Recent results on ion cyclotron and combined heating of TEXTOR

ELSEVIER

Fusion Engineering and Design 26 (1995) 103-120

Fusion Engi ng aridDesign

Recent results on ion cyclotron and combined heating of TEXTOR R. Koch a, A.M. Messiaen a, j. Ongena a, R. Van Nieuwenhove a, G. Van Oost ~, G. Van Wassenhove ~, P. Dumortier a, F. Durodie a, P.E. Vandenplas a, D. Van Esteer ~, M. Vervier a, R.R. Weynants a, K.H. Finken b, H. Euringer b, V. Philipps b, U. Samm b, B. Unterberg b, j. Winter b, G. Bertschinger b, H.G. Esser b, G. Fuchs b, B. Giesen b E. Hintz b, F. Hoenen b, p. Hiitteman b, L. K6nen b, M. Korten b, H.R. Koslowski b', A. Kr~mer-Flecken b, M. Lochter b, G. Mank b, A. Pospieszczyk b, B. Schweer b, H. Soltwisch b, G. Telesca b, R. Uhlemann b, G. Waidmann b, G.H. Wolf b, J. Boedo ~, D. Gray c, D.L. Hillis d, T. Oyevaar e, H.F. Tammen e, T. Tanabe ~, Y. Ueda r Laboratoire de Physique des Plasmas, Laboratorium voor Plasmafysica, Association "Euratom-Belgian State", Ecole Royale Militaire, Koninklijke Militaire School, B-1040 Brussels, Belgium b Institut J~r Plasmaphysik, Forschungszentrum Jiilich G.m.b.H., D-52425 Jiilich, German), ~"Institute of Plasma and Fusion Research and Department of Mechanical, Aerospace and Nuclear Engineering, University of CaliJbrnia, Los Angeles, CA, USA d Oak Ridge National Laboratory, P.O. Box X, Oak Ridge, TN 37831, USA e FOM-Instituut voor Plasmafysica 'Rijnhuizen', P.O. Box 1207, 3430 BE Nieuwegein, Netherlands f Faculty of Engineering, Osaka University, Osaka 565, Japan

Abstract

The recent experimental activity in the field of auxiliary heating and related topics on TEXTOR is reviewed. TEXTOR is equipped with up to 4 M W of ion cyclotron heating power and 3.4 M W of neutral beam injection. The combination of the radiating boundary concept with high auxiliary power has extended the improved confinement domain to the large density regime and demonstrated the viability of the radiating boundary concept for long pulse high power operation. Improved confinement was also achieved in third harmonic heating, characterised by predominant coupling of the R F to the beam ions. Operation of an unshielded antenna with insulated limiters proved that R F sheaths are taking place on the side limiters and are suppressed by insulation. Control of the helium flux by the R F was successfully demonstrated using the interaction of the R F with fast 3He ions injected by neutral beam. Preliminary tests with a high Z limiter indicate compatibility, and even a positive effect, of the RF. Experience gained in operating unshielded antennas is also commented on.

0920-3796/95/$09.50 ,~ ~'~ 1995 Elsevier Science S.A. All rights reserved SSDI 0920-3796(94) 00176-6

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R. Koch et al. / Fusion Engineering and Design 26 (1995) 103-120

I. Introduction

There has been a considerable amount of experimental activity on TEXTOR preceding the June 1993 shut-down of the machine. The machine is at present being upgraded for longer pulse operation. The main line of experimentation was the high auxiliary power and high density operation aiming at reactor-relevant tests of new coatings [ 1] and of the concept of edge radiative cooling [2]. However, a number of other r.f. experiments, which will also be discussed here, were performed, namely (i) third harmonic D heating, (ii) test of unshielded antenna with insulated limiters, (iii) attempt to stabilize sawteeth by minority current drive, (iv) 3He pumpout experiments, and (v) high Z limiter tests. Finally, as a last point, we shall discuss the yearslong experience gained on TEXTOR in using unshielded antenna(s). Improved confinement operation (I mode) has been one of the main auxiliary heating research subjects of TEXTOR [3]. The I-mode is reached (usually) without transition, but back transition from I to L mode is sometimes observed. The I mode can be obtained in a well-conditioned machine with coinjection alone, with coinjection + counterinjection and with any of these beam combinations together with ion cyclotron resonance heating (ICRH). Counterbeam alone or ICRH alone or both together remain in the L confinement mode and follow the K a y e - G o l d s t o n scaling [4]. In the earlier experiments, the enhancement factor with respect to the ITER L89-P scaling was observed to decrease substantially at higher density. The recent campaign with siliconized walls and radiating boundary has allowed the good confinement regime to be extended to the high density region with enhancement factors of the order of 1.5-1.7. Conversely, the high power high density operation has demonstrated the reactor relevance of the radiating boundary concept, implemented in TEXTOR by feedback-controlled neon gas puff [2] and/or silicon coating [1]. Third harmonic deuterium heating was attempted earlier in TEXTOR [5], with coinjection alone, with good results in terms of neutron production but unsatisfactory heating efficiency. In a new campaign, third harmonic heating was per-

