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A study of the mechanisms of metal impurity release during ICRF heating in the URAGAN-3 torsatron
458
Journal
of Nuclear
Materials
162-164
(1989) 458-461
North-Holland.
A STUDY OF THE MECHANISMS OF METAL IMPURITY RELEASE HEATING IN THE URAGAN3 TORSATRON
Amsterdam
DURING ICRF
L.I. GRIGOR’EVA, V.G. KONOVALOV, N.I. NAZAROV, V.V. PLYUSNIN, G.N. POLYAKOVA, A.I. SKIBENKO, I.P. FOMIN, V.V. CHECHKIN, A.N. SHAPOVAL and O.M. SHVETS Kharkov Institute of Physics and Technology, Ukrainian SSR Academy
Key words:
impurity,
ICRF
Possible mechanisms with the URAGAN-3 harmonics produce an antenna surface (Ti) is pulse.
heating,
torsatron,
potential,
plasma
of Sciences, 310108 Kharkov,
USSR
sheath
responsible for metal impurity release during ICRF production and heating of the plasma are studied torsatron. It is shown that the edge plasma potential oscillations at the pumping frequency and its impurity influx from the surface of the helical winding casings (Fe, Cr). The impurity release from the caused by a quasisteady - 100 eV ion flux arising in the divertor magnetic flux region during the RF
In all experiments on ICRF and Alfvkn wave heating in tokamaks an enhanced impurity release is observed (see for example ref. [l]). This can result in essential radiation losses comparable with the total power input and eventual cessation of the heating if special measures are not taken (limiters and antenna side tiles made of graphite, wall carbonization). A similar phenomenon takes place in a stellarator [2]. The identification of possible mechanisms which are responsible for the enhanced impurity influx under RF heating conditions still remains an urgent problem. A possible reason for the edge plasma-surface interaction enhanced during RF heating is a sheath potential increase due to electron heating and hence, an increase in the energy of the ions impinging on the wall [3]. Moreover, some specific mechanisms inherent in RF plasma heating at low frequencies may exist which result in a steady and/or time-varying potential acquired by the edge plasma relative to the surface and thereby cause an additional flux of charged particles onto this surface. Edge plasma potential oscillations and RF current rectification in the plasma-antenna sheath due to plasma polarization by the antenna electrostatic field [4,5] and parametric excitation of electrostatic modes by the pumping wave [6,7] could be ascribed to these mechanisms. From numerous experiments on RF heating in tokamaks and stellarators [8] it may by concluded that some pecularities in the observed behaviour of the edge plasma can be explained by an additional potential
arising in the plasma. Faraday shielding does not eliminate these peculiarities completely. The object of this work is to examine the nature of mechanisms responsible for the release of metal impurities during RF production and heating of the plasma in the Uragan-3 (U-3) torsatron. Both oscillations in the plasma potential and the quasisteady flux of ions onto the antenna surface which arises during RF heating are regarded as possible mechanisms. The U-3 machine [9] is an I = 3 torsatron with 9 periods of the magnetic field helical winding [fig. l(a) and (b)]. R = 100 cm and the inner radius of the casings of the helical windings (1, 2, 3) is 19 cm. The idea of the behaviour of the magnetic field lines in the central region of the torus cross-section where closed magnetic surfaces are formed (average minor radius Z = 9 cm) and in the periphery where the magnetic divertor flux exists, is illustrated in fig. l(b). The rotational transform in the closed magnetic surface region varies from t(O) = 0.18 to t(a) = 0.4. The whole magnetic system is enclosed in a 70 m3 vacuum tank. The experiments were carried with a steady hydrogen inlet, pH = (1.4-3.0) X lo-’ Torr. The plasma was produced by exciting an ion cyclotron wave at f= 5.4 MHz. To couple RF power to the plasma two twisted frame-type antennae without Faraday shield were installed at a minor radius r, = 16 cm on the low field side. The voltage amplitude across the antenna was - 10 kV at a 300 kW power input. As shown in fig. l(a), one antenna (“distant antenna”:K-2)
459
LI. Grigor’eva et al. / h4echanisms of metal impurity release
:m @ j*
was installed at 130° toroidally from torus section 4 were probes were placed. The edge of the other antenna (.. near antenna”: K-l) was at a distance of 30 cm (- 15 o toroidally) from the section with the probes. A schematic view of the antenna and the instantaneous voltage r? distribution along its length are shown in fig. l(c) to illustrate the role of the antenna as a possible source of oscillations in the plasma potential. The antenna was installed along the rib of the separatrix so as to follow its shape. Typical plasma parameters during the quasisteady stage of the RF discharge were n - (2-3) x 10” cme3, Ti - (600-900) eV, Te - 250 eV in the central region and n - 10” cme3, T, - 10 eV in the edge.
