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Surface damage by sheath effects and unipolar arcs
SURFACE DAMAGE BY SHEATH EFFECTS AND UNIPOLAB ARCS* F. SCHWIRZKE Departmmcntof fiysics
and CRemisby, Nawal Postgwduak
School, Monkrey,
Gaiifomia 93940, USA
and R.J. TAYLOR Cenkr for Plasma Physics and Fusion Engineering,
University of California, Los Angeles, California 90024, USA
Unipolar arcing develops if the sheath potential is high enough. A laser-produced plasma of short duration was used to study the onset and development of arcing on a stainless steel surface. The laser-produced plasma with /CT,= 100 eV expands rapidly from the focal spot on the target surface in the normal and in radial directions. After one laser shot the damage on the polished surface was observed with an optical and a scanning electron microscope. Although no external voltage was applied, about 20000 unipolar arc craters are observable on the stainless steel surface which was exposed to the radially expanding plasma for the short time of a few hundred nanoseconds. The size of the arc craters decreases with increasing distance from the focal spot. The initial cathode spot is 1 pm in diameter and 34 pm deep, acting like a hollow cathode. The arcs obviously contribute to the erosion of wall material. The experimental results also show that some of the eroded material is redeposited on stainless steel surfaces in loosely bound form.
1. Introduction Sheath effects are of importance when material surfaces are exposed to particle and photon tluxes from a plasma. Such exposure causes surface damage via physical and chemical sputtering, evaporation, and arcing. Arcing represents one of the most damaging plasmasurface interaction processes. The electric Aeld in the sheath between a plasma and the surface is of the order E = - ur/Au, where Vt is the floating or sheath potential Vf = (kT,/2e)(ln Mi/27rm,) and An = (kT,/4rrne2)ln is the IYebye length. Only electrons in the high energy tail of the distribution can overcome the sheath pptential. Ions entering the sheath are accelerated and gain energy. Since the ions are much more effective than electrons in sputtering and the sputtering rate for deuterons increases steeply with energy in the range of interest, *This work was supported by NPS Foundation Research Program and DOE Grant no. DE-AMO-76FTKKllO.
102-l@ eV, sputtering and desorption of loosely bound metal atoms [l] will become worse if the sheath potential increases due to an increase of the electron temperature near the wall [2]. Consequently, large influxes of metal atoms are observable during tokamak disruptions and wave and beam heating experiments. In each of these cases a higher temperature plasma is contacting the wall. Unipolar arcing [3,4] develops if the sheath potential is high enough to sustain an arc. Electrons are then emitted from a surface spot into the plasma. This reduces the nearby plasma potential and more electrons from the high energy tail of the Maxwellian distribution can reach the wall thus closing the current loop. Arc damage has been found on many tokamak surfaces. What causes the initial breakdown and the formation of the cathode spot is still under discussion. Thin dielectric spots or films (oil, grease) lead to enhanced arcing [5]. The erosion structure depends on the burning time of the arcs [6]. 2. Experimental procedures A laser-produced plasma was used to study the effects of the sheath potential on the onset of
Journal of Nucldru Materials 93 & 94 (1980) 780-7&4 @INorth-Holland bblishing
Company
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F. Schwirzke, R.J. Taylor I Surface damage by sheath effects
arcing. Q-switched neodymium laser pulses of 3-10 J, 25 ns HWFM were focused onto polished stainless steel targets in a vacuum chamber with a base pressure of 10”Torr. The temperature of the laser-produced plasma was measured spectroscopically [7], kT, = 1OOeV. Time of flight measurements [8] showed that the plasma expands rapidly with a velocity of lO’cm/s from the focal spot on the target surface in the normal and radial directions. After one laser shot the damage on the polished surface was observed with an optical and a scanning electron microSCQpe. The
plasma expanding in normal direction from the 304 stainless steel target disk of 0.5 in. diameter could be intercepted by a collector disk with its surface half masked (see fig. 1). After lo-40 shots a thin film of darker appearance was deposited on the polished collector plate in the form of a half circle (fig. 2). A PGT 1000 X-ray microanalyzer was used to determine the composition of the coating on various collector disks.
Fig. 2. Half-coated stainless steel target disk of 0.5 in. diameter with laser plasma damaged area at center.
3. Experimental results Such a half-coated disk was also used as a target. Fig. 2 shows at the center of the disk the
LASER BEAM
/
_
FOCUS LENS
TARGET HOLDER Fig. 1. Target and collector arrangement.
