WO2013096517A1

WO2013096517A1 - Method and apparatus for radio frequency (rf) discharge with control of plasma potential distribution

Info

Publication number: WO2013096517A1
Application number: PCT/US2012/070735
Authority: WO
Inventors: Vadim Dudnikov; Andrei DUDNIKOV
Original assignee: Muons Inc
Current assignee: Muons Inc
Priority date: 2011-12-20
Filing date: 2012-12-19
Publication date: 2013-06-27
Anticipated expiration: 2014-06-20

Abstract

In a particular embodiment, a device is disclosed that includes means for providing a radio frequency (RF) discharge plasma generator with an additional electrode for independent control of plasma potential distribution. The device also includes means for positive biasing of the additional electrode relative to end flanges and a longitudinal magnetic field, such that a confinement of fast electrons in the discharge may be improved for more reliable triggering of a pulsed RF discharge at low gas density and a rate of ion generation may be enhanced. In another particular embodiment, a method is disclosed that includes steps for providing a radio frequency (RF) discharge plasma generator with an additional electrode for independent control of plasma potential distribution. The method also includes steps for positive biasing of the additional electrode relative to end flanges and a longitudinal magnetic field.

Description

METHOD AND APPARATUS FOR RADIO FREQUENCY (RF) DISCHARGE WITH CONTROL OF PLASMA POTENTIAL DISTRIBUTION

INVENTORS:

Vadim Dudnikov, Ph.D.

and

Andrei Dudnikov, Ph.D.

J. Cross-Reference to Related Application

[0001] This application claims the benefit of U.S. Provisional Patent Application No.

61/578,220, filed December 20, 2011, which is hereby incorporated by reference in its entirety, as if set out below.

//. Field of the Disclosure

[0002] The present disclosure is generally related to a radio frequency (RF) discharge plasma generator with an additional electrode for independent control of plasma potential distribution and, in particular, to an RF discharge plasma generator with an additional ring electrode for independent control of plasma potential distribution, with positive biasing of this ring electrode relative to end flanges and a longitudinal magnetic field, such that a confinement of fast electrons in the discharge may be improved for more reliable triggering of a pulsed RF discharge at low gas density and a rate of ion generation may be enhanced.

///. Summary

[0003] An RF discharge plasma generator with one or more additional electrodes for

independent control of plasma potential distribution is described herein. With positive biasing of this ring electrode relative to end flanges and a longitudinal magnetic field, a confinement of fast electrons in the discharge may be improved for reliable triggering of a pulsed RF discharge at low gas density and a rate of ion generation may be enhanced. In various illustrative embodiments of the discharge combination, the electron energy is enhanced by an RF field and the fast electron confinement is improved by an enhanced positive plasma potential that improves the efficiency of plasma generation

significantly. This combination creates a synergistic effect significantly improving the plasma generation performance at low gas density. The discharge parameters may be optimized for enhanced plasma generation with acceptable electrode sputtering.

[0004] High-frequency (RF) discharges with capacitive and inductive energy transfer to

electrons of the plasma are used widely in ion sources, as described, for example, in M.D. Gabovich, Sov. Phys. Technical Physics 28, 814 (1958), Iwao Ogawa, Nagao Abe, Electron cyclotron resonances in a radiofrequency ion source, Nuclear Instruments and Methods 16, 227 (1962), A. Sorokin, V. Belov, V. Davydenko, P. Deichuli, A. Ivanov, A. Podyminogin, I. Shikhovtsev, G. Shulzhenko, N. Stupishin, and M. Tiunov, Rev. Sci. Instrum. 81, 02B108 (2010), M. Stockli, B. Han, S. N. Murray, T. R. Pennisi, M. Santana, and R. F. Welton, Rev. Sci. Instrum. 81, 02A729 (2010), W. Kraus, U. Fantz, and P. Franzen, Rev. Sci. Instrum. 81, 02B110 (2010), and V. Dudnikov and R. Johnson, Phys. Rev. ST - Accelerators and beams 14, 054801 (2011), and in many other technological applications of low temperature plasmas, as described, for example, in Francis F. Chen and Humberto Torreblanca, Physics of Plasmas, 16, 057102 (2009).

