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Influence of the alfvén wave spectrum on the scrape-off layer of the tca tokamak
270
Journal
INFLUENCE OF THE ALFVkN OF THE TCA TOKAMAK
of Nuclear
Materials 162- 164 (1989) 270-275 North-Holland, Amsterdam
WAVE SPECTRUIW ON THE SCRAPEOFF
LAYER
Y. MARTIN and Ch. HOLLENSTEIN Cenrre de Recherches en Physique des Plasmas, Association Euratom - Con@d&ation Suisse. Ecole Po~techniq~ Fpdprnle de ibsanne, CH-1007 Latrsanne, Switzerland
Key words:
TCA tokamak,
Alfvvtn wave, scrape-off
layer
The study of the scrape-off layer (SOL) during Alfven wave heating may lead antenna-plasma interaction. The scrape-off layer of the TCA tokamak has been widely
to a better understanding of the investigated by means of Lan~uir
probes. The aim of this work is to present measurements on the infhrence of the Alfven wave spectrum on the scrape-off layer. These experiments have shown that the plasma boundary layer is strongly affected by the wave field, in particular the ion saturation current and the floating potential. In TCA, as the spectrum evolves due to a density rise, the passage of the AlfvCn continua and their associated eigenmodes, the Discrete Alfven Wave (DAW) induces a strong depletion in the edge density of up to 70% during the continuum part and a density increase during the crossing of an eigenmode. The floating potential becomes negative during the continua and even more negative crossing the eigenmodes. In case of MHD mode activity, this behaviour changes for power exceeding 100 kW. The profiles of basic parameters are modified, depending on the wave spectrum. MHD mode activity which can occur during the RF (radio frequency) phase considerably alters the behaviour mentioned above. Finally, the modulation of the RF power allows us to characterize the coupling between RF power and typical edge parameters.
In fusion research, tokamaks are a possibility for obtaining reactor conditions. In most present day tokamaks, high-frequency additional heating schemes are installed in order to achieve high-temperature plasmas. Among the different types of waves used to heat tokamak plasmas, Alfv&r wave heating is an interesting candidate due to its potentially high efficiency and low cost. The antennae of most RF heating schemes are placed in the scrape-off layer. Heavily shielded antennae structures in the SOL are used in these experiments to reduce the antennae-pl~ma interaction. Even so, the plasma boundary interacts with the wave field, and moreover, the behaviour of this boundary plasma may be connected with the efficiency of the wave heating. In AlfvCn wave heating, no complete shielding of the antennae structure has been usually used so far. In the TCA tokamak no shielding of the antenna structure is used which leads to strong ~tennae-plasma interaction. Therefore, a good knowledge of the plasma boundary
can help to understand the coupling between the wave and the core plasma. Interactions of the RF power with the scrape-off layer are not yet as well documented as other aspects of additional heating. However, the impurity fluxes and confinement time problems may have their source in the scrape-off layer and therefore the study of this region is important. The actual great interest in the SOL behaviour underlines that this field is not completely covered and a global view of the RF interaction with the SOL is necessary. Furthermore, there is a great lack of theories dealing with RF-SOL interactions which actually makes it difficult to fully understand the various experimental results. One of the main aims of the TCA tokamak is the study of the AlfvCn wave heating (AWH). The scrape-off layer of the TCA tokamak has been widely investigated by means of Langmuir probes [1,2]. This diagnostic gives information on the principal characteristics of the SOL such as ion density, plasma potential and electron temperature. In this paper, we present results on the influence of the RF wave field on the scrape-off parameters, due to unshielded antenna structure.
