GENERATION OF RF DRIVEN CUR RENTS BY LOWER-IIYBRID WAVE INJECTION IN THE VERSATOR II TOKAMAK

Transcription

1 I GENERATION OF RF DRIVEN CUR RENTS BY LOWER-IIYBRID WAVE INJECTION IN THE VERSATOR II TOKAMAK S.C. Luckhardt, M. Porkolab, S.F. Knowlton, K-I. Chen, A.S. Fisher, F.S. McDermott, and M. Mayberry Massachusetts Institute of Technology Department of Physics, Research Laboratory of Electronics and Plasma Fusion Center Cambridge, MA PFC/JA July, 1981 By acceptance or this article, the publisher and/or recipient acknowledges the U.S. Government's right to retain a non-exclusive, royalty-frec license in and to any copyright covering this paper.

2 GENERATION OF RF DRIVEN CURRENTS BY LOWER-HYBRID WAVE INJECTION IN THE VERSATOR II TOKAMAK* by S. C. Luckhardt, M. Porkolab, S. F. Knowlton, K-I Chen, A. S. Fisher, F. S. McDermott, and M. Mayberry Massachusetts Institute of Technology Department of Physics, Research Laboratory of Electronics, and Plasma Fusion Center Cambridge, MA ABSTRACT Experimental verification of lower-hybrid RF current drive in the Versator II tokamak is presented. The experiments show that efficient current drive exists only in low density discharges (ffe < 6 X cm- 3 ) in the "slide-away" regime where a preformed supra-thermal electron tail exists prior to the application of the RF power. *Work supported by the U.S. Department of Energy. 1

3 In tokamak-type plasma confinement devices the confining toroidal current is induced in the plasma by means of a pulsed ohmic-heating transformer'. From the engineering point of view a pulsed tokamak reactor is unattractive, and therefore ways must be found to drive the toroidal current continuously by other means, such as neutral beams 2 or radio-frequency (RF) power 3. Lower-hybrid travelling waves, launched by a phased array of waveguides, have recently been proposed as an attractive means of efficiently generating toroidal currents in tokamaks 4 ' 5. At low densities and temperatures in both a linear device 6 and a torus 7 it was shown that modest currents could be generated (a few amps with a few hundred watts of RF power) with unidirectionally launched lowerhybrid waves. In lower-hybrid injection experiments in tokamaks changes in the plasma loop voltage have been reported and identified with current drive at relatively low densities (1012 _ 0'l cm-3)8-2. In some of these experiments the inferred current was attributed to generation of runaway electrons 8 " 0 "',1. However, in the JFT- 2 experiment 9 good agreement with the Fisch theory 4 was claimed even though the rf pulse length, trf, was much shorter than the L/R time, t L/R, so that observation of an RF induced change in the toroidal current was not possible. Even though in the JFT-2 experiment a weak phase dependence of the loop voltage was found, changes in the loop voltage due to surface currents, current profile modifications, electron heating, and runaway production cannot be excluded. Therefore, the agreement with theory 4 ' 5 must be regarded as fortuitous rather than proven. In the present paper we present the first unambiguous experimental demonstration of a lowerhybrid wave driven current in a tokamak (which is possible in the Versator experiment since tl/r < tr). It is also shown that under normal circumstances current generation can be obtained only in the slide-away regime' 2 where a tail on the electron distribution exists even before the RF power is applied1 3. Thus the present results show that the Fisch mechanism of efficient lower-hybrid current drive (i.e., raised plateau formation) is not the dominant mechanism in these experiments. Furthermore, all the experimental results reported until now operate in this low density "slide-away" regime; therefore, at high density some mechanism must prevent the formation of the raised plateau, which is a prerequisite for efficient current generation 4. It is likely that an explanation of the lower-hybrid current drive results observed so far will require a detailed transport code which includes both a runaway or slide-away tail, a Fokker-Planck collision calculation, radial transport and a realistic model of wave absorption and heating in a tokamak discharge. The experiments were carried out on.the MIT Versator 11 tokamak: R, = 40 cm, a. = 13 cm, Iol = 30 ka, tpe = 30 msec, f, = 2 X X 10 "cm, Ti, = 120 ev, T,, = ev. The time 2

