`
`MOSCOW ENGINEERING PHYSICS INSTITUTE
`
`(TECHNICAL UNIVERSITY)
`
`This material ma be rotected b Co ri_ht law Title 17 US. Code
`
`Manuscript
`
`Dmitriy V. MOZGRIN
`
`HIGH-CURRENT LOW-PRESSURE
`
`QUASI-STATIONARY DISCHARGE IN A MAGNETIC FIELD:
`
`EXPERIMENTAL RESEARCH
`
`04/01/08 — Plasma Physics and Chemistry
`
`Ph.D. Thesis in Physics and
`
`Mathematics
`
`Author
`
`[signature]
`
`Research advisor [signature]
`
`of the Ph.D. candidate in Physics and Mathematics
`
`Associate Professor Igor Konstantinovich Fetisov
`
`Moscow - 1994
`
`INTEL 1006
`
`
`
`TABLE OF CONTENTS
`
`TABLE OF CONTENTS .................................................................................................... 2
`
`INTRODUCTION. ...................................................... 4
`
`Chapter 1. High—current low-pressure discharges in a magnetic field and their use for
`
`generation of dense plasma and intense flows of charged particles ............ 7
`
`1.1 Experimental research of low-pressure gas discharge in a magnetic field and
`
`its application in modern technology. . ........................................................ 7
`
`1.2 Methods
`
`for generating high-power
`
`low-pressure
`
`discharges with
`
`homogeneous plasma structure. ................................................................ 25
`
`32
`Conclusions
`_
`Chapter 2. Experiment methods and techniques............................................................... 34
`
`2.1 The experimental setup................................................................................. 34
`
`2.2 The probe method for determining plasma and ion flow parameters. .......... 45
`
`2.3 Double-mode laser method for determining the density of the plasma. 54
`
`2.4 Pulsed biasing. .............................................................................................. 57
`
`Chapter 3. Parameter ranges and conditions of high-current low—pressure quasi-stationary
`
`discharge in a magnetic field of various configurations. .......................... 59
`
`3.1 Quasi-stationary discharge regimes. ............................................................. 59
`
`3.2 Application of pulse probe method to determine plasma and ion flow
`
`parameters. ................................................................................................ 73
`
`3.3 Detenninationof the discharge plasma parameters by double-mode laser
`
`interferometry............................................................................................ 82
`
`3.4 Discussion of results
`
`87
`
`Chapter 4. Emission and sputtering characteristics of the high—current low-pressure quasi-
`
`stationary discharge............................................................................
`
`100 .
`
`4.1 Features of cathode sputtering in a high-current quasi-stationary magnetron
`
`discharge and» implementation thereof in pulsed sputtering technology. 100
`
`
`
`_3-
`
`4.2 Emission properties of the high-current diffuse discharge plasma and
`
`implementation thereof in the technology of ion—stimulated etching of
`
`materials. ................................................................................................. 103
`
`CONCLUSION ............................................................................................................... 110
`
`REFERENCES ............................................................................................................... 1 1 1
`
`
`
`INTRODUCTION.
`
`Low—pressure discharges in a transverse magnetic field are the
`
`subject of intensive research due to their wide use in technological
`
`magnetron devices, closed—electron-drift plasma accelerators, and as plasma
`
`emitters in electron and ion injectors.
`
`An analysis of the literature shows that
`
`the properties of the
`
`discharge in a transverse magnetic field with closed electron drift in the area
`of transition from a magnetron discharge with a growing current—voltage
`
`characteristic to an arc. discharge are practically unknown. There are no
`
`sound theoretical and experimental data to determine the limit regimes «ofthe
`
`magnetron discharge in which most technological devices. operate. The study
`
`Qf high—power pulsed discharges used for
`
`the generation: of dense
`
`homogeneous plasma indicates the possibility ofexistence of‘-.stable form‘ssof‘
`
`discharge in a magnetic field, not transiting to a contracted phase at high
`
`currents, and generally corresponding to the arc regime of the current-
`
`voltage characteristic of such discharges.
`
`Purpose of the research:
`
`To study the current—voltage characteristics and the regimes of
`
`existence of the high-current quasi—stationary low-pressure discharge in
`
`magnetic fields of different configurations.