782
F. Schwirzke, R.J. Taylor I
‘ace damage by sheath effects
Fig. 3. Overlapping “younger” and “older” craters. The laser impact area is beyond the right-hand side of the photograph. 100 pm.) (-
Fig. 5. Removal of FeCr coating (darker areas) from highly steel surface by plasma contact: polished stainless 100 pm.) (-
laser impact crater of about 0.75 mm diameter surrounded by a plasma damaged surface area of about 3 mm diameter. Although no external voltage was applied, about 20 000 unipolar arc craters are observable on the stainless steel surface which was exposed to the radially expanding laser plasma for the short time of a few hundred nanoseconds. Figs. 3-6 show a sequence of SEM photos of the plasma-damaged surface for in-
creasing radii from the laser focal spot. Close in, fig. 3 shows a superposition of arc craters. The oldest ones, burning for a longer time during the existence of the laser-produced plasma, have larger diameter of 30-40 pm. Smaller craters within larger ones are probably initiated towards the end of the plasma surface interaction process. They have smaller crater diameters of 10 pm and frequently a dark spot of l-3 pm size representing a center core crater. The size of the arc craters also decreases with increasing distance from the focal spot (figs. 4 and 5). The
Fig. 4. Crater formation at r = 0.15 cm from laser focal spot which is located beyond left-hand side. Dark spot in the crater represents the center core crater, the cathode spot. Different crater sire results from differences in arc duration. 100 pm.) Crater density is of the order 3 x Id/cm*. (-
Fig. 6. Onset of unipolar arc crater polished stainless steel surface. (-
formation 10 km.)
on highly
F. Schwirzke, R J. Taylor / Surface damage by sheath effects
darker, grey-shaded area on the right-hand side of fig. 5 represents the Fe, Cr coating which was deposited when the disk was used as a collector plate. Thus, figs. 5 and 2 clearly show that the coating is removed by the plasma contact even though there are relatively few craters and no other damage is visible on the stainless steel surface itself. The 1230 times enlarged fig. 6 shows the onset of crater formation near the edge of the expanding plasma. Deep center core craters of 0.5-l pm diameter are clearly visible with crater rim formation just barely beginning. The depth of these fully developed center core craters is 3-6 pm. 4. Discussion The high density, 1oZ’cme3, high temperature k7’, = 100 eV laser-produced Fe-Q plasma expands rapidly in the radial direction over the surface. Plasmasurface damage and removal of the loosely bound Fe-Cr deposit is observable to a radius of r = 0.3 cm. A transition from large overlapping craters with molten rims to smaller circular craters is visible at r = 0.15 cm along a vertical center line in fig. 4. The crater density in this area is about 300000 craters/cm*. The minimum requirement for the onset of arcing is that the sheath potential is comparable to the ionization energy, which for Fe is Vi = 7.9eV and for Cr Vi = 6.8eV. The sheath potential for the Fe-plasma is tY, = 4.85 kT,/e. Thus, the minimum electron temperature for the onset of arcing is kT, = 2 eV. The binding energy of Fe is 4 eV. For an arc to develop it is also necessary that the ion density increases in front of the cathode spot in order that a larger electron current can flow into the plasma. The increased electric field strength on surface protrusions and whiskers will increase the ion flux from the plasma to these spots. Increased ion bombardment and recombination rates lead to increased surface temperature. Also, the heat conduction from such whiskers and dielectric coatings back into the metal surface is reduced. Thus, the local heating leads to desorption of gases, vaporization of an oil film, and metal evaporation. Fe atoms emit-
783