formed with balanced injection. This has allowed us to reach I mode operation in this scenario. With balanced beam injection, most of the neutron production is due to beam-target and b e a m - b e a m reactions. Nevertheless the r.f. causes a large increase in the neutron production, with respect to the beam-only neutron level. This increase is particularly remarkable at high density where the slowing-down time becomes short. In some shots a spectacular energy increase occurs when a modest amount of r.f. power is applied at the third harmonic. The earlier comparison of Faraday-shielded and unshielded antennas [6] has led to the conclusion that the sheath rectification phenomenon (and therefore the r.f. sheaths themselves) was taking place on the antenna protection limiters rather than on the Faraday shield, at least on TEXTOR antennas. Therefore it was thought that by operating the antenna with insulated limiters, negative effects, if any, associated with the r.f. sheaths could be eliminated. Such a test has been performed showing indeed that r.f. sheath rectification was suppressed by the insulation. However, the presence of the insulation weakened the antenna and the insulation had to be removed after a while. By connecting each generator to one strap of the antenna pair 1 (see below), an arbitrary phase difference was imposed between the two antenna currents. This was used to try sawtooth stabilization by minority current drive, as done earlier in JET [7]. The phase difference was measured and feedback controlled. The frequency was 32.5 MHz and the magnetic field 2.03 T to locate the cyclotron layer at the q = 1 surface (determined from the electron cyclotron emission (ECE) signals) on the high field side (HFS) of the machine. The plasma position was swept (+_2 cm around the q = 1 surface position) during the r.f. such as to move the q = 1 surface across the minority H cyclotron layer. The H:D ratio was about 3%. No significant difference in sawtooth period was obtained for the different phasings 0, r~/2, - ~ / 2 and r~. It is conjectured that this negative result may be due to the too broad r.f. deposition profile. Further analysis of the experimental data and theoretical modelling are necessary to interpret these results. They will not be further discussed in this paper.

R. Koch et al. / Fusion Engineering and Design 26 (1995) 103 120

An r.f.-based technique for pumping the helium ash out of a reactor was proposed by Chang et al. [8,9]. This method has been tested on TEXTOR using 3He neutral beam injection (NBI) with offaxis heating by the r.f. at the second harmonic of 3He. The effect of the r.f. on the fast He population is investigated using charge exchange spectroscopy of an He + line. The charge exchange neutrals are provided by D beam injection. Qualitative agreement with theory is observed. High Z limiters (Mo and W) were tested in TEXTOR. Some shots were performed with NBI and some with NBI + r.f. They indicate a pumpout by the r.f. of the central high Z impurity content. We shall end this overview of the recent activity on TEXTOR by summarizing the unshielded antenna operation results. The TEXTOR r.f. system has been constantly operated with at least one unshielded antenna for about two years. At the same time as heating was performed in a wide variety of conditions, the parameter range over which successful operation of the unshielded antenna is obtained was thus automatically extended, thereby proving the viability of unshielded antennas under a very wide variety of experimental conditions.

2. TEXTOR tokamak and heating system TEXTOR is a medium size machine with plasma radius ap <0.48 m, major radius R0 1.75 m, plasma current lp ~< 500 kA and toroidal magnetic field BT ~<2.8 T. The auxiliary heating system of TEXTOR is composed, as shown on Fig. 1, of (1) two parallel injection neutral beam lines with injection energies up to 55 keV and a maximum power of 1.7 MW for 10 s each and (2) two r.f. generators of 2 MW for 3 s each (2538 MHz). Each generator is connected to an antenna pair. Antenna pair 1 was constantly operated without a Faraday shield. As both beams are shooting in this direction, this antenna pair is always equipped with a protection against shine-through, which at the same time constitutes a covering of the feeder area. Antenna pair 2 was sometimes operated without a shield. In

105

these cases, however, either (i) there were no protection plates over the feeder area in order to test this configuration or (ii) the protection limiters were insulated. The transmission line of antenna pair 1 was equipped with an automatic real-time tuning system [10]. That of antenna 2 was not. The r.f. system is at present being upgraded for 10 s operation with arbitrary feedback-controlled phasing between the two straps of each pair and automatic tuning on both lines. The injectors will also be upgraded to deliver 2 MW at 60 kV.

3. I mode results with radiating boundary The improved confinement operation (I mode) on TEXTOR has been characterized by Ongena et al. [3] and has to be opposed to the L mode operation described earlier by Messiaen et al. [4]. Three distinct features of the I mode are (i) the enhancement of the energy content above the ITER L89-P scaling, (ii) the occurrence of transitions back to L mode, especially when operating close to the fl limit, and (iii) the tendency to obtain peaked density and temperature profiles reminiscent of the supershot regime in TFTR [ 11] in the case of balanced injection (with or without r.f.). In the lower density range ~o~<2 x 1019 m 3, the beam component of the energy (fast particles) can be significant thereby giving large enhancement factors (up to 2.2). However, in the medium density range ((3-4) x 1019m 3) the fast particle contribution becomes small and a kind of offset-linear scaling of the energy with the power is observed (Fig. 2). In this range, the heating efficiencies of the beams and of the r.f. are clearly similar. In the earlier operation with boronized walls, the enhancement factor fh over the ITER L89-P scaling decreased significantly when the density was increased, reaching the low value f h - - 1 . 2 at 4.5 x 1019m 3. This situation has changed radically since the radiative edge cooling concept was introduced in TEXTOR. The concept of radiative edge, or "cold plasma mantle", was first investigated in TEXTOR using neon puffing [13]. In later experiments, this technique was compared with wall siliconization and

106

R. Koch et al. / Fusion Engineering and Design 26 (1995) 103 120

TEXTOR HEATING SYSTEM

CO-InjectorI 1.7MWat 55kV 2.0MWat 60 kV /

Pini

Antenna pair 2

2MW

I

Antenna p a i r ~ ~ _--~I 2MW 25 -38MHz '

BT.,~.__ Counter -Injector lI 1.7MWaf55 kV 2.0MWat 60 kV

Fig. 1. Auxiliary heating system of TEXTOR, top view. For the normal Ip and B t configuration injector 1 is the coinjector and injector 2 is the counterinjector.

disilane (Si2H6) puffing [2]. We refer the reader to Refs. [3,14,15] for pure silicon wall operation. These experiments showed that the light impurities neon and silicon are valuable for generating a cold plasma mantle capable of radiating a significant fraction of the total heating power [2,13].

