraction, au lieu d'utiliser un seul moyen pour réaliser ces deux fonctions. Par exemple, on peut utiliser des ferrites pour diminuer l'amplitude du champ magnétique.In particular, it is possible to use separate means for extracting the negative ions and for canceling the amplitude of the magnetic field at the level of these extraction means, instead of using a single means for performing these two functions. For example, ferrites can be used to decrease the amplitude of the magnetic field.

Par ailleurs, le champ magnétique à symétrie axiale peut être produit par des ferrites à la place de l'utilisation de deux bobines alimentées en contre-champ et entourant la cavité hyperfréquence.Furthermore, the magnetic field with axial symmetry can be produced by ferrites instead of the use of two coils supplied in counter-field and surrounding the microwave cavity.

De même, la cavité peut présenter une autre forme qu'une forme cylindrique, par exemple une forme parallélépipédique.Likewise, the cavity may have a shape other than a cylindrical shape, for example a parallelepiped shape.

Enfin, la description a été faite dans le cas d'une production d'ions H-, D- ou T-, mais bien entendu, la source selon l'invention peut produire d'autres types d'ions négatifs et en particulier des ions d'oxygène, de sodium, de lithium et d'iode.Finally, the description has been made in the case of production of H-, D- or T- ions, but of course, the source according to the invention can produce other types of negative ions and in particular oxygen, sodium, lithium and iodine ions.

Claims (7)

1. Negative ion source comprising a closed enclosure (2) containing a gas or vapour of a material intended for forming a plasma, characterized in that it comprises means (6, 8) for injecting into the enclosure (2) a high frequency electromagnetic field forming electrons by the ionization of the gas or vapour, means (12, 14) for producing within the enclosure (2) a magnetic field of axial symmetry, whose amplitude (B) increases along the axis of symmetry (Z), said amplitude, which is at a maximum (BM) in the vicinity of and upstream of the negative ion extraction zone having in the central region (ZR) of the enclosure a value (BR) for which the electron cyclotron resonance condition is satisfied and means (18) for extracting the negative ions formed raised to a positive potential (+AV) compared with the enclosure (2).
2. Negative ion source according to claim 1, characterized in that it comprises, downstream of the extraction means (18), means (22) for accelerating the negative ions formed.
3. Negative ion source according to claims 1 or 2, characterized in that it comprises means (18) for cancelling out the amplitude of the magnetic field at the ion extraction means (18).
4. Negative ion source according to claim 3, characterized in that the cancelling means and extraction means coincide, said means (18) being formed by a ferromagnetic material plate perforated with at least one opening (20) to permit the passage of the ions.
5. Negative ion source according to any one of the claims 2 to 4, characterized in that the acceleration means (22) are formed from an electrode brought to a positive potential compared with that of the extraction means and provided with at least one opening (24) to permit the passage of the ions.
6. Negative ion source according to any one of the claims 1 to 5, characterized in that the means (6, 8) for injecting the electromagnetic field comprise a waveguide (6), whereof the end mounted on the enclosure (2) is equipped with a dielectric material window (8).
7. Negative ion source according to any one of the claims 1 to 6, characterized in that the gas is hydrogen or isotopes thereof.
EP86400726A 1985-04-11 1986-04-04 Electron cyclotron resonance negative ion source Expired EP0199625B1 (en)

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FR8505461A FR2580427B1 (en) 1985-04-11 1985-04-11 SOURCE OF NEGATIVE IONS WITH ELECTRON CYCLOTRON RESONANCE

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US4859908A (en) * 1986-09-24 1989-08-22 Matsushita Electric Industrial Co., Ltd. Plasma processing apparatus for large area ion irradiation
US4845364A (en) * 1988-02-29 1989-07-04 Battelle Memorial Institute Coaxial reentrant ion source for surface mass spectroscopy
JPH0216732A (en) * 1988-07-05 1990-01-19 Mitsubishi Electric Corp Plasma reactor
US5107170A (en) * 1988-10-18 1992-04-21 Nissin Electric Co., Ltd. Ion source having auxillary ion chamber
US5051557A (en) * 1989-06-07 1991-09-24 The United States Of America As Represented By The Secretary Of The Department Of Health And Human Services Microwave induced plasma torch with tantalum injector probe
US5106570A (en) * 1990-08-02 1992-04-21 The United States Of America As Represented By The Secretary Of The Air Force Intense negative ion source
FR2668642B1 (en) * 1990-10-25 1993-11-05 Commissariat A Energie Atomique HIGHLY CHARGED ION SOURCE WITH POLARIZABLE PROBE AND ELECTRONIC CYCLOTRON RESONANCE.
US5306921A (en) * 1992-03-02 1994-04-26 Tokyo Electron Limited Ion implantation system using optimum magnetic field for concentrating ions
JP2693899B2 (en) * 1992-10-09 1997-12-24 栄電子工業株式会社 ECR plasma processing method
DE19929278A1 (en) * 1998-06-26 2000-02-17 Nissin Electric Co Ltd Negative hydrogen ion beam injection method on substrate
US6441569B1 (en) 1998-12-09 2002-08-27 Edward F. Janzow Particle accelerator for inducing contained particle collisions

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US4447773A (en) * 1981-06-22 1984-05-08 California Institute Of Technology Ion beam accelerator system
US4486665A (en) * 1982-08-06 1984-12-04 The United States Of America As Represented By The United States Department Of Energy Negative ion source
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