0022-3115/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
271
Y. Martin, Ch. Hollenstein / Influence of Alfukn wave spectrum on the SOL 2. Experiment The TCA is a tokamak with the following parameters: R, = 0.61 m, a = 0.18 m, B, I 1.5 T. The typical densities and temperatures for the plasma core are n, = 3.0 x 1019/m3, T” = 500 eV and, for the plasma T, = 15 eV respectively. boundary n e = 2.0 x 10”/m3, The working gas is H, or D, with plasma currents of up to 130 kA, which correspond to a cylindrical safety factor qcy, = 3. The vacuum vessel is made of stainless steel and, in all the results presented here, carbon limiters are used covering approximately 70% of the poloidal circumference. The Langmuir probes are located in the outer equatorial midplane, about toroidally opposite to the limiter. The probes are made out of molybdenum wire, 0.5 mm in diameter and 5 mm long. Four of these probes are mounted on one probe shaft to form an array. The spacing between probes is 5 mm. This probe shaft can be moved in order to radially scan the plasma boundary. This experimental set-up allowed us to measure at the same time and probe position, the ion saturation current and the floating potential. A current probe with a frequency bandwidth of up to 50 MHz was used to measure the currents drawn by the probe. The RF wave field is produced by eight groups of antennae which are spaced regularly around the torus, four on the top and four on the bottom, three centimeters behind the main limiters [3]. Each group consists out of six stainless steel bars, fed in parallel and covered with 6-7 pm of titanium nitride (TiN). For all the presented experiments, no antenna screens or shielding has been used. The RF generator frequency was typically in the range between 2.0 and 2.5 MHz and RF power of the order of up to 250 kW has been delivered to the plasma. All the antennae are electrically floating with respect to the vacuum vessel. The poloidal currents, driven by the above-mentioned antenna bars, create a oscillating toroidal field. This field induces a pressure modulation which has a strong coupling to the shear AlfvCn wave. Taking account of the cyclotronic effects, the shear Alfven wave can be excited if the following dispersion relation is fulfilled:
200 bll hnl 150 -
(2.11
D4w
Fig. 1. The antennae loading reveals the Alfv& wave spectrum as a function of the central density. The excitation structure is N = 2, M = 1 and the following (n,m)modes are generated in the plasma: (2, - 1) (2,0) (2.1) (2,2). the radius with the mass density p and the safety factor 9:
where n and rn are the toroidal and poloidal mode numbers [3]. At a fixed frequency as imposed by the generator, the position of the resonant layer in the plasma depends on the mass density and the safety factor. During a density rise, the resonant layer of the shear Alfven waves (continua) originally generated at the centre is moving towards the plasma edge. Peaking on the antenna loading curve, such as shown in fig. 1, reveal the presence of other modes. These modes are called discrete Alfven waves (DAW) and they correspond to global eigenmodes which are the low-frequency counterpart of the ion cyclotron wave and have their origin in toroidal effects. These modes are superimposed on the continua, resulting in a spectrum as has been shown in previous papers on Alfven waves studies in TCA [4]. The layout of the antennae structure around the torus and the possibility of shifting individually the relative phase of the RF currents of each group of antennae allows us to excite waves with precisely defined toroidal and poloidal mode numbers (N,M). Due to toroidal effects, the exciting mode (N,M) can generate some additional modes. In fig. 1, we show the multiple modes generated by a (2,l) launching structure as a function of the central mean density.
3. Results where v, represents the Alfven velocity of the wave. Using periodic cylindrical geometry to describe the toroidal plasma, the frequency w varies as a function of
In the scrape-off layer, the Alfven wave field imposes a strong perturbation in the ion saturation current
272
Y. Martin, Ch. Hoilenstein
/ Influence of At@& wave spectrum on the SOL
and in the floating potential. Because of the accompanying density rise with the RF wave field, a shear Alfven wave appears within the plasma and the density in the scrape-off layer decreases within a few milliseconds as shown in fig. 2. The density stays at this low level during the presence of the continuum. Depending on the conditions such as RF power, excitation modes and MHD mode activity, a density depression of up to 70% of the ohmic value can be observed. During this time, the floating potential decreases continuously in time as long as the resonant layer moves towards the edge, and reaches about -30 V. As one of the global modes enters the plasma, the ion saturation current in the scrape-off layer increases again to the ohmic value. In some cases, even higher than ohmic values could be observed. The floating potential, during this period, becomes strongly negative. Values of up to -60 V can be measured for probe positions near to the limiters. The next continuum induces again an ion saturation current depletion in the plasma boundary. The different behaviour of the floating potential may be due to changes of the core plasma interactions with the wave field expressed through an observed higher antennae loading and by the fact that the previous resonant layer reaches the edge and therefore induces a change in the coupling. The total density decrease of the TCA scrapeoff layer during the continuum represents about 10” particles which probably enter the plasma core in less than 4 ms. This yields a particle flux in the order of 1Or5 particles per second per square centimeter. These numbers are deduced in considering that the whole plasma boundary situated between limiter radius and antennae
1 JSAT [au.1
I..