The thickness of the radiation belt, of the order of 20 cm, was observed to be similar with neon and silicon. However, the control of the radiation level in the silicon case is more difficult because Si sticks to the wall. Part of the Si is released by sputtering. Therefore, the maximum radiation

R. Koch et al. / Fusion Engineering and Design 26 (1995) 103-120 E (k J)

20(

/ B,-co

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Fig. 2. Dependence of the energy content of the plasma on the total auxiliary power Ptot ...... where the beam power has been corrected for charge exchange losses and shine through (from Messiaen et al. [12]). The difference between the equilibrium (or magnetohydrodynamic) energy Eequ~ and the diamagnetic energy Edi. is due to the presence of a fast particle component in the plasma, rico is the central line-averaged density.

levels reached with Si were only about 75%. In contrast, neon does not stick to the walls but is recycled, which means that in the absence of a sink the neon level cannot be controlled. In TEXTOR, the necessary sink is provided by the pumping capability of the ALT-II toroidal belt limiter system and the radiation level can be feedback controlled using the neon VIII line intensity [2,13]. Feedback control of the radiation is an essential ingredient of the cold plasma mantle concept in order to avoid the 100% radiated power limit, which leads to detachment, or at high auxiliary power to MARFEs, quite often followed by disruption. The use of controlled neon injection has allowed to radiate in stable and stationary discharges more than 90% of the heating power. First experiments with moderate auxiliary (NBI coinjection alone) heating power proved the feasibility of edge cooling with auxiliary heating [2].

More recent experiments not only have proven the viability of edge cooling with high auxiliary heating power, but also have extended the range

107

of the high confinement (I mode) regime to higher densities [16]. The basic improvement gained by operating with a radiating belt can be described as follows. Comparing a discharge without an edge radiative layer with another discharge with it (neon or silicon), and for identical heating power, one can note two main differences: (1) the radiated power fraction increases from about 20% to typically 50%-90% with radiating belt, and (2) much higher densities can be obtained in the latter case for a similar value of the electron temperature. Therefore a larger energy content is obtained with a radiating boundary. The gain in energy results from the density increase, the density profile being more peaked than without radiating layer, while the temperature profile is almost unchanged, or slightly more peaked. For shots with equal electron temperature, the neutron yield is also identical. As with neutral beam injection nearly all the neutrons are produced by beamtarget reactions, which rate is proportional to the number of deuterium ions in the plasma core; this is indicative of a maintained plasma purity, in agreement with earlier findings at moderate auxiliary power [2]. It is also noticeable that, in this improved confinement mode, there is no sign of central impurity accumulation. Fig. 3 show the dependence of the enhancement factor fh on density. The large increase in this factor at the highest densities is its most striking feature. The strong density dependence offh contrasts with the earlier I mode behaviour [3] showing only a weak density dependence of the ITER L89-P type. Note that the largest densities are reached only with large radiated power fractions, and that there is no systematic dependence, at the same density, on the radiated power fraction ~. This implies that, contrary to what might have been feared, the radiating boundary not only does not cause the confinement to deteriorate, but also gives access to the high density domain. It should be noted that there is a similarity between the present radiating boundary I mode and the socalled Z mode discovered on ISX-B and described by Lazarus et al. [18]. The significant gain in confinement resulting from the operation at high density with radiating boundary is illustrated by the record shot shown

R. Koch et al. / Fusion Engineering and Design 26 (1995) 103-120

108

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Fig. 3. Enhancement factor fh of the diamagnetic energy relative to the ITER L89-P scaling [17] for shots with radiative cooling by neon puffing, fh is plotted vs. the central chord line-averaged density ~i~o. The statistics is made for shots with about 1.45 MW of NBI coinjection + 1-2 M W of ICRH. The data discrimination is relative to , / = PRAD/Pin the ratio of the total radiated power to total input power (OH + additional). Plasma current Ip = 350 kA, minority H heating, 32.5 MHz, 2.25 T.

in Fig. 4. In this shot a total stored energy content of 200 kJ is obtained with only 4 MW of total heating power (with no connection of the beam power for shine-through etc.). This can be compared with the earlier record of 200 kJ in Fig. 2 (no neon puffing) which was obtained with more than 6 MW of additional power. Note that, in the case of Fig. 4, the density is about twice as high. The weak density dependence of the usual confinement scalings can certainly not explain such a difference in heating power. The enhancement factor fh is also shown on the figure. The ratio of the actual diamagnetic energy value to that corresponding to the Troyon limit (Troyon factor, 2.8) reaches 0.71, corresponding to a Troyon factor of 2. Near 1.4s, there is a sudden loss of confinement and, afterwards, the energy follows lower confinement regimes. It should be stresed that, in addition to the above confinement results, these experiments at high density and large heating power provide a proof of the compatibility of the radiating boundary concept with high confinement regimes.

This concept might be useful to relieve the divertar (or limiter?) of the reactor by radiating a large fraction of the power all over the plasma outer surface.