[0005] In ion sources it is important to ensure the generation of a dense plasma with a minimal density of the gas in order to prevent charge exchange of positive ions and stripping of negative ions in the target of gas escaping through the emission aperture. In the sources of negative Hydrogen ions with the emission apertures ~1 cm, a Hydrogen pressure before the discharge ignition should be below 5 mTorr. With such low gas density it is difficult to produce a reliable ignition of pulsed discharges. In the known conventional sources for the ignition of pulsed RF, triggering plasma guns (TPG) are used to initiate rapid ignition of high power RF discharges.

[0006] The triggering conditions of the discharge are determined by a balance of the generation and the loss of ionizing electrons. In the RF electric field, the electrons gain energy through a change of the velocity vector orientation to the electric field in the elastic collisions with the gas particles. With increase of the electron energy above the excitation potential, the electrons begin to lose energy and generate new electrons. The electrons are then lost from the volume due to the diffusion and drift to the walls. [0007] In a magnetic field, the electron diffusion across a magnetic field is suppressed and it is possible to ignite discharges at lower gas density. However, this gas density is still high for the normal operation of ion sources, because the electrons can quickly enough escape to the walls along the magnetic field. A further lowering of the critical gas density can be reached by suppression of the electrons escaping to the wall along the magnetic field. For this we realized that it is possible to use an electrostatic trap, such as a Penning type cell. We describe herein various illustrative embodiments that use such an electrostatic trap.

[0008] An inductive discharge with a solenoidal antenna, as shown in Figure la, for example, is used most widely in RF ion sources. With the solenoidal antenna, the induction electric field is proportional to a distance from the axis, has a maximum near the antenna, and is equal to zero on the axis. The plasma is generated near the wall of the gas -discharge chamber and diffused to the axis, creating the rather flat distribution of density required for ion sources with broad multi- aperture extraction systems. The efficiency of the generation of the extracted ions through a front plasma electrode with an extraction aperture is rather low thought to be due to losses of plasma on the walls and the fast escape of the fast electrons to the walls. The inductive electric field of the antenna is shielded by the plasma and quickly decays during a penetration into the plasma (there is a strong skin effect). For this reason, the efficiency of plasma generation falls with an increase of frequency. Therefore, it is conventional to use RF sources, such as the one shown in Figure la, with low frequencies in the range of 1-2 MHz.

[0009] Higher efficiency of ion generation on the axis can be produced by use of a saddle antenna with an additional longitudinal magnetic field, as shown on Figure lb, for example. The RF magnetic field of the saddle antenna is perpendicular to the axis of system and to a constant longitudinal magnetic field B_z. The induction electric field of the saddle antenna has a component that is maximal on the axes of the system, providing generation and acceleration of the electrons in the center of the plasma. The permanent magnetic field B_z changes the law of dispersion of an electromagnetic wave in the plasma and facilitates the transfer of energy in the central area of the plasma, and suppresses diffusion of the plasma to the lateral walls, as well. This results in a significantly greater density of plasma on the axis with the saddle antenna plasma, with a more peaked distribution, as described, for example, in M.D. Gabovich, Sov. Phys. Technical Physics 28, 814 (1958), V. Dudnikov and R. Johnson, Phys. Rev. ST - Accelerators and beams 14, 054801 (2011), and V. Dudnikov, R.P. Johnson, S. Murray, Pennisi, M. Santana, and R. F. Welton, report WEP273, PAC 2011, New York, USA , 2011 (unpublished).

[0010] The influence of the longitudinal magnetic field on the radial plasma density

distribution was tested, as described, for example, in V. Dudnikov, R.P. Johnson, S. Murray, Pennisi, M. Santana, and R. F. Welton, report WEP273, PAC 2011, New York, USA , 2011 (unpublished). The plasma density distribution was measured by collectors extracting the ion beam current through small (2 mm diameter) emission apertures located along a radius in the end plate attached to the discharge chamber. Oscilloscope traces of collector currents at a high magnetic field are shown, for example, in Figure 2.

