US3375401A - Source of negatively charged particles having positively charged particle retaining means - Google Patents

Description

March 26, 1968 TIVELY T. CONSOLI ETAL SOURCE OF NEGATIVELY CHARGED PARTICLES HAVING POSI CHARGED PARTICLE RETAINING MEANS Filed Sept. 30, 1965 United States Patent SOURCE OF NEGATIVELYCHARGED PARTICLES HAVING POSITIVELY CHARGED PARTICLE RETAINING MEANS Terenzio Consoli, La Celle-Saint-Cloud, and Richard Geller, Antony, France, assignors to Commissariat a lEnergie Atomique, Paris, France Filed Sept. 30, 1965, Ser. No. 491,700 Claims priority, application France, Oct. 14, 1964,

91,422 2 Claims. (Cl. 315111) ABSTRACT OF THE DISCLOSURE The present invention relates to source of negatively charged particles, in particular an electron source adapted to generate a very high-energy electron beam.

The known electron sources, in particular those-of the thermoionic type, usually comprise a first electrode, or'

cathode, which is raised to a certain temperature and emits electrons, said cathode being in register with a positive electrode. The electric field thus established between these two electrodes permits to accelerate, and conting ently to focus, the electrons emitted by said cathode.

The intensity of the electron beam thus obtained is limited by the emitting power of the cathode and by a space charge which tends to repel towards the latter a large proportion of the electrons emitted. The limitation due to the space charge, localized in the vicinity of the cathode and known under the name of Child-Langmuir effeet, is preponderant and due in particular to the fact that since the electrons leave the cathode with initial velocities which are both very low and randomly orientated, the electronic density is very high in the vicinity of said cathode, where the action of the electric field is very small.

Moreover, the known electron guns, which can be applied in particular to the devices for welding by electron bombardment, contain an electron source which generates a beam the intensity of which is in general largely limited by the space charge.

The present invention, which is free of the above drawbacks, relates to a source of negatively charged particles, and in particular an electron source adapted to generate a very high-energy electron beam, said source comprising means for generating an accelerated jet of positively charged particles and of negatively charged particles which is altogether electrically neutral and possessed of a very high energy, at least two electrically conductive parallel plates at right angles to the longitudinal axis of said jet, said plates being provided with openings co-axial to said jet and the areas of which are of the order of that of the cross-section of said jet, and a source of direct current adapted to establish an electric field between said plates and so mounted that its negative terminal is connected to that of said plates which is the more upstream of said jet, whereby when said jet has passed through said opening of said upstream plate, the positively charged particles of said jet, due to said electric field, are slowed down and then, sent back towards said upstream plate by ICC which they arecaptured, whereas the negatively charged particles proceed on their way and pass through the open ing of the other plate after having been accelerated by said electric field.

According to a specific form of embodiment, said jet of particles is constituted by a plasma emitted by an electro-magnetic accelerator.

Said source of negatively charged particles can comprise means located between said means for generating an accelerated jet and said two plates, and adapted to generate a magnetic field which is parallel to the longitudinal axis of said jet and the intensity of which increases in the direction of flow of said jet, said magnetic field permitting to focus the negative particles in the vicinity of the axis of said jet. 7

The features of the present invention will be disclosed hereafter reference being had to the accompanying drawing which diagrammatically represents a source according to the invention, shown in section along a plane passing through the axis of the jet of particles, said jet being assumed as constituted by an accelerated plasma emitted by an electro-magnetic accelerator.

In the figure, an accelerator of ionised gas or plasma is diagrammatically shown at 1. This accelerator emits a jet of plasma 2 possessed of a very high energy, which is altogether electrically neutral and composed of positive ions and of electrons, the velocity of the positive ions being substantially equal to the component of the electron velocity along the axis XX of the jet.

The electrons of jet 2 follow helical paths whereas the ions, driven by the electronic spacecharge, move in the direction of axis XX. The energy of the flux of particles forming jet 2 is proportional to the electro-magnetic power of accelerator 1, which is for instance of 300 w., the electron energy being then of the order of 10 ev. and the total number of particles emitted per second of the order of 10 A coil 3 co-axial to the plasma jet 2 generates a static magnetic field which is parallel to the longitudinal axis of said jet, and the intensity of which increases in the direction of flow of said jet.

Two circular openings 4 and 5 co-axial to the plasma jet 2 and the areas of Whichare of the order of that of the cross-section of said jet, are provided in two parallel conductive plates, 6 and 7 respectively, connected to the terminals of a direct current source 8, plate 6 located between coil 3 and plate 7 being connected to the negative terminal of said source 8.