4. Third harmonic heating Third harmonic heating of deuterium was tested earlier in TEXTOR, as reported by Van Wassenhove et al. [5]. In this scenario, the experimental conditions a r e B t - - 1.7 T, Ip = 350 kA, and f = 38 MHz = 3fed at the plasma center. Because of the very weak absorption by the thermal deuterium at the third harmonic, the presence of beam ions is necessary. In the earlier experiments, only the beam coinjection was operated. In this situation a total absorption per transit of 10% was predicted by the SWHAP code [19]. The main feature of these experiments was the large neutron production induced by the r.f., the r.f. power being shown in this respect to be equivalent to beam power. However, the global heating

R. Koch et al. / Fusion Engineering and Design 26 (1995) 103-120

TEXTOR .

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.

.

.

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Fig. 4. Record shot with total additional power of nearly 4 MW. Siliconized wall and neon puffing. The traces of the diamagnetic energy Eaia, of the central line-averaged density ~eo and of the beam and ICRH power are shown.

efficiency proved to be only about one-half that of the usual minority H heating scenario. Supported by neutral particle analyser measurements, these results pointed to a strong interaction of the r.f. with the beam leading to a very energetic ion tail (order of 100keV or higher). Because of the rather low current in TEXTOR, most of the fastest particles are expected to be quickly lost, thus explaining the reduced heating efficiency of this scenario. New experiments have been performed with both beams operated at reduced voltage (40 kV, P = 800 kW per beam), thus allowing the r.f. power to be distributed over more beam particles. As reported by Messiaen et al. [20] this new combination allowed us not only to obtain reasonable heating efficiency, but even to reach the I

mode with the third harmonic heating. Fig. 5 shows the strong increase in neutron yield resulting from r.f. heating. On this figure, the yield is compared with that obtained with balanced injection alone in a shot with gas puffing. At the end of the r.f. phase, the neutron yield is about 4 times higher than what it can be estimated to be in the balanced injection alone case. In Fig. 6, we compare the neutron yield for different combinations of the beams and the r.f. with different power levels of ICRH. With coinjection alone the neutrons are produced by beam-target reactions and the number of neutrons is proportional to the product of the beam density by the thermal D density, i.e. approximately t o T3e/2. As the electron temperature drops approximately inversely proportionally to the density, the neutron yield also

110

R. Koch et al. / Fusion Engineering and Design 26 (1995) 103-120

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Fig. 5. Neutron yield vs. line-averaged density ~ieo for two shots, one with balanced injection only (54903), the other with balanced injection + ICRH (54906). The different power levels applied in the different density ranges are also shown. Although gas was puffed in the first shot, the density could not rise as high as in the last shot. The first part (balanced injection only) of shot 54906 is nearly identical to that of shot 54903. - - -, guess of the neutron yield evolution at larger densities.

drops with density as shown. In the coinjection + counterinjection situation, part of the neutrons are produced by b e a m - b e a m reactions whose rate is proportional to the product of the number of fast beam ions in each beam, i.e. to T3/21n -~2, and the total rate thus drops even more e i C/ rapidly with density than in the pure coinjection case. The large neutron increase due to the r.f. is particularly remarkable at high density, where the slowing-down time becomes rather short and thence the number of fast beam ions is strongly reduced. In passing, note that shot 54906 exhibits near ~ieo~ 4.2 x 1019 m -3 an abrupt rise in neutron yield due to a spontaneous transition to higher confinement. This is a rather rare event in TEXTOR. In some instances, the addition of a modest amount of r.f. power causes a spectacular increase

in energy, as shown in Fig. 7. Crudely interpreted, these data imply an incremental confinement time of 60 ms for the first part of the r.f. pulse and 53 ms during the second r.f plateau. This is a factor 2 - 3 higher than what is normally achieved, as the comparison with the beam phase (r E = 31 ms) and Fig. 2 shows. However, in this last figure, the statistics were made at constant density while in Fig. 7 the additional power causes significant density increases. Remember, however, the weak dependence of all auxiliary heated confinement scalings on density. An alternative way of interpreting the large energy increases caused by the r.f. is to assume that the r.f. induces a change in confinement, as might be suggested by the strong density peaking that is observed after the r.f. is applied; Fig. 8. At about t = 2.2 s (Fig. 7) there is an abrupt loss of confinement which corresponds

R. Koch et al. / Fusion Engineering and Design 26 (1995) 103-120

5 TEXTOR

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Fig. 6. Statistical analysis of the neutron yield vs. central chord line-averaged density he0 for various third harmonic shots with NBI coinjection, NBI coinjection + r.f., NBI coinjection + counterinjection and NBI coinjection + counterinjection + r.f. The different symbols represent different r.f. power levels as indicated. The beam power is 0.8 M W per beam line•

to a transition back to the L mode. The reason for this transition is attributed to the fact that the plasma fl is at the Troyon limit.

5. Operation with insulated iimiters For a period of 3 months the antenna pair 2 (Fig. 1) was operated with insulated protection limiters. The insulation between the limiters and the steel frame of the antenna was constituted of a set of small, thin silicon nitride tiles, not seen by the plasma along field lines. This antenna was also equipped with feeder protection plates [6] covering the feeder area. In T E X T O R the antenna box is connected to the liner by metallic strips and the current flowing in these strips is measured [21]. With the normal (shielded or unshielded) antennas, during the r.f. the intensity of this current is

significantly increased because of the sheath rectification effect. The direct current drawn by the antenna was suppressed by the insulated limiters, thus proving that the r.f. sheaths occur on the antenna side limiters. The performances of the insulated and non-insulated antennas were found to be similar. Some variation in the results was observed but with no systematic trend. Sometimes one antenna seemed to perform better; sometimes it was the other. Fig. 9 shows a comparison where the insulated antenna performs somewhat better: at the end of the first pulse, the electron temperature stays constant but the density was somewhat higher during the first pulse. The neutron rate is also higher during the pulse with insulated antenna. This is consistent with the normal dependence of the thermal neutron rate on density, i.e. proportional to n 2 at constant temperature. On the contrary, the first antenna causes a density

112

R. Koch et al./ Fusion Engineering and Design 26 (1995) 103-120 o

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.