[0011] Evolution of the plasma density distribution with increase of the magnetic field is

shown, for example, in Figure 3 (RF frequency f = 13.56 MHz; RF power Pr_f = 1 kW; Ceramic chamber inner diameter (ID) is 6.8 cm; with magnetic coil current I_m=70 A magnetic field B=250 G). The collector current density was increased up to 5 times from 12 mA/cm² to 60 mA/cm² as the magnetic field increased from 0 G to 250 G. For low magnetic field (I_m up to 20 A), the radial distribution of plasma density is flat. For higher magnetic fields, the plasma density inside a 2 cm radius is much higher. The same increase of ion current density on the axis with increasing magnetic field was observed with saddle antenna discharges at f=5 MHz. With the magnetic field, the minimal gas density needed for pulsed RF discharge triggering without using a triggering plasma gun (TPG) was up to 5 times lower than without the magnetic field.

[0012] In both cases electrons escape from the plasma more quickly than ions and the plasma gets a positive potential for confinement of the escaping electrons and accelerating the ions to the walls that provides macroscopic quasi-neutrality. The positive plasma potential confines electrons with an energy smaller than an ambipolar potential but does not prevent the escape of the most valuable fast electrons with the greatest cross- sections for ionization.

[0013] Depth of a potential hole for the electrons can be increased by application of the

adjustable potential to an electrode, as indicated by (5) in Figure 4, contacting with the plasma, as shown, for example, in Figure 4 presenting different versions of RF plasma generators with a controllable distribution of the plasma potential. In various illustrative embodiments, an RF plasma generator with controllable distribution of plasma potential includes: the gas-discharge chamber (1) fabricated from a vacuum-tight insulating material, the saddle antenna (2), coils of the magnetic field (3, 4), the ring electrode (5), isolated feed-through (6), insulator of the feed-through (7), a plasma electrode with emission aperture (8), a back flange electrode (9), the plasma generator for discharge triggering (10), and the gas delivery system (11). The ring electrode can have a gap (12) for penetration of the RF magnetic field into the discharge chamber. The antenna is connected to the RF power generator through a matching network.

[0014] With the positive potential on a ring electrode, the potential of plasma rises and

conditions for confinement of more of the faster electrons with a higher rate of ion generation are improved. In various illustrative embodiment of the plasma generator the electrons are magnetized and slowly drifting to the ring electrode in an electric field crossed with a magnetic field.

[0015] In previous designs of RF ion sources, as described, for example, in M.D. Gabovich, Sov. Phys. Technical Physics 28, 814 (1958), and Iwao Ogawa, Nagao Abe, Electron cyclotron resonances in a radiofrequency ion source, Nuclear Instruments and Methods 16, 227 (1962), a contact of the plasma with a face electrode (probe) was used for creation of a potential difference between the plasma and the plasma electrode for extraction of ions from the plasma by this potential difference, as described, for example, in M.D. Gabovich, Sov. Phys. Technical Physics 28, 814 (1958), but conditions for the confinement of fast electrons in the plasma was not created.

[0016] Some possible versions of the saddle antenna are shown, for example, in Figure 5. In a similar way, fast electrons may be confined in RF discharges with a solenoidal antenna and in microwave sources with an electron cyclotron resonance. A prototype of the RF generator with a ring electrode similar to that shown, for example, in Figure 4a has been prepared for testing purposes.

[0017] It is attractive to use such an RF discharge with controlled potential distribution for negative ion generation in surface plasma sources, as described, for example, in V. Dudnikov and R. Johnson, Phys. Rev. ST - Accelerators and beams 14, 054801 (2011), and V. Dudnikov and R.P. Johnson, Fusion Sci. and Technol., v.59 (1), p. 277-279 (2011). The discharges with increased plasma potential can be used for cleaning of plasma electrodes by high energy positive ion bombardment, which may be important for efficient cesiation, enhancing secondary negative ion emission, as described, for example, in V. Dudnikov and R. Johnson, Phys. Rev. ST - Accelerators and beams 14, 054801 (2011).

[0018] In a particular embodiment, a device is disclosed that includes means for providing a radio frequency (RF) discharge plasma generator with an additional electrode for independent control of plasma potential distribution. The device also includes means for positive biasing of the additional electrode relative to end flanges and a longitudinal magnetic field, such that a confinement of fast electrons in the discharge may be improved for more reliable triggering of a pulsed RF discharge at low gas density and a rate of ion generation may be enhanced.

