[handwritten:] 61 95-1/593-2
`
`MOSCOW ENGINEERING PHYSICS INSTITUTE
`(TECHNICAL UNIVERSITY)
`_____________________________________________________________
`This material may be protected by Copyright law (Title 17 U.S. 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
`
`GILLETTE 1105
`
`

`
`- 2 -
`
`TABLE OF CONTENTS
`TABLE OF CONTENTS .................................................................................................... 2(cid:3)
`
`INTRODUCTION. ............................................................................................................. 4(cid:3)
`
`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(cid:3)
`
`1.1 Experimental research of low-pressure gas discharge in a magnetic field and
`its application in modern technology. ......................................................... 7(cid:3)
`
`low-pressure discharges with
`for generating high-power
`1.2 Methods
`homogeneous plasma structure. ................................................................ 25
`
`32
`Conclusions
`
`
`Chapter 2. Experiment methods and techniques. .............................................................. 34(cid:3)
`
`2.1 The experimental setup. ................................................................................ 34(cid:3)
`
`2.2 The probe method for determining plasma and ion flow parameters. .......... 45(cid:3)
`
`2.3 Double-mode laser method for determining the density of the plasma. ....... 54(cid:3)
`
`2.4 Pulsed biasing. .............................................................................................. 57(cid:3)
`
`Chapter 3. Parameter ranges and conditions of high-current low-pressure quasi-stationary
`discharge in a magnetic field of various configurations. .......................... 59(cid:3)
`
`3.1 Quasi-stationary discharge regimes. ............................................................. 59(cid:3)
`
`3.2 Application of pulse probe method to determine plasma and ion flow
`parameters. ................................................................................................ 73(cid:3)
`
`3.3 Determination of the discharge plasma parameters by double-mode laser
`interferometry. ........................................................................................... 82
`
`87
`3.4 Discussion of results
`
`Chapter 4. Emission and sputtering characteristics of the high-current low-pressure quasi-
`stationary discharge. ................................................................................ 100(cid:3)
`
`4.1 Features of cathode sputtering in a high-current quasi-stationary magnetron
`discharge and implementation thereof in pulsed sputtering technology. 100(cid:3)
`
`

`
`- 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(cid:3)
`
`CONCLUSION ............................................................................................................... 110(cid:3)
`
`REFERENCES ............................................................................................................... 111(cid:3)
`
`

`
`- 4 -
`
`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 of the
`magnetron discharge in which most technological devices operate. The study
`of high-power pulsed discharges used for the generation of dense
`homogeneous plasma indicates the possibility of existence of stable forms of
`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.
`
`

`
`- 5 -
`
`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 (cid:1540)100 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 of this type of discharge.
`We have established the existence of a high-voltage high-current
`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/cm2). 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.
`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 -
`
`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 [Moskovskiy
`inzhinerno-fizicheskiy institut (Moscow Engineering and Physics Institute)]
`and MVP [Mnogootraslevoe vnedrenheskoe predpriyatiye (Multi-sector
`Innovative Enterprise)] Different, Minsk, collaboratively).
`
`The following theses will be defended below:
`1. The results of experimental research of high-current quasi-
`stationary
`low-pressure discharge
`in magnetic
`fields of different
`configurations: a. parameter ranges and regimes of two forms of discharge:
`b. dependence of current-voltage characteristics on the
`pressure, magnetic field density, type of gas, electrode material, and type of
`discharge 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-
`stationary diffuse discharge regime in a weakly inhomogeneous magnetic
`field.
`
`The main results are published in the papers [94-104].
`
`

`
`- 7 -
`
`LOW-PRESSURE
`HIGH-CURRENT
`1.
`CHAPTER
`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 application, systems
`with field configuration, ensuring a closed electron drift, proved to be the
`most promising. These include cylindrical [1,2] and planar [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 50 ns [17,18]. The only drawback of crossatron modulator switches
`
`is the large—(cid:131)(cid:132)(cid:145)(cid:151)(cid:150) 300 V—discharge voltage drop, which ensures 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 of negative ions of hydrogen [19].
`One of the 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 (cid:1540)1 torr—is:
`
`

