[handwrittenz] 61 95-1/593-2
`
`MOSCOW ENGINEERING PHYSICS INSTITUTE
`
`(TECHNICAL UNIVERSITY)
`
`This material ma be rotected b Co riht law Title 17 U.S. Code
`
`Manuscript
`
`Dmitriy V. MOZGRIN
`
`HI§GH—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 .................................................................................................. .. 2
`
`TABLE OF CONTENTS
`
`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
`L
`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 2
`
`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 do‘uble—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 V
`
`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
`
`. . . . . . . . - . . . . - . . . . . . - . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - . . . . . . . . . . . . . . . . . . . . . . . - - « . . . . . - . . . . . . . . . . .. 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 curr_ent—voltage
`
`characteristic to an arc discharge are practically" unknown. There‘ are no
`
`‘sound theoretical and experimental data to determine: the
`
`regiimesi oifthe
`
`—m-agnetron discharge in which most technological devices operate. The study
`
`Qf
`
`.high—power Apulsed ~dis.char__ges used for
`
`the g,e11era.ti‘onr of dense
`
`homogeneous plasma iindicatejs the possibility of»existence of=.stable formssof‘
`
`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—st'ationary 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
`
`~ 1103
`
`A qualitative mo.,d.e.l is: propos:ed_, e.xp1»aining the possible mechanism
`
`of eIner‘gence= and existence of‘this type: of discharge.
`
`We have .es,,t-abl-i:s?7hed. the aexisitencle of a high.-‘voltage highecurrent
`
`of .quasi—statio‘n”ar_y“ irnaginetronl diiisehvarge
`
`(hi'gh,-icurrent. magnetron
`
`regime), characterized by high voltage (up to 1.200
`
`and high cathode
`
`,c11ITen.t 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/cmz
`and an energy of 100 eV.
`0
`
`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-
`
`

`
`-5-
`
`proizvodstvennoe obyedinenie (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
`
`[Mos/covskiy
`
`inzhyinemo-fizicheskz'y institut (Moscow Engineering and Physics Institute)]
`
`and MVP [Mnogootraslevoe vnedrenheskoe predpriyatiyet (Multi-sector
`
`Innovative Ente1prise)] Different, Minsk, collaboratively).
`
`The following theses will be defended below‘:
`
`A
`
`.1. The results. of experimental
`
`research of ‘high-current; quasi;-A
`
`stationary low-pressure .d~i—secharge
`
`in
`
`‘magnetic;
`
`zfiields of different
`
`aconfiguratilonsz. a, parameterra:I1_;geis and.-regimes of t,wo.:oifidis‘c*hargef:
`
`b. dependence of’
`
`.cu’rre.nt-vol:ta‘g‘e GhaI2IctBCl'.~i:Sti(§S
`
`Ion the
`
`‘pressure, magnetic field .density,, type ofsgas, .e'1ectrode material, and of
`
`di_scharge device.
`
`c. 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-
`
`stationa
`1'3’
`
`diffuse dischar e re ime in a weakl
`8
`g
`
`Y
`
`inhomo eneous ma netic
`3
`8
`
`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
`
`;ma;gnet:ic~ fields has led to the development of many gas] .di’s‘charge_ devices
`
`‘widely applied in various fields of science and technology, notable for the
`
`-cfonfifigtiratiion of the E and B; fields and for the magnitude of the -discharge
`
`current; voltage, and" gas. pressure. In terms of“practical application, -systems
`
`‘with field c.on.‘fi’g.ur;atilonv, ensuring a1 c“los‘e.d, electron drift, prove.d to be the
`
`most promising. These: in'cl1’1de :cyl~indrfica1 [1,2] andplanar [3,4] magnetronsf,
`
`the Penning cell
`
`[5], *com‘b‘ined devifces With] magnetron and a hollow
`
`cathode [6—8], and plasma accelerators with a closed Hall current [9—l 1]. 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
`
`

`
`-3-
`
`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
`
`and voltages» above 100 kV, and at the ‘same time to lock the device in
`
`less] than 50 ns [’17,18].. The only drawback of’ crossatron modulator switches
`
`the ,1arge———ab.ou;t’ 30. V+discharge' Veiltfclgeb drop, which ensures, the
`
`:operation of the [plasma cathode; The discharge‘ plasma. with a transverse
`
`magnetic. ‘field is re-fficiently used in the generation of"intense ion beams-—-for
`
`example, beams of‘negative. 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 2:1 torr——is:
`
`

