Plasma Generator

Abstract

An apparatus for generation of dense ion-electron plasma. Ions are accelerated from an ion source of the apparatus through an electron emitting filament thereof. Electrons from the rear of the filament are abstracted to the positive ions and the combination forms a moving plasma with ions and electrons of equal velocity, the plasma forming a concentrated beam utilized for striking a desired target.

Description

BACKGROUND OF THE INVENTION

The invention described herein was made in the course of, or under, Contract No. W-7405-ENG-48, with the United States Atomic Energy Commission.

This invention is directed to a plasma source, and more particularly to an ion-electron generator from which an energetic plasma may be obtained.

Ion sources, such as the Phillips Ion Gauge are known in the art. In such conventional ion sources free electrons are constrained by electric and magnetic fields to circulate in a chamber and ionize a gas. Also, in such conventional ion beams, the mutual repulsion between ions causes the beam to separate and disperse so that only a few ions remain to strike the target.

SUMMARY OF THE INVENTION

The invention is an electron-ion source from which an energetic plasma may be obtained. Also, the inventive device provides a very advantageous ion source in that a very high ion current can be obtained without "blow-up", that is, mutual charge repulsion between the ions is cancelled by the charge of the electrons. Thus, the present device may function as a low-energy, high-current ion source with a high density ion beam.

The means for creating the ions and electrons in the present invention is similar to that used in conventional ion sources. However, in the present device, the ions are withdrawn from the chamber through spaces between heated electron emitting filaments. The space charge of the ions causes electrons to follow the emitted ions, forming a plasma in which the electrons and ions have equal velocity. No appreciable local fields exist within the plasma and improved plasma stability is obtained.

Therefore, it is an object of this invention to provide an electron-ion plasma generator.

A further object of the invention is to provide a plasma generator which produces a plasma in which the ions and electrons are all moving at the same velocity so that local instabilities within the plasma are minimized.

Another object of the invention is to provide a device which may function as a low-energy, high-current ion source with a high density ion beam.

Another object of the invention is to provide a plasma generator wherein the electrons cancel the space charge so that the ions do not repel and separate, thereby producing a narrow, concentrated beam of plasma wherein the ions and electrons are moving at the same velocity.

Other objects of the invention, not specifically set forth above, will become readily apparent from the following description and accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

The single FIGURE illustrates an embodiment of the inventive plasma generator.

DESCRIPTION OF THE INVENTION

As pointed out above the invention is directed to a generator for producing a plasma in which the ions and electrons are all moving at the same velocity so that local instabilities within the plasma are minimized. Also, the device provides a very advantageous ion source in that a very high ion current can be obtained without "blow-up", that is, mutual charge repulsion between the ions is cancelled by the charge of the electrons. Thus, the inventive generator, in addition to producing a narrow, concentrated plasma beam, may function as a low-energy, high-current ion source with a high density ion beam.

