- Email: [email protected]
Capacitive probe measurements during RF heating on textor
Fusion Engineering and Design 12 (1990) 203-207 North-Holland
CAPACITIVE
203
PROBE MEASUREMENTS
R. VAN NIEUWENHOVE,
F. DURODIE,
Luboratoire
de Physique
- Lnboratorium
Associatie
Euratom-Belgische
des Plasmas Stoat,
Ecole
Royale
DURING
RF HEATING
ON TEXTOR
R. KOCH and G. VAN OOST uoor PlasmaJj&a,
Militaire
Association
- Koninklijke
Militaire
Yamagata
University,
Euralom-Etai School,
Beige
B-1040
Brussels,
Belgium
K. MATSUMOTO Department
of Electrical
Engineering,
Faculty
of Engineering,
Yonezawa
992. Japan
Measurements with a capacitive probe, capable of withstanding the high heat loads during auxiliary heating, have been performed on an ICRF heated tokamak. The amplitude of the signal at the generator frequency measured by the capacitive probe corresponds to values of a few hundreds volts peak to peak at 1 MW of ICRF power, thereby confirming earlier measurements, at lower power on the UBAGAN 3 [1,2,3] torsatron and on the RF test facility RFTF [4]. A theoretical model of the capacitive probe shows that these potential oscillations are probably related to ion Bernstein waves.
1. Introduction An important characteristic of ICRF heating on a tokamak is its effect on the near-wall plasma conditions. The direct interaction of the RF power with the edge plasma [S-7] leads to direct edge heating, density increases, profile modifications, and increases of physical sputtering. As these changes are by now rather well documented more effort is going toward the identification of the responsible mechanisms. One mechanism, which may have been overlooked in the past is the occurrence of plasma potential oscillations at the driving frequency of the generator, as measured by Grigor’eva et al. in 1984 [l] in a small glass tube, and later by the same group on the URAGAN 3 torsatron in 1988 [2]. In the last case, a calibrated capacitive probe was used to measure the potential oscillations. However, since the RF antenna used on URAGAN 3 was unshielded it remained unclear whether these strong potential oscillations (of the order of 200 V) would still exist when using the regular, Faraday shielded fast wave antenna. That this was indeed the case was demonstrated by Caughman et al. in 1989 [4] on an RF test
present day tokamaks. The present study extends these measurements to the case of a tokamak (TEXTOR) at RF power levels up to 4 MW.
2. Experimental
set-up
The RF power on TEXTOR is provided by two pairs of RF antennas, located toroidally at diametrically opposite positions.
In the poloidal
direction,
each antenna
extends from the outer midplane to 7S0 from the top of the machine. The capacitive probe is located at the top of an RF antenna (of pair Al), thus poloidally at the top of the machine, at 2 cm behind the radial position
of the protection
limiters,
i.e. at r = 50.5 cm. In
facility, again revealing potential oscillations as high as 300 V peak to peak. The capacitive probe measurements were in this case also confirmed by Langmuir probe measurements. These measurements were however carried out at a magnetic field of only 2 kG, which is an
the first phase of the experiments (phase I), the antennas at the location of the probe were not the usual Faraday shielded ICRF antennas such as those of pair A2; instead the two antennas of this pair had been transformed into electrostatic RF launchers [8]. This was accomplished by replacing the electrical short at the end of the central conductor by ceramic supports and by removing the Faraday shield as well. One purpose of this modification was to study direct Ion Bernstein’ Wave (IBW) launch on TBXTOR. In the second phase (phase II), these electrostatic antennas were again mod-
order of magnitude
ified to the regular Fast Wave (FW) antennas.