`
`-9-
`
`10“-10*‘ A, 10-4-10“ 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—vo1tage 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 atmobility mode with
`
`the s_peci;fic;ati‘on 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. ‘vorltage fgallrs, ‘in the I
`
`A
`
`fc;a‘thode. region; therefore, the normal and anomalous .gl:ow i.di1s,cha;rge: are
`
`discharges? with positive space charge.
`
`In the presence of a transverse magnetic field, the speed oflmovement
`
`r_o;f: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——i.e., to formation of an expressed anode layer.
`
`A Anode layer or negative space charge discharge regimes are mainly
`
`obltained at low pressures (p S 10'?‘ 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
`
`‘i 500
`
`1 009
`
`50.€3=
`
`10*‘
`
`10”"
`
`19”‘
`
`19
`
`Inn
`
`300
`
`600 rm:
`
`Fig. 1.1. Discharge CVC in Ne
`d = 1 torr [28]
`
`Fig. 1.2. Discharge CVC in magnetron
`sputtering systems [35]
`
`
`
`probe
`
`I
`
`b‘
`
`I
`
`II31.41‘
`
`nifm.
`
`I
`
`
`
`2 ‘Q§———‘-I&""“—‘
`g
`’
`1015‘
`
`gas
`
`exhaust
`
`Fig. 1.3. Diagram of a planar
`magnetron MRS [43]
`
`\I’efé'§U
`"
`
`ai
`
`Q"—-—-—-
`
`V
`
`- Ea-an
`
`5
`
`3
`1
`
`50
`
`'5 00
`
`‘$350
`
`50
`
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`
`1 50
`
`Fig. 1.4. Dependences of the electron concentration (a) and ion density (b) ‘on the
`discharge current (planar magnetron) [45]
`
`

`
`-1]-
`
`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 I6; = egg, 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
`
`applicabiility of the .anoc;l.e“
`
`layer model
`
`to describe the ’exp.e-rir‘nental
`
`characteristics at reliatively high pressure (‘1 .5 x 10" < p < 8 X. 1.07‘ torr).
`
`Deviationis from this pattern of dependence of the discharge volitage on
`
`the ‘current Were observed when ‘Working: with uncooled ;CathO'des: [36]. In
`
`irnagnetict fields with a density‘ over
`
`‘k;Gs 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 2200
`
`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+a)2r2) towards higher
`
`. currents (c0—cyclotron frequency,
`
`r———electron
`
`relaxation time), which
`
`means a corresponding increase in the normal current density. The: authors
`
`derive from this conditions of sirnilarityc of the cathode layer in the absence
`
`and in the prje‘s‘ence of a magnetic
`
`‘The theoretical model sot’ low-—
`
`pressure discharge with. closed electron
`
`realized ‘in’ a magnetron
`
`sputtering jsystem‘ with» a range’ of parameters
`
`for iteichnologficlal
`
`appltications was con‘stru.cted in paper
`
`[42]. Uniivariate,
`
`’so;luti?ons-. were
`
`obtained, describing the plasma parameters: in different parts ofithe.-disch‘arge
`
`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 U,;(Id), 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 12,- 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,- =10“) — 3.5 X l0” cm’3
`
`and electron temperatures of 20-5 eV were reached at a pressure of the
`
`working gas of 5 X l0'3 - 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 working gas at a pressure
`
`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 Velocity of electrons. in the direction of the- E and H fields.
`
`The ion teniperamrc was measured. using. an e1.ecjtroJs:tatic analyzer. The
`
`maximum electron temp'eratu’re-r was observed in the area adjacent. to the
`
`system axis at the cathode surface..
`
`The maximum -electron density
`
`was 3.5 x 1012 cm'3 and was comparable to: the ion density. It was found that
`
`in the area of perpiendicularity 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 crn/s——
`
`i.e., V3 -'2 1.5 x 108 cm/s—which. corresponds to an electron temperature of
`
`=6 eV. Measurements of the ion temperature have shown that T,- 2 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 102 torr, working gas——
`
`argon, cathode material—copper, discharge current 940.1 A. The maximum
`
`electron density and temperature achieved were:
`
`

`
`-14-
`
`1. p = 5 x 103 torr, mg 2.10” cm'3, Te 2 20 eV
`
`2. p =2 x 102 torr, 113 —~— 5xl01° cm'3, T9 = 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
`5 not enable us to draw any general conclusions. The same 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 rcrornponent of‘ the. magnetic field and a marked dip -along
`
`the
`
`axis, of the. electrode —syste;m..~..
`
`—imp‘orttantt parameter of the ;discharge. affecting‘ its current-
`
`vo‘ltageetcha_racte,r:istic 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 ii!" = 94.3?‘
`
`g1g'1a11,€5
`Sm [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 -
`
`104 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 x 104 torr, Ud = 300 V,1d = 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 corn'p’onen't B, is maxirnized in
`
`the discharge gap, and
`
`.
`B;,v>Bz;
`
`B \
`‘"
`flrflr
`
`2*» Z;
`
`B‘
`’"
`73°39"
`
`:3 1Ci,Wh6fC
`
`.7 is the electrode width, and d is the distance‘ between the electrodes-. The
`
`maximum value of the B; on the system axis at z = 0.1 m is =1,000 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
`
`

