Chen, K.I.

Luckhardt, S.C.

Mayberry, M.J.

Porkolab, M.

DSpace@MIT

PFC/JA-87-2Particle Confinement Improvement During2.45GHz Lower-Hybrid Current Drive ExperimentsM.J. Mayberry, K-I. Chen, S.C. Luckhardt, M. PorkolabJanuary 1987Plasma Fusion CenterMassachusetts Institute of TechnologyCambridge, Massachusetts 02139 USAThis work was supported by DOE Contract No. DE-AC02-78ET-51013.Submitted for publication to Physics of Fluids.Particle Confinement Improvement During2.45GHz Lower-Hybrid Current Drive ExperimentsM.J. Mayberry,") K-I. Chen, S.C. Luckhardt, M. PorkolabPlasma Fusion CenterResearch Laboratory of ElectronicsandDepartment of PhysicsMassachusetts Institute of TechnologyCambridge, Massachusetts 02139AbstractParticle confinement behavior during 2.45 GHz lower-hybrid current drive has beeninvestigated on the Versator II tokamak. It is found that during combined Ohmic and rfcurrent drive the global particle confinement time, r,, increases by up to a factor of twocompared to purely Ohmically driven discharges, as observed in earlier 800 MHz experi-ments at lower densities , <; 6 x 1012 cm- 3 [Phys. Fluids 29, 1985 (1986)]. In the presentexperiments, r, increases have been observed at densities up to i, = 2 x 1013 cm-3a) Present Address: GA Technologies Inc., San Diego, CA 92138.1Density increases have been observed previously in a number of tokamak experimentsin which lower-hybrid current drive has been combined with inductive ohmic heating tomaintain or increase the plasma current. 1 4 Such density increases can result from threepossible causes: 1) increased ionization of hydrogen (due to an increase in particle recy-cling), 2) increased influx of impurities, or 3) an increase in the particle confinement time.In previous Versator II 800 MHz experiments' it was shown that the density increases ob-served during rf current drive were due to a factor-of-two increase in the global particleconfinement time, r,, compared to purely ohmically driven discharges. However, becauseof the low rf source frequency used in those experiments, the confinement investigationswere restricted to the relatively low-density regime below the density limit for current drive(f, < 6 x 1012 cm- 3 ). 1We report here on recent particle confinement studies carried out during 2.45GHzlower hybrid current drive experiments. As reported previously,'' 7 with the higher rfsource frequency, plasma current increases, loop voltage drops and enhancements in thenonthermal electron cyclotron emission have been observed at densities up to , ~ 2 x1013 cm-3, approximately a factor of three higher than the 800 MHz density limit. Ata line-averaged density of f', ~ 1.3 x 103 cm-3 , factor-of-two increases in r, have beenobserved during the injection of 65 kW of 2.45 GHz rf power, compared to purely ohmicallydriven plasmas. The rp improvement increases linearly with the 2.45 GHz rf power, untilthe maximum line-averaged density during the rf pulse approaches i, 2: 2 x 1013 cm- 3 .Above this density, 2.45 GHz current drive effects become negligibly small, and the particleconfinement improvements also disappear.The experiments were carried out on the Versator II tokamak (R = 40.5 cm, a =13 cm, Bt <; 15 kG, I, = 30 - 40 kA). Lower-hybrid slow waves were excited in the plasmausing a stainless steel four-waveguide-array antenna, mounted on an outside port of thetokamak. The individual waveguide dimensions were: 1.0 cm width, 8.64 cm height, 0.15 cmwall thickness. With a relative waveguide phasing of +7r/2, the calculated Brambilla powerspectrum of the antenna ranges from NII = ckIj/w = 1 to NII = 5, with 75% of the totalpower launched in the direction of the electron ohmic drift velocity. With the 2.45 GHzrf system, quasi-steady-state fully rf-driven discharges have been achieved at densities up2to ie = 1 x 1013 cm-3 .6 At higher densities, flat-topping the plasma current has not beenpossible due to insufficient rf power available (P,.1 < 95kW). Therefore, in order to studythe global particle confinement properties of rf current-driven