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Measurements with an ion-sensitive probe in stationary rf-discharges of stellarator configuration
Volume 70A, number 5,6
PHYSICS LETTERS
2Apr11 1979
MEASUREMENTS WITH AN ION-SENSITIVE PROBE IN STATIONARY RF-DISCHARGES OF STELLARATOR CONFIGURATION G. BOHM, B. KAMPMANN and H. SCHLUTER Institut für Experimentaiphysik II, Ruhr-Universitdt, Bochum, Germany Received 21 December 1978
An ion-sensitive probe of the Katsumata type is applied to rf generated plasmas in stellarator configuration (Wendeistein Ila). In the range of rf power <50 W CW the radial profiles of the ion temperature are compared with those of the electron temperature, electron density, rf pump field, and rf fields due to parametric decay.
Previous investigations [1,2] of rf generated plasmas have shown the occurrence of resonant cones and parametric decay in a stellarator configuration (Wila). In connection with these and related phenomena the radial distribution of the ion temperature is of particular interest. The present study is mainly concerned with the application of a simple ion-sensitive probe of the Katsumata type [3]. Results obtained in the range up to about 50 W CW of rf power display radial variations different from those observed via Doppler broadening of Ba+ impurities under somewhat different conditions [4] (higher rf power, higher toroidal field strength, lower pressure). The WIla device has been described previously [1]. The toroidal field is presently varied up to 0.4 T, the ~
piosma
_______
height
~guard
~.
L~ner
5 1OV
electrode
I
collector
x
-
reco~ier Fig. 1. Ion-sensitive probe according to Katsumata: (a) schema, (b) probe circuit. When measuring electron densities and electron temperatures the probe circuit is the same as for an ordinary double probe.
rotational transform being in the range 0—0.2. The working gas is hydrogen at pressures of 0.007 to 0.07 Pa. Stationary plasmas are generated by rf waves at 35 to 100 MHz, coupled into the device by means of a ring-shaped electrostatic electrode [1]. For the range of energy densities considered a small shielded ion-sensitive probe [3] was tested. An outer conductor (outer diameter = 2mm) shields off the electrons due to their small Larmor radii so that the inner conductor (diameter = 1 mm) receives ion currents. Most of the outer conductor is isolated against the plasma by a quartz tube (outer diameter = 3 mm). To make sure that the electrons are sufficiently shielded off, the inner conductor has been made continuously movable against the outer one and is drawn back until the electron current is reduced by more than five orders of magnitude. Typical distances between the ends of the inner and outer conductors are I to 1 5 mm The whole probe system is radially movable and if the inner conductor is pushed forward, it can be operated as an ordinary electrostatic double probe, too. Thus electron-density and electron-temperature profiles can be conveniently measured for the same conditions and positions. In addition, the radial distributions of rf electric fields were measured by means of small coaxial capacitive antennae. The electron-density and -temperature profiles and the ion-temperature profile are measured at the same toroidal position about 120 cm away from the coupling structure. Therefore, the radial distributions of these 413
Volume 70A, number 5,6
PIIYSICS LETTERS
1
2 April 1979
/
~‘
0
50
40
200-20
-~
/
~
Fig. 2. Radial profile of the ion-temperature measured with the ion-sensitive probe (f p = 0.026 Pa, t = 0.1 8).
90 MHZ,Prf
=
9C1AH~
30W,Btor
=
0.19 T,
L.~r0,i/’ 60
~
00
~_-_-~
20
0
-zU
-00
_—_-
5U
Fig. 4. Radial field distribution of the pump wave.