`
`- 9 -
`
`10-10-10-4 A, 10-4-10-1 A, 10-1-10 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 a mobility 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
`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 of movement
`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—i.e., to formation of an expressed anode layer.
`Anode layer or negative space charge discharge regimes are mainly
`obtained at low pressures (p (cid:148) 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 -
`
` 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]
`
`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
`, where Id is the discharge current, Ud - the discharge
`the formula
`
`voltage, and (cid:2301)(cid:481) 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 10-1 torr).
`Deviations from this pattern of dependence of the discharge voltage on
`the current were observed when working with uncooled cathodes [36]. In
`magnetic fields with a density over 2 kGs 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 (cid:1540)200
`of (cid:1540)2 A. The authors explain this S-shaped curve by an increase in the
`
`V, after which Ud is almost independent of the current up to a current value
`
`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-ln x j is shifted by 3/21n(1+(cid:550)2(cid:306)2) towards higher
`currents ((cid:550)—cyclotron frequency, (cid:306)—electron relaxation time), which
`means a corresponding increase in the normal current density. The authors
`derive from this conditions 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 typical for technological
`applications was constructed in paper [42]. Univariate solutions were
`obtained, describing the plasma parameters in different parts of the 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 Ud(Id), stabilizing at 400-600 V at a
`current density of 0.04-0.1 A/cm2, which is in satisfactory agreement with
`the experiment. However, the current density in the stationary regime (up to
`0.3 A/cm2) 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 ni 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 ni (cid:1540)1010 - 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 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 drift 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
`
`was 3.5 x 1012 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 z
`
`system axis at the cathode surface ((cid:1540)8eV). The maximum electron density
`and was (cid:1540)0.05. Probe measurements have shown that VD(cid:1540) 7.3 x 106 cm/s—
`i.e., Ve(cid:1540) 1.5 x 108 cm/s—which corresponds to an electron temperature of
`(cid:1540)6 eV. Measurements of the ion temperature have shown that Ti(cid:1540) 0.6 - 1,2
`eV, and the ratio between electron and ion temperatures was (cid:1540)3 - 5.
`argon, cathode material—copper, discharge current (cid:1540)0.1 A. The maximum
`
`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—
`
`electron density and temperature achieved were:
`
`

`
`1. p = 5 x 10-3 torr, ne(cid:1540) 1010 cm-3, Te(cid:1540) 20 eV
`2. p =2 x 10-2 torr, ne(cid:1540) 5x1010 cm-3, Te(cid:1540) 5 eV
`
`- 14 -
`
`—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 ni(Id) and decrease of the Te (Id)
`dependence (Fig. 1.4), but the narrow range of currents of 50-150 mA does
`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 ni (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.
`An important parameter of the discharge affecting its current-
`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
`
` [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
`
`10-4 - 5 x 10-2 torr, Ud(cid:1540) 300 V, 1d(cid:1540) 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 component Br is maximized in
`the discharge gap, and
`
`l;
`
`, where
`
`maximum value of the BZ on the system axis at z = 0.1 m is (cid:1540)1,000 Gs, and
`the maximum value of Br in the symmetry plane of the system (cid:1540)350 Gs. The
`maximum operating voltage was (cid:1540)3 kV.
`
`l is the electrode width, and d is the distance between the electrodes. The
`
` In the discharge device described above, depending on the pressure
`of the working gas, three types of discharge were discovered: 1. 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 -
`
`70 Gs
`
`200 Gs
`
`300 Gs
`
`Fig. 1.5 Discharge device with circular
`electrodes in a quadrupole magnetic
`field [50].
`
`
`
`
`
`
`
`
`
`Fig. 1.6 Potential distribution in a
`high-current discharge regime
`Id = 50 mA, p = 2 x 10-1 torr [49].
`
`Arc
`
`Arc
`
`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].
`
`Fig. 1.9 High-power (a) and high-voltage (b) regimes [64-66].
`a. 1-Ar, p = 2 x 10-2 torr, B = 670 Gs, 2 - B = 1,360 Gs, 3 - B = 1,800 Gs, 4.5-N2, H2.
`b. He p = 1 x 10-3 torr, 1 - B = 1,360 Gs, 2 - B = 910 Gs, 3 - B = 670 Gs.
`
`

