CHAPTER 13: CURRENT DRIVE IN ASDEX UPGRADEoldweb.reak.bme.hu/fileadmin/user_upload/felhasznalok/pokol/... · CHAPTER 13: CURRENT DRIVE IN ASDEX UPGRADE ALBRECHT STÄBLER,* JÖRG HOBIRK,

CHAPTER 13: CURRENT DRIVE IN ASDEX UPGRADEALBRECHT STÄBLER,* JÖRG HOBIRK, FRITZ LEUTERER, FERNANDO MEO, andJEAN-MARIE NOTERDAEME Max-Planck-Institut für Plasmaphysik, 85748 Garching, Germany

Received March 14, 2003Accepted for Publication May 18, 2003

External current drive (CD) is an important pre-requisite for the control of the plasma current profile inadvanced tokamak scenarios as well as for the develop-ment of quasi-stationary, fully noninductivly driven to-kamak discharges. On ASDEX Upgrade, three heatingsystems, neutral beam injection, ion cyclotron resonanceheating, and electron cyclotron resonance heating, areavailable for this purpose. The status of CD modelingand the CD capability of these systems are reviewed, anda brief overview is provided of what has been achievedexperimentally with respect to CD in various dischargescenarios.

KEYWORDS: noninductive current drive, current profilecontrol, advanced tokamak scenarios

I. THE ROLE OF CURRENT DRIVE INTHE ASDEX UPGRADE PROGRAM

The study of advanced tokamak scenarios, i.e., theoptimization of plasma performance and the sustain-ment of stationary tokamak operation, requires means toexternally drive a noninductive plasma current and, inaddition, to control the current profile. Performance op-timization includes the suppression and0or avoidance oflocal instabilities, such as neoclassical tearing modes~NTMs!, as well as the improvement of confinement bycreating and maintaining internal transport barriers~ITBs!. The latter are experimentally related to the exis-tence of flat or inverted profiles of the safety factorq~r !in the plasma center, which illustrates the need for localcurrent control. Experiments concerning advanced toka-mak scenarios have been an important part of the ASDEXUpgrade program in the past~see Chap. 4! and, based onthe improved current drive~CD! capabilities describedlater, will become an even more important issue in thefuture.

The possibility of achieving quasi-stationary, fullynoninductively driven plasmas in ASDEX Upgrade witha plasma currentIP of ;1 MA has been assessed byperforming transport simulations.1,2 For these simula-tions, ITB discharges were taken that offer the advantageof a high fraction of the internal diffusion-driven boot-strap current~'70% of Ip! due to the steep pressuregradient associated with the transport barrier. Stationarysustainment of reversed or low magnetic shear with aminimum value ofq~r ! . 1 was shown to require anexternally driven current of;250 kA peaked at halfplasma radius~off-axis CD!. These considerations trig-gered the modifications of the neutral beam injection~NBI ! system as described in Sec. III.A. The pulse lengthcapability of 10 s now available on ASDEX Upgrade isapproximately twice a typical resistive diffusion time-scale and, therefore, allows meaningful CD studies.

In addition to discharge optimization by local CD,basic studies of ion cyclotron current drive~ICCD! are afurther aspect of CD within the ASDEX Upgrade pro-gram. ICCD requires launching of an asymmetric powerspectrum by an appropriate antenna phasing and damp-ing of the wave energy onto the plasma electrons. Thereare two main ICCD techniques: Current can be drivenin the plasma center by fast waves@fast wave currentdrive ~FWCD!# in the absence of ion cyclotron reso-nances inside the plasma as well as after mode conver-sion to ion Bernstein waves@mode conversion currentdrive ~MCCD!# in a plasma with an admixture of a sec-ond ion species such as3He. In the latter case, off-axisCD is possible, depending on the operating condition~vandB! and on the relative concentration of the two ionspecies. ICCD is being used to encompass both FWCDand MCCD.

This paper is organized as follows: First, a shortsummary is given of the tools available to model thedifferent CD methods and to analyze discharges with asignificant part of the plasma current being drivennoninductively. For external CD on ASDEX Upgrade,three heating systems as described in Chap. 2 are avail-able. Their CD features are discussed next. The remain-ing section is then devoted to the experimental resultsobtained so far and gives a brief outlook on futureexperiments.*E-mail: [email protected]

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II. CURRENT DRIVE MODELING

ASDEX Upgrade discharges are routinely analyzedusing the 112

_-dimensional transport code ASTRA~Ref. 3!,which takes into account the actual plasma geometry andmeasured density and temperature profiles. In addition,ASTRA is used to predict the plasma behavior by apply-ing various transport models. This code includes a neu-tral beam package with a simplified beam depositionmodel, a Fokker-Planck treatment of fast ion slowingdown, and a determination of neutral beam current drive~NBCD! according to Ref. 4. Beam deposition can bechecked against the more detailed but time-consumingcalculations by the FAFNER code,5 which takes intoaccount the exact beam geometry as well as the ionoptics of the beam sources and follows individual ionorbits during slowing down using Monte Carlo tech-niques. ASTRA simulations of discharges with ion cy-clotron resonance heating~ICRH! require introductionof heating power deposition and CD profiles calculatedwith the stand-alone codes described later as an addi-tional input, whereas for simulations of electroncyclotron resonance heating~ECRH! discharges, anECRH package has recently been linked to the ASTRAcode.