ted at the melting temperature of 1526°C for example, leave the surface with a thermal velocity of 9 x 104cm/s. For a sheath potential of 10eV and an electron density of 1013cm-3, the sheath width is about lo-‘cm and the time of flight for Fe atoms to enter the plasma would be lOWas, well within the duration of the laser pulse. If only 10% of a monolayer of typically 2 x 10’5/cm2 is suddenly released from a surface spot, the neutral density in the sheath would locally increase to 101’cm-‘. Assuming an ionization cross section of 10-l’ cm*, a small fraction, 2 x 10d4, of the neutral atoms would be ionized within the sheath, i.e. Iti = 2 X 10’3/cm3. This would effectively triple the existing plasma density near the surface. For this example an Fe+ ion produced in the sheath would need about 3 ns to fall back to the surface. This locally increased ion flwr further increases the surface temperature, the vaporization rate, and electron emission from the hot cathode spot results. The emitted electrons are accelerated by Uf and thus are able to ionize Fe atoms if Ur > 7.9 V. The ion current to the surface obeys the Child-Langmuir law i a Ufn/A&. Since the mean free path length for electrons A,$ An, we may assume that kT, = constant while ni locally increases. Consequently, the ion current increases as i a ni. The locally increased ionization reduces hn and thus the sheath width while the arc driving electric field in the sheath in V/cm, IEI = &/An = 6.6 x 1O-3 (kT,nJ1” actually increases with the local plasma pressure as E a p”‘, where kT, is in eV and ntein cme3. Field emission should occur for E > 10’ V/cm or for (n,kT,)“* > 1.5 x 10’. The locally increased plasma pressure above the cathode spot also leads to an electric field E, = in the radial direction, -(kT,/e)(l/n)(dn/dr) tangential to the surface (fig. 7). This reduces the potential in a ring-like area surrounding the higher pressure region above the arc spot. The sheath potential will be lowered there and more electrons from the high energy tail in the Maxwellian distribution can reach the surface, thus closing the current loop. The1 short lifetime of the laser-produced plasma allows the study of the temporal development of the arcs. The initial breakdown
F. Schwirzke, R.J. Taylor 1 Surface damage by sheath
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6
k 2
Fig. 7. Schematic of potential distribution and electric fields near cathode spot.
phase of unipolar arcing is documented by the small but deep arc craters formed furthest away from the focal spot (fig. 6), where the plasma conditions for arc formations lasted only for the short time of about lo-50 ns. The craters have a diameter of 1 pm and a depth up to 6 pm. Assuming a cylindrical volume with 3 pm depth, the crater volume is V = 2.4 x lo-l2 cm3 and the removed mass is 1.9 x 10e5 pg, corresponding to 2 X 10” Fe atoms. Minimum arc current and voltage are probably of the order 10 A and 10 V, respectively. For a 30 ns arc, 3 x lo-’ C are flowing, or 5 x 10” charges. Thus, the erosion rate is approximately 4 x lo-* atoms/charge. The depth of the initial arc crater of 3-6 pm is larger than the diameter of 1 pm. This indicates that the pinch magnetic field of 40000 G for a current of 10 A and r = 0.5 pm contributes to the concentration of the arc energy onto the small cathode spot. If k7’, = 10 eV, this field can confine a plasma with a density up to 4~ 1018cme3. At this density hr, = lob6 cm 4 1 pm and it is justified to assume that a high density plasma exists in the arc hole. This represents now a hollow cathode configuration and the ionization rate should be high. 5. Conclusions A laser-produced plasma was used to study the onset of unipolar arcing. The initial arc crater on
a stainless steel surface has a typical dimension of 1 pm diameter and a depth of 3-6 pm. If allowed to burn long enough, the metal on the exit melts is pushed outwards in the radial direction and a crater rim of lo-4Opm diameter is formed. For a sufliciently high electron temperature of 30-1OOeV an enormous number of unipolar micro-arcs, about 300 OOO/cm*,develop in a rather short time of a few hundred nanoseconds. These arcs obviously contribute to the erosion of wall material. The experimental results also show that some of the eroded material is redeposited on stainless steel surfaces in loosely bound form. References [l] F. Schwirzke, L. Oren, S. Talmadge and R.J. Taylor, Phys. Rev. Letters 40 (1978) 1181. [2] L. Oren, R.J. Taylor and F. Schwirzke, J. Nucl. Mater. 76/ 77 (1978) 412. [3] A.E. Robson and P.C. Thonemann, Proc. Phys. Sot. 73 (1959) 508. [4] G.H. Miley, J. Nucl. Mater. 63 (1976) 331. [S] P. Mioduszewski, R.E. Clausing and L. Heatherly, in: Proc. IEEE Intern. Conf. on Plasma Science, Montreal, Canada, 1979, Conference Record-Abstract (1979) p. 119. [6] E. Hantzsche, B. Juttner, V.F. Puchkarov, W. Rohrbech and H. Wolff, J. Phys. D (Appl. Phys.) 9 (1976) 1771. [7] S.A. Shewchuck, M.S. Thesis, Naval Postgraduate School, Monterey, CA (1976). [8] R.S. Bird, L.L. McKee, F. Schwirzke and A.W. Cooper, Phys. Rev. A7 (1973) 1328.