`
`-15-
`
`
`r .
`
`
`0
`5
`_
`2 cl! cm}
`Fig. 1.6 Potential distribution in a
`high-current discharge regime
`Id= 50 mA, p = 2 x 10" torr [49].
`
`Fig. 1.5 Discharge device with circular
`electrodes in a quadrupole magnetic
`field [50].
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`Fig. 1.7 CVC of a discharge in a quadrupole
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`
`Fig. 1.8 High-current discharge
`regime [63].
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`Fig. 1.9 High-power (a) and high—voltage (b) regimes [64-66].
`a. l-Ar, p = 2 X lO'2 torr, B = 670 G5, 2 - B = 1,360 Gs, 3 — B = 1,800 Gs, 4.5-N2, H2.
`b.Hep=1xl0'3to1r, l -B=1,360Gs,2 -B=91O Gs,3 -B=670 Gs.
`
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`
`-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 B,. The size of the anode layer does not depend on the B,. The
`
`study of the discharge voltage» de_per1_der‘r_ce of the current ’Ud(Id) [49] in the
`
`region. -of high (stationary) currents has shown the presence of’ discharge
`
`;r‘e_gi-mes, whose. the current-voltage characteristics.
`
`are ‘shown in
`
`.1 in the area (pd) corresponding "to. the min‘imum~ and the irirghtv branch of
`
`the Paschen curve‘. For Ar, this area is in the rangeieof Pressures .2x.10l"1i < P‘ <
`2. torr. Specific. to this group’ of curves Udfld) is that the slope of the c‘lU/df
`
`CVC and the discharge volt"age— decrease with increasing cross-co.mponen‘t
`
`density of the magnetic field. The influence of the magnetic field on the
`
`characteristics is in this case the same as that found in hol1ow—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 B, 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 =0-.‘l - 1 torr,
`
`‘high*-current
`
`stationary
`
`discharge
`
`regimes were
`
`obtained, whose
`
`characteristics are similar to those obtained. in a device Withl disk ‘electrodes;
`
`at currents exceeding ;1A the discharge was more stable ‘than.
`
`the device
`
`with‘ planar’ electrodes-. In s'tud31ying;‘the- dep‘end_e'nCe of the :dis‘eha1:g‘e:cu1‘rent
`
`on the pressure, the existence of a ;.nIe‘w’ hi;gh—.cjurr_ent regime. at low pressuress
`
`was found. With a decrease in pressure‘ to 1.073‘ torr in Ar, an antomalcous
`
`increase of the dischargeicurrent 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 =1 70 A at Ud 2: 400 V with a pulse duration Tpuise =60 us. The plasma
`
`density was 21,- -— 1012 cm'3 [54].
`
`The attempt to obtain a homogeneous plasma with a density of 1013 —
`
`10” 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 sudh a discharge a plasma column with a
`
`density n,- 2 5 X 1014 cm'3. Experiments were conducted on an “INAR”
`
`device made of two cathodes placed at a distance 2.4 m 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 at pressure of 10" — 104 tom
`
`the initial voltage was .60 kV [55] and .270. kV [56], and the magnetic. field
`
`strength was 25 kG:s and 12 kGs, respectively. The .discharge current had an
`oscililatory .cha”racte1" with éampilitudes of‘ 60and
`and decay‘ times of.80
`
`and. 70 us; The discharge conditiensiv ensured an i‘onization_‘e1ectron free path
`
`much shorter than the length of the discharge gap. The diagnosis.» was
`
`;performed using‘ laser m‘ethods_,i double ‘tprobes, and the flux of neutrals. The
`result is a maximum ion. density n,- 2 7 X 1015 cms; 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 — 1 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 2:50
`
`

`
`-20-
`
`A/cm2—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—ionized gas appears to
`
`be of interest [61,62]. The study of the reaction of low—cu1rent (0.5 A)
`
`St.ati‘OnaIy ‘glw discharge, With sectioned hollow cathode [62] in argon at
`pressure p = 1.0“ .- 104 torr, to a. thigh-lvoltage pulse‘ which turnsthe dischar e
`_
`_
`8
`
`into a .ihig.h=-current (up: to 2001
`
`.quasi—stationary ‘regime has lshown that an
`
`A
`
`increase in: the threshold amphtudez (‘=43-— kV). leads: to anomalous» regimes
`
`intense: fluctuations of the -current intensity and Voltage. A 131-ijgh.-‘current
`
`form in the lower pressure range p = 104 - —10‘2 torr is realized only
`
`anomalous regimes, which the authors attribute to the explosive phenomena
`
`at the cathode.
`
`A
`
`Of particular interest is the study of high—current and high-voltage
`
`varieties of the quasi—stati0nary 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
`
`and eoperating pres.su.r.e.;p = 5 x.1.0.'3
`
`— 5 :x, 102 torr in Ar, N2, and H2. The current pulse ‘duration didnot exceed
`
`100 us. The typical icurrentevoltage charact.eri‘