Fig. 2. Time dependent evolution of: (a) the floating potential; (b) the ion saturation current; (c) the antennae loading and the central line average density.
DAW
o ILimiter v 0
I ‘O
- 20 Positron
Ante&& -30 [mm1
Wall 40
E
:
Fig. 3. Ion saturation current profile in the scrape-off layer for: ohmic standard conditions; - - continuum Alfven wave field spectrum the upper curve with MHD mode activity, and the lower curve without; * * - - - - global eigenmode, without MHD mode activity.
radius loses 70% of its ohmic density. However, this number is not sufficient to explain the observed central density rise of the core plasma. It represent only roughly 10% of the total density increase. During the density increase due to the appearance of a new global mode, the outward flux into the scrape-off layer is estimated to be of the same order. The usual interpretation of the Langmuir currentvoltage characteristic to obtain the electronic temperature is only valid if the electrons are in a maxwellian distribution. We found that it is the case during the ohmic heating phase, but is surely not the case when the AlfvCn wave field perturbs the edge plasma. Phenomena like possible driven electron beam may lead to a nonmaxwellian distribution of the electrons [l]. The usual treatment of the Langmuir characteristic during the AlfvCn wave heating may therefore lead to faulty measurements of the electron temperature. Additional information on the plasma boundary should be obtain to clarify the existence of the RF driven electron beam and to understand its important role in the RF-SOL interaction. The ion saturation current profile is presented in the fig. 3. For probe location behind he antennae structure, we always found small ion saturation current. In addition at this location only small variations due to the changes in the AlfvCn wave spectrum have been detected. This is probably due to the fact that the antennae behave like back-up limiters. These ion saturation current variations induced by the wave field are most pronounced in front of the antennae, as shown in fig. 3,
213
Y. Martin, Ch. Hollenstein / Influence of A&&n wave spectrum on the SOL 2 ;;” b=f I, s r. I- l-
0 .
.
.
3
I ;; o___-----_____ 0 l l L! +
-1
--------
.’
c);f
. 0
50 RF
I 100 Power
I
2
150 [kWl
Fig. 5. Variation of ion saturation current between two times (75 ms and 105 ms) as a function of the RF power. The conditions are: I, = 107 kA and MHD mode activity present. Fig. 4. The time-dependent evolution of the antennae loading (a) and the floating potential (b) showing the sensitivity of the scrape-off layer to the small irregularities on the loading curve. (c) shows the constant relative phase delay between the antenna loading and the floating potential.
where interactions with the wave field and the scrape-off layer are strongest. The data taken during the (2,0) continuum reveal two cases. The first shows ion saturation currents with higher values than preceding ohmic ones. These data come from shots where strong MHD mode activity induced by the RF wave field is present. The second case corresponds to data of shots without any MHD mode activity during the entire discharge. In the TCA tokamak, during the Alfven wave heating, some oscillations are seen on the antenna loading curve and on some other diagnostics such as magnetic coils just after the passage of a global eigenmode. Such oscillations are attributed to an inward propagation of the kinetic Alfven wave and to its consequent possible radial standing waves. These waves could change the antennae-plasma coupling as shown by Hasegawa and Chen [5]. Further related experimental data from TCA are discussed in ref. 6. Our measurements in fact reveal these oscillations with a constant relative phase delay as shown in fig. 4 (c). That proves that the edge plasma is very sensitive to fine-scale RF induced effects which take place in the plasma core. It is well known that the presence of MHD mode activity usually increases the density in the scrape-off layer. In TCA we observe this phenomenon in ohmic conditions and during AWH as seen on fig. 5. More precisely, we find, with a plasma current of 107 kA, two types of behaviour of the scrape-off layer showing the competition between RF wave field and mode activity
depending on the RF power in the antennae. For powers above 100 W, the mode activity induced by the RF wave field is strongly established and the density continuously increases. For powers lower than 100 kW the density decreases, slightly influenced by the mode activity. This is illustrated in fig. 5, where we show the difference of the density taken at two times in the RF pulse as a function of the power delivered to the antennae. We show in fig. 6 the density and the floating potential as a function of the power delivered to the antennae, for shots without any modes. Collins et al. [3] observed a non-constant antenna loading curve as a
Ion Saturation
Current
.