Numerical modeling of electron cyclotron currentdrive ~ECCD! is done with the TORBEAM code.6 Inthis code, the beam tracing technique7 is used to de-scribe propagation and absorption of a Gaussian wavebeam. As in the standard ray-tracing approach, Max-well’s equations are reduced to a set of first-order ordi-nary differential equations. The beam-tracing method,however, allows for diffraction effects, neglected by thegeometric optics procedure. The whole beam can be spec-ified in terms of the trajectory of the beam axis and theevolution of a set of parameters connected with the cur-vature of the wave front and the width of the radiationfield profile. The calculation of a large number of raysthus becomes unnecessary. Moreover, the influence ofthe diffractive broadening of the beam on the power de-position and driven current profiles is taken automati-cally into account.

Two three-dimensional finite element full-wave codesFELICE and TORIC are used to gain insight into thephysics of ICCD. Both codes solve the finite Larmorradius~FLR! equations in the ion cyclotron frequencyrange. They describe the compressional and shear Alfvénwaves and the lowest ion Bernstein~IB! waves excitedby mode conversion. Both include electron Landau damp-ing of IB waves. FELICE is a full-wave code in a slabgeometry. The code solves the wave equations over annf spectrum in either the whole plasma with a metallicboundary or with an outward radiation condition at somepoint in the plasma. The latter procedure helps to inves-tigate single-pass absorption. FELICE also contains aself-consistent antenna evaluation section, which can cal-culate~in simple geometry! the power spectra and an-

tenna loading resistance. TORIC~Ref. 8! solves the FLRequations in an arbitrary toroidal geometry at one valueof 1nf. It calculates the CD in two ways. The Ehst-Karney parameterization9 uses thek5 calculated by TORICand takes into account toroidal effects such as trappingand assumes that each toroidal wave number drives cur-rent independent of the others. As a second way, the CDcalculation uses a Fokker-Planck equation for the elec-trons.10 The TORIC code has been benchmarked forFWCD to DIII-D experiments11 shown in Fig. 1. Thefigure shows little discrepancy between CD efficiencyfrom Ehst-Karney parameterization and Fokker-Planckcalculation. At higher temperatures and higher phasevelocities, which have not yet been explored, the differ-ences may be greater. Figure 2 shows the radial CD den-sity for the present ion cyclotron resonance frequency~ICRF! system on ASDEX Upgrade for the same plasmaconditions.

Fig. 1. Calculated fast-wave radial profiles of the CD density~top! and the power density~bottom! for DIII-D. Thesuperimposed dashed lines constitute the experimen-tally determined profile. For details on parameters,see Ref. 11.

Stäbler et al. CURRENT DRIVE IN ASDEX UPGRADE

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III. CURRENT DRIVE SYSTEMS ON ASDEX UPGRADE

Recently, one of the existing neutral beam injectorswas optimized with respect to off-axis CD, and an up-grading of the ECRH system was initiated. On this basis,the CD capabilities of the ASDEX Upgrade tokamak asdescribed in the following are well suited to focus theexperimental program more strongly toward studies ofadvanced scenarios.

III.A. NBI System Optimized for Off-Axis Current Drive

The northwest injector~NI-2! operated at 93 kV~seeChap. 2, Sec. B! has recently been modified12 to providethe aforementioned off-axis CD by NBI. The reorienta-tion of this injector in a more tangential injection direc-tion avoids generation of fast ions on trapped orbits andincreasesv5 of the circulating ions—both resulting in ahigher CD efficiency. Tangential injection, however, wasrestricted by technical boundary conditions to a radius of

tangencyRT 51.29 m for the two more tangential beams~CD beams!, which is smaller than the major plasmaradiusR0 5 1.65 m. Off-axis deposition around a nor-malized flux radius~ r! of 0.5 was therefore achieved byincreasing the vertical inclination angle~b, measuredwith respect to the midplane! of these two beams fromb 5 64.9 to66.7 deg and by shifting the injector fartherfrom the plasma by;0.5 m. In addition, the inclinationof the two CD beams can be varied byDb 5 60.5 degvia remote beam steering between pulses correspondingto a shift of the deposition maximum byDr ' 60.1. Theremaining two perpendicular beams of this injector~RT 50.84 m! are still available for central heating. An upgrad-ing of this injector to 100-kV operation is foreseen forthe future. All these modifications required a new port tobe welded into the tokamak vessel. The new beam ge-ometry compared with that of the southeast injector~NI-1!is illustrated in Fig. 3.