parameters can be directly compared. The small coaxial antenna used for measuring the radial distributions of the rf electric field strength had to be located at a different toroidal position, about 35 cm away from the coupler. For this reason shifts in the radial distribution structures of the plasma parameters and the electric field strength may occur. A typical result for the radial ion temperature distribution is shown in fig. 2. The ion temperature exhibits a maximum at about the center of the tube, not observed previously [4]. The absolute values close to the limiters (r 50 mm) are close to the values measured at the edges of the plasma by means of an electrostatic analyzer which favours ion motion along the magnetic field lines whereas the probe used here favours the perpendicular ion motion. The finer structures to be seen (fig. 2) are of the order of the accuracy of the measurements; however, it should be pointed out that they appear to .3
4
10 cm
2
eV
found to be slightly above the ion cyclotron frequency. Theclosely radial distribution thethe low-frequency mode resembles thatof of pump wave, decay fig. 6,
0 0 0 00
0 0
4
60
40
20
0
-20
-40
-60
mm
Fig. 3. Radial profiles of the relative electron density and deetron temperature measured with the Katsumata probe when operated as usual double probe.
414
be rather reproducible and that the overall radial distribution appears to be correlated to the rf field distribution of the upper decay mode shown below. Curves of measurements in the geometrical shadow of the limiter are dashed. With extended inner conductor the probe is operated to yield axial distributions of electron density and electron temperature. The results are depicted in figs. 3a,b. For the range of parameters considered here, the electron temperature does not exhibit a distinct minimum at the center either [4]. Fig. 3a shows the distribution of the electron density. This plasma parameter has a typical double peak structure. The distribution of electron density may be understood in view of the distribution of the plasma-generating rf field as demonstrated by fig. 4. The field structure obviously reflects the occurrence of resonance cones in the lower-hybrid regime, the peak densities being limited by the lower-hybrid densities. For rf powers surpassing threshold values in the order of 10W high-frequency and corresponding low-frequency side bands are observed, indicative of parametric decay. Fig. 5 shows a typical decay spectrum; the peak frequency of the lower-frequency spectrum is always
whereas for the conditions investigated so far (rf power range < SOW, Btor <0.4 T) and particularly for the high-pressure values (0.007—0.07 Pa) the character of the high-frequency decay product is different, exhibiting no cone structure (fig. 7). Similar observations have been made for different values of t in the range 0.12< t <0.18 and for a pump frequency of 60 MHz.
Volume 70A, number 5,6
PHYSICS LETTERS
2 April 1979
a) -a
a)
a-
-a.
E
0 a
MHz 86
88
92
MHz
94
2
4
6
(a)
8
10
(b)
Fig. 5. Decay spectrum: (a) pump wave and high-frequency sideband, (b) low-frequency spectrum.
C) 00
2
fo23MHz
F 00
S 60
1.0
20
0
-20
-40
-60
,
mm
// / 60
1.0
20
0
-20
-40
-60
mm
Fig. 6. Radial field distribution of the low-frequency decay mode,
Fig. 7. Radial field distribution of the high-frequency sideband mode.
As comparison with fig. 2 demonstrates, the radial field distribution of the higher-frequency decay mode is of a character similar to that of the ion temperature suggesting a particular influence of this mode on the resulting ion temperature. As mentioned above, the small shifts of profiles against each other may be attributed to the different toroidal positions. Integrations of the higher-frequency decay mode versus frequency yield rf powers contained in this spectrum locally up to 15% of the power contained in the pump spectrum. So far the applicability of the probe measurements is mainly given by the thermal load of the probe material; current efforts are directed towards the construction of probes using different materials, like molybdenum, and towards pulse operation so that an extended range of energy density may become accessible. Rf field measurements at higher rf powers and
lower pressures indicate a transition to a cone-type radial distribution even of the high-frequency decay mode. Preliminary ion-temperature measurements reveal a corresponding development of double peaked radial profiles for these conditions, too. References [1] P. Javel, G. MUller, U. Weber and R.R. Weynants, Report 1pP2/229 (1976).
[21 P. Javel, G. MUller, U. Weber and R.R. Weynants, Plasma Phys. 18(1976)51. Katsumata, Ion sensitive probe, Symp. Probe measurements in gaseous plasmas (Institute ical Research, Tokyo, Japan, 1968).of Physical and Chem[4] P. Javel, G. MUller, AvH. van Oordt, U. Weber and R.R. Weynants, Proc. 7th European Conf. on Controlled fusion and plasma physics, Vol. 1(1975) p. 147.