`
`To determine the local plasma characteristics of the above-mentioned
`
`forms of discharge.
`
`To study the possibility of using a high—current discharge plasma to
`
`generate dense plasma formations and intense flows of charged particles.
`
`
`
`Scientific novelty and practical value of the work:
`
`In carrying out this work, we have investigated a range of parameters
`
`and regimes of high-current
`
`forms of quasi-stationary low-pressure
`
`discharge in magnetic fields of different configurations.
`
`A new stable form of quasi~stationary discharge in a transverse
`
`magnetic field was discovered, which is an intermediate stage of transition
`
`from the magnetron discharge to the arc discharge (high-current diffuse
`
`mode) and has the following main characteristics: Voltage 2100 V, duration
`
`of several milliseconds, and current intensity range between 10' A and 2 x
`
`~ 103 A. A qualitative model is: proposed, explaining the possible mechanism
`
`of emergence and existence ofthis type: of discharge.
`
`We have ,establ-iShed- the existence of :a highe‘voltage highecurrent
`
`form of quasi—stationary magnetron discharge (high-current. magnetron
`regime), characterized by high voltage (up to 1200 V) and high cathode
`
`current density (up. to 25 A/cmz). The duration of this type of discharge can
`
`exceed 20 ms.
`
`We have indicated the possibility of obtaining plasma formations
`
`with a density of up to 1.5 X 1015 cm‘3 in a volume of up to 1000 cm3, which
`
`ensures the emission of an ion beam with a density of more than 10 A/cm2
`and an energy of 100 eV.
`V
`
`We have also demonstrated the possibility of intensive cathode
`
`sputtering and creation of high density flows of sputtered material particles.
`
`The results obtained are used:
`
`—
`
`In new plasma technology for ion—stimulated etching and in
`
`building a plasma reactor
`
`for rapid layer etching (NPO [Nauchnoe—
`
`
`
`-6—
`
`proz'zvodsz‘vennoe obyedinem'e (Scientific Development and Production
`
`Center)] Submicron);
`
`— In technology of pulsed sputtering (NPO Energomash, Samara);
`
`- In applying defect-free layers, particularly thin films, on heat
`
`sensitive ionizing radiation scintillation detectors
`
`(MIFI
`
`[Maskovskz'y
`
`inzhinemo-fizz‘cheskzy institut (Moscow Engineering and Physics Institute)]
`
`and, MVP [Mnogootraslevoe vnedrenheskoe predprz'yatz’ye (Multi-sector
`
`Innovative Enterprise)] Different, Minsk, collaboratively).
`
`The following theses Will be defended below:
`
`.
`
`l. The results, of experimental
`
`research of ‘highecurrent; quasgie
`
`stationary low-pressure discharge
`
`in magnetic fields of different
`
`cenfigurations: a. parameter ranges and-regimes of two. forms :of‘diisehar‘gei:
`
`b. dependence of current-voltage characteristics on,
`
`the
`
`pressure, magnetic field density, type ofsga-s, electrode material, and type of
`
`discharge device.
`
`0. qualitative model explaining a possible mechanism of
`
`emergence and existence of the high—current diffuse form of discharge.
`
`2. The method of pulsed probe diagnostics of dense plasma.
`
`3. The results of research, of the plasma parameters of a high quasi-
`
`stationary diffuse discharge regime in a weakly inhomogeneOus magnetic
`
`field.
`
`The main results are published in the papers [94-104].
`
`
`
`-7-
`
`CHAPTER
`
`1.
`
`HIGH-CURRENT
`
`LOW-PRESSURE
`
`DISCHARGES IN A MAGNETIC FIELD AND THEIR USE FOR
`
`GENERATION OF DENSE PLASMA AND INTENSE FLOWS OF
`
`CHARGED PARTICLES.
`
`1.1 Experimental research of low—pressure gas discharge in a
`
`magnetic field and its application in modern technology.
`
`Existing types of devices based on a discharge in crossed E and H
`
`fields.