The deposition profiles of the fast ions for a stan-dard ASDEX Upgrade equilibrium and measured den-sity and temperature profiles have been calculated usingthe aforementioned FAFNER code. For the chosen ge-ometry of the CD beams, a broad deposition with a max-imum atr > 0.5 is obtained~see Fig. 4!. The ASTRAcode was applied to determine the current profile drivenby these beams. Beam deposition and driven current pro-files were shown to be rather similar12 with some devi-ations close to the plasma edge, where trapped particlesand orbit losses are expected to play a role. At 100 kV,D0 injection into a D1 plasma withTe~ r 5 0.5! ' 2 keVand ne~ r 5 0.5! ' 4{1019 m23, a total driven currentINBCD ' 250 kA is calculated forPNBI 5 5 MW, whichmeets the aforementioned requirement. This value scaleswith ~10ne! but increases less than linear withTe becausefor higher temperatures, higher injection energies are re-quired for optimum CD.

Plasma equilibria in ASDEX Upgrade are possiblewith significantly different vertical positions of the mag-netic axis~zp, measured with respect to the midplane!.The influence of the vertical plasma position on the de-position of the CD beams has therefore been studied~DzP 5 22 cm a 112 cm!. The result is shown inFig. 4. Whereas the deposition maximum of individualbeams varies strongly withDzP, the deposition of bothCD beams together is remarkably robust against verticalplasma shifts: the peak of the profiles stays atr > 0.5and only forDzP . 8 cm is nonnegligible central depo-sition observed.

The described modifications have recently been com-pleted; the injector is back in operation and now avail-able for experiments.

III.B. ECCD Capabilities on ASDEX Upgrade

Electron cyclotron current drive can be achievedby launching the microwave beam in a direction with atoroidal component. This is achieved using steerable

Fig. 2. Calculated fast-wave radial profiles of the CD density~top! and the power density~bottom! for ASDEX Up-grade for the same plasma parameters as DIII-D exper-iment of Ref. 11.



mirrors, which allow toroidal launching angles in the630-deg range and, thus, both co- and counter-CD. Po-larizer mirrors in the transmission lines allow excitingthe strongly damped second harmonic X-mode with theproper elliptical polarization. In the present ECRH sys-tem~which is described in more detail in Chap. 2 of thisissue!, the launching angles are changed between pulses,but in the future system, it will be possible to adjust themduring a pulse. This modification will be part of the up-

grading of the ECRH system described in Chap. 2, Sec. V.So far, 43 0.5 MW of ECRH power at 140 GHz isavailable from four gyrotrons for a 2-s pulse length. Even-tually, this will be 43 1 MW for up to 10 s at variousfrequencies between 105 and 140 GHz. These systemenhancements will make ECCD an even more versatiletool for CD and current profile control.

The CD efficiency and the driven current profiledepend on the launching angle, on the location of the

Fig. 3. Beam axes of the new tangential injector~NI-2! for off-axis CD. Left: projected into the poloidal plane. Right: projectedinto the horizontal midplane. For comparison~thin lines!, geometry of NI-1.

Fig. 4. Neutral beam deposition profiles H~ r! for individual beams~left! and for top plus bottom beam~right! for varying thevertical plasma position~PNBI 5 2.5 MW0beam!.



deposition center in the plasma, and on electron densityand temperature. The efficiency is maximum in the cen-ter and decreases in the off-axis case due to trapped elec-trons. For the present beam geometry and for central CDwith a toroidal launching angle of620 deg, a drivencurrent of IECCD ' 5 PECCD Te0ne ~kA, MW, keV,1020 m23! has been calculated.