[31 I.
415
PHYSICS LETTERS
2Apr11 1979
MEASUREMENTS WITH AN ION-SENSITIVE PROBE IN STATIONARY RF-DISCHARGES OF STELLARATOR CONFIGURATION G. BOHM, B. KAMPMANN and H. SCHLUTER Institut für Experimentaiphysik II, Ruhr-Universitdt, Bochum, Germany Received 21 December 1978
An ion-sensitive probe of the Katsumata type is applied to rf generated plasmas in stellarator configuration (Wendeistein Ila). In the range of rf power <50 W CW the radial profiles of the ion temperature are compared with those of the electron temperature, electron density, rf pump field, and rf fields due to parametric decay.
Previous investigations [1,2] of rf generated plasmas have shown the occurrence of resonant cones and parametric decay in a stellarator configuration (Wila). In connection with these and related phenomena the radial distribution of the ion temperature is of particular interest. The present study is mainly concerned with the application of a simple ion-sensitive probe of the Katsumata type [3]. Results obtained in the range up to about 50 W CW of rf power display radial variations different from those observed via Doppler broadening of Ba+ impurities under somewhat different conditions [4] (higher rf power, higher toroidal field strength, lower pressure). The WIla device has been described previously [1]. The toroidal field is presently varied up to 0.4 T, the ~
piosma
_______
height
~guard
~.
L~ner
5 1OV
electrode
I
collector
x
-
reco~ier Fig. 1. Ion-sensitive probe according to Katsumata: (a) schema, (b) probe circuit. When measuring electron densities and electron temperatures the probe circuit is the same as for an ordinary double probe.
rotational transform being in the range 0—0.2. The working gas is hydrogen at pressures of 0.007 to 0.07 Pa. Stationary plasmas are generated by rf waves at 35 to 100 MHz, coupled into the device by means of a ring-shaped electrostatic electrode [1]. For the range of energy densities considered a small shielded ion-sensitive probe [3] was tested. An outer conductor (outer diameter = 2mm) shields off the electrons due to their small Larmor radii so that the inner conductor (diameter = 1 mm) receives ion currents. Most of the outer conductor is isolated against the plasma by a quartz tube (outer diameter = 3 mm). To make sure that the electrons are sufficiently shielded off, the inner conductor has been made continuously movable against the outer one and is drawn back until the electron current is reduced by more than five orders of magnitude. Typical distances between the ends of the inner and outer conductors are I to 1 5 mm The whole probe system is radially movable and if the inner conductor is pushed forward, it can be operated as an ordinary electrostatic double probe, too. Thus electron-density and electron-temperature profiles can be conveniently measured for the same conditions and positions. In addition, the radial distributions of rf electric fields were measured by means of small coaxial capacitive antennae. The electron-density and -temperature profiles and the ion-temperature profile are measured at the same toroidal position about 120 cm away from the coupling structure. Therefore, the radial distributions of these 413
Volume 70A, number 5,6
PIIYSICS LETTERS
1
2 April 1979
/
~‘
0
50
40
200-20
-~
/
~
Fig. 2. Radial profile of the ion-temperature measured with the ion-sensitive probe (f p = 0.026 Pa, t = 0.1 8).
90 MHZ,Prf
=
9C1AH~
30W,Btor
=
0.19 T,
L.~r0,i/’ 60
~
00
~_-_-~
20
0
-zU
-00
_—_-
5U
Fig. 4. Radial field distribution of the pump wave.
parameters can be directly compared. The small coaxial antenna used for measuring the radial distributions of the rf electric field strength had to be located at a different toroidal position, about 35 cm away from the coupler. For this reason shifts in the radial distribution structures of the plasma parameters and the electric field strength may occur. A typical result for the radial ion temperature distribution is shown in fig. 2. The ion temperature exhibits a maximum at about the center of the tube, not observed previously [4]. The absolute values close to the limiters (r 50 mm) are close to the values measured at the edges of the plasma by means of an electrostatic analyzer which favours ion motion along the magnetic field lines whereas the probe used here favours the perpendicular ion motion. The finer structures to be seen (fig. 2) are of the order of the accuracy of the measurements; however, it should be pointed out that they appear to .3
4
10 cm
2
eV
found to be slightly above the ion cyclotron frequency. Theclosely radial distribution thethe low-frequency mode resembles thatof of pump wave, decay fig. 6,
0 0 0 00
0 0
4
60
40
20
0
-20
-40
-60
mm
Fig. 3. Radial profiles of the relative electron density and deetron temperature measured with the Katsumata probe when operated as usual double probe.