0920-3796/90/$03.50
lower
than the magnetic
field on
0 1990 - Elsevier Science Publishers B.V. (North-Holland)
R. Van Nieuwenhove et al. / Capacitive probe nreasuremenfs
204 2.1. Probe design
In previous designs of capacitive probes [g-12], the probe consisted of a metallic electrode, which is connected to the inner conductor of a coaxial line. The electrode, as well as the coaxial line are then surrounded by a glass tube. The outside surface of the glass tube is at the plasma floating potential, and the detecting electrode couples capacitively to this potential [9]. Such probes cannot be used on a tokamak because the high heat loads during auxiliary heating, or the thermal shocks caused by a major plasma disruption would easily break the insulating glass or ceramic of the probe. Therefore a new type of capacitive probe has been designed in which a compromise between physical and mechanical requirements has been realised. A schematic drawing of this probe is shown in fig. 1. The capacitor is formed by the plasma, a circular ceramic (A1,O1) disk and a circular conductor (the electrode) at 3 mm behind the surface of the ceramic disk. A metallic cylinder, connected to the shield of the coaxial tube, serves as a protection for the ceramic disk. The thickness of the ceramic disk has been chosen to obtain a probe capacitance (C = 0.22 pF) which is much smaller than the sheath capacitance between the probe and the plasma (C, = 6 pF), allowing sheath effects to be neglected. The input impedance of this probe is 24.95 kSZ (at Y = 29 MHz), thus being much larger than the 30 s2resistance on which the coaxial line is terminated. Therefore, only a small, known fraction of the voltage at the probe is detected. 2.2. Probe theory
Instead of using the terminology of “RF potential oscillations at the generator frequency” in evaluating
capacitive probe data [4,11] it is more appropriate, and less ambiguous, to talk about plasma waves. In a hot plasma only three types of waves come into consideration; the fast wave (F), the slow wave (S), and the ion Bernstein wave (B). Since the slow wave is evanescent in normal conditions, the ion Bernstein wave is the only propagating electrostatic wave in the system and it can therefore be expected that the electrostatic plasma potential oscillations are linked to this type of wave. To get an idea of the sensitivity of RF probe to the different waves, one can first determine the emission properties of such a probe, considering it as an RF launcher, and then by reciprocity [13] deduce the reception properties. A coupling model as described in ref. [14] has been applied to the particular geometry of the capacitive probe used in the experiments. In this model a uniform plasma is assumed, and the plasma description is only valid below the 2nd cyclotron harmonic. No further approximations were made. The model calculations show that the probe excites only (99.9%) Bernstein waves. This result is very similar to the case of an IBW waveguide launcher [14] where the plasma touches the waveguide mouth. Using reciprocity theorems it is in principle possible to obtain a relation between the measured probe voltage-sand the electric fields (instead of potentials) in the plasma. Note however that the model neglects the possible presence of evanescent SW fields generated at the nearby antenna Al. Since these calculations have not yet been completely carried out, we will in the following still use the calibrated voltage (RF amplitude) at the probe surface, while keeping in mind that, according to the coupling model, essentially Bernstein waves are detected. Until now however, no experimental testing of this result has yet been performed and therefore it cannot be completely ruled out that the probe also detects other wave field components. 3. Measurements 3. I. Measurements 3.1.1. Parametric
Fig. 1. Schematic drawing of the capacitive probe.
in phase I dependencies
A power scan in which only the electrostatic antenna pair (Al) is excited shows (fig. 2) an almost linear dependence of probe signal on power, reaching RF amplitudes of 125 V at 450 kW of applied power. Another power scan (over a much larger power range), in which only the FW antennas (A2) are excited, reveals (fig. 3) a linear dependence on power above P,, = 200 kW, when A2 is positioned at r= 47.5 cm, but a
R. Van Nieuwenhove
ef al. / Capacitive
probe
measurements
I
vRF[vl 125-
IOO-
120
240
.
380 450
RF power[kW]
f
0
14.5
29
43.5
56
Fig. 2. Dependence of the calibrated RF amplitude of the capacitive probe signal versus RF power for the electrostatic antenna pair Al (#35030, 35031, 35040, 35047). The plasma parameters are: ZP= 340 kA, B, = 2.4 T, ii, = 2X 10” cmd3, H/D ratio = 4.5%. The generator frequency is 29 MHz, and the two antennas (of a single pair) are fed in B phasing. Limiter positions, 46 cm; antenna position, 47 cm.
Fig. 4. Frequency spectrum of the capacitive probe signal for 300 kW on Al (#35735). The frequency peak at the generator frequency is saturated. The variation of the probe calibration factor with frequency has not been taken into account. The plasma parameters are: ZP= 343 kA, B, = 2.4 T, ii, = 1.57 X lOI cm-‘; other parameters as in fig. 2.
