EP0300932B1 - Electron source - Google Patents

Electron source Download PDF

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Publication number
EP0300932B1
EP0300932B1 EP88420256A EP88420256A EP0300932B1 EP 0300932 B1 EP0300932 B1 EP 0300932B1 EP 88420256 A EP88420256 A EP 88420256A EP 88420256 A EP88420256 A EP 88420256A EP 0300932 B1 EP0300932 B1 EP 0300932B1
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EP
European Patent Office
Prior art keywords
anode
cathode
plasma
electron source
electron
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Expired - Lifetime
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EP88420256A
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German (de)
French (fr)
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EP0300932A1 (en
Inventor
Jacques Menet
Olivier De Gabrielli
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Centre National de la Recherche Scientifique CNRS
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Centre National de la Recherche Scientifique CNRS
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J3/00—Details of electron-optical or ion-optical arrangements or of ion traps common to two or more basic types of discharge tubes or lamps
    • H01J3/02—Electron guns
    • H01J3/025—Electron guns using a discharge in a gas or a vapour as electron source

Definitions

  • the present invention relates to an electron source for an electron gun.
  • thermoelectronic source those with thermoelectronic source and those in which the source is a plasma.
  • FIG. 1 very schematically represents an electron gun with plasma source.
  • This electron gun comprises a cathode plate K opposite an anode plate A.
  • the conditions of electric field and gas pressure between the cathode and the anode are chosen so that a luminescent discharge is obtained in the area between these plates. In practice, this means that there must be a minimum field of the order of a few hundred V / cm and a pressure of the order of 1 to 10 Pa.
  • a plasma P is created between cathode and anode and electrons strike the anode plate.
  • This anode plate A is provided with a central opening through which the electrons can escape, which are then accelerated by various means towards a collecting plate C. If one wishes to have a length acceleration zone sufficient, it is necessary that the pressure in the area between the anode plate A and the collecting plate C is sufficiently low. For example, to have an average free path of electrons of 20 cm, the pressure must be less than 0.1 Pa.
  • FIG. 1 The structure of Figure 1 is extremely schematic. In practical devices, numerous improvements are provided, for example electrodes intermediate between cathode and anode. These electrodes can take the form of grids. Similarly, to maintain good focusing of the electron beam in the acceleration zone, it is sometimes planned to use magnetic fields parallel to the direction of propagation of the electrons.
  • thermoelectronic sources have the advantage over thermoelectronic sources of a longer service life and the possibility of operating at higher pressure (1 to 10 Pa instead of 10- 4 to 10- 3 Pa).
  • a first drawback lies in the fact that, since a plasma is created in the entire area between the cathode and the anode, the efficiency of the device is necessarily less than unity since part of the electrons will strike the anode plate A. This loss can be reduced by using, instead of a simple opening as shown in FIG. 1, a transparent anode system provided with magnetic focusing. However, we are left with yields lower than one, for example of the order of 70%.
  • an object of the present invention is to provide a new type of plasma electron source overcoming the drawbacks exposed above of conventional plasma electron sources.
  • the present invention provides an electron source comprising, in a low pressure enclosure, an anode, a cathode and means for applying a magnetic field, in which the cathode is consisting of an equipotential cavity provided with an opening on the anode side and a magnetic field parallel to the anode-cathode direction is applied at the opening.
  • the pressure in the enclosure is of the order of a few tenths of pascal.
  • the voltage applied between the anode and the cathode is of the order of a few hundred volts / cm.
  • the magnetic field is of the order of a few hundredths of a tesla.
  • the opening is a very long slot.
  • the electron source according to the present invention comprises a cathode K and an anode A.
  • the cathode K consists of an equipotential cavity 10 provided with an opening 11 on the side of the anode A.
  • Means, for example, permanent magnets, are provided to apply a magnetic field B in the cathode-anode direction at the opening 11.
  • FIG. 3 represents a plasma electron source according to the present invention associated with an accelerating cavity.
  • the cathode K consisting of a cavity 10 and the anode A.
  • the anode A is provided with an opening 20 to let the electrons pass to an accelerating electrode C.
  • a transparent anode could be provided, that is to say for example an anode constituted by a grid.
  • the advantage of the present invention in relation to an accelerating cavity lies in particular in the fact that the entire system can operate at a single pressure and therefore that it is no longer necessary to provide sophisticated pumping systems.
  • the cathode cavity can be rectangular with a width of 50 mm and a depth of 10 mm, the slot having a width of a few mm and a length of 100 mm.
  • the anode-cathode voltage is of the order of 400 V and the magnetic field of 8 x 10 -2 tesla, the intensity of the electron beam being 1.5 A.
  • the present invention is susceptible of numerous variants consisting notably in adapting to it the conventional variants of known plasma electron sources.
  • intermediate grids can be provided in the cathode-anode zone, more sophisticated focusing systems can be provided in the accelerating zone, and, if desired, a pressure difference can be provided between the plasma creation zone and the accelerating zone and this pressure difference will be easier to apply than in the prior art because of the lower pressure in the plasma creation zone.