. Floating
Potential .
x\x, Antenna x\\
0
20 RF
40 Power
Loadmg X-X
60
sb
1’ 10
[kWl
Fig. 6. Ion saturation current, floating potential and antennae loading during continuum as functions of the RF power, in standard conditions (without MHD modes). For the first two, we subtract the ohmic value from the continuum value.
274
Y. Martin, Ch. Hollenstein
/ Influence
function of the RF power in TCA without any lateral screen. They considered the following explanation: the scrape-off layer, interacting with the local wave field of the antennae, provides a sink for the direct dissipation of the antenna current. The RF power modifies the SOL, in particular it decreases its density. Once the edge plasma density is swept away, the lack of the dissipative medium damps the effects for direct dissipation of the currents and leads to a saturation-like behaviour. Our results seems to be in agreement with this model since we can see a similar type of behaviour in the important edge parameters. However, further experiments in order to complete the set of data on a wide range of power are necessary and underway. Recently, some experiments were carried out in the TCA tokamak to study the dynamic response of the plasma [7]. The aim of this type of experiment is to measure the reaction of the plasma columns to an external pulsed force. The dynamic response of a system is completely characterized by the transfer function. The transfer function can be obtained from the perturbating input signal and the resulting output signal of the system. The transfer function is entirely described by a gain and a phase. As one of the many possible external perturbations on TCA, the RF power has been chosen. In this present study, the applied RF power was
ofA&I&I
wuve spectrum on the SOL
modulated at the frequency of about 500 Hz with an amplitude of 10%. Information on the plasma boundary response to the AlfvCn wave heating is obtained from the transfer function between the RF input signal and the Langmuir probe data. The calculated gain and phase of the transfer function from the PF power as input and of the ion saturation current as output, is shown in fig. 7. This result shows a great difference in the gain during the continuum and the discrete wave spectrum phase of the RF pulse. This implies that there is a strong interaction between the applied RF wave field and the scrape-off layer density during the continuum. However, when a global mode enters the plasma, the gain is less important and therefore the coupling to the edge density is less efficient. The application of this modulation techniques may be therefore a powerfui tool in order to understand more about the different and complicated SOL-RF antenna interactions. Moreover, this type of experiment may lead to a description of the energy deposition in the SOL.
4. Conclusion All the measurements reveal a strong interaction between the wave field and the scrape-off layer. The presented data clearly show the important changes in the behaviour of the plasma boundary, depending on the Alfven wave spectrum. The measurements also showed that the SOL is very sensitive to RF phenomena occurring in the hot core plasma. The description of the SOL during intense RF heating is therefore not only determined by the direct antenna-SOL interactions but also by the RF induced effects of the heated plasma core. We find an important decrease of the edge density due to strong RF interaction during the continuum part of the spectrum. The boundary plasma density can be very strongly modified by the RF power, the modulation of the RF power and the mode activity.
Acknowledgements We wish to thank the whole TCA team for its excellent support. The present work was partially supported by the Fond National Suisse de la Recherche Scientifique. -180 Time
[msl
Fig. 7. Results of the dynamical response (modulated at 50 Hz); (b) ion saturation gain and phase, respectively.
study: (a) RF power current; (c) and (d) of the transfer function.
References [I] A. de Chambrier 310.
et al., J. Nucl. Mater. 128 & 129 (1984)
Y. Martin, Ch. Hollenstein / Influence ofAljv~% wave spectrum on the SOL [2] Ch. Hollenstein et al., Proc. XII EPS Conf. Controlled Fusion and Plasma Physics, Budapest, 1985 Vol. II, p. 601. (31 G.A. Collins et al., Phys. Fluids 29 (1986) 2260. [4] G. Besson et al., Plasma Phys. Contr. Fusion 28 (1986) 1291.
215
[5] A. Hasegawa and L. Chen, Phys. Fluids 19 (1976) 1924. [6] K. Appert et al., Int. Conf. on Plasma Physics, Kiev, 1987, invited paper. [7] B. Joye et al., Plasma Phys. Contr. Fusion 30 (1988) 743.