`
`The study of characteristics of the discharge in crossed electric and
`
`magnetic fields has led to the develOpment of many gas discharge devices
`
`widely applied in various fields of science and technology, notable for the
`
`configuration of the» E- and. B" fields and for the: magnitude of the discharge
`
`current; voltage, and gas, pressure. In terms of‘practical appliCatiion, systems
`
`with field configuration, ensuring .a closed, electron drift, proved to be the
`
`most. promising. These: include cylindrical [1,2] andplanar [3,4] magnetrons‘,
`
`the Penning cell
`
`[5], Combined devices with. magnetron and a hollow
`
`cathode [6—8], and plasma accelerators with a closed Hall current [9—11]. In
`
`the existing literature, the properties of discharges for almost all possible
`
`configurations of the axially symmetric E and B fields have been studied.
`
`Studies of discharges With non-closed electron drift were conducted to
`
`determine the possibility of obtaining a plasma flux accelerated by crossed
`
`electric and magnetic fields, the magnetic confinement of the plasma, and
`
`the use of such discharges in plasma chemical reactors [12,13]. Discharges
`
`in a transverse magnetic field have been widely used for the development of
`
`plasma emitters and plasma cathodes. In the technique for generation of
`
`
`
`—8-
`
`poWer beams, an arc discharge is usually used as plasma emitter, which is
`
`obtained by transition from a glow discharge, but at low pressure, necessary
`
`for the generation of the electron beam and its transportation, a glow
`
`discharge cannot be ignited; therefore, a discharge with closed electron drift
`
`is used as an auxiliary discharge, which then transits into an arc regime
`
`[14,15]. Attempts have been made to use directly the contracted discharge in
`
`crossed E and H fields as an electron emitter. [16]. The use of plasma
`
`cathodes based on uncontracted discharge in a magnetic field in both plane
`
`and cylindrical geometry to develop fully controllable switching devices has
`
`enabled the construction of valves able to Switch currents higher than 150
`
`kA and voltages above 100 kV, and, at the same time to lock the device in
`
`less than 50113- ['17.,18].. The only drawback of’ crossatron modulator switches
`
`is the large—«about 30! V+discharge voltage drop, which, ens-Utes the
`
`operation :of’ the "plasma cathode: The discharge plasma, with a tranSVerse
`
`magnetic. field is efficiently used in the generation of"intense ion beams—for
`
`example, beams ofnegative- ions ofhydrogen [19].
`
`One ofthe most important practical applications of the discharges in a
`
`magnetic field is the development of magnetron sputtering systems, which
`
`are used to apply thin films [20—27].
`
`Properties of the discharges in a transverse magnetic field.
`
`According to paper [28], in the absence of a magnetic field, depending
`
`on the pressure and the discharge current, there are four stable types of cold
`
`cathode discharge: dark [Townsend discharge], normal glow, anomalous
`
`glow, and arc. The range of currents for each type—for example, for a
`
`discharge in Ne between parallel plate electrodes at a pressure of =1 torr—is:
`
`
`
`_9-
`
`1040-10“ A, 104—10“ A, 104—10 A, >10 A, respectively. The qualitative
`
`current-voltage characteristic of the discharge is shown in Fig. 1.1. High-
`
`current forms include the anomalous glow discharge, which covers the entire
`
`surface of the cathode and is characterized by an increasing current-voltage
`
`characteristic (CVC), and the arc discharge, which is a contracted discharge
`
`with decreasing current-voltage characteristic. The main mechanism for
`
`maintaining a glow discharge is the electron impact ionization in. the cathode
`
`layer and the electron emission from the cathode, caused by the secondary
`
`ion—electron emission and the photoelectric effect. The motion of charges in
`
`the cathode layer- of the glow discharge takes place in amOb‘ility mode with
`
`the specification that the velocity of: the ions is much smaller than that of the
`
`electrons, so that in the cathode space there is an excess of' positive charge,
`
`which distorts the electric field. Most of the discharge. voltage falls; in the I
`
`_ cathode. region; therefore, the normal and anomalous glow discharge?- are
`
`discharges with positive space charge.
`
`In the presence of a transverse magnetic field, the speed ofmovement
`
`of the electrons towards» the anode is reduced, which leads to a decrease in
`
`the cathode voltage drop and an increase in the negative space charge near
`
`the anode—Le, to formation of an expressed anode layer.