[0019] In another particular embodiment, a method is disclosed that includes steps for

providing a radio frequency (RF) discharge plasma generator with an additional electrode for independent control of plasma potential distribution. The method also includes steps for positive biasing of the additional electrode relative to end flanges and a longitudinal magnetic field, such that a confinement of fast electrons in the discharge may be improved for more reliable triggering of a pulsed RF discharge at low gas density and a rate of ion generation may be enhanced.

IV. Brief Description of the Drawings

[0020] The following figures form part of the present specification and are included to further demonstrate certain aspects of the present invention. The present invention may be better understood by reference to one or more of these drawings in combination with the description of embodiments presented herein.

[0021] Consequently, a more complete understanding of the present disclosure and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which the leftmost significant digit(s) in the reference numerals denote(s) the first figure in which the respective reference numerals appear, wherein: [0022] Figure 1 is a diagram illustrating a radio frequency (RF) plasma generator antenna, with Figure la showing an ordinary conventional solenoid antenna, and Figure lb showing a saddle antenna, where an external magnetic field is along the axis of the cylindrical discharge chamber;

[0023] Figure 2 is a diagram illustrating an oscilloscope trace of collector current I_c at high magnetic field (I_m=70A);

[0024] Figure 3 is a diagram illustrating a radial distribution of current density of extracted positive ions (plasma density) for different magnetic fields (coil current I_m), as determined from the 7 collectors, as described, for example, in V. Dudnikov, R.P.

Johnson, S. Murray, Pennisi, M. Santana, and R. F. Welton, report WEP273, PAC 2011, New York, USA , 2011 (unpublished);

[0025] Figure 4 is a diagram illustrating various designs of RF plasma generators with a

controllable distribution of the plasma potential: Figure 4a shows an axial electric feed- though; Figure 4b shows a radial electric feed-though; and Figure 4c shows a metallic insert, where 1 shows ceramic gas-discharge chambers, 2 shows a saddle antenna, 3, 4 show magnetic field coils, 5 shows a ring electrode, 6 shows an isolated feed-though, 7 shows insulators of a feed-though, 8 shows a plasma electrode with the emission aperture, 9 shows a back face flange, 10 shows a plasma generator for triggering of RF discharge, 11 shows a gas delivery system, and 12 shows a gap for ac magnetic field penetration;

[0026] Figure 5 is a diagram illustrating versions of the saddle antenna: Figure 5a showing a multiturn saddle antenna, made from isolated Litz wires; and Figure 5b showing a one turn saddle antenna, made from copper tube;

[0027] Figure 6 is a diagram illustrating an embodiment of an apparatus including means for providing a radio frequency (RF) discharge plasma generator with an additional electrode for independent control of plasma potential distribution and means for positive biasing of the additional electrode relative to end flanges and a longitudinal magnetic field, such that a confinement of fast electrons in the discharge may be improved for more reliable triggering of a pulsed RF discharge at low gas density and a rate of ion generation may be enhanced; and [0028] Figure 7 is a flow diagram of an illustrative embodiment of a method including steps for providing a radio frequency (RF) discharge plasma generator with an additional electrode for independent control of plasma potential distribution and steps for positive biasing of the additional electrode relative to end flanges and a longitudinal magnetic field, such that a confinement of fast electrons in the discharge may be improved for more reliable triggering of a pulsed RF discharge at low gas density and a rate of ion generation may be enhanced.

V. Detailed Description

[0029] Illustrative embodiments of the present invention are described in detail below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of the present disclosure.

[0030] Particular embodiments of the present disclosure are described with reference to the drawings. In the description, common features are designated by common reference numbers.

[0031] Referring to Figure 1, a diagram illustrating a simplified conceptual scheme of a radio frequency (RF) plasma generator antenna is depicted and indicated generally, for example, at 100. Figure la shows an ordinary conventional solenoid antenna 110, and Figure lb shows a saddle antenna 120, where an external magnetic field is along the axis of the cylindrical discharge chamber.