:m 0
5
10
ms
Fig. 2. RF voltage (d), magnetic probe signal (i), capacitive probe signal (i), electric probe signal from the casing surface (i), electric probe signal from the spacing between the casings (I,).
--R +--
_-.
-- --__.__~,__---’ “CM Fig. 1. (a) Schematic view of the U-3 torsatron with some edge plasma diagonistics and antennae: 1, 2, 3 helical winding casings; 4 cross-section of the torus where the probes are placed; 5 monochromator; 6 microwave cavity. (b) Cross-section 4 of the torus with electric probes mounted normal to the casing surface (7), along the helical winding (8) and between the casings (I-V). Magnetic field lines which form closed magnetic surfaces in the central region and those which form the divertor flwr at the periphery are separated by the separatrix. (c) Schematic view of the antenna: parts C and D face the rib of the separatrix and have zero RF potential.
Oscillations in the edge plasma potential at the pumping frequency f and its harmonics were detected by a movable capacitive probe (CP) its insulating envelope diameter being 6 mm (see for example ref. [lo]). The charged particle flux onto the helical winding casing was detected with three earthed shielded electric probes (EPs). The EPs were mounted on the inner surface of the casing. One of them [7 in fig. l(b)] was directed normal to the surface, the other two (8) were parallel to the helical winding and were facing in opposite directions to each other. A set of 5 EPs (I-V in fig. l(b)) was installed radially between the casings along an arc with a minor radius of 23 cm to detect the charged particle flux in the divertor magnetic flux region. Impurity atom lines were detected with two monochromators: one sensed the radiation [Cr I (359.3 nm); Fe I (302.0 nm)] from the probe-carrying section 4, the other [S in fig. l(a)] faced the distant antenna region [Ti I (364.2 nm)]. The edge plasma density in the divertor magnetic flux region was estimated using an open microwave cavity installed on the outer side of the casing [6 in fig.
WI.
In fig. 2 typical oscillograms are shown of the RF antenna voltage (c), RF magnetic probe signal (d) indicating the ion cyclotron wave excitation, CP signal (i), charged particle current (r”) to one of the EPs mounted on the casing, and charged particle current (Z,) to one of the EPs placed between the casings. As seen in fig. 2, the excitation of intense potential oscillations starts at the onset of the discharge while the
460
L. I. Grigor’eua et al. / Mechanisms of metal impurity release
Fig. 3. RF voltage and edge plasma potential oscillations at 3 different positions of CP.
plasma density is still insufficient for the electromagnetic (ion cyclotron) wave excitation. Time variations of the edge plasma potential 6 are coherent oscillations at the pumping frequency f and its harmonics as has been observed earlier in simulation experiments [4,5]: The shape of the oscillations is position dependent. Three oscillograms of potential oscillations 6 taken at different CP positions together with antenna voltage c are shown by way of example in fig. 3. The amplitude of the potential oscillations excited by the distant antenna decreases along the radius (fig. 4: (1) onset of the discharge; (2) the end of the RF pulse). This means that the distant antenna excites oscillations which are transferred to the torus section discussed through the plasma core rather than the periphery. The value of the oscillation amplitude at the periphery (r - 15-20 cm) was 20-50 V with the distant antenna and 200 V with the near antenna energized. The potential oscillations in the edge plasma result in the electron and ion currents to the casing surface
01
5
4
10
15
,
r, cm
Fig. 4. Potential oscillation amplitude versus minor radius (measured along the major radius). 1: onset of the discharge; 2: end of the RF pulse. A dotted line indicates the casing inner radius (19 cm).