.

.

.

.

12.

P [MW]

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Fig. 7. A comparison of two shots, one with balanced injection only (55268), the other with, in addition, a small amount of r.f. (55271). The diamagnetic energy signal Edna, the central line-averaged density fi~oand the auxiliary power (r.f. and NBI) are shown. Boronized wall conditions.

increase while the other does not. All in all, no significant difference could be detected between the insulated and the non-insulated antennas whether with respect to heating efficiency, impurity generation, edge effects etc: The insulated antenna was then damaged at the top. The exact cause of the damage is unknown. The problem may have been caused (or worsened) by a large d.c. voltage build-up due to the r.f. sheath rectification effect. Nevertheless, the damage seems to have found its origin in the operation of the machine with an error in the vertical positioning of the plasma which was strongly pushed to the top, with the r.f. antennas acting as top limiters. The insulated antenna was monitored by a television camera and a strong interaction with the top part of the antenna could be observed, even in the absence of r.f. Since that day, the antenna could no longer be safely operated as

arcing was taking place, mainly at the top, with melting of antenna parts and UFOs falling through the plasma. Moving arcs short circuiting the insulating layers could also be observed. The silicon nitride tiles broke owing to the thermal stresses. Although the origin of the damage is unclear, it is, however, to be concluded from these experiments that the presence of the insulator weakens the antenna without bringing otherwise any clear and significant improvement in the r.f. operation.

6. Helium pump-out experiments

A method to expel the lower energy part of the fast Qt particle population in a reactor was proposed by Chang et al. [8,9]. This method consists of modifying the grad B drift of these

R. Koch et al. / Fusion Engineering and Design 26 (1995) 103 120

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fast particles by changing their perpendicular energy with the r.f. in such a way as to cause an overall inward flow of the higher energy part of the a particle population accompanied by a concomitant outward flow of the lower energy as. In this situation, computation shows that the total r.f.-induced flux (fast + s l o w particles) is outward [9]. In order to obtain a net flux, the r.f. must interact off axis with passing particles in one toroidal direction only (otherwise, by symmetry the flows cancel). The asymmetry is obtained by phasing the r.f. such as to interact only with particles having a given sign of v//. The discrimination between as and D ions is obtained from the Doppler shift of the as, which positions their cyclotron layer away from the D cyclotron resonance. In order to test such a scheme in TEXTOR and in the absence of a fast a population in this machine, we model the process by using parallel

injection of 3He. This gives an asymmetric population (in v//) of fast helium ions with a resonance layer distinct from that of the deuterium. Under the usual conditions in TEXTOR (see Fig. 1), the injector II which is equipped for 3He is the counterinjector. In this configuration, the grad B drift

Zev~ B × VB

vo- 2~

~o~

(1)

is downwards. In the present experiments, both B v and Ip were reversed. This leaves the pitch of the magnetic field unchanged but changes the grad B drift direction which is now upwards. In this situation, the 3He injector (II) becomes the coinjector. Fig. 10 shows the location of the various cyclotron resonances for different magnetic fields in TEXTOR. The r.f. is operated at 38 MHz in order to interact with the 3He at the second harmonic, which gives a better coupling to the faster ions. The different magnetic fields at which

114

R. Koch et al. I Fusion Engineering and Design 26 (1995) 103-120

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)

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oo

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1-[ME IS] Fig. 9. Comparison of the antenna with insulated limiters (A2, first pulse) with the normal unshielded antenna (AI, second pulse). Minority H scenario, 32.5 MHz, 2.25 T. T,o is the central electron temperature (from ECE) and PRAD is the total radiated power (from bolometry). The neutron yield (arbitrary units) is measured by two detectors which give nearly the same signal.

the experiment was performed are indicated. For the higher BT, the second harmonic of 3He is located outboard and the coions which move downward along field lines at the cyclotron layer get a kick in vi and therefore a larger inward drift in the lower part of their trajectory. In the upper part of their trajectory the drift is outward but smaller, because these ions have slowed down (reduced v±) as a result of collisions. In total, in this configuration, theory predicts an overall inward flux of the high energy helium ions due to the r.f. In this configuration, interaction with the D beam at the HFS second harmonic is not expected to play any significant role, because this resonance is too far away from the plasma core, located near r ~ + 7 cm owing to the Shafranov shift. When the cyclotron layer is positioned at the inboard side of the machine, the converse happens and the fast He flux is outward. However

in this case the third harmonic heating of D is competing with the second harmonic heating of 3He. For a central position of the second harmonic He layer, no net flux should be induced. As this mechanism works only with passing particles, the r.f. should not be too strong to avoid trapping by r.f. heating. In the T E X T O R case, this limits the r.f. power to about 1 MW. The expected r.f. induced flux computed from Chang et al.'s formulae for 0.5 M W of r.f. is of the same order as the rate of particle injection by NBI at the level of 1 MW. The experiments are performed with a 40keV, 0.8 M W 3He coinjection beam and a 50 keV 0.8 M W D beam serving as diagnostic beam as explained below. Two typical shots are shown on Fig. 11, one with r.f and the other without r.f. The large density excursions reflect the high recycling of the helium (only a small fraction of the helium is

R. Koch et al. / Fusion Engineering and Design 26 (1995) 103 120

2.6 BT

ITl

!