III.C. ICCD Capabilities on ASDEX Upgrade

The ICRF system at ASDEX Upgrade~see Chap. 2,Sec. IV! has four generators~30 to 120 MHz!, each ca-pable of delivering up to 2 MW of power via coaxialfeeding lines to four double-strap antennas.13 The sys-tem has been upgraded with crossover feeding lines pro-viding 90-deg phasing between straps necessary for CD~see Fig. 5!. At present, CD is set up at 30 MHz. Thesymmetric power spectrum~heating case! peaks atnf 512, while the asymmetric power spectrum~CD case! peaksatnf 5 6, wherenf 5 k5R5 Rvn5c21. The directionality~i.e., ratio of the power in either direction! of the asym-metric power spectrum at 30 MHz is;66%. Figure 6shows the asymmetric power spectrum and the singlepass absorption~SPA! as a function of toroidal wavenumbernf. The SPA shown has been calculated for dif-

ferent plasma temperatures at a central density of 4.331019 m23. The predicted FWCD for ASDEX Upgradewith its present antenna at 30 MHz atTe~0! 5 6.5 keVand central density of 4.33 1019 m23 is 49 kA0MW ofabsorbed FW power. This value scales withTe0ne in thisparameter range. A calculation of the MCCD is difficultat present due to simulation difficulty of resolving thesmall wavelengths of the IB waves. The work is stillongoing.

IV. CURRENT DRIVE EXPERIMENTS

For some of the ASDEX Upgrade experiments pre-sented elsewhere in this paper, an externally driven non-inductive plasma current is an essential ingredient of therespective discharge scenario. A prominent example forthis is the stabilization of~3,2! NTMs. Here, it has beenshown that the current driven within the saturated island~;15 kA! by toroidal launching of ECCD~co-CD,215 deg! plays an important role in the stabilizationprocess.14 Pure ECRH~i.e., no toroidal launching! at theposition of the island has only a minor effect on theNTM amplitude. A further example is obtained whenapplying central co- or counter-CD by ECRH in an ITBdischarge produced by programmed rampup of currentand NBI heating power,15 leading to reversed centralshear. In case of central counter-ECCD during the cur-rent ramp, the centralq value was kept atq0 ' 2, andhigh ion and electron temperatures were obtained simul-taneously during this transient phase. Both experimentsmentioned so far are described in detail elsewhere: NTMin Chap. 9 and ITB in Chap. 4. The remainder of thissection deals with significant CD by neutral beams in the

Fig. 5. Schematic of ICRF system showing the crossover feed-ing lines providing the 90-deg phasing between straps.

Fig. 6. Present asymmetric antenna spectrum~maximum nor-malized to 1.0! and SPA for ASDEX Upgrade for dif-ferent temperatures as a function of toroidal modenumber.



presence of a high bootstrap current fraction, with~co-and counter-! ECCD in low-density plasmas and withinvestigations of the CD scenarios when applying ICRH.

IV.A. Toward Steady-State NoninductiveCurrent Drive Scenarios with NBCD

As already mentioned, one of the aims of the afore-mentioned enhancements of the ASDEX Upgrade CDcapability is to establish and to investigate fully non-inductively driven tokamak discharges. For this purpose,the restricted efficiency of the different CD methods re-quires that the plasma current be driven mostly by theintrinsic bootstrap current,16 which is, at fixedIp, pro-portional to the poloidalb ~bpol!. Fully noninductivelydriven high-bpol have been demonstrated on JT60-U~Refs. 17 and 18! with bpol ' 3 and ^ne&0nGW , 0.5,nGW 5 Ip0pa2 ~1020 m23, Ip in mega-amperes,a in me-tres! being the Greenwald density.

First experiments in this direction were started onASDEX Upgrade prior to the change of the NBI systemtoward more CD~Ref. 19!. High-bpol values wereachieved by injectingPNBI 5 10 MW in a plasma withIP 5 400 kA and a toroidal field~BT! of 2 T, correspond-ing to an edge safety factorq95 ' 9. Time traces of themajor plasma parameters are shown in Fig. 7. The pa-rameters reached att 5 2.6 s arebpol 5 3, a normalizedbvalue ofbN 5 2.7, and a confinement factor20 of H89P51.8 at a Greenwald fraction of^ne&0nGW ' 1. Fromt 5

2.5 s onward, the magnetic flux in the ohmic transformerwas kept constant, leading to a slow decay of the plasmacurrent~Fig. 7!, which indicates that the external CD isnot quite sufficient to sustain the 400-kA current. Thelow but positive toroidal loop voltage~,0.05 V! for t .2.8 s is due to a slightly changing vertical magnetic fieldnecessary to maintain the plasma equilibrium. TheDa-trace given in Fig. 7 clearly shows bursts from edge-localized modes, which illustrates that the discharge is inthe high-confinement mode~H-mode!. In addition, it hasbeen shown that an ion ITB is present atrtor ' 0.25~Ref. 21!. The bpol trace results from the equilibriumreconstruction code CLISTE~Ref. 22! using MotionalStark Effect23 ~MSE! measurements.

Figure 8 shows theq and current profiles and itsdifferent components fort 5 2.6 s as calculated by neo-classical current diffusion and bootstrap current usingthe ASTRA transport code. For comparison, the profiles

Fig. 7. Time traces of the main plasma parameters of a high-bpol discharge. In the topmost part, the plasma currentand the neutral beam heating power are shown. In thesecond trace, the line-averaged density and the Green-wald factor are shown, followed by the loop voltage,theDa trace, and the poloidal beta trace at the bottom.The dashed line indicates where a more detailed analy-sis is done.