plasmas at high densities,we have injected rf power into ohmically sustained plasmas, comparing the global particleconfinement of the plasma immediately before and during the rf injection. For all the datapresented, the rf pulse length (7-10 ms) was long compared to the estimated bulk particleconfinement time (1 - 2 ms).The typical density behavior during ohmically heated discharges with and withoutrf injection is shown in Fig. 1. The gas feed rate, controlled with a piezoelectric puff valve,was held constant during the entire time interval shown. The line-averaged density beforethe rf firing time at t = 20ms is f, = 1.1 x 1013 cm- 3 . Without rf, the density remainsnearly constant. In contrast, when 65 kW of rf power is injected with +r/2 waveguidephasing, the density rises by nearly a factor of two, to ff, = 2.0 x 1013 cm- 3. The densitytypically starts to rise approximately 1 ms after the beginning of the rf pulse. Once themaximum value is reached, in this case 6 ms later, sawtooth oscillations with slow-rise andfast-fall characteristics appear on the density trace. During the remainder of the rf pulse,the density decays slightly or remains constant, depending on the in/out plasma motion.Following the termination of the rf pulse, however, the density always decays rapidly.The ionization rate of hydrogen is determined from the brightness of the H, lineemission. For the plasma conditions in these experiments, the Ha emissivity is proportionalto the volumetric hydrogen ionization rate.' As shown in Fig. 1, the Ha brightness levelmeasured near the edge of the plasma decreases during the density rise. In the ohmicdischarge, with the same rate of gas feed, the H, brightness level remains nearly constant.The maximum reduction in the H, brightness level during the rf pulse is approximately30%. Note, however, that once the density reaches its maximum level of fi, = 2.0 x1013 cm~ 3 , the Ha emission quickly returns to its original level.The behavior of impurity species was also monitored during rf injection using theVUV monochrometer. As shown in Fig. 1, the brightness of the 0 V impurity line (emittedfrom the outer region of the plasma) remains constant during the first 7 ms of the rf pulse,during the density rise. Once the density maximum is reached, however, the 0 V brightness3increases slightly. Other impurity lines surveyed, including C V (emitted near the plasmacenter) and C III (emitted near the edge) exhibit similar temporal behavior. We concludefrom these measurements that the observed density rise during current drive cannot bedue to an influx of impurities, nor to an increase of hydrogen ionization, but rather to animprovement in the global particle confinement time r.. A quantitative estimate of the r,increase requires spatial profile measurements of the density and the H0 emission.The global particle balance equation is given by:N,Ne= (1)S dN,/dt'where N, is the total number of electrons, and S is the total ionization source term. Thetotal number of electrons is given by the volume integral of the density profile,N,(t) = 47r2R rn,(r, t) dr, (2)Towhere n. (r, t) is obtained from Abel-inverted 4 mm interferometer data. The H' emissionwas monitored at various times from three different toroidal locations. Two availabletangential side ports afforded a direct view of the limiter and the rf antenna. A third port,located below the tokamak, was used for radial scans of the plasma cross section. Thetime dependence of the H0 signal was approximately the same at each toroidal location.Hence, following Ref. 5, we have assumed that the temporal evolution of S is independentof the toroidal location, deducing relative changes in S from the Abel-inverted H, profilemeasurements. The ionization source term is then given by:S(t) xo rE,(r, t) dr, (3)where it has been assumed that the H0 emissivity, Ec, is proportional to the volumetrichydrogen ionization rate." The source term S(t) is then normalized, using Eq. (1), bychoosing a reasonable value for r, just prior to rf injection which is consistent