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  • Plasma Technology (AREA)
  • Electron Sources, Ion Sources (AREA)
  • Particle Accelerators (AREA)

Description

La présente invention concerne une source d'électrons pour canon à électrons.The present invention relates to an electron source for an electron gun.

Dans l'art antérieur on trouve deux types principaux de canons à électrons classés selon le type de source d'électrons, à savoir ceux à source thermoélectronique et ceux dans lesquels la source est un plasma.In the prior art there are two main types of electron guns classified according to the type of electron source, namely those with thermoelectronic source and those in which the source is a plasma.

Dans les canons à électrons à source thermoélectronique, des électrons sont créés par un solide porté à haute température. L'inconvénient principal de ce type de dispositif est que la durée de vie du filament est relativement faible. Pour augmenter la durée de vie, on est amené, d'une part, à prévoir des matériaux très sophistiqués utilisés par exemple dans les cathodes de tubes électroniques et, d'autre part, à réduire autant que possible la pression dans la zone de création d'électrons. On cherche ainsi à obtenir des pressions inférieures à 10-5 Torr (environ 10-3 Pa) ce qui oblige à prévoir des installations de vide très sophistiquées.In electron guns with thermoelectronic source, electrons are created by a solid brought to high temperature. The main drawback of this type of device is that the life of the filament is relatively short. To increase the service life, we are led, on the one hand, to provide very sophisticated materials used for example in cathodes of electronic tubes and, on the other hand, to reduce as much as possible the pressure in the creation zone of electrons. The aim is to obtain pressures below 10 -5 Torr (about 10 -3 Pa) which requires the provision of sophisticated vacuum systems.

La figure 1 représente très schématiquement un canon à électrons à source à plasma. Ce canon à électrons comprend une plaque de cathode K en regard d'une plaque d'anode A. Les conditions de champ électrique et de pression de gaz entre la cathode et l'anode sont choisies pour que l'on obtienne une décharge luminescente dans la zone comprise entre ces plaques. En pratique, ceci signifie qu'il doit exister un champ minimal de l'ordre de quelques centaines de V/cm et une pression de l'ordre de 1 à 10 Pa. Ainsi, un plasma P se crée entre cathode et anode et des électrons viennent frapper la plaque d'anode. Cette plaque d'anode A est munie d'une ouverture centrale par laquelle peuvent s'échapper les électrons qui sont ensuite accélérés par divers moyens vers une plaque collectrice C. Si l'on veut pouvoir disposer d'une zone d'accélération de longueur suffisante, il faut que la pression dans la zone comprise entre la plaque d'anode A et la plaque collectrice C soit suffisamment faible. Par exemple, pour avoir un libre parcours moyen des électrons de 20 cm, il faut que la pression soit inférieure à 0,1 Pa.FIG. 1 very schematically represents an electron gun with plasma source. This electron gun comprises a cathode plate K opposite an anode plate A. The conditions of electric field and gas pressure between the cathode and the anode are chosen so that a luminescent discharge is obtained in the area between these plates. In practice, this means that there must be a minimum field of the order of a few hundred V / cm and a pressure of the order of 1 to 10 Pa. Thus, a plasma P is created between cathode and anode and electrons strike the anode plate. This anode plate A is provided with a central opening through which the electrons can escape, which are then accelerated by various means towards a collecting plate C. If one wishes to have a length acceleration zone sufficient, it is necessary that the pressure in the area between the anode plate A and the collecting plate C is sufficiently low. For example, to have an average free path of electrons of 20 cm, the pressure must be less than 0.1 Pa.