Any electron contained in plasma jet 2 undergoes, when passing through coil 3, the action of the magnetic field generated by said coil, and if this magnetic field, which is co-axial to jet 2, is gradually increasing, it causes a decrease of the radius of the helical path followed by said electron. Therefore the major portion of the electrons which pass through said coil are concentrated in the vicinity of the axis of jet 2, thus forming a beam which converges towards opening 4 and passes through said opening. As stated above, the electrons move in the direction of axis XX and therefore in the direction of the magnetic field. Accordingly, the magnetic field exerts no action upon the electrons and they are not deviated and all of the electrons pass through opening 4.

The ions of the jet are not deflected by the magnetic field, since their velocity is parallel to said field; therefore they also pass through opening 4 and they are then submitted to the decelerating electric field established between plates 6 and 7. This electric field, the lines of force of which are substantially parallel and directed from plate 7 to plate 6, slows down the ions and then sends them back towards plate 6 by which they are captured. Plate 6 thus permits to set apart, within beam 2, the ions from the electrons, and only the latter are able to pass through opening 5.

One can then obtain, beyond plate 7, an electron beam 9 the electrons of which have their energy still increased by the acceleration which they undergo between plates 6 and 7. The ions captured by plate 6 are driven by conduction to source 8.

Since the energy of the electrons contained in beam 2 is already very high before they pass through opening 4, the space charge created in the vicinity of said opening has no substantial action on electron beam 9 and the drawback arising from the child-Langmuir effect is therefore avoided.

Accelerator 1 and plates 6 and 7 are located in a tight enclosure (not shown), inside which the air pressure is maintained at about atmosphere.

The intensity of the electron beam delivered by the source according to the present invention is, for instance, of the order of 1 A./cm. the diameter of openings 4 and 5 being 1 cm. and the voltage delivered by source 8 being of 500 v.

Such a source associated to an accelerating and focusing device of a known type, is capable of generating a very strong electronic beam permitting to carry out welding by electronic bombardment, and it can advantageously replace the thermoionic sources usually associated to the electron guns provided for this purpose.

What is claimed is:

1. A device for sorting out the electrons and positive ions from an accelerated jet of positively charged particles and negatively charged particles which is altogether electrically neutral and possessed of a veiy high energy for retaining positive ions and emitting electrons including a source for said jet, at least two electrically conductive parallel plates at right angles to the longitudinal axis of said jet, said plates being provided with openings co-axial to said jet and the areas of which are of the order of that of the cross-section of said jet, a source of direct current adapted to'establish an electric field between said plates and so mounted that its negative terminal is connected to that of said plates which is the more upstream of said jet, and a coil located entirely between said means for generating an accelerated jet and said two plates and generating a magnetic field which is parallel to the longitudinal axis of said jet and the intensity of said magnetic field increases continuously in the direction of fiow of said jet, whereby said negatively-charged particles are caused to converge towards said longitudinal axis and, once said jet has passed through said opening of said upstream plate, the positively-charged particles of said jet, due to said electric field, are slowed down and then sent back towards said upstream plate by which they are captured, whereas the negatively-charged particles proceed on their way and pass through the opening of the other plate after having been accelerated by said electric field.

2. A source of electrons adapted to generate a very high energy electron beam, said source comprising an electromagnetic accelerator adapted to emit a jet of plasma possessed of a very high energy, at least two electrically conductive parallel plates at right angles to the longitudinal axis of said jet, said plates being provided with openings co-axial to said jet and the areas of which are of the order of that of the cross-section of said jet, a source of direct current adapted to establish an electric field between said plates and so mounted that its negative terminal is connected to that of said plates which is the more upstream of said jet, and a coil located entirely between said means for generating an accelerated jet and said two plates and generating a magnetic field which is parallel to the longitudinal axis of said jet and the intensity of said magnetic field increases continuously in the direction of flow of said jet, whereby the electrons of said plasma are caused to converge towards said longitudinal axis and once said jet has passed through said opening of said upstream plate, the positive ions of said plasma, due to said electric-field, are slowed down and then sent back towards said upstream plate by which they are captured, whereas the electrons of said plasma proceed on their way and pass through the opening of the other plate after having been accelerated by said electric field.

References Cited UNITED STATES PATENTS 2,489,082 11/1949 De Forest 3l385 X 2,975,279 3/1961 Craig 25041.9 3,082,326 3/1963 Arnold 3l3-6l 3,275,867 9/1966 Tsuchimoto 3 l363 3,287,582 11/1966 Baldwin 313-63 JAMES W. LAWRENCE, Primary Examiner.

STANLEY D. SCHLOSSER, Examiner.

S. A. SCHNEEBERGER, Assistant Examiner.

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