with theobserved density behavior in Ohmic discharges.Abel-inverted profiles of the density n,(r) and the H0 emissivity E0 (r) are shownin Fig. 2. The line-averaged density just before the rf injection (t = 19 ms) is f, = 1.3 x41013 cm- A. As the density rises during the rf pulse to a maximum of H, = 1.9 x 1013 cm- 3,the density profile steadily broadens. This is in contrast to the low density 800 MHzexperiments on Versator5 where the density profile become more peaked during rf injection.The H, emissivity profile, which is localized to the outer region of the plasma cross section,remains nearly unchanged.The time histories of N,, S andr, shown in Fig. 3 are very similar to those measuredin the earlier 800 MHz experiments.' While the total number of electrons in the plasmaincreases by 50%, the total ionization source term decreases by more than 30%. Hence,the global particle confinement time r,, calculated from Eq. (1), increases by a factor oftwo during the 2.45 GHz rf pulse. The improvement in r, appears to saturate and evendecline somewhat as the line averaged density rises to a level near R. = 2 x 1013 Cm- 3 .The particle confinement improvement during 2.45 GHz current drive varies withthe injected rf power level as shown in Fig. 4. In this power scan the initial density wasW,. = 1.3 x 1013 cm- 3 . With +7r/2 current drive phasing, the rise in the density, Ae, andthe drop in the H, brightness increase linearly with the rf power level, except at the highestpower levels where the effects begin to saturate. The global particle confinement timeestimated from these density and H, measurements shows a similar behavior, increasingwith the applied rf power level until AR, becomes so large that the line-averaged densityexceeds li, = 1.8 x 1013 cm- 3 . The saturation effect could be due to the increase in theinjected rf power level, or to the increase in density. It is interesting to note, however, thatthe saturation of r, coincides with the reduction of other current drive effects at densitiesnear , ~ 2 x 1013 cm-3. For two of the data points shown in Fig. 4, the antenna phasingwas reversed to A4 = -7r/2, such that the waves were launched in the anti-current drivedirection. In these cases, the increase in r, was less than half of that obtained with +7r/2phasing at a comparable power level.The dependence of the density rise during the rf pulse, AK., on the target plasmadensity level just prior to rf injection is shown in Fig. 5. The rf power level was in therange Pf = 60 - 70kW, except at the lowest densities where it was reduced slightlyin order to avoid excessive outward plasma motion caused by large rf induced currentincreases. As shown in Fig. 5, the density rise during rf injection can be as large as a5factor of two when the initial density is sufficiently low (We < 1 x 10 3 cm-). At higherdensities, the effects become smaller. There also appears to be an upper density limit forimproved particle confinement. For a toroidal field strength of 11kG, this limit occursat a density of f, ~ 2.1 x 1013 cm-'. Below this limit, density rises are accompaniedby decreases in the H, emission. Above the limit, slight density decreases are observed,accompanied by small increases in the H, emission. These latter observations indicate thatthe bulk particle confinement may actually be degraded by the rf at the higher densities. Asimilar transition in particle confinement behavior was observed at the upper density limitfor strong wave-electron interaction (ft, ~ 5 x 1013 cm- 3 ) on the FT tokamak,' where a2.45 GHz rf frequency was used as well. As shown in Fig. 5, when the toroidal field strengthis lowered from 11 kG to 9 kG, the density rise observed during rf injection is substantiallyreduced. In addition, the upper density limit for improved particle confinement decreasesto 5 ~ 1.5 x 1013 cm 3 . This dependence on the toroidal magnetic field suggests that poorwave accessibility may be responsible for the upper density limit for improved r, during2.45 GHz current drive. Note that at the interception points (Af,, = 0), w,(0)/w2,(0) ~2.6, a relatively high value of the dielectric constant.From our results, it appears that the improvement in global particle confinement ob-served during 2.45 GHz lower hybrid current drive is due to the rf generation of a quiescentsuperthermal electron tail. The dependences of r, on rf power, antenna phasing and plasmadensity resemble those of other current drive effects observed such as the incremental cur-rent increase7 and the nonthermal cylotron emission' (see Fig. 1). We note that in theearlier 800 MHz experiments at low densities, -r improvements were observed during cur-rent drive only when a relaxation instability of the fast electron tail due to the anomolousdoppler effect1 0 was suppressed by the injection of sufficient rf power levels. Under certainconditions, the threshhold power level required for complete tail mode suppression was aslow as 4kW. However, increasing the rf power level well above this threshhold producedno further improvement in r,, despite significant increases in the rf-driven current. For allthe 2.45 GHz data presented here, the level of plasma hard x-ray emission prior to the rfinjection was negligible, indicating that few energetic (E > 20 keV) electrons were present.In addition, there was no evidence of bursting rf emission either before or during the rfpulse which would signify tail mode oscillations. Improvements in r, scaled linearly with6the rf power level, with no evidence of a power thresshold. Despite these contrasts, thefollowing conclusion appears to be consistent with the results from both experiments: thegeneration of a quiescent superthermal electron tail with lower hybrid waves is correlatedwith an improvement in the global particle confinement.In order to explain the factor-of-two density increases observed in either experiment,a mechanism which links the presence of a stable superthermal electron tail to bulk particleconfinement must be established. Possible candidates include: 1) a reduction in magneticfluctuations due to suppression of MHD activity during current drive, 2) a modification ofthe electrostatic potential profile of the plasma, and 3) a reduction in low frequency driftwave activity. So far, we have investigated only the first of these possibilities. Preliminarymeasurements of the MHD activity during current drive have been carried out using Bloops to measure poloidal field fluctuations and a gas-filled proportional counter to measuresoft x-ray fluctuations. Typically, during the period of density rise, the level of poloidalfield fluctuations appears to decrease as indicated by a reduction in the amplitude of the besignal. Once the density reaches the maximum level, however, the poloidal field fluctuationsreturn to their original level. Also at this time, the sawtooth oscillations appear on thedensity trace as well as on the soft x-ray signal from the center of the plasma. Whilethese effects may indicate a change in the MHD behavior, it is not known whether thisis the cause of the sudden degradation in particle confinement when the density reachesH, ~ 2 x 1013 cm- 3 (see the H, trace in Fig. 1). More detailed studies of the MHD activityduring current drive are clearly warranted.In summary, we have studied the particle confinement behavior during combinedohmic/LHCD discharges with a 2.45 GHz source frequency. This has allowed us to inves-tigate lower-hybrid current drive at relatively high densities (fi. > 8 x 1012 cm- 3), andto extend the density regime over which improved particle confinement was obtained inearlier 800 MHz experiments. During lower hybrid current drive, the particle confinementtime increases typically by a factor of two with 65kW of rf injection and A0 = +7r/2waveguide phasing. The r, increases observed during rf injection disappear at densitiesabove 5i, ~ 2 x 1013 cm-3, where other current drive effects also diminish. At these densi-ties, Wo(0)/w2 > 2, and the accessibility of low-N waves, which are the most efficient in7driving current, is expected to be severely restricted. Although