La structure de la figure 1 est extrêmement schématique. Dans des dispositifs pratiques de nombreux perfectionnements sont prévus, par exemple des électrodes intermédiaires entre cathode et anode. Ces électrodes peuvent prendre la forme de grilles. De même, pour maintenir une bonne focalisation du faisceau d'électrons dans la zone d'accélération, il est parfois prévu d'utiliser des champs magnétiques parallèles à la direction de propagation des électrons.The structure of Figure 1 is extremely schematic. In practical devices, numerous improvements are provided, for example electrodes intermediate between cathode and anode. These electrodes can take the form of grids. Similarly, to maintain good focusing of the electron beam in the acceleration zone, it is sometimes planned to use magnetic fields parallel to the direction of propagation of the electrons.

Ces sources d'électrons à plasma présentent par rapport aux sources thermoélectroniques l'avantage d'une plus grande durée de vie et d'une possibilité de fonctionnement à plus haute pression (1 à 10 Pa au lieu de 10-4 à 10-3 Pa).These plasma electron sources have the advantage over thermoelectronic sources of a longer service life and the possibility of operating at higher pressure (1 to 10 Pa instead of 10- 4 to 10- 3 Pa).

Toutefois, ces sources à plasma présentent encore plusieurs inconvénients.However, these plasma sources still have several drawbacks.

Un premier inconvénient réside dans le fait que, un plasma étant créé dans toute la zone comprise entre la cathode et l'anode, le rendement du dispositif est nécessairement inférieur à l'unité puisqu'une partie des électrons viendra frapper la plaque d'anode A. Cette perte peut être réduite en utilisant, au lieu d'une simple ouverture comme cela est schématisé en figure 1, un système à anode transparente muni d'une focalisation magnétique. On se retrouve néanmoins avec des rendements inférieurs à l'unité, par exemple de l'ordre de 70%.A first drawback lies in the fact that, since a plasma is created in the entire area between the cathode and the anode, the efficiency of the device is necessarily less than unity since part of the electrons will strike the anode plate A. This loss can be reduced by using, instead of a simple opening as shown in FIG. 1, a transparent anode system provided with magnetic focusing. However, we are left with yields lower than one, for example of the order of 70%.

Un autre inconvénient réside dans le fait que la zone de création de plasma entre cathode et anode et la zone d'accélération entre l'anode A et la plaque accélératrice C doivent nécessairement être à des pressions différentes, la deuxième zone devant se trouver une pression plus faible que la première. Ceci oblige à utiliser des systèmes de pompage différentiel sophistiqués pour optimiser les pressions dans chacune des zones. Ceci est généralement effectué en injectant un gaz dans la zone cathode-anode et en pompant dans la zone anode- plaque accélératrice.Another drawback lies in the fact that the plasma creation zone between cathode and anode and the acceleration zone between the anode A and the accelerator plate C must necessarily be at different pressures, the second zone having to be at a pressure weaker than the first. This requires the use of sophisticated differential pumping systems to optimize the pressures in each of the zones. This is generally done by injecting a gas into the cathode-anode area and pumping into the anode-accelerator plate area.

Ainsi, un objet de la présente invention est de prévoir un nouveau type de source d'électrons à plasma palliant les inconvénients exposés ci-dessus des sources d'électrons à plasma classiques.Thus, an object of the present invention is to provide a new type of plasma electron source overcoming the drawbacks exposed above of conventional plasma electron sources.