4 dependence of the density is controllable by gas puffing. The ohmic heating power is supplied by a 100 kjoule capacitor bank. The stainless steel vacuum chamber walls are conditoned by low density hydrogen discharges13 and by periodic localized titanium deposition which yields Zeff = 2 discharges. The RF source is an 800 MHz, 150 kw klystron with a modulator controlled pulse duration of 10 msec, or 30 msec at reduced power. The power is split into four or six well isolated channels and fed through continuously variable phase shifters into a four or a six waveguide grill. Present results were obtained using the six waveguide grill which has 0.8 cm gaps separated by 0.17 cm thickness walls, and 25.4 cm height. Prior to installation the stainless steel grill was baked in a vacuum oven at 350* C and during operation frequent RF processing, and periodic titanium coating ensured operation with up to 50 kw of total RF power (0.5 kw/cm 2 ) without RF breakdown or plasma formation in the guides. The grill is normally positioned 1 cm behind the limiter and changing the relative phase between adjacent waveguides, A#, from 600 to 120' moves the antenna spectrum peak from Nz = 4 to 11 with ANz/Nz ~ 0.8. If toroidal effects are taken into account16 with Te 0 ~ 250eV the waves should always damp on electrons in the regime neo < 1.5 X 1013 cm- 3 Furthermore, collisional absorption on the cold surface plasma accounts for no more than 25% of the injected power. Scattering on drift waves at the plasma surface does not significantly modify these results' 6. The basic effect associated with the injection of lower-hybrid power is shown in Fig. la. Total current and loop voltage signals are superimposed for two discharges with and without RF power injection. During the RF pulse there is a large increase in the total current above that due to ohmic heating only. The difference between the two currents, AIt, exceeds 10 ka at maximum..the decrease in the total current after about 15 msec results from a loss of radial equilibrium caused by vertical field supply limitations. For times between the electric field diffusion time TDE (-.5-1 msec) and the L/R time TL/R ( msec) simple circuit analysis indicates that the quantity Al/At is approximately proportional to the RF current where At is the RF pulse duration. 'The loop voltage signal initially decreases by.15 volts, then shows sudden spikes consisting of a rapid increase followed by a slower decrease. It can be seen in Fig. la that these spikes occur at the same time as sudden small decreases in the time derivative of the total'current, dil/dt. These voltage spikes generally become larger and more frequent at higher RF power. This voltage behavior has been reported in other lowerhybrid experiments' 0,1. Also evident in Fig. la is an interesting time lag of several miliseconds between initiation of the RF power pulse and the beginning of the RF driven increase in total current. Although at present it is not well-understood, it is likely that the time delay results from a slow evolution of the slide-away 3

5 tail on the electron distribution function, and can usually be eliminated by applying the RF power later in the discharge. The continuous slow drop in the loop voltage in both discharges is caused by limitations in the capacitor driven ohmic heating power supply. When a short RF pulse is applied at the time when the ohmically driven current is beginning to drop, the total current can be maintained at a constant level, dit/dt = 0 (Fig. 1b). The single turn loop voltage drops through zero and is maintained at a small negative level, V volt for the duration of the RF pulse. Interestingly, the loop voltage drop occurs rapidly, in approximately 100 pzsec; an RF current generated near the magnetic axis is expected to cause loop voltage changes on the classical electric field penetration time scale, =_ 1 msec. Therefore, it appears that in the case shown, Fig. 1b, the RF driven current extends, at least in part, to the plasma edge. Alternatively, anomalous electric field penetration could be responsible. Independent of electric field penetration effects, the observation of negative resistive loop voltage,with di/dt = 0, eliminates edge heating as an interpretation of the RF effects; increasing the plasma edge conductivity by heating could cause a voltage drop, but clearly could not by itself produce a negative loop voltage. Such an explanation would also be difficult in view of the observation of the large temperature drop (Fig. 3c) in the plasma center and the observed increase in total current. The incremental current, AIt, normalized to constant transmitted power shows a strong asymmetrical dependence on A# (see Fig. ic). The largest current increments are generated when A# = -90*, corresponding to a wave spectrum travelling in the direction of the electron ohmic drift. This value of the array phasing launches a wave packet with a spectrum peaked at Nz = 8 with a width ANz = 6, and approximately 90% of the power is directed along the drift direction with 10% going the opposite way. Interestingly, when waves are launched opposite to the electron ohmic drift, A# > 0, no decrease in total current is observed: instead, only a smaller current rise occurs. The results indicated in Fig. 1 are consistent with the generation of a toroidal current in the plasma by the travelling lower-hybrid waves. However, possible electron heating by the RF could result in conductivity increases and it may also cause an increase in the toroidal current. To eliminate this possibility it is necessary to measure the electron temperature with Thomson scattering, Figs. 2a, 3c. The central electron temperature during RF injection with waveguide phasing optimized for current drive, AO = -90' is not found to increase, Fig. 3c; instead, there is a cooling trend due to a decrease in the ohmic heating power, Fig. 3b. The cooling effect disappears for positive values of A# (anti-current drive injection) 4