[0032] An inductive discharge with a solenoidal antenna, as shown in Figure la, for example, is used most widely in RF ion sources. With the solenoidal antenna, the induction electric field is proportional to a distance from the axis, has a maximum near the antenna, and is equal to zero on the axis. The plasma is generated near the wall of the gas -discharge chamber and diffused to the axis, creating the rather flat distribution of density required for ion sources with broad multi- aperture extraction systems. The efficiency of the generation of the extracted ions through a front plasma electrode with an extraction aperture is rather low thought to be due to losses of plasma on the walls and the fast escape of the fast electrons to the walls. The inductive electric field of the antenna is shielded by the plasma and quickly decays during a penetration into the plasma (there is a strong skin effect). For this reason, the efficiency of plasma generation falls with an increase of frequency. Therefore, it is conventional to use RF sources, such as the one shown in Figure la, with low frequencies in the range of 1-2 MHz.

[0033] Higher efficiency of ion generation on the axis can be produced by use of a saddle

antenna with an additional longitudinal magnetic field, as shown on Figure lb, for example. The RF magnetic field of the saddle antenna is perpendicular to the axis of system and to a constant longitudinal magnetic field B_z. The induction electric field of the saddle antenna has a component that is maximal on the axes of the system, providing generation and acceleration of the electrons in the center of the plasma. The permanent magnetic field B_z changes the law of dispersion of an electromagnetic wave in the plasma and facilitates the transfer of energy in the central area of the plasma, and suppresses diffusion of the plasma to the lateral walls, as well. This results in a significantly greater density of plasma on the axis with the saddle antenna plasma, with a more peaked distribution, as described, for example, in M.D. Gabovich, Sov. Phys. Technical Physics 28, 814 (1958), V. Dudnikov and R. Johnson, Phys. Rev. ST - Accelerators and beams 14, 054801 (2011), and V. Dudnikov, R.P. Johnson, S. Murray, Pennisi, M. Santana, and R. F. Welton, report WEP273, PAC 2011, New York, USA , 2011 (unpublished).

[0034] Referring to Figure 2, a diagram illustrating an oscilloscope trace of collector current I_c at high magnetic field (I_m=70A) is depicted and indicated generally, for example, at 200.

[0035] The influence of the longitudinal magnetic field on the radial plasma density

[0036] Referring to Figure 3, a diagram illustrating a radial distribution of current density of extracted positive ions (plasma density) for different magnetic fields (coil current I_m), as determined from the 7 collectors, as described, for example, in V. Dudnikov, R.P.

Johnson, S. Murray, Pennisi, M. Santana, and R. F. Welton, report WEP273, PAC 2011, New York, USA , 2011 (unpublished), is depicted and indicated generally, for example, at 300.

[0037] Evolution of the plasma density distribution with increase of the magnetic field is

[0038] In both cases electrons escape from the plasma more quickly than ions and the plasma gets a positive potential for confinement of the escaping electrons and accelerating the ions to the walls that provides macroscopic quasi-neutrality. The positive plasma potential confines electrons with an energy smaller than an ambipolar potential but does not prevent the escape of the most valuable fast electrons with the greatest cross- sections for ionization.

[0039] Referring to Figure 4, a diagram illustrating various designs of RF plasma generators with a controllable distribution of the plasma potential is depicted and indicated generally, for example, at 400. Figure 4a shows an axial electric feed-though 410.

Figure 4b shows a radial electric feed-though 420. Figure 4c shows a metallic insert 430, where 1 shows ceramic gas-discharge chambers, 2 shows a saddle antenna, 3, 4 show magnetic field coils, 5 shows a ring electrode, 6 shows an isolated feed-though, 7 shows insulators of a feed-though, 8 shows a plasma electrode with the emission aperture, 9 shows a back face flange, 10 shows a plasma generator for triggering of RF discharge, 11 shows a gas delivery system, and 12 shows a gap for ac magnetic field penetration.

[0040] Depth of a potential hole for the electrons can be increased by application of the

[0041] With the positive potential on a ring electrode, the potential of plasma rises and

[0042] In previous designs of RF ion sources, as described, for example, in M.D. Gabovich, Sov. Phys. Technical Physics 28, 814 (1958), and Iwao Ogawa, Nagao Abe, Electron cyclotron resonances in a radiofrequency ion source, Nuclear Instruments and Methods 16, 227 (1962), a contact of the plasma with a face electrode (probe) was used for creation of a potential difference between the plasma and the plasma electrode for extraction of ions from the plasma by this potential difference, as described, for example, in M.D. Gabovich, Sov. Phys. Technical Physics 28, 814 (1958), but conditions for the confinement of fast electrons in the plasma was not created.