saturated dependence on power when A2 is at r = 48.5 cm. In the last case the amplitude of the signal is also much higher. Due to the large scatter in the probe data it was not possible to find clear systematic dependences of probe signal versus density, magnetic field, plasma current, etc.. . . The largest probe signals (using A2)
were obtained at low plasma current (Zn = 220-260 kA), with amplitudes which are 3-4 times higher than at In = 340 kA (at a given power level). No significant difference was however observed between In = 340 kA and Zn = 480 lcA. An important parameter seems to be the minority (H) concentration; an increase of this
v,,tvl
’
FREQUENCY
[MHz]
0
260-
'0 0
200-
/
/
/
/
------
0
r,=485cm
/'
/'
I
I
loo-
/I o ,/
I’
I’
,I’
r,,=47.5cm
x
4 I Ir
/-'
20-y
ti~ti0
@iI
loo0
lsbo
RF PowerLkWl
Fig. 3. Dependence of the calibrated RF amplitude of the capacitive probe signal versus power for two different antenna positions of A2 (rA2 =47.0 cm, #35549-35559; r,z = 48.5 cm, #35562-35570). The plasma parameters are: I,, = 340 kA, B, = 2.0 T, ii, = 3.1 x 10” cm-‘; other parameters as in fig. 2.
206
R. Van Nieuwenhooe
et al. / Capacitive
probe
measurements
concentration from 2% to 5% decreases the signal amplitude by a factor 3-4 (using A2). Surprisingly, experiments in which Al and A2 were excited in the same shot at comparable RF power revealed no significant difference in probe signal amplitude regardless of the very different type of excitation and very different emitter-probe distances.
PROBE
SIGNAL
3.1.2. Frequency spectrum
The frequency spectrum of the probe signal, when exciting the electrostatic antenna Al, contains, besides the peak at the generator frequency (0) also peaks at the harmonics and half integer harmonics (fig. 4). The other, smaller peaks in fig. 4 are probably related to parametric decay in the SOL of the fast wave (which is also excited by Al) into an ion Bernstein wave and an ion quasimode [16,17]. The amplitude of the 20 peak is typically 8-10% of the peak at the generator frequency, compared to only l-2% for the o/2 peak, taking into account the variation of the probe sensitivity with frequency. The harmonics, as the half harmonic, were found to be present over a wide range of plasma parameters. It is important to note here that at the position of the probe, w/2 < wci (oci is the ion cyclotron frequency of the main species)when the toroidal magnetic field is higher than 2 T, which is usually the case. Therefore, the w/2 signal is likely to correspond to a propagating slow wave, in agreement with the theory that these waves are parametrically excited in the edge of the plasma [15]. The frequency spectrum for the usual fast wave antenna (A2) is very similar and has already been described before [16]. 3.2. Measurements
in phase II
Since in phase II, two nearly identical pairs of FW antennas were used, it was possible to examine the toroidal propagation of the waves by exciting separately Al and A2. At low hydrogen concentrations (5 3%) RF amplitudes as high as 520 V were observed for Al (at 400 kW) and 120 V for A2 (at 1 MW),thus showing a significant toroidal decay of the waves. After hydrogen injection (H/D = 12%), but in otherwise similar conditions, much lower signals were observed (fig. 5); 66 V for Al (at 500 kW) and 6.8 V for A2 (at 500 kW). A limited power scan in which both antennas were excited simultaneously was performed under the following plasma conditions; B, = 2.26 T, Ir, = 470 kA, lid) = 3.0 X lOI cms3, H/D = 12%. The combined power of Al and A2 was varied between 2.5 MW (1200 kW on Al, 1400 kW on A2) and 4 MW (2020 kW on Al, 1960 kW on A2). In this power range no significant variation
.-r
I 0.4ooa
0.6000
A2
Al ;
:
: I
:
I
1.200
1.6W TIME
I 2.400
[Sl
Fig. 5. Comparisonof the capacitiveprobe signalat equal RF (500 kW) power on Al and A2 (#36758). The plasma conditions are: IP = 340 lc4, B, = 2.25 T, i&,,= 2.0~ 10” cmm3, H/D concentration=12%. The generator frequency is 32.5 MHz. Limiter position,46 cm; antenna position,47 cm.
of the probe signal (at the fundamental, the harmonics and w/2) was observed. The RF amplitude over this power range was about 145 V. Even at these high power levels, the amplitude of the frequency peak at o/2 was at least 40 dB lower than the peak at the generator frequency. It is therefore unlikely that this type of parametric decay can lead to significant heating in the edge of a tokamak. In all the experiments described above, no correlation between the measured RF amplitudes and impurity production or edge changes has been observed. The absence of such a correlation could however simply be due to the fact that the observed amplitudes at the probe location are not representative for the scrape-off layer as a whole.