`i Anode layer or negative space charge discharge regimes are mainly
`
`obtained at low pressures (p S 10'3 torr). A more comprehensive research of
`
`the properties of the anode layer discharge in a cylindrical magnetron was
`
`conducted in the papers [29—32]. At higher pressures, and not too strong of
`
`magnetic fields, a cathode layer (positive space charge) discharge is realized.
`
`
`
`-10-
`
`2000
`
`‘3 500
`
`“f 009
`
`509:
`
`in”
`
`10"“ m“1 w Ira:
`
`300
`
`600 Um
`
`Fig. 1.1. Discharge CVC in Ne
`d = 1 torr [28]
`
`Fig. 1.2. Discharge CVC in magnetron
`sputtering systems [35]
`
`probe
`
`
`
`gas
`
`exhaust
`
`Fig. 1.3. Diagram of a planar
`magnetron MRS [43]
`
`(1‘
`
`nits:
`
`J
`
`b.
`
`
`
`were?!
`5 N
`
`4
`
`3
`a
`1
`
`§\.
`
`V
`
`- Earn
`
`5 .
`4 ;
`
`2 I...tar—M I
`,
`'
`1015'
`
`Kim}
`
`50
`
`'3 00
`
`1‘50
`
`50
`
`‘i €30
`
`1 50
`
`Fig. 1.4. Dependences of the electron concentration (a) and ion density (b) on the
`discharge current (planar magnetron) [45]
`
`
`
`-11-
`
`Experimental studies of discharges in a transverse magnetic field of different
`
`configurations showed that, according to [33,34], for any magnetron system
`
`with closed electron drift the current-voltage characteristic is described by
`
`the formula 1,; = «$113, where Id is the discharge current, Ud — the discharge
`
`voltage, and k, n (n > 1) are values depending on the design of the device,
`
`type of gas, pressure, and magnetic field. The paper [35]
`
`shoWs the
`
`experimental dependences of the current on the discharge voltage, which
`
`characterize, in the opinion of the authors, any magnetron system (Fig. 1.2).
`
`It can. be noticed that, with increasing. strength of the magnetic field at a
`
`fixed current,
`
`the voltage decreases. The authors draw a conclusion of
`
`applicability of. the anode layer model
`
`to describe the experimental
`
`characteristics at relatively high pressure (‘1 .5 x 10"1 < p < 8 x. 1071 tort).
`
`Deviations from: this pattern of dependence of. the discharge voltage on
`
`the current, Were observed when working: With uncool-ed :CathO'des; [36]. In
`
`magnetic fields with a density over 2st and-a pressure over 3 x 10"2 torr,
`
`CVCs of the discharge were obtained where the monotonic increase: of the
`
`voltage is interrupted, and a steep decreaSe: thereof occurs to a level of 3200
`
`V, after which Ud is almost independent of the current up to a current value
`
`of :2 A. The authors explain this S-shaped curve by an increase in the
`
`plasma density following the ionization of the sputtered atoms of the cathode
`
`material.
`
`The research regarding the dependence of the discharge current on
`
`magnetic field strength is reflected in papers [37-40]. It is shown that the
`
`discharge current either increases with an increasing magnetic field—such as
`
`discharges in a cylindrical magnetron and in a combined hollow cathode
`
`
`
`-12-
`
`system [37,38]——or the dependence Id(H) has an insignificant maximum
`
`value [39,40].
`
`The model of the cathode layer of the glow discharge in a transverse
`
`magnetic field is presented in paper [41]. It is shown that the inclusion of a
`
`magnetic field increases the cathode voltage drop at low current densities
`
`and decreases it at high current densities. As a result, the CVC constructed
`
`on the coordinates UK-In x j is shifted by 3/2In(1 +w212) towards higher
`
`. currents (co—cyclotron frequency, r-electron relaxation time), which
`
`means a corresponding increase in the normal current density. The authors
`
`derive from this cenditions of similarity of the cathode layer in the absence
`
`and in the presence of a magnetic field The theoretical model of low—
`
`pressure discharge with. closed electron drift realized in a magnetron
`
`sputtering: system with a range of parameters typrcal .for’
`
`itechnomgfieal
`
`{ applications was constructed in paper
`
`[42]. UniVariate "solutions: were
`
`obtained, describing the plasma parameters: in different parts ofthe-discharge
`
`gap:
`
`the dark. cathode layer and, the cathode glow region. A method of
`
`calculation of the current-voltage characteristic of the discharge was
`
`developed.