[0043] Referring to Figure 5, a diagram illustrating versions of the saddle antenna is depicted and indicated generally, for example, at 500. Figure 5 a shows a multiturn saddle antenna, made from isolated Litz wires 510, and Figure 5b shows a one turn saddle antenna, made from copper tube 520.

[0044] Some possible versions of the saddle antenna are shown, for example, in Figure 5. In a similar way, fast electrons may be confined in RF discharges with a solenoidal antenna and in microwave sources with an electron cyclotron resonance. A prototype of the RF generator with a ring electrode similar to that shown, for example, in Figure 4a has been prepared for testing purposes.

[0045] It is attractive to use such an RF discharge with controlled potential distribution for negative ion generation in surface plasma sources, as described, for example, in V. Dudnikov and R. Johnson, Phys. Rev. ST - Accelerators and beams 14, 054801 (2011), and V. Dudnikov and R.P. Johnson, Fusion Sci. and Technol., v.59 (1), p. 277-279 (2011). The discharges with increased plasma potential can be used for cleaning of plasma electrodes by high energy positive ion bombardment, which may be important for efficient cesiation, enhancing secondary negative ion emission, as described, for example, in V. Dudnikov and R. Johnson, Phys. Rev. ST - Accelerators and beams 14, 054801 (2011).

[0046] Referring to Figure 6, a diagram illustrating an embodiment of an apparatus is depicted and indicated generally, for example, at 600. The apparatus 600 includes means for providing a radio frequency (RF) discharge plasma generator with an additional electrode for independent control of plasma potential distribution 610 and means for positive biasing of the additional electrode relative to end flanges and a longitudinal magnetic field, such that a confinement of fast electrons in the discharge may be improved for more reliable triggering of a pulsed RF discharge at low gas density and a rate of ion generation may be enhanced 620.

[0047] Referring to Figure 7, a flow diagram of an illustrative embodiment of a method is depicted and indicated generally, for example, at 700. The method 700 includes steps for providing a radio frequency (RF) discharge plasma generator with an additional electrode for independent control of plasma potential distribution 710 and steps for positive biasing of the additional electrode relative to end flanges and a longitudinal magnetic field, such that a confinement of fast electrons in the discharge may be improved for more reliable triggering of a pulsed RF discharge at low gas density and a rate of ion generation may be enhanced 720.

[0048] Attached herewith as an Appendix to this specification is a document describing more details about various illustrative embodiments, which Appendix to this specification is incorporated by reference as if set forth below. More details about various illustrative embodiments may be found by referring to the Appendix.

[0049] The present invention is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as those that are inherent therein. While the present invention has been depicted, described and is defined by reference to exemplary embodiments of the present invention, such a reference does not imply a limitation of the present invention, and no such limitation is to be inferred. The present invention is capable of considerable modification, alteration, and equivalency in form and function as will occur to those of ordinary skill in the pertinent arts having the benefit of this disclosure. The depicted and described embodiments of the present invention are exemplary only and are not exhaustive of the scope of the present invention.

Consequently, the present invention is intended to be limited only by the spirit and scope of the appended claims, giving full cognizance to equivalents in all respects.

[0050] The particular embodiments disclosed above are illustrative only, as the present

invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of composition or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and intent of the present invention. In particular, every range of values (of the form, "from about a to about b," or, equivalently, "from approximately a to b," or, equivalently, "from approximately a-b") disclosed herein is to be understood as referring to the power set (the set of all subsets) of the respective range of values, in the sense of Georg Cantor. Accordingly, the protection sought herein is as set forth in the claims below.