which oscillate at the pumping frequency and its harmonics and are phased with the potential oscillations. When RF power is launched by the distant antenna, the ion current amplitude is - 1 mA/cm’ as detected by the EP installed normal to the surface, and - 10 mA/cm* to each EP parallel to the helical winding. In the latter case the two currents are phased 1X0 o with respect to each other, indicating an appreciable contribution of the electric drift to these currents. Both the electron and the ion current increase by an order of magnitude as the near antenna is energized. The EPs I-V mounted between the casings detect a quasisteady ion current I, (see fig. 2). The I, distribution along the arc generally agrees with the density distribution in the divertor flux region [9]. The average ion energy as estimated from the ion current vs the retarding potential is - 100 eV though a considerable fraction of the ions have energy exceeding 300 eV. A specific feature of the I, current is that it rapidly decreases ( < 100 ps) when the RF pulse is switched off. The rise of a quasisteady ion flux on an earthed surface intersected by the divertor magnetic flux during an RF pulse is possibly caused by the rise of some quasisteady potential of positive polarity in the edge plasma. Such a potential can arise from the RF rectification in the edge plasma - antenna sheath as was mentioned above. To elucidate possible effects of the edge plasma potential oscillations and the I,, ion current on the
Fig. 5. Potential oscillation amplitude (G), Cr I intensity, ion current plasma
in the divertor region (I,,), Ti I intensity density (n) versus the hydrogen pressure. represent neutral atom concentrations.
and divertor Dotted lines
L. I. Grigor’eva et al. / Mechanisms of metal impurity release
461
frequency range which eventually result in impurity release. These are the RF plasma potential oscillations which give rise to the impurity influx from the casings of the magnetic field helical winding and the quasisteady ion flux with ion energy of >, 100 eV which arises in the divertor magnetic flux region and hits the antenna surface. Both the amplitude of the potential oscillations in the edge plasma and the ion current in the divertor magnetic flux region are at least an order of magnitude greater in the vicinity of the working antenna than at a distance from it.
Fig. 6. The same quantities as in fig. 5 versus the magnetic field
References
strength. impurity release, simultaneous measurements of these characteristics and the impurity atom line intensities versus the hydrogen pressure p (fig. 5) and confining magnetic field B (fig. 6) were performed. The dotted curves are Cr and Ti atom concentrations estimated by normalking the corresponding intensity to the edge plasma density n. It follows from the data shown that the concentration of the atoms released from the casing surface (Cr, Fe) correlates with the variations of the potential oscillation amplitude 4, while the concentration of the Ti atoms released from the antenna surface correlates with the variations of the I, ion current. The plasma contamination with titanium is believed to be due to the ion sputtering of the antenna components facing a rib of the separatrix (C and D in fig. 1). Visual examination revealed the greatest erosion of these areas. The average Ti atom velocity estimated from the Ti I Doppler broadening (- 6 x lo5 cm/s) is found to be close to typical ion sputtering values. To summarize, the experiments carried out on the URAGAN-3 torsatron show that there are two mechanisms inherent in RF plasma heating in the low
[l] Equipe TFR, in: Heating in Toroidal Plasmas, Proc. 3rd Joint Grenoble-Varenna Symp., Grenoble, 1982, Vol. 3 (CEC, Brussels, 1982) p. 1177. [2] E.D. Andryukhina, KS. Dyablin and 0.1. Fedyanin, Fiz. Plazmy 12 (1986) 898, in Russian. [3] V.S. Vojtsenya, S.I. Solodovchenko and V.I. Tereshin, Kharkov Inst. Phys. Techn. Preprint KFTI 81-24, Kharkov (1981). [4] L.I. Grigor’eva, A.V. Pashchenko, B.I. Smerdov and V.V. Chechkin, J. Nucl. Mater. 128 & 129 (1984) 317. [5] L.I. Grigor’eva, B.I. Smerdov and V.V. Chechkin, Kharkov Inst. Phys. Techn. Preprint KFTI 86-13 (TsNIIAtominform, Moscow, 1986). [6] A.A. Ivanov and V.V. Parail, Zh. Exp. Teor. Fiz. 62 (1972) 932, in Russian. [7] A.B. Kitsenko, V.I. Panchenko and K.N. Stepanov, Fiz. Plazmy 1 (1975) 268, in Russian. [8] L.I. Grigor’eva, B.I. Smerdov and V.V. Chechkin, K voprosy o vliyanii elektrostaticheskogo polya antenny na povedenie periferijnoj plazmy pri VCh nagreve: Obzor (TsNIIAtominform. Moscow, 1986) in Russian. [9] V.V. Bakaev et al., in: Plasma Physics and Controlled Nuclear Fusion Research 1984, Proc. 10th Int. Conf., London, 1984, Vol. 2 (IAEA, Vienna, 1985) p. 397. [lo] R.J. Roth and W.M. Krawczonek, Rev. Sci. Instr. 42 (1971) 589.