6 and there may be a small temperature increase, Figs. 2a, b. Bulk electron heating is therefore eliminated as an explanation of the RF induced changes in current and loop voltage. Transport code simulations of the Versator current drive experiment have predicted a qualitatively similar electron temperature dependence on A014. The profile changes in the electron temperature can produce, through conductivity increases, a modification of the current profile and therefore an inductive voltage proportional to IdL/dt. However, such current profile changes due to heating will occur on the electron energy confinement time scale, of the order msec in Versator II. Voltage transients of one millisecond duration can be expected from this process. In the experiments constant incremental.voltage drops for the duration of the RF pulse, msec have been observed and therefore cannot be explained as profile changes. A model circuit calculation can easily be made to obtain an approximate value for the RF generated current. Considering the mutual inductive coupling between the plasma current, RF driven current, and OH transformer a simple expression for the loop voltage can be obtained: VL - R,(IT - IR - IRUN) + "IT where VL is the loop voltage, IT is the total plasma current, IpR is the RF driven current, IRUN is the current carried by runaway or slide-away electrons, R, is the Spitzer plasma resistance, and PIt is the inductive voltage component. Using this expression and the data in Fig. 3a, we find IRp ~ 10-15kA; and under optimized conditions IR kA has been obtained. We shall now present experimental evidence which shows that RF current can be initiated only in a low density regime characterized by the presence of an energetic electron tail before RF injection. The dependence of AIt on the line average electron density at the beginning of the RF pulse is shown in Fig. 2c for a series of deuterium discharges. The current rise, AIt is approximately independent of density for ie < 6 X 1012 cm-3. Above this density, AI ~ 0 and apparently RF current cannot be driven. Also indicated in Fig 2c is an experimental point obtained with lie e 4 X 1012 cm- 3 where AI = 0. In this type of discharge little or no RF current is driven, even though the density was low. Discharges of this type, which we call type II, are obtained by reducing the electric field applied to the plasma during the first millisecond of the shot and by increasing the initial hydrogen gas feed rate. At the time the RF is applied, the discharge indicated in Fig. 2c has nominally the same parameters, It, BO, ie, and plasma position as the discharges, type I, supporting current drive; however, the two discharges differ in that type I has a larger preformed supra-thermal electron tail than type II. Soft x-ray spectroscopy using a Kevex lithium doped silicon Si(Li) detector and pulse height analysis indicates greatly enhanced soft x-ray emission in the energy range of 2-5 kev, and an elevated tail temperature of typically 1.7 kev in type I discharges compared to a tail temperature of 900 ev in a type II discharge. Type 5

7 II discharges generally exhibit fractionally larger loop voltage levels than type I with other conditions of current and density the same. Therefore, on the basis of these data it appears that a preformed suprathermal electron tail of medium energy is a necessary prerequisite for efficient RF current generation. Other observations indicated in Fig. 3 are a sudden decrease in hard x-ray emission during RF current drive with a subsequent sudden increase at the end of the RF pulse. The line average density (Fig. 3d) shows a tendency to increase during RF current drive without a significant change in light impurity line radiation. When RF is injected into type II discharges no current is driven, and there is little or no increase in density. These phenomena are not well understood at present; however, more efficient recycling and/or ionization may occur during RF current drive. We recall that in several past lower-hybrid experiments production of the RF current has been attributed to RF enhanced runaway production 8 " 0 "'. However, such a runaway current cannot account for many of the present experimental results, particularly the generation of a negative loop voltage in Fig. 1b. Similarly, a large increase in the number of runaway electrons would be expected to cause an enhancement in the plasma electron cyclotron radiation at the second harmonic; however, the 2 wce radiation is found to decrease during RF current drive. The runaway production picture may also be difficult to reconcile with the observed decrease in the hard x-ray emission. Based on these results, plus the soft x-ray measurements, we conclude that the RF driven runaway effect alone does not explain the present experimental results. Rf current generation has also been obtained with a four waveguide antenna with n, < 4. This antenna is expected to interact with higher energy electrons so that RF induced runaway production may be more important. In summary, observations in the Versator II current drive experiment can be consistently described if we hypothesize that efficient absorption of the lower-hybrid power occurs only by damping on a preformed suprathermal electron tail in the energy range 1-5 kev (f/kte c- 4-20). This range of energies is equivalent to the "plateau" electron population in the conventional current drive theory 4. Such a slide-away electron tail is a well known feature of tokamak discharges of sufficiently low density. The tail absorption picture therefore accounts for the observed density limit, and the different behavior of discharges of types I and II with regard to current drive efficiency. Discharges with insufficient resonant tail electrons do not support efficient current generation. Since the slide-away tail is asymmetrically weighted in the direction of the electron drift, waves launched in the opposite direction to the electron ohmic drift do not lead to efficient RF drive in agreement with experiment (see Fig. Ic and 2b). At present we do not understand what prevents efficient current gener- 6