US20160133426A1

US20160133426A1 - Linear duoplasmatron

Info

Publication number: US20160133426A1
Application number: US14/897,229
Authority: US
Inventors: John E. Madocks
Original assignee: General Plasma Inc
Current assignee: General Plasma Inc
Priority date: 2013-06-12
Filing date: 2014-06-12
Publication date: 2016-05-12
Also published as: WO2014201285A1; US20160148775A1; US10134557B2; WO2014201292A1

Abstract

A duoplasmatron is provided having a cathode, an anode with linear slit, and an intermediate electrode (IE) between the cathode and the anode where the IE has an opening that is aligned with the anode slit. A magnet forms a magnetic field that passes through the anode slit. A discharge passes from the cathode to the anode through the IE opening and the anode slit. The discharge is constricted through the IE opening and the magnetic field in the anode slit. An extractor external to the anode accelerates ions through an ion emitting slit aligned with the anode slit. A process of generating an accelerated ion beam is provided that includes flowing a gas into the IE and then energizing at least one power supply to induce electron flow to the anode. Ionizing the gas in the gap between the IE and anode. The ions are accelerated from the anode through the extractor ion emitting slit forming a linear ion beam.

Description

RELATED APPLICATIONS

This application claims priority benefit of U.S. Provisional Application Ser. No. 61/834,351 filed Jun. 12, 2013; the contents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates in general to ion sources and in particular to ion sources for industrial applications and ion thrusters for space travel.

BACKGROUND OF THE INVENTION

The duoplasmatron ion source was invented in 1956 by Manfred von Ardenne and is still commercially available today. Prior art drawings, FIGS. 1 and 1A show simplified 3D and section view of the source attributed to Ardenne. This source 100, as with all prior art duoplasmatron ion sources, is round with ions extracted through a pin hole aperture. The source functions when a low pressure arc discharge 119 is formed between a thermionic cathode 105 and an anode 107. The discharge is constricted by an intermediate electrode (IE) 106 placed between the two main electrodes. The discharge is further constricted at the anode 107 aperture by a strong magnetic field 120 between the IE 106 and anode 107. This can be seen in the detail view, prior art FIG. 1B. The magnetic field is created by electromagnet 101. The IE 106 and anode 107 are constructed of ferromagnetic material. Outer ring 102 provides a magnetic field return path. Source gas 112 flows into cathode cavity 118 through bottom plate 113 and must pass through the constricted discharge 119 to reach the lower pressure of the vacuum chamber (not shown). The gas 112 is efficiently ionized in the constricted discharge. Both gas ions and electrons pass through the anode 107 aperture to the ‘low pressure’ side of the source. Outside the anode aperture, ions 110 experience the strong electric field between the extractor 108 and anode 107 and are accelerated out of the source. Four power supplies are required: PS1 heats cathode 105 into thermionic emission. PS 2 generates and sustains the arc discharge between cathode 105 and anode 107. PS 3 is the high voltage power supply to accelerate the ions 110 out of the source. PS4 drives electromagnet 101. Resistors R1 and R2 bias IE 106 to encourage discharge 111 electrons toward anode 107. The resistors also limit current to the IE 106.
Significant advantages of the duoplasmatron include the high efficiency and high brightness derived from a single ion optic. With a single optic, the shape of each optic component can be optimized and a magnetic field can effectively be used to increase source efficiency. In the case of the conventional duoplasmatron shown in source 100, the optic components are the IE 106, anode 107 and extractor 108. With a single ion optic it is possible to use high acceleration voltages without excessive extractor erosion.
While a single ion optic is advantageous, the maximum ion current extracted through the aperture can be less than desired. Additionally, when driven to high currents, space charge effects in the resulting ion beam can cause beam spreading and increased impingement on the extractor. In prior art, many variants to the conventional duoplasmatron ion source have been proposed to increase ion current and control beam spreading. In all prior art however, these sources have a round shape with a pin hole ion optic. In several prior art sources, the hole size was enlarged and ions were extracted through grids. Other prior art sources (Brooks et al. U.S. Pat. No. 3,137,801) use a ferromagnetic extractor and a non-magnetic anode to reduce beam spreading at high currents. Other variants implement an additional reflector electrode between the anode and extractor. While these variants have met with limited success, they tend to add complexity and cost and fundamentally, these sources remain single optic ion sources with limited output current. Therefore, there is a need for a high efficiency duoplasmatron ion source that can deliver high ion currents without excessive beam spreading.