Pour atteindre cet objet ainsi que d'autres, la présente invention prévoit une source d'électrons comprenant, dans une enceinte à basse pression, une anode, une cathode et des moyens d'application d'un champ magnétique, dans laquelle la cathode est constituée d'une cavité équipotentielle munie d'une ouverture du côté de l'anode et un champ magnétique parallèle à la direction anode-cathode est appliqué au niveau de l'ouverture.To achieve this object as well as others, the present invention provides an electron source comprising, in a low pressure enclosure, an anode, a cathode and means for applying a magnetic field, in which the cathode is consisting of an equipotential cavity provided with an opening on the anode side and a magnetic field parallel to the anode-cathode direction is applied at the opening.

Selon un mode de réalisation de l'invention, la pression dans l'enceinte est de l'ordre de quelques dixièmes de pascal.According to one embodiment of the invention, the pressure in the enclosure is of the order of a few tenths of pascal.

Selon un mode de réalisation de l'invention, la tension appliquée entre anode et cathode est de l'ordre de quelques centaines volt/cm.According to one embodiment of the invention, the voltage applied between the anode and the cathode is of the order of a few hundred volts / cm.

Selon un mode de réalisation de l'invention, le champ magnétique est de l'ordre de quelques centièmes de tesla.According to one embodiment of the invention, the magnetic field is of the order of a few hundredths of a tesla.

Selon un mode de réalisation de l'invention, l'ouverture est une fente de grande longueur.According to one embodiment of the invention, the opening is a very long slot.

Ainsi, le dispositif selon la présente invention permet de disposer d'un canon à électrons présentant notamment les avantages suivants :

  • - rendement théorique égal à 1,
  • - délimitation du faisceau d'électrons indépendamment de la structure d'anode et des structures de focalisation,
  • - possibilité de fonctionner à une pression de l'ordre de quelques dixièmes de pascal dans la zone de création du plasma, ce qui est compatible avec la pression permise dans la zone d'accélération.
Thus, the device according to the present invention makes it possible to have an electron gun having in particular the following advantages:
  • - theoretical yield equal to 1,
  • - delineation of the electron beam independently of the anode structure and the focusing structures,
  • - Ability to operate at a pressure of the order of a few tenths of pascal in the plasma creation zone, which is compatible with the pressure allowed in the acceleration zone.

Ces objets, caractéristiques et avantages ainsi que d'autres de la présente invention seront exposés plus en détail dans la description suivante de modes de réalisation particuliers faite en relation avec les figures jointes parmi lesquelles :

  • la figure 1 destinée à illustrer une source à plasma de l'art antérieur a été décrite précédemment ;
  • la figure 2 représente très schématiquement un mode de réalisation d'une source d'électrons selon la présente invention ; et
  • la figure 3 représente très schématiquement un mode de réalisation d'un canon à électrons selon la présente invention.
These objects, characteristics and advantages as well as others of the present invention will be explained in more detail in the following description of particular embodiments made in relation to the attached figures, among which:
  • FIG. 1 intended to illustrate a plasma source of the prior art has been described previously;
  • FIG. 2 very schematically represents an embodiment of an electron source according to the present invention; and
  • FIG. 3 very schematically represents an embodiment of an electron gun according to the present invention.

La source d'électrons selon la présente invention comprend une cathode K et une anode A. La cathode K est constituée d'une cavité équipotentielle 10 munie d'une ouverture 11 du côté de l'anode A. Des moyens, par exemple, des aimants permanents, sont prévus pour appliquer un champ magnétique B dans la direction cathode-anode au niveau de l'ouverture 11.The electron source according to the present invention comprises a cathode K and an anode A. The cathode K consists of an equipotential cavity 10 provided with an opening 11 on the side of the anode A. Means, for example, permanent magnets, are provided to apply a magnetic field B in the cathode-anode direction at the opening 11.