Journal
of Nuclear
Materials
162-164
(1989) 458-461
North-Holland.
A STUDY OF THE MECHANISMS OF METAL IMPURITY RELEASE HEATING IN THE URAGAN3 TORSATRON
Amsterdam
DURING ICRF
L.I. GRIGOR’EVA, V.G. KONOVALOV, N.I. NAZAROV, V.V. PLYUSNIN, G.N. POLYAKOVA, A.I. SKIBENKO, I.P. FOMIN, V.V. CHECHKIN, A.N. SHAPOVAL and O.M. SHVETS Kharkov Institute of Physics and Technology, Ukrainian SSR Academy
Key words:
impurity,
ICRF
Possible mechanisms with the URAGAN-3 harmonics produce an antenna surface (Ti) is pulse.
heating,
torsatron,
potential,
plasma
of Sciences, 310108 Kharkov,
USSR
sheath
responsible for metal impurity release during ICRF production and heating of the plasma are studied torsatron. It is shown that the edge plasma potential oscillations at the pumping frequency and its impurity influx from the surface of the helical winding casings (Fe, Cr). The impurity release from the caused by a quasisteady - 100 eV ion flux arising in the divertor magnetic flux region during the RF
In all experiments on ICRF and Alfvkn wave heating in tokamaks an enhanced impurity release is observed (see for example ref. [l]). This can result in essential radiation losses comparable with the total power input and eventual cessation of the heating if special measures are not taken (limiters and antenna side tiles made of graphite, wall carbonization). A similar phenomenon takes place in a stellarator [2]. The identification of possible mechanisms which are responsible for the enhanced impurity influx under RF heating conditions still remains an urgent problem. A possible reason for the edge plasma-surface interaction enhanced during RF heating is a sheath potential increase due to electron heating and hence, an increase in the energy of the ions impinging on the wall [3]. Moreover, some specific mechanisms inherent in RF plasma heating at low frequencies may exist which result in a steady and/or time-varying potential acquired by the edge plasma relative to the surface and thereby cause an additional flux of charged particles onto this surface. Edge plasma potential oscillations and RF current rectification in the plasma-antenna sheath due to plasma polarization by the antenna electrostatic field [4,5] and parametric excitation of electrostatic modes by the pumping wave [6,7] could be ascribed to these mechanisms. From numerous experiments on RF heating in tokamaks and stellarators [8] it may by concluded that some pecularities in the observed behaviour of the edge plasma can be explained by an additional potential
arising in the plasma. Faraday shielding does not eliminate these peculiarities completely. The object of this work is to examine the nature of mechanisms responsible for the release of metal impurities during RF production and heating of the plasma in the Uragan-3 (U-3) torsatron. Both oscillations in the plasma potential and the quasisteady flux of ions onto the antenna surface which arises during RF heating are regarded as possible mechanisms. The U-3 machine [9] is an I = 3 torsatron with 9 periods of the magnetic field helical winding [fig. l(a) and (b)]. R = 100 cm and the inner radius of the casings of the helical windings (1, 2, 3) is 19 cm. The idea of the behaviour of the magnetic field lines in the central region of the torus cross-section where closed magnetic surfaces are formed (average minor radius Z = 9 cm) and in the periphery where the magnetic divertor flux exists, is illustrated in fig. l(b). The rotational transform in the closed magnetic surface region varies from t(O) = 0.18 to t(a) = 0.4. The whole magnetic system is enclosed in a 70 m3 vacuum tank. The experiments were carried with a steady hydrogen inlet, pH = (1.4-3.0) X lo-’ Torr. The plasma was produced by exciting an ion cyclotron wave at f= 5.4 MHz. To couple RF power to the plasma two twisted frame-type antennae without Faraday shield were installed at a minor radius r, = 16 cm on the low field side. The voltage amplitude across the antenna was - 10 kV at a 300 kW power input. As shown in fig. l(a), one antenna (“distant antenna”:K-2)
459
LI. Grigor’eva et al. / h4echanisms of metal impurity release
:m @ j*
was installed at 130° toroidally from torus section 4 were probes were placed. The edge of the other antenna (.. near antenna”: K-l) was at a distance of 30 cm (- 15 o toroidally) from the section with the probes. A schematic view of the antenna and the instantaneous voltage r? distribution along its length are shown in fig. l(c) to illustrate the role of the antenna as a possible source of oscillations in the plasma potential. The antenna was installed along the rib of the separatrix so as to follow its shape. Typical plasma parameters during the quasisteady stage of the RF discharge were n - (2-3) x 10” cme3, Ti - (600-900) eV, Te - 250 eV in the central region and n - 10” cme3, T, - 10 eV in the edge.