SUMMARY OF THE INVENTION

A duoplasmatron is provided having a cathode, an anode with linear slit, and an intermediate electrode (IE) between the cathode and the anode where the IE has an opening that is aligned with the anode slit. A magnet forms a magnetic field that passes through the anode slit. A discharge passes from the cathode to the anode through the IE opening and the anode slit. The discharge is constricted through the anode slit by the magnetic field. An extractor external to the anode accelerates ions through an ion emitting slit aligned with the anode slit.
A process of generating an accelerated ion beam is provided that includes flowing a gas into the IE and then energizing at least one power supply to induce electron flow to the anode. Ionizing the gas in the gap between the IE and anode. The ions are accelerated from the anode through the extractor ion emitting slit forming a linear ion beam.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter that is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 shows a 3D view of a conventional prior art duoplasmatron ion source;

FIG. 1A shows section view of the conventional prior art duoplasmatron ion source;

FIG. 1B shows a detail view of the aperture in the conventional prior art duoplasmatron;

FIG. 1C shows a top view of the anode aperture in the prior art duoplasmatron ion source;

FIG. 2 shows a 3D view of a linear duoplasmatron ion source (Lineatron);

FIG. 2A shows a section view of the Lineatron ion source;

FIG. 2B shows an expanded section view of the slit area in a Lineatron ion source;