/ /

f

/

2.0

L8

t.4

R

lml

Fig. 10. Location of the D and 3He cyclotron layer in the tokamak midplane for various magnetic fields (central BT). The horizontal lines show the different values of the magnetic field for which the experiment was performed. The dots indicate the position of the cylotron resonance layers for these different magnetic fields.

implanted into the walls). The global He content is therefore dominated by recycling and plasmawall interaction rather than by the r.f. interaction. We have thus investigated the effect of r.f. on the fast He component by using charge exchange spectroscopy of a 3He+ line. The 3He+ ions are produced by charge exchange with the D-injected neutrals and the line spectrum gives (in principle) a picture of the He distribution function by Doppler shift. Observed in the parallel direction, th e spectrum exhibits a large peak associated with the thermalized He 2+ and a tail at Dopplershifted frequencies corresponding to energies around the injection energy of 3He. No tail is observed in the perpendicular direction whether with or without r.f. The diagnostic has 10 lines of sight in the equatorial plane with distance to the magnetic axis at the tangency points of - 6 , - 2 , 0, 4, 8, 11, 16, 20, 26 and 30 cm. As shown in Fig. 12 this provides a radial profile of the emission line. The interpretation of the line shape is, how-

115

ever, not evident as this signal is the sum of the light emitted by the charge exchange 3He+ ions themselves (which we shall call hereafter the CX signal) and of the light emitted by the 3He+ beam ions once ionized just after injection (this light will be called the just injected ion signal or JI signal; it does not reflect the distribution of the He 2* slowing-down population). The latter ions can survive in the first ionization stage the time of at least one toroidal revolution around the plasma before becoming twice ionized. Furthermore, the charge exchange and excitation cross-sections, together with the complicated beam injection geometry and line integration by the diagnostic must be taken into account for a detailed interpretation of the emission line shape. The required theoretical interpretative modelling is underway. Nevertheless, we have investigated the most global aspects of the measurement, namely the influence of the r.f. on the global line intensity, in the wavelength range corresponding to fast particles. Fig. 13 shows four profiles of the line intensity. Each of these profiles summarizes the information similar to that displayed in Fig. 12, at a given time, for different shots. The global intensity of each channel is plotted vs. the closest distance of the line of sight to the magnetic axis (see caption of Fig. 12). Only the large red shift part of the spectrum, corresponding to the higher energy range of the beam ions, is taken into account; the wing of the maxwellian, when present, is neglected in the computation of the global intensity. The profiles shown in Fig. 13 are not profiles of a physical plasma parameter, but rather the combined result of geometry and of several physical processes, such as beam profile and attenuation, and cross-sections for ionization and charge exchange. For example, the sharp drop in intensity around the magnetic axis (r ~ 0) is due to the strong attenuation of the D beam and thence of the local decrease in the neutral D density. On the contrary, the drop at the low field side edge is a consequence of the decrease in the fast He 2 + density. Finally, it is to be kept in mind that each data point is the superposition of the CX and JI signals. However, for all the shots discussed here, the intensity of the JI signal prior to D beam injection was identical, enabling us to

116

R. Koch et al./ Fusion Engineering and Design 26 (1995) 103-120

,TEXTOR

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,

,

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Fig. 11. Two discharges with (54858) and without (54859) r.f. The 3He beam (40 kV, 0.8 MW) is from 1 to 2 s, the D beam (50 kV, 0.8 MW) from 1.4 s to 1.9 s. In shot 59858, the r.f. is added on top of the beam(s). The density and radiated power evolutions are compared.

consider that the variations in the intensities mainly result from variations in the CX signal and reflect the variations in the total number of 3He2+ slowing-down ions. The four profiles of Fig. 13 therefore are indicative of the dependence of the fast He 2+ density on the magnetic field, i.e. on the location of the cyclotron layer (Fig. 10). The profile for BT ~ 1.9 T is nearly identical to that obtained without r.f., indicating that the variations in light intensity are not the result of a pure heating process of the He 2+. When the cyclotron layer is located outboard (2 T and 2.1 T), the light intensity is higher indicating a higher central concentration of He 2+ ions. On the contrary, for the Ba-~ 1.7 T case, the lower intensity reflects the expulsion of the fast He 2-- ions by the r.f. Indeed, neither a pure He 2÷ heating effect nor the partial absorption of the r.f. by the D beam in the centre (Fig. 10), which reduces the He-r.f. interaction,

can explain a decrease in the light intensity with respect to the case without r.f. Further analysis, ongoing, is needed to explain the details but, globally, we conclude that the results of Fig. 13 indeed point to the existence of an r.f. induced He 2+ transport process acting in the direction predicted by Chang et al.'s theory [8,9].

7. Operation with high Z limiter The effect of high Z materials in contact with the plasma was tested in TEXTOR by introducing molybdenum and tungsten test limiters in the plasma. The results of these experiments, which fall outside the scope of the present paper, will be presented elsewhere [22], but some shots, performed with additional heating, NBI and/or ICRH exhibited an interesting behaviour that we

R. Koch et al. / Fusion Engineering and Design 26 (1995) 103-120

40.