414
be rather reproducible and that the overall radial distribution appears to be correlated to the rf field distribution of the upper decay mode shown below. Curves of measurements in the geometrical shadow of the limiter are dashed. With extended inner conductor the probe is operated to yield axial distributions of electron density and electron temperature. The results are depicted in figs. 3a,b. For the range of parameters considered here, the electron temperature does not exhibit a distinct minimum at the center either [4]. Fig. 3a shows the distribution of the electron density. This plasma parameter has a typical double peak structure. The distribution of electron density may be understood in view of the distribution of the plasma-generating rf field as demonstrated by fig. 4. The field structure obviously reflects the occurrence of resonance cones in the lower-hybrid regime, the peak densities being limited by the lower-hybrid densities. For rf powers surpassing threshold values in the order of 10W high-frequency and corresponding low-frequency side bands are observed, indicative of parametric decay. Fig. 5 shows a typical decay spectrum; the peak frequency of the lower-frequency spectrum is always
whereas for the conditions investigated so far (rf power range < SOW, Btor <0.4 T) and particularly for the high-pressure values (0.007—0.07 Pa) the character of the high-frequency decay product is different, exhibiting no cone structure (fig. 7). Similar observations have been made for different values of t in the range 0.12< t <0.18 and for a pump frequency of 60 MHz.
Volume 70A, number 5,6
PHYSICS LETTERS
2 April 1979
a) -a
a)
a-
-a.
E
0 a
MHz 86
88
92
MHz
94
2
4
6
(a)
8
10
(b)
Fig. 5. Decay spectrum: (a) pump wave and high-frequency sideband, (b) low-frequency spectrum.
C) 00
2
fo23MHz
F 00
S 60
1.0
20
0
-20
-40
-60
,
mm
// / 60
1.0
20
0
-20
-40
-60
mm
Fig. 6. Radial field distribution of the low-frequency decay mode,
Fig. 7. Radial field distribution of the high-frequency sideband mode.
As comparison with fig. 2 demonstrates, the radial field distribution of the higher-frequency decay mode is of a character similar to that of the ion temperature suggesting a particular influence of this mode on the resulting ion temperature. As mentioned above, the small shifts of profiles against each other may be attributed to the different toroidal positions. Integrations of the higher-frequency decay mode versus frequency yield rf powers contained in this spectrum locally up to 15% of the power contained in the pump spectrum. So far the applicability of the probe measurements is mainly given by the thermal load of the probe material; current efforts are directed towards the construction of probes using different materials, like molybdenum, and towards pulse operation so that an extended range of energy density may become accessible. Rf field measurements at higher rf powers and
lower pressures indicate a transition to a cone-type radial distribution even of the high-frequency decay mode. Preliminary ion-temperature measurements reveal a corresponding development of double peaked radial profiles for these conditions, too. References [1] P. Javel, G. MUller, U. Weber and R.R. Weynants, Report 1pP2/229 (1976).
[21 P. Javel, G. MUller, U. Weber and R.R. Weynants, Plasma Phys. 18(1976)51. Katsumata, Ion sensitive probe, Symp. Probe measurements in gaseous plasmas (Institute ical Research, Tokyo, Japan, 1968).of Physical and Chem[4] P. Javel, G. MUller, AvH. van Oordt, U. Weber and R.R. Weynants, Proc. 7th European Conf. on Controlled fusion and plasma physics, Vol. 1(1975) p. 147.
[31 I.
415