4. Discussion
Experimental ion Bernstein wave studies with toroidal loop antennas [18] have shown that the ion Bernstein wave is only directly excited immediately below the harmonics of the cyclotron frequency of the plasma ion species. In all the experiments described in this paper the 2+, layer crosses the RF antenna at a few cm from the top, and therefore some direct IBW launch can, in principle, be expected. The physical
R. Van Nieuwenhooe
et al. / Capacitive
meaning of this coupling condition for IBW is that just below 20,~ the wavelength of the ion Bernstein wave becomes large enough to fulfill wavelength matching to the (large) exciting antenna. Such a good matching condition could be met near the antenna top which in normal FW conditions is crossed by the 2+-, layer. If, in the other hand, the excitation of IBW is caused by short scale length electric fields, such as fringing fields, or fields in the sheath surrounding the plasma exposed parts of the RF antenna, it is no longer necessary for w to be just below 20,-c, and IBW could then be excited over a much wider range of frequencies (or toroidal magnetic field values). This type of excitation mechanism of the IBW and the ensuing difference in launched k,, spectra, for which the capacitive probe has a different sensitivity, could explain the high probe signal amplitude when using the FW antenna as compared to the electrostatic antenna excitation.
References [l] L.1. Grigor’eva et al., Plasma potential formation under the influence of an external RF field and the effect of this potential on particle transport, J. Nucl. Mater. 128-129 (1984) 317-318. [2] L.I. Grigor’eva et al., A study of the mechanisms of metal impurity release during ICRF heating in the URAGAN-3 Torsatron, J. Nucl. Mater. 162-164 (1989) 458-461. [3] V.V. Chechkin et al., Edge plasma potential and associated ion fluxes to the surface during ICRF plasma production and heating in the URAGAN-3 Torsatron, Fu sion Engrg. Des. 12 (1990) 171-178, in these Proceedings. [4] J.B.O. Caughman et al., Ion energy and plasma measurements in the near field of an ICRF antenna, 8th Topical Conf. on RF power in Plasmas, May l-3, 1989, Irvine, California; and J.B.O. Caughman et al., Experimental evidence of increased electron temperature, plasma potential and ion energy near an ICRF antenna Faraday shield, Fusion Engrg. Des. 12 (1990) 179-183, in these Proceedings.
probe measurements
207
[5] A.S. Wan et al., The effect of ICRF on the ALCATOR C scrape-off layer plasma, J. Nucl. Mater. 162-164 (1989) 292-299. [6] R. Van Nieuwenhove, Influence of ion cyclotron resonance heating on the edge plasma of tokamaks, Doctoral Thesis, U.I.A. University of Antwerp, B 2610 Antwerp, April 1989. (71 G. Van Oost et al., ICRF/edge physics research on TEXTOR, invited paper, Fusion Engrg. Des. 12 (1990) 149170, in these Proceedings. [8] R. Koch et al., Study of coupling and edge effects during ICRF heating experiments using an electrostatic antenna on TEXTOR, Fusion Engrg. Des. 12 (1990) 15-23, in these Proceedings. [9] J.A. Schmidt, High impedance Langmuir probes, Rev. Sci. Instr. 39 (9) (1968) 1297-1299. [lo] J.R. Roth and W.M. Krawczonek, Paired comparison tests of the relative signal detected by capacitive and floating Langmuir probes in turbulent plasma from 0.2 to 10 MHz, Rev. Sci. Instr., 42 (5) (May 1971) 589-594. [ll] M. Yatsuuka et al., Measurement of rf potential in a magnetoplasma by a capacitive probe, Jpn. J. Appl. Phys. 24 (12) (1985) 1724-1725. [12] N. Benjamin, High-impedance capacitive divider probe for potential measurements in plasmas, Rev. Sci. Instr. 53(10) (1982) 1541-1543. [13] S.A. Schelkunoff and H.T. Friis, Antennas Theory and Practice, (John Wiley & Sons, New York, 1952) p. 299. [14] R. Koch et al., Full hot plasma ray tracing, waveguide coupling and application to ion Bernstein wave heating on JET, LPP-ERM/KMS Report No. 89, Brussels (1989). [15] SC. Chiu, Parametric decay of a fast wave to two electromagnetic slow waves at half the pump frequency, Phys. Fluids 31(11) (1988) 3295-3298. [16] R. Van Nieuwenhove et al., Parametric decay in the edge plasma of ASDEX during fast wave heating in the Ion Cyclotron Frequency range, Nucl. Fus. 28 (9) (1988) 1603-1609. 1171 R. Van Nieuwenhove et al., Observations of harmonics and parametric decay instabilities during ICRF heating on TEXTOR, Europ. Conf. Abstr. 12B (1988) 778. 1181 Y. Takase, et al., Study of directly launched ion Bernstein waves in a tokamak, Phys. Rev. Lett. 59(11) (1987) 12011204.