`
`- 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
`lK decreases. It is known that the lK 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 Br. The size of the anode layer does not depend on the Br. The
`study of the discharge voltage dependence of the current Ud(Id) [49] in the
`region of high (stationary) currents has shown the presence of discharge
`regimes, whose the current-voltage characteristics (CVCs) are shown in Fig.
`1.7 in the area (pd) corresponding to the minimum and the right branch of
`the Paschen curve. For Ar, this area is in the range of pressures 2x10-1 < p <
`2 torr. Specific to this group of curves Ud(Id) is that the slope of the dU/dI
`CVC and the discharge voltage 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 Br increases, the
`critical current at which the discharge transits into an arc increases; for
`example, in nitrogen at Br (cid:149) 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 ((cid:1540)300 - 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 (cid:1540)(cid:882).1 - 1 torr,
`
`regimes were obtained, whose
`stationary discharge
`high-current
`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 studying the dependence of the discharge current
`on the pressure, the existence of a new high-current regime at low pressures
`was found. With a decrease in pressure to 10-3 torr in Ar, an anomalous
`increase of the discharge current 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(cid:1540) 70 A at Ud(cid:1540) 400 V with a pulse duration (cid:306)pulse(cid:1540)60 (cid:541)s. The plasma
`density was ni(cid:1540)(cid:3)1012 cm-3 [54].
`The attempt to obtain a homogeneous plasma with a density (cid:145)(cid:136) 1013 -
`density ni(cid:1540) 5 x 1014 cm-3. Experiments were conducted on an “INAR”
`
`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
`
`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 a pressure of 10-1 - 10-4 torr,
`the initial voltage was 60 kV [55] and 20 kV [56], and the magnetic field
`strength was 25 kGs and 12 kGs, respectively. The discharge current had an
`oscillatory character with amplitudes of 60 and 20 kA and decay times of 80
`and 70 (cid:541)s. The discharge conditions ensured an ionization electron free path
`much shorter than the length of the discharge gap. The diagnosis was
`performed using laser methods, double probes, and the flux of neutrals. The
`
`result is a maximum ion density ni(cid:1540) 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 10-2 - 1 torr in a pulsed
`
`regime at currents of up to 2 kA and pulse duration of 50 (cid:586)s. The
`abovementioned discharge is characterized by a current density of (cid:1540)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-current (0.5 A)
`stationary glow discharge, with sectioned hollow cathode [62] in argon at
`pressure p = 10-4 - 10-1 torr, to a high-voltage pulse which turns the discharge
`into a high-current (up to 200 A) quasi-stationary regime has shown that an
`
`increase in the threshold amplitude ((cid:1540)3 kV) leads to anomalous regimes
`
`with intense fluctuations of the current intensity and voltage. A high-current
`form in the lower pressure range p = 10-4 - 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 of the 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
`
`

`
`symmetric magnetic and electric fields, powerful ((cid:1540)1 MW) high-current and
`
`- 21 -
`
`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 x 10-3
`- 5 x 10-2 torr in Ar, N2, and H2. The current pulse duration did not exceed
`100 (cid:541)s. The typical current-voltage characteristics of the discharge are
`shown in Fig. 1.9a. The dependence Ud(Id) is the same for different
`parameters p, d, B0, and types of gas. First, with the increase of current to a
`critical value I
`, the voltage increases monotonically by 50 - 100
`
` the discharge voltage is practically independent of
`
`V, and then at Id > I
`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 Id entails an almost
`linear increase in the dis