Journal
INFLUENCE OF THE ALFVkN OF THE TCA TOKAMAK
of Nuclear
Materials 162- 164 (1989) 270-275 North-Holland, Amsterdam
WAVE SPECTRUIW ON THE SCRAPEOFF
LAYER
Y. MARTIN and Ch. HOLLENSTEIN Cenrre de Recherches en Physique des Plasmas, Association Euratom - Con@d&ation Suisse. Ecole Po~techniq~ Fpdprnle de ibsanne, CH-1007 Latrsanne, Switzerland
Key words:
TCA tokamak,
Alfvvtn wave, scrape-off
layer
The study of the scrape-off layer (SOL) during Alfven wave heating may lead antenna-plasma interaction. The scrape-off layer of the TCA tokamak has been widely
to a better understanding of the investigated by means of Lan~uir
probes. The aim of this work is to present measurements on the infhrence of the Alfven wave spectrum on the scrape-off layer. These experiments have shown that the plasma boundary layer is strongly affected by the wave field, in particular the ion saturation current and the floating potential. In TCA, as the spectrum evolves due to a density rise, the passage of the AlfvCn continua and their associated eigenmodes, the Discrete Alfven Wave (DAW) induces a strong depletion in the edge density of up to 70% during the continuum part and a density increase during the crossing of an eigenmode. The floating potential becomes negative during the continua and even more negative crossing the eigenmodes. In case of MHD mode activity, this behaviour changes for power exceeding 100 kW. The profiles of basic parameters are modified, depending on the wave spectrum. MHD mode activity which can occur during the RF (radio frequency) phase considerably alters the behaviour mentioned above. Finally, the modulation of the RF power allows us to characterize the coupling between RF power and typical edge parameters.
In fusion research, tokamaks are a possibility for obtaining reactor conditions. In most present day tokamaks, high-frequency additional heating schemes are installed in order to achieve high-temperature plasmas. Among the different types of waves used to heat tokamak plasmas, Alfv&r wave heating is an interesting candidate due to its potentially high efficiency and low cost. The antennae of most RF heating schemes are placed in the scrape-off layer. Heavily shielded antennae structures in the SOL are used in these experiments to reduce the antennae-pl~ma interaction. Even so, the plasma boundary interacts with the wave field, and moreover, the behaviour of this boundary plasma may be connected with the efficiency of the wave heating. In AlfvCn wave heating, no complete shielding of the antennae structure has been usually used so far. In the TCA tokamak no shielding of the antenna structure is used which leads to strong ~tennae-plasma interaction. Therefore, a good knowledge of the plasma boundary
can help to understand the coupling between the wave and the core plasma. Interactions of the RF power with the scrape-off layer are not yet as well documented as other aspects of additional heating. However, the impurity fluxes and confinement time problems may have their source in the scrape-off layer and therefore the study of this region is important. The actual great interest in the SOL behaviour underlines that this field is not completely covered and a global view of the RF interaction with the SOL is necessary. Furthermore, there is a great lack of theories dealing with RF-SOL interactions which actually makes it difficult to fully understand the various experimental results. One of the main aims of the TCA tokamak is the study of the AlfvCn wave heating (AWH). The scrape-off layer of the TCA tokamak has been widely investigated by means of Langmuir probes [1,2]. This diagnostic gives information on the principal characteristics of the SOL such as ion density, plasma potential and electron temperature. In this paper, we present results on the influence of the RF wave field on the scrape-off parameters, due to unshielded antenna structure.