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Fig. 12. A succession of charge exchange spectra along different equatorial lines of sight. Channels 1 10 correspond respectively to lines of sight with minimum distances (centimetres) to the toroidal axis of - 6 , - 2 , 0, 4, 8, 11, 16, 20, 26 and 30. The data are taken from shot 54858 at t ~ 1.5 s. Only the part of each spectrum corresponding to large Doppler red shifts is represented. The wavelength range corresponds to energies from 8.5 to 40 keV as shown. The sharp increase in light intensity at the right of some spectra corresponds to the wing of the thermal 3He maxwellian.

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200

200

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i

i

i

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Fig. 13. Global intensity of the light coming from the high energy He tail. The global intensity corresponding to each channel does not include the maxwellian wing (if any) and is plotted vs. the radius of closest distance to the magnetic axis corresponding to each channel (see Fig. 12). The intensity profiles are plotted for four shots with different magnetic fields (©, 54854 (1.7T); ×, 54856 (1.9T); D, 54857 (2T); A, 54858 (2.1 T)) at the same time in the discharge (about 1.5 s) in a phase where both beams and the r.f. are operated (see Fig. l l for the timing).

shall briefly mention here. Only the case o f tungsten is considered here. In most cases with H injection and high Z limiter no strong central radiation was observed. However, in some shots o f a series with D injection, a central p e a k appeared in the radiation profile indicating the presence o f high Z impurities in the centre. As shown in Fig. 14, the central radiation appears with some delay with respect to the start o f the heating pulse corresponding to a penetration time o f the high Z impurity. R e m a r k a b l y (Fig. 14), as the r.f. was added to N B I , the central radiation disappeared after some hundreds o f milliseconds. A similar expulsion o f the high Z impurities from the centre by the r.f. was observed in another shot with ohmic target plasma. The interpretation o f these results is not obvious because there is a significant variation in the radiation behaviour in shots with beam only. Some o f them exhibit no central peak in the radiation and, in some cases, the central peak disappears as sawtoothing sets in. Both shots with r.f. were sawtoothing. On the contrary, in the D beam shot shown in Fig. 14 sawtoothing stops shortly after the start o f the N B I pulse. One can w o n d e r whether the disappearance o f the central

118

R. Koch et al. / Fusion Engineering and Design 26 (1995) 103-120

TEXTOR

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,

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i

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radiation is not linked to a modification of the sawtoothing behaviour caused by the r.f. Alternatively, one might wonder whether the cause of the change in sawtoothing is not the reduction in central radiation due to the expulsion of the high Z impurities by the r.f. Other examples can be found, with H beam alone and without central radiation in spite of the absence of sawteeth, which seem to contradict the hypothesis of a direct relation between sawtoothing and central radiation. This set of shots has as yet only been partially analysed and, in view of the rather limited database, further experiments will probably be necessary to resolve the issue.

8. Experience with the unshielded antenna

Since the initial [23] tests, the r.f. system of T E X T O R has now been operated for more than two years with at least one antenna without a Faraday shield. Comparison between shielded and unshielded antennas has shown no significant difference, whether with respect to heating efficiency, impurity production or interaction with the edge. The voltage stand-off is generally similar and was in some cases found to be better without a Faraday shield [6]. N o particular technical problems were encountered in tuning the r.f. gen-

erator to the unshielded antenna. A detailed comparison of shielded-unshielded antennas has been given by Van Nieuwenhove et al. [6,24]. The experiments described in the latter papers covered already a wide range of plasma parameters. Today it can be stated that the unshielded antenna has been operated in a very wide range of experimental conditions. These vary from the lowest density to the highest achievable density on TEXTOR. A wide variety of scenarios has been tested, including minority H, third harmonic D, second harmonic 3He, on- and off-axis heating, weak or strong damping scenarios. In no case could a significant difference in performance be observed between the two types of antennas. This antenna was operated both with ~, 0 and + To/2 phasing without noticeable effect. The unshielded antenna was operated under boronized and siliconized wall conditions, with the radiating belt resulting from neon puffing and with the high Z test limiters. The improved performance operation, with confinement enhancement of the same order as in elmy H mode, is in no way hampered by the use of the unshielded antenna. The comparison of shielded, unshielded and insulated antennas showed that, at least for TEXT O R antennas, the r.f. sheaths occur on the protection limiters of the antenna and not on the screen. These sheaths, however, do not seem very

R. Koch et al. / Fusion Engineering and Design 26 (1995) 103-120

harmful as their absence on the antenna with insulated limiters did not bring improvements of the antenna performance. A Langmuir probe was used to measure the residual density in the antenna box, at the level of the central conductor in the absence of screen. It was found that this low density plasma is blown away by the ponderomotive force as soon as a few kilowatts are applied to the antenna. With respect to possible future applications of the unshielded antenna concept the following points should be noted. The presence of a covering of the feeder area appears to be beneficial, at least with purely ohmic target plasmas. In the design of an unshielded antenna, care has to be taken to have a sufficiently low density (in the absence of r.f.) at the central conductor level such as to avoid, at least according to theory, the formation of r.f. sheaths on the central conductor. We also consider that direct parallel flow along field lines is to be strictly avoided as the failure of an earlier experiment with an electrostatic antenna [25] was ascribed to such a flow.