:m 0
5
10
ms
Fig. 2. RF voltage (d), magnetic probe signal (i), capacitive probe signal (i), electric probe signal from the casing surface (i), electric probe signal from the spacing between the casings (I,).
--R +--
_-.
-- --__.__~,__---’ “CM Fig. 1. (a) Schematic view of the U-3 torsatron with some edge plasma diagonistics and antennae: 1, 2, 3 helical winding casings; 4 cross-section of the torus where the probes are placed; 5 monochromator; 6 microwave cavity. (b) Cross-section 4 of the torus with electric probes mounted normal to the casing surface (7), along the helical winding (8) and between the casings (I-V). Magnetic field lines which form closed magnetic surfaces in the central region and those which form the divertor flwr at the periphery are separated by the separatrix. (c) Schematic view of the antenna: parts C and D face the rib of the separatrix and have zero RF potential.
Oscillations in the edge plasma potential at the pumping frequency f and its harmonics were detected by a movable capacitive probe (CP) its insulating envelope diameter being 6 mm (see for example ref. [lo]). The charged particle flux onto the helical winding casing was detected with three earthed shielded electric probes (EPs). The EPs were mounted on the inner surface of the casing. One of them [7 in fig. l(b)] was directed normal to the surface, the other two (8) were parallel to the helical winding and were facing in opposite directions to each other. A set of 5 EPs (I-V in fig. l(b)) was installed radially between the casings along an arc with a minor radius of 23 cm to detect the charged particle flux in the divertor magnetic flux region. Impurity atom lines were detected with two monochromators: one sensed the radiation [Cr I (359.3 nm); Fe I (302.0 nm)] from the probe-carrying section 4, the other [S in fig. l(a)] faced the distant antenna region [Ti I (364.2 nm)]. The edge plasma density in the divertor magnetic flux region was estimated using an open microwave cavity installed on the outer side of the casing [6 in fig.
WI.
In fig. 2 typical oscillograms are shown of the RF antenna voltage (c), RF magnetic probe signal (d) indicating the ion cyclotron wave excitation, CP signal (i), charged particle current (r”) to one of the EPs mounted on the casing, and charged particle current (Z,) to one of the EPs placed between the casings. As seen in fig. 2, the excitation of intense potential oscillations starts at the onset of the discharge while the
460
L. I. Grigor’eua et al. / Mechanisms of metal impurity release
Fig. 3. RF voltage and edge plasma potential oscillations at 3 different positions of CP.
plasma density is still insufficient for the electromagnetic (ion cyclotron) wave excitation. Time variations of the edge plasma potential 6 are coherent oscillations at the pumping frequency f and its harmonics as has been observed earlier in simulation experiments [4,5]: The shape of the oscillations is position dependent. Three oscillograms of potential oscillations 6 taken at different CP positions together with antenna voltage c are shown by way of example in fig. 3. The amplitude of the potential oscillations excited by the distant antenna decreases along the radius (fig. 4: (1) onset of the discharge; (2) the end of the RF pulse). This means that the distant antenna excites oscillations which are transferred to the torus section discussed through the plasma core rather than the periphery. The value of the oscillation amplitude at the periphery (r - 15-20 cm) was 20-50 V with the distant antenna and 200 V with the near antenna energized. The potential oscillations in the edge plasma result in the electron and ion currents to the casing surface
01
5
4
10
15
,
r, cm
Fig. 4. Potential oscillation amplitude versus minor radius (measured along the major radius). 1: onset of the discharge; 2: end of the RF pulse. A dotted line indicates the casing inner radius (19 cm).