CAPACITIVE
203
PROBE MEASUREMENTS
R. VAN NIEUWENHOVE,
F. DURODIE,
Luboratoire
de Physique
- Lnboratorium
Associatie
Euratom-Belgische
des Plasmas Stoat,
Ecole
Royale
DURING
RF HEATING
ON TEXTOR
R. KOCH and G. VAN OOST uoor PlasmaJj&a,
Militaire
Association
- Koninklijke
Militaire
Yamagata
University,
Euralom-Etai School,
Beige
B-1040
Brussels,
Belgium
K. MATSUMOTO Department
of Electrical
Engineering,
Faculty
of Engineering,
Yonezawa
992. Japan
Measurements with a capacitive probe, capable of withstanding the high heat loads during auxiliary heating, have been performed on an ICRF heated tokamak. The amplitude of the signal at the generator frequency measured by the capacitive probe corresponds to values of a few hundreds volts peak to peak at 1 MW of ICRF power, thereby confirming earlier measurements, at lower power on the UBAGAN 3 [1,2,3] torsatron and on the RF test facility RFTF [4]. A theoretical model of the capacitive probe shows that these potential oscillations are probably related to ion Bernstein waves.
1. Introduction An important characteristic of ICRF heating on a tokamak is its effect on the near-wall plasma conditions. The direct interaction of the RF power with the edge plasma [S-7] leads to direct edge heating, density increases, profile modifications, and increases of physical sputtering. As these changes are by now rather well documented more effort is going toward the identification of the responsible mechanisms. One mechanism, which may have been overlooked in the past is the occurrence of plasma potential oscillations at the driving frequency of the generator, as measured by Grigor’eva et al. in 1984 [l] in a small glass tube, and later by the same group on the URAGAN 3 torsatron in 1988 [2]. In the last case, a calibrated capacitive probe was used to measure the potential oscillations. However, since the RF antenna used on URAGAN 3 was unshielded it remained unclear whether these strong potential oscillations (of the order of 200 V) would still exist when using the regular, Faraday shielded fast wave antenna. That this was indeed the case was demonstrated by Caughman et al. in 1989 [4] on an RF test
present day tokamaks. The present study extends these measurements to the case of a tokamak (TEXTOR) at RF power levels up to 4 MW.
2. Experimental
set-up
The RF power on TEXTOR is provided by two pairs of RF antennas, located toroidally at diametrically opposite positions.
In the poloidal
direction,
each antenna
extends from the outer midplane to 7S0 from the top of the machine. The capacitive probe is located at the top of an RF antenna (of pair Al), thus poloidally at the top of the machine, at 2 cm behind the radial position
of the protection
limiters,
i.e. at r = 50.5 cm. In
facility, again revealing potential oscillations as high as 300 V peak to peak. The capacitive probe measurements were in this case also confirmed by Langmuir probe measurements. These measurements were however carried out at a magnetic field of only 2 kG, which is an
the first phase of the experiments (phase I), the antennas at the location of the probe were not the usual Faraday shielded ICRF antennas such as those of pair A2; instead the two antennas of this pair had been transformed into electrostatic RF launchers [8]. This was accomplished by replacing the electrical short at the end of the central conductor by ceramic supports and by removing the Faraday shield as well. One purpose of this modification was to study direct Ion Bernstein’ Wave (IBW) launch on TBXTOR. In the second phase (phase II), these electrostatic antennas were again mod-
order of magnitude
ified to the regular Fast Wave (FW) antennas.