`
`In addition, a model of interaction between the fluxes of
`
`sputtered material and working gas, which leads to a reduction of the gas
`
`concentration in the cathode glow region and in increase in the discharge
`
`voltage, is proposed. The calculation of the CVC using this model also gives
`
`a monotonically increasing dependence Udfld), stabilizing at 400—600 V at a
`
`current density of 0.04—0.1 A/cmz, which is in satisfactory agreement with
`
`the experiment; However, the current density in the stationary regime (up to
`
`0.3 A/cmz) used by the authors for comparison with the calculation raises
`
`some doubts.
`
`
`
`-13-
`
`A number of papers are dedicated to the measurement of the local
`
`characteristics of the stationary discharge plasma—such as the plasma
`
`density n,- and the electron temperature Te—and to the radial distributions of
`
`these parameters in axially symmetric systems [43-45]. For example,
`
`in
`
`planar magnetron systems [43,44], ion densities n; 21010 - 3.5 x 1012 cm'3
`
`and electron temperatures of 20-5 eV were reached at a pressure of the
`working gas of 5 X 10"3 - 2.5 torr, respectively. The diagram of the
`
`experiment [43] is shown in Fig. 1.3. The magnetic field at the surface of the
`magnet reached 1,600 Gs. Argon was used as the werking gas at a pressure I
`
`of 2.5 x 10’2- 2.2 torr. The electron density and temperature were measured
`
`With a probe.- The current passing through the: probe was also used, to
`
`calculate the dI‘lft Velocity of electrons in the: direction of the E and H fields.
`
`The ion temperature was measured, using. an electroStatic analyzer. The
`
`maximum electron temperature was observed in the area adjacent to the
`
`system axis at the. cathode surface. (=8eV) The maximum electrOn density
`
`Was 3.5 x 10'12 cm'3 and was comparable to: the ion density. It was found that
`
`in the area of perpendicularity of the electric and magnetic fields, the ratio
`
`between the drift, velocity VD to the heat velocity Ve remains constant along 2
`
`and was 20.05. Probe measurements have shown that VD '2 7.3 x 106 cm/s—
`
`i.e., V3 :2 1.5 x 103 cm/s—which. corresponds to an electron temperature of
`
`26 eV. Measurements of the ion temperature have shown that T,- = 0.6 — 1,2
`
`eV, and the ratio between electron and ion temperatures was —~—3 — 5.
`
`In a similar system presented in paper [44], the discharge was ignited in the
`
`following conditions: pressure p = 5 x 10'3 — 3 x 10'2 torr, working gas——
`
`argon, cathode material—copper, discharge current 20.1 A. The maximum
`
`electron density and temperature achieved were:
`
`
`
`-14-
`
`1, p = 5 X10'3torr,ne 2.10“) cm'3, Te 2 20 eV
`
`2. p =2 x 10'2 torr, me —~— 5xlO10 cm'3, Te 2 5 eV
`
`—i.e. the pressure increase along with the increase in ion concentration leads
`
`to a decrease in electron temperature.
`
`The studies on the dependences of the ion concentration and electron
`
`temperature on the discharge current in a planar magnetron [45] have shown
`
`a practically exponential
`
`increase of n,(1d) and decrease of the Te (Id)
`
`dependence (Fig. 1.4), but the narrow range of currents of 50—150 mA does
`I not enable us to draw any general. conclusions. The sanie studies present the
`radial distributions of the abovementioned parameters obtained by probe
`
`measurements. The distribution 22; (r) has a maximum in the region of the
`
`maximum radial component of the. magnetic field and a marked dip along
`
`the, symmetry axis of the. electrode ~system..--.
`
`'
`
`voltage characteristic is the thickness of the cathode layer. According to; the
`
`authors of paper [46.], the dependence of this value on the discharge
`
`parameters can be represented in the form in" = 94.?