which oscillate at the pumping frequency and its harmonics and are phased with the potential oscillations. When RF power is launched by the distant antenna, the ion current amplitude is - 1 mA/cm’ as detected by the EP installed normal to the surface, and - 10 mA/cm* to each EP parallel to the helical winding. In the latter case the two currents are phased 1X0 o with respect to each other, indicating an appreciable contribution of the electric drift to these currents. Both the electron and the ion current increase by an order of magnitude as the near antenna is energized. The EPs I-V mounted between the casings detect a quasisteady ion current I, (see fig. 2). The I, distribution along the arc generally agrees with the density distribution in the divertor flux region [9]. The average ion energy as estimated from the ion current vs the retarding potential is - 100 eV though a considerable fraction of the ions have energy exceeding 300 eV. A specific feature of the I, current is that it rapidly decreases ( < 100 ps) when the RF pulse is switched off. The rise of a quasisteady ion flux on an earthed surface intersected by the divertor magnetic flux during an RF pulse is possibly caused by the rise of some quasisteady potential of positive polarity in the edge plasma. Such a potential can arise from the RF rectification in the edge plasma - antenna sheath as was mentioned above. To elucidate possible effects of the edge plasma potential oscillations and the I,, ion current on the
Fig. 5. Potential oscillation amplitude (G), Cr I intensity, ion current plasma
in the divertor region (I,,), Ti I intensity density (n) versus the hydrogen pressure. represent neutral atom concentrations.
and divertor Dotted lines
L. I. Grigor’eva et al. / Mechanisms of metal impurity release
461
frequency range which eventually result in impurity release. These are the RF plasma potential oscillations which give rise to the impurity influx from the casings of the magnetic field helical winding and the quasisteady ion flux with ion energy of >, 100 eV which arises in the divertor magnetic flux region and hits the antenna surface. Both the amplitude of the potential oscillations in the edge plasma and the ion current in the divertor magnetic flux region are at least an order of magnitude greater in the vicinity of the working antenna than at a distance from it.
Fig. 6. The same quantities as in fig. 5 versus the magnetic field
References
strength. impurity release, simultaneous measurements of these characteristics and the impurity atom line intensities versus the hydrogen pressure p (fig. 5) and confining magnetic field B (fig. 6) were performed. The dotted curves are Cr and Ti atom concentrations estimated by normalking the corresponding intensity to the edge plasma density n. It follows from the data shown that the concentration of the atoms released from the casing surface (Cr, Fe) correlates with the variations of the potential oscillation amplitude 4, while the concentration of the Ti atoms released from the antenna surface correlates with the variations of the I, ion current. The plasma contamination with titanium is believed to be due to the ion sputtering of the antenna components facing a rib of the separatrix (C and D in fig. 1). Visual examination revealed the greatest erosion of these areas. The average Ti atom velocity estimated from the Ti I Doppler broadening (- 6 x lo5 cm/s) is found to be close to typical ion sputtering values. To summarize, the experiments carried out on the URAGAN-3 torsatron show that there are two mechanisms inherent in RF plasma heating in the low
[l] Equipe TFR, in: Heating in Toroidal Plasmas, Proc. 3rd Joint Grenoble-Varenna Symp., Grenoble, 1982, Vol. 3 (CEC, Brussels, 1982) p. 1177. [2] E.D. Andryukhina, KS. Dyablin and 0.1. Fedyanin, Fiz. Plazmy 12 (1986) 898, in Russian. [3] V.S. Vojtsenya, S.I. Solodovchenko and V.I. Tereshin, Kharkov Inst. Phys. Techn. Preprint KFTI 81-24, Kharkov (1981). [4] L.I. Grigor’eva, A.V. Pashchenko, B.I. Smerdov and V.V. Chechkin, J. Nucl. Mater. 128 & 129 (1984) 317. [5] L.I. Grigor’eva, B.I. Smerdov and V.V. Chechkin, Kharkov Inst. Phys. Techn. Preprint KFTI 86-13 (TsNIIAtominform, Moscow, 1986). [6] A.A. Ivanov and V.V. Parail, Zh. Exp. Teor. Fiz. 62 (1972) 932, in Russian. [7] A.B. Kitsenko, V.I. Panchenko and K.N. Stepanov, Fiz. Plazmy 1 (1975) 268, in Russian. [8] L.I. Grigor’eva, B.I. Smerdov and V.V. Chechkin, K voprosy o vliyanii elektrostaticheskogo polya antenny na povedenie periferijnoj plazmy pri VCh nagreve: Obzor (TsNIIAtominform. Moscow, 1986) in Russian. [9] V.V. Bakaev et al., in: Plasma Physics and Controlled Nuclear Fusion Research 1984, Proc. 10th Int. Conf., London, 1984, Vol. 2 (IAEA, Vienna, 1985) p. 397. [lo] R.J. Roth and W.M. Krawczonek, Rev. Sci. Instr. 42 (1971) 589.