FIG. 2C shows a top view of the anode aperture in a Lineatron ion source; and

FIG. 3 shows a 3D section view of an annular Lineatron.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention overcomes ion current limitations of prior art duoplasmatrons without overdriving the source and causing beam spreading. The present invention realizes that the electric and magnetic crossed field effects that exist in the duoplasmatron aperture would remain in effect if the round aperture is extended into a slit. In a slit configuration, all the advantages of the duoplasmatron are maintained while output ion current can be readily scaled by extending the length of the slit. The inventive linear duoplasmatron ion source is termed synonymously a ‘Lineatron’ herein. As will be evident to others skilled in the art, this invention opens many opportunities to use the duoplasmatron ion source configuration where, due to ion current limitations, it was not possible before.
It is to be understood that in instances where a range of values are provided that the range is intended to encompass not only the end point values of the range but also intermediate values of the range as explicitly being included within the range and varying by the last significant figure of the range. By way of example, a recited range of from 1 to 4 is intended to include 1-2, 1-3, 2-4, 3-4, and 1-4.
FIG. 2 shows an outside 3D view of the present invention, Lineatron ion source 200 and FIG. 2A shows a section view of the source. The 3D view illustrates the basic improvement to the duoplasmatron ion source—the round ion emitting aperture of prior art duoplasmatrons is now a slit from which a uniform, high current ion beam 10 emanates.
As used herein, a slit is defined as an opening having a dimensional ratio in the x:y directions as defined in FIG. 2 of at least 3. It is appreciated that a slit can be linear, curved, or annular.
FIG. 2A is a section view of FIG. 2 that shows the inside of Lineatron ion source 200. Housing 2 surrounds the outside of the source and is secured to bottom plate 3. Cathode core 4 is supported inside housing 2 on insulators not shown. Core 4 is fluid cooled by known methods using fluids such as water or air. Target 5 is seated in core 4 such that they are in good electrical and thermal contact. Magnets 1 surround target 5 both along the straight sections of housing 2 and at the rounded ends of housing 2. Housing 2 and bottom plate 3 are made of magnetic steel. Cathode core 4 and cathode target 5 are made of non-magnetic materials. Core 4 is made of copper. Target 5 is made of isomolded graphite. Intermediate electrode (IE) 6 is centered over target 5 by insulators not shown. IE 6 is made of low carbon steel or other highly permeable material. Anode 7 is also made of low carbon steel and is supported by insulators not shown over IE 6. Extractor 8 is non-magnetic and is held away from anode 7 by insulators not shown. IE 6, anode 7 and extractor 8 are positioned so that their respective slit openings are aligned over each other and in alignment with the center of target 5. As documented in prior art, the anode pin hole diameter 122 of conventional duoplasmatrons and now the anode slit 22 width of the present invention are chosen by several parameters including cathode operating pressure and extraction voltage. Generally however, the slit 22 width is limited by the ability of the electric field from extractor 8 to penetrate the plasma exiting the anode slit. The anode slit 22 width of the Lineatron should be 3 mm or less and preferably less than 1.5 mm.
Cathode core 4 and target 5 are connected to the negative terminal of power supply PS2. The positive terminal of PS2 is connected to anode 7. The IE 6 is biased between anode 7 and cathode 4 by resistors R1 and R2. Extractor 8 is connected across PS3 to anode 7. PS3 is a high voltage power supply. In some inventive embodiments, housing 2 with bottom plate 3 and magnets 1 are supported in the vacuum chamber such that they are electrically floating. Source gas 12 is directed into cavity 18 through bottom plate 3, cathode core 4 and target 5.
In operation, PS2 is turned on and a magnetron plasma lights inside cavity 18. This configuration of inward facing magnets forming a magnetron discharge in a cavity is based upon prior art from the present inventor. A plasma source with this configuration is called the Plasma Beam Source (PBS). Exemplary forms of these are detailed in U.S. Pat. Nos. 7,327,089 B2 and 7,411,352 B2. As described in these patents, this magnetron plasma effectively makes a linear electron source with electrons passing out of the magnetron racetrack and exiting the cavity through the magnetic mirror 17 at the cavity exit. In the case of the present invention, the PBS is used to supply electrons uniformly along the length of the source cavity 18 to anode 7. As with a conventional duoplasmatron, a low pressure discharge 19 (formed by PBS electrons) is constricted physically by the IE 6 slit 24 and magnetically by field lines 20 between the IE 6 slit 24 and anode 7 slit 22. The slit shown in FIG. 2 is shown expanded section view in FIG. 2B. FIG. 2B shows the concentrated magnetic field 20 between the IE slit 24 and anode slit 22. Since electrons are blocked from crossing magnetic field lines, the only option for the electrons is to pass through the center of the slit 22. This is seen as a narrow plasma beam exiting anode slit 22 during source operation. Electron and plasma flow through slit 22 is also impeded by the converging magnetic mirror 17 in the IE 6 slit 24.
While electrons and plasma are flowing through slit 22, coincidently, source gas 12 is also flowing through the slit and gas 12 is efficiently ionized. Once outside the anode 7 slit 22, ions 10 experience the electric field imposed between the anode 7 and extractor 8 by PS3 and ions 10 are accelerated out of the extractor 8 slit region 21 into the vacuum chamber, forming a linear, uniform ion beam 10 as depicted in FIG. 1.
The functions and operation described above for the IE, anode and extractor are identical to prior art duoplasmatron ion sources. As stated earlier and as shown in FIGS. 2, 2A, 2B and 2C, the present invention, contrary to conventional thinking that a circular aperture is needed to inhibit beam spreading at high currents, realizes that these functions and operation can be extended from a round aperture 121 to a linear slit 21 without undue beam spreading at high currents. FIG. 2C shows a top view of the anode 7 slit 22 as an analog to FIG. 1C. In FIG. 2C, the magnetic field lines 20 emanating out of anode 7 slit 22 are shown. As in the pin hole aperture of the conventional duoplasmatron (Prior art FIG. 1C), E×B forces, where E is the electric field, B is the magnet field and E×B denotes the cross product therebetween, cause electrons 23 to move orthogonally to the B field in a radial direction. In the case of the Lineatron, the electrons 23 move around slit 22, uniformly ionizing gas along the slit 22 length. In addition to E×B forces, the gradient in the magnetic field induces motion in the same Hall direction.
Extending the aperture of a duoplasmatron from a pin hole to a slit has several important features and benefits:

- The linear or annular slit of the Lineatron enables the Lineatron to overcome the inherent ion current limitations of the Duoplasmatron. This opens applications like ion thrusters for space applications to duoplasmatrons.
- Like other magnetically confined ion and plasma sources, for example anode layer ion sources or planar magnetron sputter cathodes, the length of the Lineatron can extend to several meters.
- Using a PBS as the electron source to ‘feed’ the Lineatron is a robust and economical solution. By supplying a uniform, linear electron stream inside the source cavity, the uniformity of the resulting source ion beam is enhanced. Additionally, the PBS is not sensitive to oxygen as filaments or hollow cathodes tend to be. The referenced patent for the PBS describes the design and operation of this source in detail.
- The ion current density out of the source can be reduced in the Lineatron because the slit length can be made longer to address high current requirements. In the Duoplasmatron, since the orifice size is limited by electric field considerations, ion current density in the orifice must be maximized to attain high output currents. Running the Duoplasmatron at maximum current can cause operational instabilities and ion beam spreading due to space charge effects can be severe (as described in the Background section). With the Lineatron, the current density along the slit can be reduced to a stable operating regime while the total output current still far exceeds the output current possible from prior art duoplasmatron sources.

FIG. 3 shows a section view of a Lineatron ion source embodiment that is particularly well-suited for space ion engine applications. This source 300 is round with an annular slit. Electromagnets 301 and 304 replace the function of permanent magnets in the above detailed embodiments to produce a strong B field 309 in the gap between the extractor 302 (composed of outer and inner parts 302 a and 302 b) and IE 305 (composed of outer and inner parts 305 a and 305 b). An anode 303, with outer and inner parts 303 a and 303 b, is positioned so an anode slit 323 is formed and is aligned on axis with the IE slit 336 and top pole top opening 337. The anode is electrically insulated from the other source parts and is connected to power supply 321. Two electron emitting filaments, 313, 314 are shown inside the IE 305 and power supplies 311 and 312 connect to these filaments and drive sufficient current to heat the filaments to thermionic emission. Gas 322 is conducted into the IE 305 by a conduit not shown. Back plate 317 prevents gas leakage, forcing all the gas to flow through the IE slit 336 and anode slit 323.
Note that Lineatron source 300 illustrates how the present invention is readily adaptable to the many variants on the basic duoplasmatron. In source 300, the anode 303 is non-magnetic, for instance copper or isomolded graphite and the extractor 302 is a high permeability material. Though this magnetic field creates more of a Penning style electron confinement than a classic duoplasmatron, the fundamental aspect of an impeded, constricted discharge through an anode aperture remains the same.
The operation of Lineatron 300 is as follows:

- Gas flows into the IE.
- The filament power supplies are turned on and electrons flow to the anode—and to a lesser degree to the IE. Resistors 319 and 320 limit the current to the IE and bias the IE to be between the anode and cathode. Typical values for these resistors are 1000 ohm each. As the electrons move toward the anode they pass through the IE slit. Coincident with these electrons, the source gas is also passing through this slit and the gas is ionized in the process.
- The plasma formed in the cathode/IE cavity and slit then flows through the anode slit 323 where the strong E field between the anode and extractor is encountered. Reacting to this electric field, ions are accelerated out of the source.

Although the present invention has been described with reference to preferred embodiments, numerous modifications and variations can be made and still the result will come within the scope of the invention. No limitation with respect to the specific embodiments disclosed herein is intended or should be inferred.

Claims

1. A linear ion source comprising:

a cathode;

an anode with a linear slit

an intermediate electrode (IE) between said cathode and said anode with an opening aligned with the anode slit

a magnet forming a magnetic field wherein a portion of the magnetic field passes through the anode slit

a discharge passing from said cathode to said anode through said IE opening and said anode slit, said discharge being constricted at the anode slit by the magnetic field;

an extractor external to said anode with an ion emitting slit aligned with said anode slit for accelerating ions

2. The ion source of claim 1 wherein said cathode is a magnetron sputter cathode

3. The ion source of claim 1 wherein said cathode is a magnetron cathode that emits electrons over a linear length similar to the ion emitting slit length

4. The ion source of claim 1 wherein said cathode is a heated filament

5. The ion source of claim 1 wherein said linear ion source ion emitting slit is longer than 100 mm

6. The ion source of any of claims 1 wherein said ion emitter slit is linear, curved, or annular.

7. A process of generating an accelerated ion beam comprising:

flowing a gas into said IE of claim 1;

energizing at least one power supply to induce electron flow to said anode of claim 1 and ionize the gas to form ions; and

accelerating the ions from said anode through the ion emitter slit of claim 1 to generate the accelerated ion beam.

8. The process of claim 7 further comprising resistively biasing said IE.