`
`gym11ft;
`sets
`
`[cm]. In paper
`
`[47] an attempt is made, by calculations of the Poisson equation for the
`cathode layer and taking into account the changes in the lateral mobility of
`
`electrons in a magnetic, field,
`
`to calculate the cathode voltage drop UK,
`
`depending on the discharge current and voltage in a range of parameters
`
`typical for magnetron sputtering systems: Ar, B = 700 - 1300 Gs, p = 10'4 -
`
`10‘1 torr, Ud = 250-600 V. Numerical calculations and measurements of the
`
`ion energy spectrum have shown that the cathode voltage drop accounts for
`
`75-90% of the total discharge voltage.
`
`
`
`.15~
`
`An overview of ion sources based on magnetron systems [48]
`
`confirms the typical parameters of discharge for magnetrons operating in a
`
`stationary regime——for example, for planar magnetrons the pressure range is
`
`104- 5 x10'2torr,Ud z 300 V, rd 2 0.1 A.
`
`A number of papers are dedicated to studying discharges in special
`
`devices. For example, the discharger in Fig. 1.5 [49,50,51] is a system made
`
`of two flat circular electrodes between which a uniform electric field is
`
`formed. The magnetic field has a “cusp” configuration in line with the
`
`geometry of the electrodes so that the radial comp'Onent B, is maximized in
`
`the discharge gap, and
`
`,
`Baafiz;
`
`
`
`B B
`1'"
`”
`:5 Z;
`first?
`Vega"
`
`:> rt, where
`
`,7 is the electrode width, and d is the distance between the electrodes. The
`
`maximum value of the El on the system axis at z = 0.1 m is 21,000 Gs, and
`
`the maximum value of B, in the symmetry plane of the system 2:350 Gs. The
`
`maximum operating voltage was ~~—3 kV.
`
`In the discharge device described above, depending on the pressure
`
`of the working gas, three types of discharge were discovered: I. at high
`
`pressures (right branch of the Paschen curve)———high-current discharge with
`
`voltage weakly dependent on the current; 2. at pressures corresponding to
`
`the minimum area of the Paschen curve, the transverse magnetic field exerts
`
`a strong influence on the voltage and slope of the current-voltage
`
`characteristic (CVC); and 3. at lower pressures, where in the absence of a
`
`magnetic field the discharge does not burn, the slope of the CVC does not
`
`
`
`-16—
`
`
`
`B \\\\—_._ m\“
`
`
`
`
`""G’
`
`Fig. 1.5 Discharge device with circular
`electrodes in a quadrupole magnetic
`field [50].
`_
`
`0
`*t
`,
`2 {lion}
`Fig. 1.6 Potential distribution in a
`high-current discharge regime
`Id: 50 mA,p = 2 x 10" torr [49].
`
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`
`Fig. 1.7 CVC of a discharge in a quadrupole
`magnetic field (high-current stationary
`regime) [51].
`
`Fig. 1.8 High-current discharge
`regime [63].
`
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`' x
`HA} ~
`'3 ‘
`.
`I‘m
`5
`"
`390 “W oW
`500
`, 1000
`1
`2
`3
`Fig. 1.9 High-powezr (a) and high—voltage (b) regimes [64-66].
`a l-Ar,=p 2X1302torr,B=670Gs,2—B=l3,60Gs,3—B=1,800Gs,4.5-N2,H2.
`b. Hep=lx10'3torr lB=1,,360Gs 2- B: 910Gs, 3- B= 670Gs.
`
`
`
`_17_
`
`depend on the magnetic field. The potential distribution in the electrode gap
`
`was measured using the floating probe method for all three regimes. Fig. 1.6
`
`shows the distribution of the potential in a high—current discharge regime. It
`
`is obvious that as the magnetic field increases, the region of the cathode drop
`
`[K decreases. It is known that the [K for the anomalous glow discharge
`
`decreases with increasing pressure, so we can assume that for the high—
`
`current regime of the stationary discharge an increase in the magnetic field is
`
`equivalent to an increase in the pressure of the working gas. The cathode
`
`voltage drop is also reduced while the anode drop slightly increases with the
`
`increaSe in the 8,. The size of the anode layer does not depend on the 3,. The
`
`study of the discharge voltage» dependence of the current ’Ucfld) [49-] in the
`
`region. of high. (stationary) currents has shown the presence of' discharge
`
`regimes, Whose. the; current-Voltage characteristics. (CVCs) are show in Fig.