[0051] The particular embodiments of the present invention described herein are merely

exemplary and are not intended to limit the scope of this present invention. Many variations and modifications may be made without departing from the intent and scope of the present invention. Applicants intend that all such modifications and variations are to be included within the scope of the present invention as defined in the appended claims and their equivalents. While the present invention has been illustrated by a description of various

embodiments and while these embodiments have been described in considerable detail, it is not the intention of the Applicants to restrict, or any way limit the scope of the appended claims to such detail. The present invention in its broader aspects is therefore not limited to the specific details, representative apparatus, methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the scope of Applicants' general inventive concept.

Met od and Apparatus for RF Discharge with Contra! of Plasma Potential Distribution

Appendix to the Specification

RF Discharge with control of plasma potential distribution

Vadim Dudnikov^, A. Dudnikov²

'Muons, Inc., Batavia, 1L 60510 USA

2BINP, Novosibirsk 63090, RUSSIA

(Presented XXXXX, received XXXXX, accepted XXXXX, published online XXXXX)

An RF discharge plasma generator with additional electrodes for independent control of plasma potential

distribution is proposed. With positive biasing of this ring electrode relative end flanges and longitudinal

magnetic field a confinement of fast electrons in the discharge will be improved for reliable triggering of

pulsed RF discharge at low gas density and rate of ion generation will be enhanced. In the proposed discbarge combination the electron energy is enhanced by RF field and the fast electron confinement is improved by enhanced positive plasma potential which improves the efficiency of plasma generation significantly. This combination creates a synergetic effect with a significantly improving the plasma generation performance at low gas density. The discharge parameters can be optimized for enhance plasma generation with acceptable electrode sputtering. possible to use an electrostatic trap, such as a Penning

I INTRODUCTION

type cell. Lest possibility is considered in this article.

High-frequency (RF) discharges with capacitor and

inductive energy transfer to electrons of plasma are used II FEATURES OF RF DISCHARGES

widely in ion sources¹'^{2 ,3}'⁴'⁵'⁶ and in many other

technological applications of low temperature plasmas ¹. An inductive discharges with the solenoidal antenna shown In ion sources it is necessary to ensure the generation of on Fig. la is used most widely in RF ion sources. At the dense plasma with a minimal density of the gas in order to solenoidal antenna the induction electric field is

prevent charge exchange of positive ions and stripping of proportional to a distance from the axis, has a maximum negative ions in the target of gas escaping through the near the antenna and is equal to zero on axis. The plasma is emission aperture. In the sources of negative Hydrogen generated near the wall of the gas -discharge chamber and ions with the emission apertures ~ 1 cm a Hydrogen diffused to the axis, creating rather flat distribution of pressure before the discharge ignition should be below 5 density required for ion sources with broad multiaperture MTorr. With such low gas density it is difficult to produce extraction systems. Efficiency of the extracted ions a reliable ignition of pulsed discharges. In the known generation through a front plasma electrode with extraction sources for the ignition of pulsed RF are used the aperture is rather low thought a losses of plasma on the triggering plasma guns (TPG) for initiate rapid ignition of walls and fast escaping of the fast electrons to the walls.

high power RF discharges. The inductive electric field of the antenna is shielded by

The triggering conditions of the discharge is determined by the plasma and quickly decay during a penetration into the balance of generation and the loss of ionizing electrons. In plasma (strong skin effect). By this reason, an efficiency of the RF electric field the electrons gain energy through a plasma generation falls with increase of frequency.

change of the velocity vector orientation to the electric Therefore it is necessary to use RF sources with low field in the elastic collisions with the gas particles. With frequencies of 1-2 MHz.

increasing the electron energy above the excitation Higher efficiency of ions generation on the axis can be potential, they begin to lose energy and generate new produced by use of the saddle antenna with an additional electrons. The electrons are lost from the volume due to longitudinal magnetic field shown on Fig. lb.

the diffusion and drift to the walls.