0022-3115/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
271
Y. Martin, Ch. Hollenstein / Influence of Alfukn wave spectrum on the SOL 2. Experiment The TCA is a tokamak with the following parameters: R, = 0.61 m, a = 0.18 m, B, I 1.5 T. The typical densities and temperatures for the plasma core are n, = 3.0 x 1019/m3, T” = 500 eV and, for the plasma T, = 15 eV respectively. boundary n e = 2.0 x 10”/m3, The working gas is H, or D, with plasma currents of up to 130 kA, which correspond to a cylindrical safety factor qcy, = 3. The vacuum vessel is made of stainless steel and, in all the results presented here, carbon limiters are used covering approximately 70% of the poloidal circumference. The Langmuir probes are located in the outer equatorial midplane, about toroidally opposite to the limiter. The probes are made out of molybdenum wire, 0.5 mm in diameter and 5 mm long. Four of these probes are mounted on one probe shaft to form an array. The spacing between probes is 5 mm. This probe shaft can be moved in order to radially scan the plasma boundary. This experimental set-up allowed us to measure at the same time and probe position, the ion saturation current and the floating potential. A current probe with a frequency bandwidth of up to 50 MHz was used to measure the currents drawn by the probe. The RF wave field is produced by eight groups of antennae which are spaced regularly around the torus, four on the top and four on the bottom, three centimeters behind the main limiters [3]. Each group consists out of six stainless steel bars, fed in parallel and covered with 6-7 pm of titanium nitride (TiN). For all the presented experiments, no antenna screens or shielding has been used. The RF generator frequency was typically in the range between 2.0 and 2.5 MHz and RF power of the order of up to 250 kW has been delivered to the plasma. All the antennae are electrically floating with respect to the vacuum vessel. The poloidal currents, driven by the above-mentioned antenna bars, create a oscillating toroidal field. This field induces a pressure modulation which has a strong coupling to the shear AlfvCn wave. Taking account of the cyclotronic effects, the shear Alfven wave can be excited if the following dispersion relation is fulfilled:
200 bll hnl 150 -
(2.11
D4w
Fig. 1. The antennae loading reveals the Alfv& wave spectrum as a function of the central density. The excitation structure is N = 2, M = 1 and the following (n,m)modes are generated in the plasma: (2, - 1) (2,0) (2.1) (2,2). the radius with the mass density p and the safety factor 9:
where n and rn are the toroidal and poloidal mode numbers [3]. At a fixed frequency as imposed by the generator, the position of the resonant layer in the plasma depends on the mass density and the safety factor. During a density rise, the resonant layer of the shear Alfven waves (continua) originally generated at the centre is moving towards the plasma edge. Peaking on the antenna loading curve, such as shown in fig. 1, reveal the presence of other modes. These modes are called discrete Alfven waves (DAW) and they correspond to global eigenmodes which are the low-frequency counterpart of the ion cyclotron wave and have their origin in toroidal effects. These modes are superimposed on the continua, resulting in a spectrum as has been shown in previous papers on Alfven waves studies in TCA [4]. The layout of the antennae structure around the torus and the possibility of shifting individually the relative phase of the RF currents of each group of antennae allows us to excite waves with precisely defined toroidal and poloidal mode numbers (N,M). Due to toroidal effects, the exciting mode (N,M) can generate some additional modes. In fig. 1, we show the multiple modes generated by a (2,l) launching structure as a function of the central mean density.
3. Results where v, represents the Alfven velocity of the wave. Using periodic cylindrical geometry to describe the toroidal plasma, the frequency w varies as a function of
In the scrape-off layer, the Alfven wave field imposes a strong perturbation in the ion saturation current
272
Y. Martin, Ch. Hoilenstein
/ Influence of At@& wave spectrum on the SOL
and in the floating potential. Because of the accompanying density rise with the RF wave field, a shear Alfven wave appears within the plasma and the density in the scrape-off layer decreases within a few milliseconds as shown in fig. 2. The density stays at this low level during the presence of the continuum. Depending on the conditions such as RF power, excitation modes and MHD mode activity, a density depression of up to 70% of the ohmic value can be observed. During this time, the floating potential decreases continuously in time as long as the resonant layer moves towards the edge, and reaches about -30 V. As one of the global modes enters the plasma, the ion saturation current in the scrape-off layer increases again to the ohmic value. In some cases, even higher than ohmic values could be observed. The floating potential, during this period, becomes strongly negative. Values of up to -60 V can be measured for probe positions near to the limiters. The next continuum induces again an ion saturation current depletion in the plasma boundary. The different behaviour of the floating potential may be due to changes of the core plasma interactions with the wave field expressed through an observed higher antennae loading and by the fact that the previous resonant layer reaches the edge and therefore induces a change in the coupling. The total density decrease of the TCA scrapeoff layer during the continuum represents about 10” particles which probably enter the plasma core in less than 4 ms. This yields a particle flux in the order of 1Or5 particles per second per square centimeter. These numbers are deduced in considering that the whole plasma boundary situated between limiter radius and antennae
1 JSAT [au.1
I..