Quand un champ électrique est appliqué entre l'anode et la cathode, du fait de l'existence d'une cavité, les lignes de champ électrique 12 s'incurvent vers l'intérieur au niveau de l'ouverture, comme cela est représenté dans la figure. Ainsi, il existe des zones où le champ électrique, du fait de la courbure des lignes de champ est perpendiculaire à la direction anode-cathode, c'est-à-dire perpendiculaire au champ magnétique B appliqué. Il en résulte la création d'un plasma pour des pressions gazeuses beaucoup plus faibles qu'en l'absence de tels champs électrique et magnétique croisés. En effet, ce sont les effets de bords (courbure du champ électrique due à la présence de l'ouverture dans la cavité) conjugués avec le champ magnétique croisé en certains emplacements qui sont à l'origine de la création du plasma. Il se crée ainsi, pour des pressions de gaz supérieures à quelques dixièmes de pascal une zone de plasma 13 sensiblement telle que délimitée par des pointillés en figure 2 au voisinage de l'ouverture 11. Des électrons de ce plasma sont ensuite attirés par l'anode A sous forme d'un faisceau creux.When an electric field is applied between the anode and the cathode, due to the existence of a cavity, the electric field lines 12 curve inward at the opening, as shown in the figure. Thus, there are areas where the electric field, due to the curvature of the field lines is perpendicular to the anode-cathode direction, that is to say perpendicular to the magnetic field B applied. This results in the creation of a plasma for gas pressures much lower than in the absence of such crossed electric and magnetic fields. Indeed, it is the edge effects (curvature of the electric field due to the presence of the opening in the cavity) combined with the crossed magnetic field in certain locations which are at the origin of the creation of the plasma. There is thus created, for gas pressures greater than a few tenths of pascal, a plasma zone 13 substantially as delimited by dotted lines in FIG. 2 near the opening 11. Electrons from this plasma are then attracted by the anode A in the form of a hollow beam.

Il en résulte pour la source selon la présente invention de nombreux avantages :

  • - le plasma se crée quelles que soient la forme et la dimension de l'ouverture. On peut ainsi envisager des ouvertures de très grandes dimensions, par exemple des fentes allongées ;
  • - le rendement entre le courant d'électrons et le courant de décharge est voisin de 100 % ;
  • - le plasma peut être créé à une pression de l'ordre de quelques dixièmes de pascal ;
  • - le système est d'une structure très simple et devrait pouvoir être fabriqué à faible coût par rapport aux canons à électrons existants.
This results in numerous advantages for the source according to the present invention:
  • - the plasma is created whatever the shape and size of the opening. One can thus envisage very large openings, for example elongated slots;
  • - The efficiency between the electron current and the discharge current is close to 100%;
  • - the plasma can be created at a pressure of the order of a few tenths of pascal;
  • - the system is very simple in structure and should be able to be manufactured at low cost compared to existing electron guns.

La figure 3 représente une source d'électrons à plasma selon la présente invention associée à une cavité accélératrice. On retrouve en figure 3 la cathode K constituée d'une cavité 10 et l'anode A. Cette fois ci, l'anode A est munie d'une ouverture 20 pour laisser passer les électrons vers une électrode accélératrice C. Bien entendu, au lieu d'une ouverture 20 de forme similaire à l'ouverture dans la cathode 10, on pourrait prévoir une anode transparente, c'est-à-dire par exemple une anode constituée d'une grille.FIG. 3 represents a plasma electron source according to the present invention associated with an accelerating cavity. We find in Figure 3 the cathode K consisting of a cavity 10 and the anode A. This time, the anode A is provided with an opening 20 to let the electrons pass to an accelerating electrode C. Of course, at instead of an opening 20 of similar shape to the opening in the cathode 10, a transparent anode could be provided, that is to say for example an anode constituted by a grid.

L'avantage de la présente invention en relation avec une cavité accélératrice réside notamment dans le fait que l'ensemble du système peut fonctionner à une pression unique et donc qu'il n'est plus nécessaire de prévoir des systèmes de pompage sophistiqués.The advantage of the present invention in relation to an accelerating cavity lies in particular in the fact that the entire system can operate at a single pressure and therefore that it is no longer necessary to provide sophisticated pumping systems.