Fig. 8. In the upper part, the current profile as derived fromthe transport code ASTRA and from an equilibriumreconstruction by CLISTE using MSE measurementsis presented. In the lower part of the figure, the corre-spondingq profiles are shown.



inferred from the equilibrium reconstruction based onMSE measurements are also given. As indicated by thedotted lines, no MSE measurements are available insidertor 5 0.08. The current diffusion calculation is started att 5 0.2 s. Att > 1 s, the loop voltage is spatially almostconstant, which means that the starting conditions do notstrongly influence the resulting current profile. After theheating power ramp, the plasma is still in a nonstation-ary state~mostly because the bootstrap current dependson the current profile itself, providing a kind of feedbackloop! until the end of the discharge. The major contribu-tions to theIp are the bootstrap current~IBS 5 200 kA!and the beam-driven current~INBI 5166 kA!, the remain-ing ohmic current~IOH5 25 kA! is negligible. The beam-driven current profile is rather broad but peaked towardthe plasma center due to the central beam deposition.The bootstrap current essentially determines the shapeof the total current profile, which is in fair agreementwith the one derived from MSE. The presence of aq51surface as deduced from MSE is supported by the obser-vation of~1,1! fishbone instabilities associated with thatmagnetic surface.

IV.B. Beam Current Drive

The previously discussed rearrangement of theNBI system allows reassessment of the neutral CDefficiency, especially for the case of off-axis beamdeposition. So far, most of the work has been done foron-axis deposition24 or without explicit current profilemeasurements.25,26

In a first step, two nearly identical discharges, onewith the more-normal on-axis beams and one with thetangential off-axis beams, have been compared. As shownin Fig. 9, the slope of the current in the ohmic trans-former changes when changing between normal and tan-gential injection. This difference in slope of the ohmictransformer current can be used to infer the total currentdriven by the off-axis tangential beams if all other cur-rent sources are given. In this analysis, the slight varia-tion ~,5%! in the plasma parameters~Te, Zeff, ne! betweenthe two discharges has been taken into account in ASTRAto determine the difference in resistivity. Using the boot-strap current from the ASTRA calculation, the drivencurrent by the tangential off-axis beams is calculated tobe INBI 5 247 kA from the difference in slope of theohmic transformer current, which is in reasonably goodagreement with theINBI 5 300 kA calculated by ASTRA.This shows that a total current of the order of the ex-pected one is driven by the tangential beams.

Although the expected total driven current by thetangential beams could be verified, these off-axis beamswere found to have less effect on the measuredq profilethan expected from code calculations~ASTRA!. As aconsequence, an experiment was performed to maxi-mize the effect on the current profile with the new injec-tion geometry. The most important time traces of this

experiment are given in Fig. 10. The beam deposition isswitched from on-axis deposition~t 5 1.3 to 3.3 s! tooff-axis deposition~t 5 3.3 to 5.3 s! then back to on-axisdeposition again with all externally controlled plasmaparameters kept constant. No significant change is foundin most other plasma parameters. The 2-s time durationis long enough for building up stationary profiles in tem-perature, density, and current according to the ASTRAmodeling of this discharge. For this experiment, MSEmeasurements are also recorded~restricted to the phasesof on-axis deposition because MSE relies on an on-axisbeam!; unfortunately, the diagnostic was not calibratedand thus no equilibrium can be reconstructed from thesemeasurements, and noq or current profile can be in-ferred. The measured MSE angles plotted in Fig. 11 areoffset corrected to the angles calculated by ASTRA att 5 2.8 s. Some differences between the calculated andthe measured angles are visible before the start of theinjection phase. These differences can be attributed to anincorrectly modeled plasma energy in this phase. Thelarger differences at the end of the discharge~t . 6 s! aredue to the fact that the current rampdown is not includedin the modeling and shows that the diagnostic is sensi-tive to the changes in poloidal field. The calculation givesa neutral beam–driven current ofINBI 5 61 kA centrallypeaked for the first 2-s interval andINBI 5 192 kA fromt 5 3.3 s tot 5 5.3 s, the latter being peaked off-axisat rtor ' 0.54 but with a rather broad distribution overalmost the full minor radius. The calculated beam depo-sition has been compared with a more sophisticated cal-culation by the FAFNER code, and the differences are

Fig. 9. Comparison of the currents in the ohmic transformeron ASDEX Upgrade between two very similar dis-charges at low density andIp 5 800 kA. The currentshave been adjusted to each other att 5 2.5 s because ofa different startup phase. The 15887 line represents adischarge with only on-axis NBI, and the 15884 one,with off-axis NBI.