0920-3796/90/$03.50
lower
than the magnetic
field on
0 1990 - Elsevier Science Publishers B.V. (North-Holland)
R. Van Nieuwenhove et al. / Capacitive probe nreasuremenfs
204 2.1. Probe design
In previous designs of capacitive probes [g-12], the probe consisted of a metallic electrode, which is connected to the inner conductor of a coaxial line. The electrode, as well as the coaxial line are then surrounded by a glass tube. The outside surface of the glass tube is at the plasma floating potential, and the detecting electrode couples capacitively to this potential [9]. Such probes cannot be used on a tokamak because the high heat loads during auxiliary heating, or the thermal shocks caused by a major plasma disruption would easily break the insulating glass or ceramic of the probe. Therefore a new type of capacitive probe has been designed in which a compromise between physical and mechanical requirements has been realised. A schematic drawing of this probe is shown in fig. 1. The capacitor is formed by the plasma, a circular ceramic (A1,O1) disk and a circular conductor (the electrode) at 3 mm behind the surface of the ceramic disk. A metallic cylinder, connected to the shield of the coaxial tube, serves as a protection for the ceramic disk. The thickness of the ceramic disk has been chosen to obtain a probe capacitance (C = 0.22 pF) which is much smaller than the sheath capacitance between the probe and the plasma (C, = 6 pF), allowing sheath effects to be neglected. The input impedance of this probe is 24.95 kSZ (at Y = 29 MHz), thus being much larger than the 30 s2resistance on which the coaxial line is terminated. Therefore, only a small, known fraction of the voltage at the probe is detected. 2.2. Probe theory
Instead of using the terminology of “RF potential oscillations at the generator frequency” in evaluating
capacitive probe data [4,11] it is more appropriate, and less ambiguous, to talk about plasma waves. In a hot plasma only three types of waves come into consideration; the fast wave (F), the slow wave (S), and the ion Bernstein wave (B). Since the slow wave is evanescent in normal conditions, the ion Bernstein wave is the only propagating electrostatic wave in the system and it can therefore be expected that the electrostatic plasma potential oscillations are linked to this type of wave. To get an idea of the sensitivity of RF probe to the different waves, one can first determine the emission properties of such a probe, considering it as an RF launcher, and then by reciprocity [13] deduce the reception properties. A coupling model as described in ref. [14] has been applied to the particular geometry of the capacitive probe used in the experiments. In this model a uniform plasma is assumed, and the plasma description is only valid below the 2nd cyclotron harmonic. No further approximations were made. The model calculations show that the probe excites only (99.9%) Bernstein waves. This result is very similar to the case of an IBW waveguide launcher [14] where the plasma touches the waveguide mouth. Using reciprocity theorems it is in principle possible to obtain a relation between the measured probe voltage-sand the electric fields (instead of potentials) in the plasma. Note however that the model neglects the possible presence of evanescent SW fields generated at the nearby antenna Al. Since these calculations have not yet been completely carried out, we will in the following still use the calibrated voltage (RF amplitude) at the probe surface, while keeping in mind that, according to the coupling model, essentially Bernstein waves are detected. Until now however, no experimental testing of this result has yet been performed and therefore it cannot be completely ruled out that the probe also detects other wave field components. 3. Measurements 3. I. Measurements 3.1.1. Parametric
Fig. 1. Schematic drawing of the capacitive probe.
in phase I dependencies
A power scan in which only the electrostatic antenna pair (Al) is excited shows (fig. 2) an almost linear dependence of probe signal on power, reaching RF amplitudes of 125 V at 450 kW of applied power. Another power scan (over a much larger power range), in which only the FW antennas (A2) are excited, reveals (fig. 3) a linear dependence on power above P,, = 200 kW, when A2 is positioned at r= 47.5 cm, but a
R. Van Nieuwenhove
ef al. / Capacitive
probe
measurements
I
vRF[vl 125-
IOO-
120
240
.
380 450
RF power[kW]
f
0
14.5
29
43.5
56
Fig. 2. Dependence of the calibrated RF amplitude of the capacitive probe signal versus RF power for the electrostatic antenna pair Al (#35030, 35031, 35040, 35047). The plasma parameters are: ZP= 340 kA, B, = 2.4 T, ii, = 2X 10” cmd3, H/D ratio = 4.5%. The generator frequency is 29 MHz, and the two antennas (of a single pair) are fed in B phasing. Limiter positions, 46 cm; antenna position, 47 cm.