and CRemisby, Nawal Postgwduak
School, Monkrey,
Gaiifomia 93940, USA
and R.J. TAYLOR Cenkr for Plasma Physics and Fusion Engineering,
University of California, Los Angeles, California 90024, USA
Unipolar arcing develops if the sheath potential is high enough. A laser-produced plasma of short duration was used to study the onset and development of arcing on a stainless steel surface. The laser-produced plasma with /CT,= 100 eV expands rapidly from the focal spot on the target surface in the normal and in radial directions. After one laser shot the damage on the polished surface was observed with an optical and a scanning electron microscope. Although no external voltage was applied, about 20000 unipolar arc craters are observable on the stainless steel surface which was exposed to the radially expanding plasma for the short time of a few hundred nanoseconds. The size of the arc craters decreases with increasing distance from the focal spot. The initial cathode spot is 1 pm in diameter and 34 pm deep, acting like a hollow cathode. The arcs obviously contribute to the erosion of wall material. The experimental results also show that some of the eroded material is redeposited on stainless steel surfaces in loosely bound form.
1. Introduction Sheath effects are of importance when material surfaces are exposed to particle and photon tluxes from a plasma. Such exposure causes surface damage via physical and chemical sputtering, evaporation, and arcing. Arcing represents one of the most damaging plasmasurface interaction processes. The electric Aeld in the sheath between a plasma and the surface is of the order E = - ur/Au, where Vt is the floating or sheath potential Vf = (kT,/2e)(ln Mi/27rm,) and An = (kT,/4rrne2)ln is the IYebye length. Only electrons in the high energy tail of the distribution can overcome the sheath pptential. Ions entering the sheath are accelerated and gain energy. Since the ions are much more effective than electrons in sputtering and the sputtering rate for deuterons increases steeply with energy in the range of interest, *This work was supported by NPS Foundation Research Program and DOE Grant no. DE-AMO-76FTKKllO.
102-l@ eV, sputtering and desorption of loosely bound metal atoms [l] will become worse if the sheath potential increases due to an increase of the electron temperature near the wall [2]. Consequently, large influxes of metal atoms are observable during tokamak disruptions and wave and beam heating experiments. In each of these cases a higher temperature plasma is contacting the wall. Unipolar arcing [3,4] develops if the sheath potential is high enough to sustain an arc. Electrons are then emitted from a surface spot into the plasma. This reduces the nearby plasma potential and more electrons from the high energy tail of the Maxwellian distribution can reach the wall thus closing the current loop. Arc damage has been found on many tokamak surfaces. What causes the initial breakdown and the formation of the cathode spot is still under discussion. Thin dielectric spots or films (oil, grease) lead to enhanced arcing [5]. The erosion structure depends on the burning time of the arcs [6]. 2. Experimental procedures A laser-produced plasma was used to study the effects of the sheath potential on the onset of
Journal of Nucldru Materials 93 & 94 (1980) 780-7&4 @INorth-Holland bblishing
Company
780
F. Schwirzke, R.J. Taylor I Surface damage by sheath effects
arcing. Q-switched neodymium laser pulses of 3-10 J, 25 ns HWFM were focused onto polished stainless steel targets in a vacuum chamber with a base pressure of 10”Torr. The temperature of the laser-produced plasma was measured spectroscopically [7], kT, = 1OOeV. Time of flight measurements [8] showed that the plasma expands rapidly with a velocity of lO’cm/s from the focal spot on the target surface in the normal and radial directions. After one laser shot the damage on the polished surface was observed with an optical and a scanning electron microSCQpe. The
plasma expanding in normal direction from the 304 stainless steel target disk of 0.5 in. diameter could be intercepted by a collector disk with its surface half masked (see fig. 1). After lo-40 shots a thin film of darker appearance was deposited on the polished collector plate in the form of a half circle (fig. 2). A PGT 1000 X-ray microanalyzer was used to determine the composition of the coating on various collector disks.
Fig. 2. Half-coated stainless steel target disk of 0.5 in. diameter with laser plasma damaged area at center.
3. Experimental results Such a half-coated disk was also used as a target. Fig. 2 shows at the center of the disk the
LASER BEAM
/
_
FOCUS LENS
TARGET HOLDER Fig. 1. Target and collector arrangement.