,10%. In particular, the basic shape of the deposition isthe same.

The ASTRA-calculated MSE angles as given inFig. 11 show the slow change in theq profile in thesecond interval~off-axis beams! due to current diffu-sion. The measured MSE angles follow the calculatedones reasonably well before switching on the off-axisbeam sources, but they have not changed when switch-ing back to on-axis beams. This is in clear contradictionto the modeling. After turning off the off-axis currentsources, the modeled MSE angles decrease to their for-mer values on the current diffusion timescale. The val-ues at the end of the decay again reach the measuredvalues at the beam switching time, which confirms thatthe measured MSE angles do not indicate any off-axispeaked driven current by the off-axis tangential beamdeposition. Theq profiles as calculated by ASTRA canbe found in Fig. 12. In the ohmic phase, a monotonicqprofile with a low centralq of ;1 results from the cal-culation started att 5 0.3 s. The central NBI predictsno significant change in theq profile as seen from theq profile at t 5 3.3 s. Finally, the calculatedq profile att 5 5.3 s shows that the off-axis CD is expected to resultin an increase of the centralq and a significant changein the radius of theq51 surface. The calculated toroidalloop voltage profile is constant at this time, whichmeans that the current diffusion process has resulted ina stationary current profile at the end of the off-axis

Fig. 10. Time traces of an ASDEX Upgrade discharge atIP 5800 kA and NBI fromt 51.3 s tot 5 6.5 s. In the topgraph, the plasma current and the stored energy areplotted. In the middle, the line-averaged electron den-sity and theR coordinate of the magnetic axis arerepresented. In the bottom graph, the NBI power andthe plasma radiation are illustrated. The neutral beamdeposition is fromt 5 1.3 s tot 5 3.3 s andt 5 5.3 sto t 5 6 s on-axis~co! and fromt 5 3.3 s tot 5 5.3 soff-axis ~co!.

Fig. 11. The MSE polarization angles for ASDEX Upgrade discharge. Fromt 5 3.3 s tot 5 5.3 s, no data are available. Thediagnostic covers the plasma betweenrtor 5 0.1 andrtor 5 0.8 with a spatial resolution of;3 cm. Channel 1 at thebottom is the most outside channel, and No. 10, the most inside. The peak of the off-axis NBI is at channel 5. The noisytime traces are the measurement. ASTRA calculations are shown as thick lines.



injection. Further modeling~not shown here! with tak-ing into account a very simple reconnection model forthe sawteeth gave qualitatively the same result, i.e., asignificant broadening of theq profile, but the positionof the minimumq value is different because of the re-connection. For this calculation, the sawtooth model wasturned off at the start of the off-axis injection followingthe experimental observations described later.

The MHD activity in this discharge shows sawteethand fishbones at theq 5 1 surface~ rtor ' 0.36! duringon-axis deposition. With off-axis tangential injection, thesawtooth oscillation stops but a continuous~1,1! modestarts at the same previous radial location of the saw-tooth precursors. This shows that the position of theq 5 1 surface does not change. Frequency and positionof this ~1,1! mode is stable for the whole of the 2-sinterval. After switching back to the central deposition,the sawteeth reappear in,10 ms, which is too fast to bea result of a current diffusion process over a larger spa-tial range. The constant location of theq 5 1 surfacedetermined by the MHD activity is in full agreementwith the given interpretation of the measured MSEangles but again in contradiction to the modeling.

The experiments discussed here are not fully conclu-sive. The current driven by the neutral beams can beverified by analyzing the ohmic transformer current, butthe predicted influence of this driven current on the cur-rent profile is not confirmed by the MSE measurementnor by the magnetohydrodynamic~MHD! activity. Thesefindings might indicate that a fast transport process movesthe fast particles from the off-axis position to some-where else. To be more specific, a transport process isneeded that results in a neutral beam current profile verysimilar to the ohmic current profile, as in the on-axis

injection case where no significant change in theq pro-file between the ohmic phase and the on-axis injectionphase could be found. A likely candidate would be anAlfvén-like MHD activity, but so far, no indications arefound for this kind of MHD. The sawteeth could be an-other candidate for a fast particle transport, but they dis-appear~together with the fishbones! when changing theneutral beam deposition and they are, therefore, not ableto influence the fast particle distribution anymore. Thusfar, no single discharge on ASDEX Upgrade has clearlyindicated an influence of the off-axis injection on theMSE measurements, even though very different MHDactivities may be present~like tearing modes!. Apartfrom MHD, one could speculate about an anomaloustransport leading to a redistribution of the fast particles~like ion temperature gradient modes! or about a kind ofstiffness in the current profile. The latter explanation,however, is very unlikely because ECCD~and lower hy-brid current drive! clearly has an influence on the plasmacurrent profile. Further experimental investigations areneeded to clarify these findings.