9. Conclusion In conclusion, the latest experiments on TEXTOR have allowed us to extend the improved confinement mode (I mode) to the high density region using the radiation belt concept. At the same time it was shown that, in this regime, stable long-pulse operation with high auxiliary power could be achieved with up to 90% of the power radiated in the outer plasma region. The high performance (I mode) conditions were also achieved with third harmonic D heating and balanced injection. In this case, a high neutron production is obtained even at high density. First experiments of 3He pump-out indicate that the expected interaction between the r.f. and the fast He ions is indeed taking place. Expulsion of central high Z impurities by the r.f. has been observed in high Z limiter tests. Finally we stress that all these experiments were constantly conducted using one unshielded antenna pair without any problem.

119

References [1] J. Winter et al., TEXTOR operation with silicon covered walls, 20th EPS on Controlled Fusion and Plasma Physics, Europhysics Conf. Abstracts, Vol. 17C, Part I, 1993, pp. 279 282. [2] U. Samm et al., Radiative edges under control by impurity fluxes, Plasma Phys. Control. Fusion 35 (1993) B167 B175. [3] J. Ongena et al., Improved Confinement in TEXTOR, Nucl. Fusion 33 (1993) 283 300. [4] A.M. Messiaen et al., High power ICRH and NB heating results in TEXTOR, Plasma Phys. Control. Fusion 32 (1990) 889-902. [5] G. Van Wassenhove et al., Study of third harmonic ICR heating of the beam heated deuterium plasma of TEXTOR, Europhysics Top. Conf. on Radiofrequency Heating and Current Drive of Fusion Devices, Europhysics Conf. Abstracts, Vol. 16E, 1992, pp. 141 144. [6] R. Van Nieuwenhove et al., Comparison of the performance of ICRF antennas with and without Faraday shield on TEXTOR, Nucl. Fusion 32 (1992) 1913 1925. [7] J. Jacquinot et al., JET recent results on wave heating and current drive. Consequences for future devices, Plasma Phys. Control. Fusion 35 (1993) A35 A52. [8] C.S. Chang, Control of energetic ion confinement by ion cyclotron range of frequency waves, Phys. Fluids B 3 (1991) 259 261. [9] C.S. Chang, J.-Y. Lee and H. Weitzner, Theory of energetic ion transport induced by waves of ion cyclotron range of frequencies in a tokamak plasma, Phys. Fluids B 3 (1991) 3429-3447. [10] F. Durodi6 and M. Vervier, Design of an automatic matching device for TEXTOR's ICRH system, Europhysics Top. Conf. on Radiofrequency Heating and Current Drive of Fusion Devices, Europhysics Conf. Abstracts, Vol. 16E, 1992, pp. 80-84. [11] J.D. Strachan et al., High temperature plasmas in the tokamak fusion test reactor, Phys. Rev. Lett. 58 (1987) 1004-1007. [12] A.M. Messiaen et al., Review of recent advances in heating and current drive on TEXTOR, Plasma Phys. Control. Fusion 35 (1993) 309- 315. [13] U. Samm et al., Plasma edge cooling by impurity radiation in a tokamak, Plasma Phys. Control Nucl. Fusion Res. 1 (1993)309 315. [14] J. Ongena et al., Improved confinement at high density with an all silicon wall in TEXTOR, 20th EPS on Controlled Fusion and Plasma Physics, Europhysics Conf. Abstracts, Vol. 17C, Part I, 1993, pp. 127 130. [15] J. Winter et al., Improved plasma performance in TEXTOR with silicon coated surfaces, Phys. Rev. Lett. 71 (1993) 1549-1552. [16] A.M. Messiaen et al., Edge radiative cooling with improved confinement at high densities and high heating power in TEXTOR, Nucl. Fusion 34 (1994) 825 836.

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[17] P.N. Yushmanov et al., Scalings for tokamak energy confinement, Nucl. Fusion 30 (1990) 1999 2006. [18] Lazarus et al. Confinement in beam-heated plasmas: the effects of low-Z impurities, Nucl. Fusion 25 (1985) 135149. [19] D. Van Eester and R. Koch, SWHAP: a code for modeling ICRF heating of nonmaxwellian populations, Europhysics Top. Conf. on Radiofrequency Heating and Current Drive of Fusion Devices, Europhysics Conf. Abstracts, Vol. 16E, 1993, pp. 129-132. [20] A.M. Messiaen et al., Review of combined ICRH-NBI results in TEXTOR, 10th Top. Conf. on Radio Frequency Power in Plasmas, Boston, MA, AIP Conf. Proc. 289 (1993) 32 35. [21] R. Van Nieuwenhove and G. Van Oost, Experimental study of sheath currents in the scrape-off layer during ICRH on TEXTOR, Plasma Phys. Control. Fusion 34 (1992) 525 532.

[22] Y. Ueda et al., Effects of impurities released from high-Z test limiter on plasma performance on TEXTOR, submitted for presentation at the l l t h Int. Conf. on PlasmaSurface Interactions, in Controlled Fusion Devices, 1 lth PSI, 1994. [23] R. Van Nieuwenhove et al., Ion Cyclotron heating of a tokamak plasma using an antenna without Faraday shield, Nucl. Fusion 31 (1991) 1770-1774. [24] R. Van Nieuwenhove et al., Evaluation of the ICRH performance using antennas without Faraday shield on TEXTOR, Europhysics Top. Conf. on Radiofrequency Heating and Current Drive of Fusion Devices, Europhysics Conf. Abstracts, Vol. 16E, 1992, pp. 137-140. [25] R. Koch et al., Study of coupling and edge effects during ICRF heating experiments using an electrostatic antenna on TEXTOR, Fusion Eng. Des. 12 (1990) 15 23.