In a magnetic field the electrons diffusion across a

magnetic field is suppressed and it is possible to ignite

discharges at lower gas density. However, this gas density

is still high for normal ion sources operation, because the

electrons can quickly enough escape to the walls along the

magnetic field. A further lowering of the critical gas

density can be reached by suppression of the electrons FIG. 1. (Color on line) RF plasma generator antennas, a-ordinar escaping to the wall along the magnetic field. For this it is solenoid antenna; b-saddle antenna. An external magnetic field is along the axis of the cylindrical discharge chamber.

t od and Apparatus for RF Discharge with Contra! of Plasma Potential Distribution

RF magnetic field of the saddle antenna is perpendicular to distribution of plasma density is flat. For higher magnetic the axis of system and to a constant longitudinal magnetic fields, the plasma density inside 2 cm radius is much field Bz. The induction electric field of the saddle antenna higher. The same increase of ion current density on the has a component maximal on the axes of the system, axis with increasing magnetic field was observed with providing generation and acceleration of the electrons in saddle antenna discharges at f=5 MHz. With magnetic the center. A permanent magnetic field will change the law field the minimal gas density necessary ro pulsed RF of dispersion of an electromagnetic wave in plasma and discharge triggering without TPG was up to 5 times lower facilitates transfer of energy in central area of the plasmas, then without magnetic field.

as well as suppresses diffusion of plasma on lateral walls.

In both cases electrons escapes from the plasma more In results with the saddle antenna plasma with significantly

quickly than ions and plasma gets positive potential for greater density of plasma on axis with more picked

confinement of the leaving electrons and accelerating the distribution ^⁶'⁸.

ions to the walls that provides macroscopic quasineutrality.

Influence of the longitudinal magnetic field to the radial The positive plasma potential keeps electrons with energy plasma density distribution was tested in ⁸. The plasma smaller then an ambipolar potential but does not prevent density distribution was measured by collectors extracting leaving of the most valuable fast electrons with the greatest the ion beam current through small (2 mm diameter) cross sections of ionization.

emission apertures locating along a radius in the end plate

attached to the discharge chamber.

Oscilloscope traces of collector currents at high magnetic

field are shown in Fig. 2.

Evolution of plasma density distribution with increase of

the magnetic field is shown in Fig. 3 (RF frequency

f=13.56 MHz; RF power Prf=l kW; Ceramic chamber ID

is 6.8 cm; with magnetic coil current I_m=70 A magnetic

field B=250 G). The collector current density was

increased up to 5 times from 12 mA/cm² to 60 mA cm² as

the magnetic field increased from 0 to 250 G. For low

magnetic field (I_m up to 20A) the radial

version of the RF plasma generators with a controllable electrode for extraction of ions from plasma by this distribution of the plasma potential. Proposed RF plasma potential difference \ but conditions for the confinement generator with controllable distribution of plasma of the fast electrons in the plasma was not created.

potential contains: the gas-discharge chamber (1) Some possible versions of the saddle antenna are shown on fabricated from a vacuum tight insulating material, the Fig. 5.

saddle antenna (2), coils of the magnetic field (3, 4), the Similar way the fast electrons can be confinement in RF ring electrode (5), isolated feed through (6), insulator of discharges with the solenoidal antenna and in microwave the feed through (7), plasma electrode with emission sources with an electron cyclotron cyclotron resonance.

aperture (8), a back flange electrode (9), the plasma A prototype of the RF generator with ring electrode similar

made from isolated Litz wires; b- one turn, made from copper tube.

With the positive potential on a ring electrode the potential

of plasma raises and conditions for confinement of more

faster electrons with higher rate of the ions generation are

improved, fn the proposed plasma generator the electrons

are magnetized and slowly drifting to the ring electrode in

an electric field across a magnetic field.

In previous designs of RF ion sources ¹² a contact of

plasma with face electrode (probe) was used for creation of

a potential difference between plasma and the plasma

Claims

1. A device comprising:

means for providing a radio frequency (RF) discharge plasma generator with an additional electrode for independent control of plasma potential distribution; and

means for positive biasing of the additional electrode relative to end flanges and a longitudinal magnetic field, such that a confinement of fast electrons in the discharge may be improved for more reliable triggering of a pulsed RF discharge at low gas density and a rate of ion generation may be enhanced.

2. A method comprising:

steps for providing a radio frequency (RF) discharge plasma generator with an additional electrode for independent control of plasma potential distribution; and

steps for positive biasing of the additional electrode relative to end flanges and a longitudinal magnetic field, such that a confinement of fast electrons in the discharge may be improved for more reliable triggering of a pulsed RF discharge at low gas density and a rate of ion generation may be enhanced.