782
F. Schwirzke, R.J. Taylor I
‘ace damage by sheath effects
Fig. 3. Overlapping “younger” and “older” craters. The laser impact area is beyond the right-hand side of the photograph. 100 pm.) (-
Fig. 5. Removal of FeCr coating (darker areas) from highly steel surface by plasma contact: polished stainless 100 pm.) (-
laser impact crater of about 0.75 mm diameter surrounded by a plasma damaged surface area of about 3 mm diameter. Although no external voltage was applied, about 20 000 unipolar arc craters are observable on the stainless steel surface which was exposed to the radially expanding laser plasma for the short time of a few hundred nanoseconds. Figs. 3-6 show a sequence of SEM photos of the plasma-damaged surface for in-
creasing radii from the laser focal spot. Close in, fig. 3 shows a superposition of arc craters. The oldest ones, burning for a longer time during the existence of the laser-produced plasma, have larger diameter of 30-40 pm. Smaller craters within larger ones are probably initiated towards the end of the plasma surface interaction process. They have smaller crater diameters of 10 pm and frequently a dark spot of l-3 pm size representing a center core crater. The size of the arc craters also decreases with increasing distance from the focal spot (figs. 4 and 5). The
Fig. 4. Crater formation at r = 0.15 cm from laser focal spot which is located beyond left-hand side. Dark spot in the crater represents the center core crater, the cathode spot. Different crater sire results from differences in arc duration. 100 pm.) Crater density is of the order 3 x Id/cm*. (-
Fig. 6. Onset of unipolar arc crater polished stainless steel surface. (-
formation 10 km.)
on highly
F. Schwirzke, R J. Taylor / Surface damage by sheath effects
darker, grey-shaded area on the right-hand side of fig. 5 represents the Fe, Cr coating which was deposited when the disk was used as a collector plate. Thus, figs. 5 and 2 clearly show that the coating is removed by the plasma contact even though there are relatively few craters and no other damage is visible on the stainless steel surface itself. The 1230 times enlarged fig. 6 shows the onset of crater formation near the edge of the expanding plasma. Deep center core craters of 0.5-l pm diameter are clearly visible with crater rim formation just barely beginning. The depth of these fully developed center core craters is 3-6 pm. 4. Discussion The high density, 1oZ’cme3, high temperature k7’, = 100 eV laser-produced Fe-Q plasma expands rapidly in the radial direction over the surface. Plasmasurface damage and removal of the loosely bound Fe-Cr deposit is observable to a radius of r = 0.3 cm. A transition from large overlapping craters with molten rims to smaller circular craters is visible at r = 0.15 cm along a vertical center line in fig. 4. The crater density in this area is about 300000 craters/cm*. The minimum requirement for the onset of arcing is that the sheath potential is comparable to the ionization energy, which for Fe is Vi = 7.9eV and for Cr Vi = 6.8eV. The sheath potential for the Fe-plasma is tY, = 4.85 kT,/e. Thus, the minimum electron temperature for the onset of arcing is kT, = 2 eV. The binding energy of Fe is 4 eV. For an arc to develop it is also necessary that the ion density increases in front of the cathode spot in order that a larger electron current can flow into the plasma. The increased electric field strength on surface protrusions and whiskers will increase the ion flux from the plasma to these spots. Increased ion bombardment and recombination rates lead to increased surface temperature. Also, the heat conduction from such whiskers and dielectric coatings back into the metal surface is reduced. Thus, the local heating leads to desorption of gases, vaporization of an oil film, and metal evaporation. Fe atoms emit-
783