Referring now to the drawing, a magnet 11 provides an axial magnetic field, indicated at 12, which passes through the central portion of a conductive cylinder 13. The magnet 11, for example, may be constructed of iron, with the cylinder 13 made of stainless steel. A heated filament 14, at negative potential and made of tungsten, for example, is positioned at one end of cylinder 13 and emits electrons, which are attracted into the cylinder 13 by a positive potential applied thereto by a power supply 15, power supply 15 being for example at 200 volts. Filament 14 is heated by a power supply 16 via leads 17 and 18, lead 18 being connected to ground. The axial magnetic field 12 is created by a coil 19 wrapped around one pole 20 of magnet 11 and connected to a power supply 21, the other pole 22 of magnet 11 being wrapped with a coil 19' and connected as indicated to power supply 21 such that the coils are in phase to produce the axially-directed magnetic field 12. The axially-directed magnetic field 12 keeps the electrons from reaching the cylinder 13 so that the electrons emitted by filament 14 oscillate axially along magnetic field lines. Gas, such as hydrogen, from a supply 23 passes through a tube or conduit 24 and is introduced into cylinder 13 wherein it is ionized. The gas supply tube 24 extends through the end piece of magnet 11 and terminates in a cold cathode 25 which is connected to ground and at negative potential. While not shown, a collar or insulating sleeve may be positioned about cold cathode 25 and the associated end of cylinder 13 to prevent leakage of the hydrogen gas therefrom. When the hydrogen gas is ionized in cylinder 13, the ions are attracted toward either the relatively negative cold cathode 25 and are lost, or toward the heated filament 14. The magnetic field 12 keeps most of the ions from striking the filament 14 so that the ions pass by the filament to a target 26 within an energy dependent upon the difference in the potentials on the filament 14 and cylinder 13. Target 26 may be constructed of carbon and is held at a negative 200 volt potential, for example, via connection with a power supply 27. The ions that pass the filament 14 create a positive space charge which causes electrons from the filament to also flow toward the target along with ions. Thus a plasma beam, indicated at 28, is formed. Tests have shown that the positive space charge is almost completely cancelled by the electrons. The various potentials set forth above are typical, and with -200 volts on the target 26, only ions will strike. The target 26 may be of any type and construction or it can be removed and the plasma beam 28 utilized for various applications.

It has thus been shown that while in conventional ion beams, the mutual repulsion between ions causes the beam to separate and disperse so that only a few ions remain to strike the target, with the inventive device the electrons cancel the space charge so that the ions do not repel and separate. Thus, as described above and illustrated in the drawing, nearly all the ions passing through the filament 14 stay in the narrow, concentrated plasma beam 28 and will strike the target 26. Accordingly, the present invention provides an ion-electron generator or source capable of producing an energetic plasma, thereby substantially advancing the state of the art.

The inventive generator may be utilized for various applications. For example, it may be used as a low energy ion source to provide, for instance, surface treatment of various materials by sputtering. Since the emitted beam is electrically neutral, it can be used to propel an ion rocket in space.

While a particular embodiment of the invention has been illustrated and described, modifications and changes will become apparent to those skilled in the art, and it is intended to cover in the appended claims all such modifications and changes as come within the spirit and scope of the invention.

Claims

What I claim is:

1. A plasma generator comprising: a conductive cylinder-like means, means for creating an axial magnetic field centrally through said cylinder-like means, means for maintaining said cylinder-like means at a positive electrical potential, electron emitting filament means maintained at a negative potential positioned at one end of said cylinder-like means, means for heating said filament means, cold cathode means maintained at a negative potential positioned at the opposite end of said cylinder-like means, and means for supplying to said cylinder-like means a gas to be ionized, whereby ions are attracted toward said filament due to the negative potential thereof and pass through said filament means due to the magnetic field and create a positive space charge which causes electrons from said filament means to flow in the same direction and at the same velocity as the ions thereby producing a narrow, concentrated plasma beam.

2. The plasma generator defined in claim 1, in combination with a target means maintained at a negative potential, said target means being positioned in spaced relationship with respect to said filament means.

3. The plasma generator defined in claim 1, wherein said axial magnetic field creating means comprises a magnet means, coil means wrapped about at least one portion of said magnetic means, and power supply means operatively connected to said coil means.

4. The plasma generator defined in claim 3, wherein said magnet means comprises a pair of end pieces and a pair of interconnecting pole pieces, said coil means being wrapped around at least one of said pole pieces; said cylinder-like means, said filament means, and said cold cathode means being positioned intermediate said end piece and said pole pieces of said magnet means.

5. The plasma generator defined in claim 4, wherein said gas supply means includes a gas supply source, and conduit means connected at one end thereof to said gas supply source and with the opposite end thereof extending through one of said end pieces of said magnetic means and through said cold cathode means so as to direct gas to be ionized into said cylinder-like means at the portion thereof adjacent said cold cathode means.

6. The plasma generator defined in claim 1, wherein said cylinder-like means is at a positive potential of about 200 volts.

7. The plasma generator defined in claim 1, wherein said gas supply means supplies hydrogen gas to said cylinder-like means.