IV.C. Significant Central Co- and Counter-ECCDin a Low-Density L-Mode Plasma

This section provides a detailed description of anexperiment, where by means of central ECCD, an exter-nally driven current of the order of the plasma currenthas been achieved. Dependent on whether co- or counter-ECCD was applied, significantly different current den-sity profiles were observed; as a consequence, MHDbehavior and confinement of the plasma differ as wellfor the two cases. The experiment therefore is an exam-ple of the way that current profile control by external CDcan be used for plasma optimization.

In this experiment, ECCD has been studied in plas-mas with a low current of 0.4 MA and low density ofne ' 1.5{1019 m23 ~Ref. 27!. The power of three gyro-trons ~1.2 MW! was launched in either co- or counter-current direction~toroidal launching angle:620 deg!.The remaining 0.4 MW of the fourth gyrotron was usedfor heating only. Central deposition was chosen requir-ing BT 5 2.35 T and resulting inq95 ' 10. Under theseconditions the plasma remained in low confinement mode~L-mode!.

In Fig. 13, the time evolutions of heating power,central electron temperature, and line-averaged densityof the co- and counter-current drive discharges are com-pared. With the onset of ECRH,Te rises sharply in bothcases. However, the temperature profiles, obtained bycombining electron cyclotron emission~ECE! and Thom-son scattering data as shown in Fig. 13, are distinctlydifferent. While for co-ECCD,¹T in the plasma coredoes not show a radial variation, an electron ITB devel-ops for counter-ECCD with a reduction of the gradientlength~T0¹T ! from LT ~5 T0¹T ! '15 cm toLT ' 8 cm.This difference is also reflected in the MHD behavior of

Fig. 12. Profiles of the safety factorq as calculated by ASTRAfor the discharge shown in Fig. 10 att 51.3 s~ohmicphase!, t 5 3.3 s~end of central NBI!, andt 5 5.3 s~end of off-axis CD!.



the two cases. While for co-ECCD sawteeth and thus aq 5 1 surface continue to exist, sawteeth disappear incase of counter-ECCD. Instead, irregular collapses ofthe electron temperature are observed, which are associ-

ated with the occurrence of a~m5 2,n51! mode, prob-ably a pressure-driven ideal mode of double kink structure.In both discharges, the line-averaged density drops at theonset of ECRH or ECCD by;10%, caused by a flatten-ing of the ne profile. Due to the low collisional heattransfer from electrons to ions,Ti ,, Te is observed~seeFig. 14!. A short neutral beam pulse of 2.5 MW has beeninjected at 3.4 s to measure the ion temperature and thecurrent profiles. As the NBI power exceeds the ECRHpower, we may consider the plasma as undisturbed onlyfor times shorter than the slowing-down time of the fastions. Although only early time points within the beampulse have been chosen for the data evaluation, an influ-ence of the beam on the result cannot be excluded entirely

The current diffusion has been simulated with theASTRA code. For the ECCD current, we use the resultof the TORBEAM code with a current profile approxi-mated by a Gaussian distribution and assuming a timedependence of the current density according tojECCD@PECCDTe0ne. To consider the sawtooth activity, ASTRAuses a Kadomtsev reconnection model, redistributing thecentral current as soon asq drops below one. The calcu-lated toroidal loop voltage was in agreement with themeasured one during the ohmic phase~Uloop 5 0.55 V!,and this remains the case during the ECCD phase~Uloop50.1 V!, provided an external driven current as deter-mined by TORBEAM is taken into account. For co-ECCD, this current is 82% of the total current, theremaining part being divided up into 12% bootstrap cur-rent and 6% ohmic current. In the counter-ECCD case,convergence problems in the ASTRA equilibrium solver,

Fig. 13. Time traces of ECRH power, line-averaged density,and central temperature for~a! cocurrent drive and~b! countercurrent drive.

Fig. 14. Temperature profiles from ECE, Thomson scattering, and charge exchange resonance spectroscopy for~a! cocurrentdrive, averaged for 800 ms, and~b! countercurrent drive, averaged for 50 ms.



due to the narrow deposition profile, did not permit aquantitative assessment of the current fractions. FromTORBEAM, we would expect that in resistive equilib-rium, a fraction of 80% is driven opposite to the plasmacurrent. Consequently, the ohmic current has to exceedthe plasma current significantly.