ted at the melting temperature of 1526°C for example, leave the surface with a thermal velocity of 9 x 104cm/s. For a sheath potential of 10eV and an electron density of 1013cm-3, the sheath width is about lo-‘cm and the time of flight for Fe atoms to enter the plasma would be lOWas, well within the duration of the laser pulse. If only 10% of a monolayer of typically 2 x 10’5/cm2 is suddenly released from a surface spot, the neutral density in the sheath would locally increase to 101’cm-‘. Assuming an ionization cross section of 10-l’ cm*, a small fraction, 2 x 10d4, of the neutral atoms would be ionized within the sheath, i.e. Iti = 2 X 10’3/cm3. This would effectively triple the existing plasma density near the surface. For this example an Fe+ ion produced in the sheath would need about 3 ns to fall back to the surface. This locally increased ion flwr further increases the surface temperature, the vaporization rate, and electron emission from the hot cathode spot results. The emitted electrons are accelerated by Uf and thus are able to ionize Fe atoms if Ur > 7.9 V. The ion current to the surface obeys the Child-Langmuir law i a Ufn/A&. Since the mean free path length for electrons A,$ An, we may assume that kT, = constant while ni locally increases. Consequently, the ion current increases as i a ni. The locally increased ionization reduces hn and thus the sheath width while the arc driving electric field in the sheath in V/cm, IEI = &/An = 6.6 x 1O-3 (kT,nJ1” actually increases with the local plasma pressure as E a p”‘, where kT, is in eV and ntein cme3. Field emission should occur for E > 10’ V/cm or for (n,kT,)“* > 1.5 x 10’. The locally increased plasma pressure above the cathode spot also leads to an electric field E, = in the radial direction, -(kT,/e)(l/n)(dn/dr) tangential to the surface (fig. 7). This reduces the potential in a ring-like area surrounding the higher pressure region above the arc spot. The sheath potential will be lowered there and more electrons from the high energy tail in the Maxwellian distribution can reach the surface, thus closing the current loop. The1 short lifetime of the laser-produced plasma allows the study of the temporal development of the arcs. The initial breakdown
F. Schwirzke, R.J. Taylor 1 Surface damage by sheath
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effects
6
k 2
Fig. 7. Schematic of potential distribution and electric fields near cathode spot.
phase of unipolar arcing is documented by the small but deep arc craters formed furthest away from the focal spot (fig. 6), where the plasma conditions for arc formations lasted only for the short time of about lo-50 ns. The craters have a diameter of 1 pm and a depth up to 6 pm. Assuming a cylindrical volume with 3 pm depth, the crater volume is V = 2.4 x lo-l2 cm3 and the removed mass is 1.9 x 10e5 pg, corresponding to 2 X 10” Fe atoms. Minimum arc current and voltage are probably of the order 10 A and 10 V, respectively. For a 30 ns arc, 3 x lo-’ C are flowing, or 5 x 10” charges. Thus, the erosion rate is approximately 4 x lo-* atoms/charge. The depth of the initial arc crater of 3-6 pm is larger than the diameter of 1 pm. This indicates that the pinch magnetic field of 40000 G for a current of 10 A and r = 0.5 pm contributes to the concentration of the arc energy onto the small cathode spot. If k7’, = 10 eV, this field can confine a plasma with a density up to 4~ 1018cme3. At this density hr, = lob6 cm 4 1 pm and it is justified to assume that a high density plasma exists in the arc hole. This represents now a hollow cathode configuration and the ionization rate should be high. 5. Conclusions A laser-produced plasma was used to study the onset of unipolar arcing. The initial arc crater on
a stainless steel surface has a typical dimension of 1 pm diameter and a depth of 3-6 pm. If allowed to burn long enough, the metal on the exit melts is pushed outwards in the radial direction and a crater rim of lo-4Opm diameter is formed. For a sufliciently high electron temperature of 30-1OOeV an enormous number of unipolar micro-arcs, about 300 OOO/cm*,develop in a rather short time of a few hundred nanoseconds. These arcs obviously contribute to the erosion of wall material. The experimental results also show that some of the eroded material is redeposited on stainless steel surfaces in loosely bound form. References [l] F. Schwirzke, L. Oren, S. Talmadge and R.J. Taylor, Phys. Rev. Letters 40 (1978) 1181. [2] L. Oren, R.J. Taylor and F. Schwirzke, J. Nucl. Mater. 76/ 77 (1978) 412. [3] A.E. Robson and P.C. Thonemann, Proc. Phys. Sot. 73 (1959) 508. [4] G.H. Miley, J. Nucl. Mater. 63 (1976) 331. [S] P. Mioduszewski, R.E. Clausing and L. Heatherly, in: Proc. IEEE Intern. Conf. on Plasma Science, Montreal, Canada, 1979, Conference Record-Abstract (1979) p. 119. [6] E. Hantzsche, B. Juttner, V.F. Puchkarov, W. Rohrbech and H. Wolff, J. Phys. D (Appl. Phys.) 9 (1976) 1771. [7] S.A. Shewchuck, M.S. Thesis, Naval Postgraduate School, Monterey, CA (1976). [8] R.S. Bird, L.L. McKee, F. Schwirzke and A.W. Cooper, Phys. Rev. A7 (1973) 1328.