Fig. 4. Frequency spectrum of the capacitive probe signal for 300 kW on Al (#35735). The frequency peak at the generator frequency is saturated. The variation of the probe calibration factor with frequency has not been taken into account. The plasma parameters are: ZP= 343 kA, B, = 2.4 T, ii, = 1.57 X lOI cm-‘; other parameters as in fig. 2.
saturated dependence on power when A2 is at r = 48.5 cm. In the last case the amplitude of the signal is also much higher. Due to the large scatter in the probe data it was not possible to find clear systematic dependences of probe signal versus density, magnetic field, plasma current, etc.. . . The largest probe signals (using A2)
were obtained at low plasma current (Zn = 220-260 kA), with amplitudes which are 3-4 times higher than at In = 340 kA (at a given power level). No significant difference was however observed between In = 340 kA and Zn = 480 lcA. An important parameter seems to be the minority (H) concentration; an increase of this
v,,tvl
’
FREQUENCY
[MHz]
0
260-
'0 0
200-
/
/
/
/
------
0
r,=485cm
/'
/'
I
I
loo-
/I o ,/
I’
I’
,I’
r,,=47.5cm
x
4 I Ir
/-'
20-y
ti~ti0
@iI
loo0
lsbo
RF PowerLkWl
Fig. 3. Dependence of the calibrated RF amplitude of the capacitive probe signal versus power for two different antenna positions of A2 (rA2 =47.0 cm, #35549-35559; r,z = 48.5 cm, #35562-35570). The plasma parameters are: I,, = 340 kA, B, = 2.0 T, ii, = 3.1 x 10” cm-‘; other parameters as in fig. 2.
206
R. Van Nieuwenhooe
et al. / Capacitive
probe
measurements
concentration from 2% to 5% decreases the signal amplitude by a factor 3-4 (using A2). Surprisingly, experiments in which Al and A2 were excited in the same shot at comparable RF power revealed no significant difference in probe signal amplitude regardless of the very different type of excitation and very different emitter-probe distances.
PROBE
SIGNAL
3.1.2. Frequency spectrum
The frequency spectrum of the probe signal, when exciting the electrostatic antenna Al, contains, besides the peak at the generator frequency (0) also peaks at the harmonics and half integer harmonics (fig. 4). The other, smaller peaks in fig. 4 are probably related to parametric decay in the SOL of the fast wave (which is also excited by Al) into an ion Bernstein wave and an ion quasimode [16,17]. The amplitude of the 20 peak is typically 8-10% of the peak at the generator frequency, compared to only l-2% for the o/2 peak, taking into account the variation of the probe sensitivity with frequency. The harmonics, as the half harmonic, were found to be present over a wide range of plasma parameters. It is important to note here that at the position of the probe, w/2 < wci (oci is the ion cyclotron frequency of the main species)when the toroidal magnetic field is higher than 2 T, which is usually the case. Therefore, the w/2 signal is likely to correspond to a propagating slow wave, in agreement with the theory that these waves are parametrically excited in the edge of the plasma [15]. The frequency spectrum for the usual fast wave antenna (A2) is very similar and has already been described before [16]. 3.2. Measurements
in phase II
Since in phase II, two nearly identical pairs of FW antennas were used, it was possible to examine the toroidal propagation of the waves by exciting separately Al and A2. At low hydrogen concentrations (5 3%) RF amplitudes as high as 520 V were observed for Al (at 400 kW) and 120 V for A2 (at 1 MW),thus showing a significant toroidal decay of the waves. After hydrogen injection (H/D = 12%), but in otherwise similar conditions, much lower signals were observed (fig. 5); 66 V for Al (at 500 kW) and 6.8 V for A2 (at 500 kW). A limited power scan in which both antennas were excited simultaneously was performed under the following plasma conditions; B, = 2.26 T, Ir, = 470 kA, lid) = 3.0 X lOI cms3, H/D = 12%. The combined power of Al and A2 was varied between 2.5 MW (1200 kW on Al, 1400 kW on A2) and 4 MW (2020 kW on Al, 1960 kW on A2). In this power range no significant variation
.-r
I 0.4ooa
0.6000
A2
Al ;
:
: I
:
I
1.200
1.6W TIME
I 2.400
[Sl
Fig. 5. Comparisonof the capacitiveprobe signalat equal RF (500 kW) power on Al and A2 (#36758). The plasma conditions are: IP = 340 lc4, B, = 2.25 T, i&,,= 2.0~ 10” cmm3, H/D concentration=12%. The generator frequency is 32.5 MHz. Limiter position,46 cm; antenna position,47 cm.
of the probe signal (at the fundamental, the harmonics and w/2) was observed. The RF amplitude over this power range was about 145 V. Even at these high power levels, the amplitude of the frequency peak at o/2 was at least 40 dB lower than the peak at the generator frequency. It is therefore unlikely that this type of parametric decay can lead to significant heating in the edge of a tokamak. In all the experiments described above, no correlation between the measured RF amplitudes and impurity production or edge changes has been observed. The absence of such a correlation could however simply be due to the fact that the observed amplitudes at the probe location are not representative for the scrape-off layer as a whole.