Fig. 2. Time dependent evolution of: (a) the floating potential; (b) the ion saturation current; (c) the antennae loading and the central line average density.
DAW
o ILimiter v 0
I ‘O
- 20 Positron
Ante&& -30 [mm1
Wall 40
E
:
Fig. 3. Ion saturation current profile in the scrape-off layer for: ohmic standard conditions; - - continuum Alfven wave field spectrum the upper curve with MHD mode activity, and the lower curve without; * * - - - - global eigenmode, without MHD mode activity.
radius loses 70% of its ohmic density. However, this number is not sufficient to explain the observed central density rise of the core plasma. It represent only roughly 10% of the total density increase. During the density increase due to the appearance of a new global mode, the outward flux into the scrape-off layer is estimated to be of the same order. The usual interpretation of the Langmuir currentvoltage characteristic to obtain the electronic temperature is only valid if the electrons are in a maxwellian distribution. We found that it is the case during the ohmic heating phase, but is surely not the case when the AlfvCn wave field perturbs the edge plasma. Phenomena like possible driven electron beam may lead to a nonmaxwellian distribution of the electrons [l]. The usual treatment of the Langmuir characteristic during the AlfvCn wave heating may therefore lead to faulty measurements of the electron temperature. Additional information on the plasma boundary should be obtain to clarify the existence of the RF driven electron beam and to understand its important role in the RF-SOL interaction. The ion saturation current profile is presented in the fig. 3. For probe location behind he antennae structure, we always found small ion saturation current. In addition at this location only small variations due to the changes in the AlfvCn wave spectrum have been detected. This is probably due to the fact that the antennae behave like back-up limiters. These ion saturation current variations induced by the wave field are most pronounced in front of the antennae, as shown in fig. 3,
213
Y. Martin, Ch. Hollenstein / Influence of A&&n wave spectrum on the SOL 2 ;;” b=f I, s r. I- l-
0 .
.
.
3
I ;; o___-----_____ 0 l l L! +
-1
--------
.’
c);f
. 0
50 RF
I 100 Power
I
2
150 [kWl
Fig. 5. Variation of ion saturation current between two times (75 ms and 105 ms) as a function of the RF power. The conditions are: I, = 107 kA and MHD mode activity present. Fig. 4. The time-dependent evolution of the antennae loading (a) and the floating potential (b) showing the sensitivity of the scrape-off layer to the small irregularities on the loading curve. (c) shows the constant relative phase delay between the antenna loading and the floating potential.
where interactions with the wave field and the scrape-off layer are strongest. The data taken during the (2,0) continuum reveal two cases. The first shows ion saturation currents with higher values than preceding ohmic ones. These data come from shots where strong MHD mode activity induced by the RF wave field is present. The second case corresponds to data of shots without any MHD mode activity during the entire discharge. In the TCA tokamak, during the Alfven wave heating, some oscillations are seen on the antenna loading curve and on some other diagnostics such as magnetic coils just after the passage of a global eigenmode. Such oscillations are attributed to an inward propagation of the kinetic Alfven wave and to its consequent possible radial standing waves. These waves could change the antennae-plasma coupling as shown by Hasegawa and Chen [5]. Further related experimental data from TCA are discussed in ref. 6. Our measurements in fact reveal these oscillations with a constant relative phase delay as shown in fig. 4 (c). That proves that the edge plasma is very sensitive to fine-scale RF induced effects which take place in the plasma core. It is well known that the presence of MHD mode activity usually increases the density in the scrape-off layer. In TCA we observe this phenomenon in ohmic conditions and during AWH as seen on fig. 5. More precisely, we find, with a plasma current of 107 kA, two types of behaviour of the scrape-off layer showing the competition between RF wave field and mode activity
depending on the RF power in the antennae. For powers above 100 W, the mode activity induced by the RF wave field is strongly established and the density continuously increases. For powers lower than 100 kW the density decreases, slightly influenced by the mode activity. This is illustrated in fig. 5, where we show the difference of the density taken at two times in the RF pulse as a function of the power delivered to the antennae. We show in fig. 6 the density and the floating potential as a function of the power delivered to the antennae, for shots without any modes. Collins et al. [3] observed a non-constant antenna loading curve as a
Ion Saturation
Current
.