`
`.17 in the area (pd) corresponding "to. the. minimum and the right branch of
`
`the ,Paschen curve. For Ari, this area is in the rangeof pressures 2X.107‘1'<‘ p <
`2. tom Specific to this group of curves Udfld) is that the slope of the dU/dI‘
`
`CVC and the discharge velt’age- decrease with increasing cross-component
`
`density of the magnetic field. The influence of the magnetic field on the
`
`characteristics is in this case the same as that found in hollow-cathode
`
`systems in a magnetic field [52].. It is also noted that, as the B, increases, the
`
`critical current at which the discharge transits into an arc increases; for
`
`example, in nitrogen at Br 2 140 Gs the arc does not occur in the entire range
`
`of measured currents. In such a discharge, low-frequency vibrations appear
`
`which in some cases are stabilized by the magnetic field.
`
`An example of stationary combined systems are devices [53]
`
`combining the discharge in a transverse magnetic field with a hollow—
`
`
`
`—18-
`
`cathode discharge. A high current density is achieved at pressures of the
`
`working gas corresponding to the effect of the hollow cathode. The
`
`possibility of obtaining high-current forms of stationary discharge while
`
`maintaining, a sufficiently high voltage (2300 - 700 V) was studied in a
`
`device with hollow shaped electrodes immersed in a cusp-shaped magnetic
`
`field. The electrode profile followed the force line of the magnetic field,
`
`which enabled the electric field to be perpendicular to the magnetic field
`
`along the electrode surface. The perpendicularity of the fields near the whole
`
`surface of the cathode facilitated the breakdown and increase of the
`
`discharge current. The working gas was argon. At pressures 2:041 - 1 torr,
`
`high—current
`
`stationary
`
`discharge
`
`regimes were
`
`obtained, whose
`
`characteristics are similar to. those obtained in a device with. disk electrodes;
`
`at; currents exceeding 1A the discharge was. more stable than. in the device
`
`with planar electrodes. In? ‘s‘tudyingithe dependenCe of the discharge. current
`
`0n the pressirre, the existence of a new high-current regime at low preSSures
`
`was, feund. With a decrease in pressure to 1073‘ torr in Ar, an anomalous
`
`increase of the dischargecurrent without transition» to the arc regime was
`
`observed. The discharge was concentrated in the center of the system, and
`
`the discharge current was up to 2 A.
`
`The need to increase the discharge power and the plasma density
`
`required the use of a pulsed Or quasi-stationary regime. The experiments
`
`conducted on magnetron systems of various geometries have shown that
`
`there are discharge regimes which do not transit to arcs at significant current
`
`values. In the super—dense glow discharge obtained in a device of “reversed
`
`magnetron” type of coaxial cylindrical geometry, the discharge current was
`
`
`
`-19-
`
`Id 2'— 70 A at Ud 2'— 400 V with a pulse duration tpulse 2=60 us. The plasma
`
`density was 71; —~— 1012 cm"3 [54].
`
`The attempt to obtain a homogeneous plasma with a density of 1013 —
`
`1014 cm‘3 with a breakdown of the electron and ion temperatures using a
`
`pulsed high-current discharge, such as a Penning discharge [55,56], has
`
`shown the possibility to create in such a discharge a plasma column with a
`
`density n,- = 5 x 1014 cm'3. Experiments were conducted on an “INAR”
`
`device made of two cathodes placed at a distance 2.4 In in the plugs of a
`
`mirror system magnetic field. The circular anode was placed in the center of
`
`"the system. The working gas was hydrogen at a pressure of 10'1 — 104 torr,
`
`the initial voltage Was .60 kV [55.] and 5270. ,kV [56‘], and the magnetic. field
`
`strength was 25 His and '12 kG’s, respectively. The discharge current had an
`oscillatory character with, amplitudes of‘ 60and 20. kA and :decay times of.80
`
`and. 70 use The: discharge conditions, ensured an ionization. electron free path
`
`much shorter than the length of the discharge gap. The diagnosis was
`
`'gperformed using laser methods, double “probes, and the flux of neutrals. The
`result is a maximum ion. density n,- z 7 x 1015 ‘cm'3; however, the electron
`
`temperature in this case did not exceed 1.3 eV.