the particle confinementimprovement appears to be correlated with the amount of rf driven current, further inves-tigations will be required to determine the physical mechanism linking the generation of afast electron tail to bulk plasma confinement.AcknowledgmentsThis work was supported by the U. S. Department of Energy under Contract No. DE-AC02-78ET-51013.8References1. S.C. Luckhardt, M. Porkolab, S.F. Knowlton, K-I. Chen, A.S. Fisher, F.S. McDer-mott, M.J. Mayberry, Phys. Rev. Lett. 48, 152 (1982).2. K. Ohkubo, S. Takamura, and the JIPPT-II Group, Proceedings of the 3rd JointVarenna-Grenoble International Symposium on Heating of Toroidal Plasmas, editedby C. Gormezano, G.G. Leotta, and E. Sindoni (Commission of the European Com-munities, Brussels, 1982), Vol. 2, p. 543.3. G. Tonon, G. Gormezano, C. Cardinalli, M. El Shaer, W. Hess, G. Ichtchenko,R. Maone, G. Melin, D. Moreau, G.W. Pacher, H.D. Pacher, F. Soeldner, andJ.G. Wegrowe, Proceedings of the 3rd Joint Varenna-Grenoble International Sym-posium on Heating of Toroidal Plasmas, edited by C. Gormezano, G.G. Leotta,and E. Sindoni (Commission of the European Communities, Brussels, 1982), Vol. 2,p. 623.4. K. Uehara and T. Nagashima, Proceedings of the 3rd Joint Varenna-Grenoble In-ternational Symposium on Heating of Toroidal Plasmas, edited by C. Gormezano,G.G. Leotta, and E. Sindoni (Commission of the European Communities, Brussels,1982), Vol. 2, p. 485.5. S.C. Luckhardt, K-I. Chen, M.J. Mayberry, M. Porkolab, Y. Terumichi, G. Bekefi,F.S. McDermott, R. Rohatgi, Phys. Fluids 29, 1985 (1986).6. M.J. Mayberry, M. Porkolab, K-I. Chen, A.S. Fisher, D. Griffin, R.D. Kaplan,S.C. Luckhardt, J. Ramos, R. Rohatgi, Phys. Rev. Lett. 55, 829 (1985).7. M.J. Mayberry, M. Porkolab, K-I. Chen, R.D. Kaplan, S.C. Luckhardt, R. Rohatgi,Radiofrequency Plasma Heating (Sixth Topical Conference, Callaway Gardens, GA,1985), edited by D.G. Swanson (American Institute of Physics, New York, 1982),p. 162.8. L.C. Johnson and E. Hinnov, J. Quant. Spectrosc. Radiat. Transfer 13, 333 (1973).9. V. Pericoli-Ridolfini, Plasma Phys. and Controlled Fusion 27, 709 (1985).10. V.V. Parail and O.P. Pogutse, Nucl. Fusion 19, 785 (1979).9Figure CaptionsFig. 1. Traces showing the temporal evolution of the line-average density, H, brightness, 0V impurity line brightness, and the 2we emission during ohmically heated dischargeswith and without 2.45 GHz rf injection. In both cases the gas feed rate was fixedduring the entire time interval shown.Fig. 2. Abel-inverted density and Ha emissivity profiles before and during rf injection. Theline-averaged density rises from ff, = 1.3 x 1013 cM- 3 just before rf injection (t =19 ms) to a maximum level during the rf pulse of W, = 1.9 x 103 cm- 3 at t = 26.5 ms.Fig. 3. Temporal evolution of the total electron number N,, the ionization source term S,and the global particle confinement time r, calculated from the profiles of Fig. 2.Fig. 4. Density rise An,, H, brightness level, and global particle confinement time versusthe rf power level for an initial density of W, = 1.3 x 1013 Cm-3 and A4 = +7r/2(current drive phasing). Also shown are two points (triangles) with the waveguidephasing reversed to A4 = -r/2.Fig. 5. Dependence of the density rise during rf injection, Anit, on the density of the ohmictarget plasma just prior to the rf injection.102.0-Eu 1.51.0 Prf =65kW1.0 -cn0.5203 0.503--- - -r---- - ----- ---15 20 25 30 35TIME (ms)I,19 ms (Ohmic):.-.-.:a,---24 ms (Rl-s --- 265 ms (F'.15r(cm)F)RF)103H-0f-l10CIIniC) II2.--% 2.0d* 1.5-1 2o \0-z -8C-,0U2 23E20 25 30TIME (msec)I I I I ~F~FTE0C+900-900I I p III I I iAA- . *20 40 60P (kW)rfI--0.60.40.2 0A1.51.41.3,EE1.61.41.21.00 80I I II'.4III*1IIci0060 00.S 00II0 0~0* * I00d 0 00In0(00I-1 6S7(qP3 2101) au-v

Particle confinement improvement during 2.45 GHz lower hybrid current drive experiments

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Particle confinement improvement during 2.45 GHz lower hybrid current drive experiments

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