. Floating
Potential .
x\x, Antenna x\\
0
20 RF
40 Power
Loadmg X-X
60
sb
1’ 10
[kWl
Fig. 6. Ion saturation current, floating potential and antennae loading during continuum as functions of the RF power, in standard conditions (without MHD modes). For the first two, we subtract the ohmic value from the continuum value.
274
Y. Martin, Ch. Hollenstein
/ Influence
function of the RF power in TCA without any lateral screen. They considered the following explanation: the scrape-off layer, interacting with the local wave field of the antennae, provides a sink for the direct dissipation of the antenna current. The RF power modifies the SOL, in particular it decreases its density. Once the edge plasma density is swept away, the lack of the dissipative medium damps the effects for direct dissipation of the currents and leads to a saturation-like behaviour. Our results seems to be in agreement with this model since we can see a similar type of behaviour in the important edge parameters. However, further experiments in order to complete the set of data on a wide range of power are necessary and underway. Recently, some experiments were carried out in the TCA tokamak to study the dynamic response of the plasma [7]. The aim of this type of experiment is to measure the reaction of the plasma columns to an external pulsed force. The dynamic response of a system is completely characterized by the transfer function. The transfer function can be obtained from the perturbating input signal and the resulting output signal of the system. The transfer function is entirely described by a gain and a phase. As one of the many possible external perturbations on TCA, the RF power has been chosen. In this present study, the applied RF power was
ofA&I&I
wuve spectrum on the SOL
modulated at the frequency of about 500 Hz with an amplitude of 10%. Information on the plasma boundary response to the AlfvCn wave heating is obtained from the transfer function between the RF input signal and the Langmuir probe data. The calculated gain and phase of the transfer function from the PF power as input and of the ion saturation current as output, is shown in fig. 7. This result shows a great difference in the gain during the continuum and the discrete wave spectrum phase of the RF pulse. This implies that there is a strong interaction between the applied RF wave field and the scrape-off layer density during the continuum. However, when a global mode enters the plasma, the gain is less important and therefore the coupling to the edge density is less efficient. The application of this modulation techniques may be therefore a powerfui tool in order to understand more about the different and complicated SOL-RF antenna interactions. Moreover, this type of experiment may lead to a description of the energy deposition in the SOL.
4. Conclusion All the measurements reveal a strong interaction between the wave field and the scrape-off layer. The presented data clearly show the important changes in the behaviour of the plasma boundary, depending on the Alfven wave spectrum. The measurements also showed that the SOL is very sensitive to RF phenomena occurring in the hot core plasma. The description of the SOL during intense RF heating is therefore not only determined by the direct antenna-SOL interactions but also by the RF induced effects of the heated plasma core. We find an important decrease of the edge density due to strong RF interaction during the continuum part of the spectrum. The boundary plasma density can be very strongly modified by the RF power, the modulation of the RF power and the mode activity.
Acknowledgements We wish to thank the whole TCA team for its excellent support. The present work was partially supported by the Fond National Suisse de la Recherche Scientifique. -180 Time
[msl
Fig. 7. Results of the dynamical response (modulated at 50 Hz); (b) ion saturation gain and phase, respectively.
study: (a) RF power current; (c) and (d) of the transfer function.
References [I] A. de Chambrier 310.
et al., J. Nucl. Mater. 128 & 129 (1984)
Y. Martin, Ch. Hollenstein / Influence ofAljv~% wave spectrum on the SOL [2] Ch. Hollenstein et al., Proc. XII EPS Conf. Controlled Fusion and Plasma Physics, Budapest, 1985 Vol. II, p. 601. (31 G.A. Collins et al., Phys. Fluids 29 (1986) 2260. [4] G. Besson et al., Plasma Phys. Contr. Fusion 28 (1986) 1291.
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[5] A. Hasegawa and L. Chen, Phys. Fluids 19 (1976) 1924. [6] K. Appert et al., Int. Conf. on Plasma Physics, Kiev, 1987, invited paper. [7] B. Joye et al., Plasma Phys. Contr. Fusion 30 (1988) 743.