Dans un exemple de réalisation de l'invention, la cavité de cathode peut être rectangulaire avec une largeur de 50 mm et une profondeur de 10 mm, la fente ayant une largeur de quelques mm et une longueur de 100 mm. La tension anode-cathode est de l'ordre de 400 V et le champ magnétique de 8 x 10-2 tesla, l'intensité du faisceau d'électrons étant de 1,5 A.In an exemplary embodiment of the invention, the cathode cavity can be rectangular with a width of 50 mm and a depth of 10 mm, the slot having a width of a few mm and a length of 100 mm. The anode-cathode voltage is of the order of 400 V and the magnetic field of 8 x 10 -2 tesla, the intensity of the electron beam being 1.5 A.

Bien entendu, la présente invention est susceptible de nombreuses variantes consistant notamment à y adapter les variantes classiques des sources d'électrons à plasma connues. Notamment, des grilles intermédiaires peuvent être prévues dans la zone cathode-anode, des systèmes de focalisation plus sophistiqués peuvent être prévus dans la zone accélératrice, et, si on le souhaite on peut prévoir une différence de pression entre la zone de création de plasma et la zone accélératrice et cette différence de pression sera plus facile à appliquer que dans l'art antérieur du fait de la pression plus faible dans la zone de création de plasma.Of course, the present invention is susceptible of numerous variants consisting notably in adapting to it the conventional variants of known plasma electron sources. In particular, intermediate grids can be provided in the cathode-anode zone, more sophisticated focusing systems can be provided in the accelerating zone, and, if desired, a pressure difference can be provided between the plasma creation zone and the accelerating zone and this pressure difference will be easier to apply than in the prior art because of the lower pressure in the plasma creation zone.

Claims (5)

1. An electron source comprising, in a low pressure chamber, an anode, a cathode and means for applying a magnetic field, characterized in this that
- the cathode (K) is constituted by an equipotential cavity (10) provided with an aperture (11) on the side of the anode (A); and
- a magnetic field parallel to the anode-cathode direction is applied at the aperture.
2. An electron source according to claim 1, characterized in this that the pressure in the chamber is in the range of a few tenths of pascal.
3. An electron source according to claim 2, characterized in this that the electrical field applied between the anode and the cathode is in the range of a few hundredths volts/cm.
4. An electron source according to any of claims 1 to 3, characterized in this that said magnetic field is exceeding a few hundredths tesla.
5. An electron source according to any of claims 1 to 4, characterized in this that said aperture is a very elongated slit.
EP88420256A 1987-07-22 1988-07-20 Electron source Expired - Lifetime EP0300932B1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
FR8710642 1987-07-22
FR8710642A FR2618602B1 (en) 1987-07-22 1987-07-22 SOURCE OF ELECTRON

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EP0300932A1 EP0300932A1 (en) 1989-01-25
EP0300932B1 true EP0300932B1 (en) 1992-01-29

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JP (1) JPH01126598A (en)
DE (1) DE3868169D1 (en)
FR (1) FR2618602B1 (en)

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FR2639756B1 (en) * 1988-11-30 1994-05-13 Centre Nal Recherc Scientifique SOURCE OF VAPORS AND IONS
JP5378723B2 (en) * 2007-07-27 2013-12-25 矢崎総業株式会社 Electron beam irradiation apparatus and coated wire manufacturing method

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FR1281964A (en) * 1960-11-18 1962-01-19 Csf New cathode with high emissive power
US3381157A (en) * 1964-12-10 1968-04-30 United Aircraft Corp Annular hollow cathode discharge apparatus
JPS4928718A (en) * 1972-07-17 1974-03-14
JPS5349B2 (en) * 1972-07-19 1978-01-05
FR2498010A1 (en) * 1981-01-12 1982-07-16 Kreindel July Ring type electron gun - has ring anode aperture displaced with respect to cathode aperture axis
US4633129A (en) * 1985-04-30 1986-12-30 International Business Machines Corporation Hollow cathode

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EP0300932A1 (en) 1989-01-25
JPH01126598A (en) 1989-05-18
US4886992A (en) 1989-12-12
FR2618602B1 (en) 1990-01-05
FR2618602A1 (en) 1989-01-27
DE3868169D1 (en) 1992-03-12

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