`
`‘
`
`The increase in the discharge current caused the anomalous glow
`
`discharge to transit
`
`into an arc discharge, but
`
`the presence of an
`
`inhomogeneous electric field created by the hollow cathode as well as the
`
`presence of a transverse magnetic field enabled the authors to obtain a high-
`
`current non-arc form, a so—called super-dense discharge [57-60]. The super~
`
`dense discharge was studied at a pressure of 5 x 102 — l torr in a pulsed
`
`regime at currents of up to 2 kA and pulse duration of 50 us. The
`
`abovementioned discharge is characterized by a Current density of =50
`
`
`
`-20_
`
`A/cmZ—i.e., much higher than in the anomalous glow discharge—voltage of
`
`300 - 700 V, and axial symmetry of the glowing circular layer on the
`
`cathode. Measurements of the distribution of ion current density on the
`
`cathode surface [59.] have shown that most of the current is discharged on
`
`the cylindrical surface of the cathode cavity. No transition of the glow
`
`discharge into super~dense discharge between the flat electrodes has been
`
`observed.
`
`The method of obtaining high—current discharges by applying a high
`
`voltage pulse to the discharge gap filled up with pre—ioniz‘ed gas appears to
`
`be of interest. [61,62]. The study of the reaction of low—current (0.5 A)
`
`stati‘OIlary glow discharge, with sectioned hollow cathode. [62] in argon at
`fp’ressUrep, = 10’4 ,-‘ 10'1 torr, to a. 'highwvo'ltage pulse which turnsthe discharge
`
`into a high-Current (up: to 2001 A) quasi-stationary regime has shown that an
`
`.
`
`increase in: the threshold amplitude (2:13- k‘V). leads: to anomalous regimes
`
`with intense: fluctuations of. the current intensity and Voltage. A highecurrent
`
`form in the lower pressure range p = 104 - ‘10'2 torr is realized only In
`
`anomalous regimes, which the authors attribute to the explosive phenomena
`
`at the cathode.
`
`.
`
`Of particular interest is the study of high-current and high-voltage
`
`varieties of the quasi-stationary discharge in closed— and open—electron drift
`
`systems in heterogeneous E and H fields [63—66]. Fig. 1.8 shows the current-
`voltage characteristics ofthe high-current regime discovered in a closed drift
`
`coaxial device [63]. The experimental curves were described quite well
`
`based on an assumption of existence of a collisionless cathode layer.
`
`The research related to the design of injectors for thermonuclear
`
`plants [64-66;] has shown that,
`
`in devices with inhomogeneous. axially
`
`
`
`-21-
`
`symmetric magnetic and electric fields, powerful (:1 MW) high-current and
`
`high-voltage pulsed discharges can be obtained, which transit into arcs at
`
`significant current intensities. The discharge ignited between the parallel flat
`
`electrodes in a transverse highly inhomogeneous magnetic field of the
`
`cylindrical electromagnet located behind the cathode (planar magnetron).
`
`The
`
`research was
`
`carried out
`
`in a pulsed regime. The temporal
`
`characteristics of the pulse magnetic field and of the discharge current were
`
`chosen so that
`
`the times of changes in the discharge voltage were
`
`significantly longer than the time of ionization and de-ionization of the
`
`charged particles in the discharge gap. The high—current discharge was
`
`studied in a current range of 5 A to 2 kA and operating pressure-p = 5 x1073
`
`- 5, :X, 10‘2 torr in Ar, N2, and H2. The enrrent pulse duration didnot exceed
`
`1.00 us. The typical currentevoltage characteristics of the discharge are
`
`‘ shown in Fig. 1.9a. The dependence Ugfld)
`
`is the same for different
`
`parameters 19', 0’, Bo, and types of gas. First, with the increase of current to a-
`critical value 1%“ fi 528 A, the voltage increases monotonically by 50 — 100
`
`V, and then at Id > I £3 the discharge voltage is practically independent of
`
`the current up to 2 kA and maintains a constant value of 300 - 500 V, which
`
`is substantially independent of the magnetic field and the type of gas. The
`decrease in the magnetic field density at a given current 1,; entails an almost
`
`linear increase in the discharge voltage Ud. The slope of the Ud(B) axis
`
`remains the same at different pressures but