In Fig. 15, the ASTRAq profiles are compared tothose inferred from CLISTE~including MSE data!. Inthe ASTRA calculations, the width of the electron cyclo-tron current distribution had to be increased above thepredicted values from TORBEAM to obtain conver-gence in the equilibrium solver. This has only little in-fluence on the result in the case of co-ECCD, since thedeposition lies inside theq 5 1 radius and the reconnec-tion model redistributes the current generated inside theq 5 1 surface

For counter-ECCD, aq 5 1 surface is not present,and this argument is no longer valid. Therefore, a mean-ingful q profile from ASTRA cannot be presented here.Qualitatively, it is clear that a strong central counter-current drive produces reversed shear, as deduced fromthe CLISTE code~Fig. 15c!. Associated with the re-versed shear in the plasma center, an internal electrontransport barrier forms atrpol ' 0.3 ~Fig. 14!. In case ofco-ECCD, the CLISTE results confirm the ASTRAqprofile ~Fig. 15b!, which predicts monotonicq with alargeq51 radius atrpol ' 0.45. This is supported by thepreservation of the sawtooth activity during the ECCDphase. Inside theq 5 1 radius, due to the assumption offull reconnection in the ASTRA sawtooth model, theASTRA q does not drop to such low values as observedby CLISTE. During the ohmic phase, ASTRA andCLISTE q profiles do not match well~Fig. 15a!. WhileASTRA indicates a smallq 5 1 radius insiderpol 5 0.2,which is consistent with the presence of sawteeth, theqfrom CLISTE stays above one. This is attributed to thedisturbance of the plasma by the NBI, as described pre-

viously. Nevertheless, the CLISTEq profile shows thatthe initially high central magnetic shear in the ohmicphase of the discharge turns into reduced shear for co-and reversed shear for counter-ECCD.

At higher density and then also lower electron tem-perature, the driven current is much less, but here too themeasured and calculated loop voltages do agree, provid-ing confidence in the TORBEAM results.

IV.D. Results from ICCD Experiments

For studies of FWCD on ASDEX Upgrade, the sce-nario chosen wasB0 5 2.9 T, fICRF 5 30 MHz. In therange of the available frequencies and relevant magneticfields, this choice provides with the present antenna theclosest match of phase velocity to thermal velocity~vphase; vthermal!, as required for optimum single pathabsorption. In addition, it minimizes the absorption dueto ion damping mechanisms because the fundamentalresonance layer of deuterium lies in the plasma periph-ery on the high-field side, the H resonance on the low-field side outside the plasma. In earlier campaigns, usinga symmetric antenna spectrum, strong heating of the elec-trons could be observed.28 In the most recent experi-ments to test the current drive, edge parasitic effectsprevented even the reproduction of the previous heatingresults. Further work is needed to perform the experi-ments under optimal conditions~reduced parasitic ab-sorption, better single path absorption, e.g., through highercentral temperatures!.

For MCCD, off-axis power deposition has beenclearly measured in a deuterium plasma with3He admix-ture using ICRF modulation experiments. The strong he-lium puff needed to put the mode conversion layer off-axis led to high density and an estimated current drivebelow the measurement level on the internal inductance

Fig. 15. Comparison of experimental and calculatedq profiles for ~a! ohmic phase,~b! cocurrent drive, and~c! countercurrentdrive.



or the loop voltage. Unfortunately, the MSE was notavailable for these higher field experiments.

The ICCD experiments performed so far are of apreliminary nature. They will be continued in the futureto assess the contribution of ICCD to the various dis-charge scenarios on ASDEX Upgrade, which require CDfor optimizing plasma performance.

V. SUMMARY

The study of externally driven plasma current byinjecting neutral beams and launching toroidally asym-metric IC or EC wave spectra plays an important role inthe physics program of ASDEX Upgrade. Adequate toolsare available to analyze discharges with a significant partof the plasma current driven noninductively. Experimen-tally, NBCD was found to contribute;40% to the cur-rent of a highbpol discharge with a noninductive plasmacurrent of more than 90%. Whereas the total beam-driven current is generally in agreement with the simu-lations, the calculated off-axis peaking of the currentprofile because of the new off-axis tangential beams couldnot be verified. The reason for this is still unclear andneeds further investigations. In low-current, low-densitydischarges, central counter-ECCD results in a reversedcentral shear and, associated with this, an ITB for theelectrons. Local ECCD was successful in suppressingNTMs. The physics of ICCD has been studied for bothscenarios, fast wave and mode conversion current drive.

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CHAPTER 13: CURRENT DRIVE IN ASDEX UPGRADEoldweb.reak.bme.hu/fileadmin/user_upload/felhasznalok/pokol/... · CHAPTER 13: CURRENT DRIVE IN ASDEX UPGRADE ALBRECHT STÄBLER,* JÖRG HOBIRK,