4. Discussion
Experimental ion Bernstein wave studies with toroidal loop antennas [18] have shown that the ion Bernstein wave is only directly excited immediately below the harmonics of the cyclotron frequency of the plasma ion species. In all the experiments described in this paper the 2+, layer crosses the RF antenna at a few cm from the top, and therefore some direct IBW launch can, in principle, be expected. The physical
R. Van Nieuwenhooe
et al. / Capacitive
meaning of this coupling condition for IBW is that just below 20,~ the wavelength of the ion Bernstein wave becomes large enough to fulfill wavelength matching to the (large) exciting antenna. Such a good matching condition could be met near the antenna top which in normal FW conditions is crossed by the 2+-, layer. If, in the other hand, the excitation of IBW is caused by short scale length electric fields, such as fringing fields, or fields in the sheath surrounding the plasma exposed parts of the RF antenna, it is no longer necessary for w to be just below 20,-c, and IBW could then be excited over a much wider range of frequencies (or toroidal magnetic field values). This type of excitation mechanism of the IBW and the ensuing difference in launched k,, spectra, for which the capacitive probe has a different sensitivity, could explain the high probe signal amplitude when using the FW antenna as compared to the electrostatic antenna excitation.
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[5] A.S. Wan et al., The effect of ICRF on the ALCATOR C scrape-off layer plasma, J. Nucl. Mater. 162-164 (1989) 292-299. [6] R. Van Nieuwenhove, Influence of ion cyclotron resonance heating on the edge plasma of tokamaks, Doctoral Thesis, U.I.A. University of Antwerp, B 2610 Antwerp, April 1989. (71 G. Van Oost et al., ICRF/edge physics research on TEXTOR, invited paper, Fusion Engrg. Des. 12 (1990) 149170, in these Proceedings. [8] R. Koch et al., Study of coupling and edge effects during ICRF heating experiments using an electrostatic antenna on TEXTOR, Fusion Engrg. Des. 12 (1990) 15-23, in these Proceedings. [9] J.A. Schmidt, High impedance Langmuir probes, Rev. Sci. Instr. 39 (9) (1968) 1297-1299. [lo] J.R. Roth and W.M. Krawczonek, Paired comparison tests of the relative signal detected by capacitive and floating Langmuir probes in turbulent plasma from 0.2 to 10 MHz, Rev. Sci. Instr., 42 (5) (May 1971) 589-594. [ll] M. Yatsuuka et al., Measurement of rf potential in a magnetoplasma by a capacitive probe, Jpn. J. Appl. Phys. 24 (12) (1985) 1724-1725. [12] N. Benjamin, High-impedance capacitive divider probe for potential measurements in plasmas, Rev. Sci. Instr. 53(10) (1982) 1541-1543. [13] S.A. Schelkunoff and H.T. Friis, Antennas Theory and Practice, (John Wiley & Sons, New York, 1952) p. 299. [14] R. Koch et al., Full hot plasma ray tracing, waveguide coupling and application to ion Bernstein wave heating on JET, LPP-ERM/KMS Report No. 89, Brussels (1989). [15] SC. Chiu, Parametric decay of a fast wave to two electromagnetic slow waves at half the pump frequency, Phys. Fluids 31(11) (1988) 3295-3298. [16] R. Van Nieuwenhove et al., Parametric decay in the edge plasma of ASDEX during fast wave heating in the Ion Cyclotron Frequency range, Nucl. Fus. 28 (9) (1988) 1603-1609. 1171 R. Van Nieuwenhove et al., Observations of harmonics and parametric decay instabilities during ICRF heating on TEXTOR, Europ. Conf. Abstr. 12B (1988) 778. 1181 Y. Takase, et al., Study of directly launched ion Bernstein waves in a tokamak, Phys. Rev. Lett. 59(11) (1987) 12011204.