Gantenbein, G.

Giruzzi, G.

Günter, S.

Leuterer, F.

Maraschek, M.

Meskat, J.

Zohm, H.

English

MPG.PuRe

27th EPS Conference on Contr. Fusion and Plasma Phys.  Budapest, 12-16 June 2000    ECA Vol. 24B (2000) 29-3229Stabilisation of Neoclassical Tearing Modesin ASDEX Upgrade with ECRHM. Maraschek , H. Zohm , G. Gantenbein, G.Giruzzi, S. Günter , F. Leuterer , J. Meskat,Q. Yu , ASDEX Upgrade-Team , ECRH-Group (AUG)  Max-Planck-Institut für Plasmaphysik, EURATOM Association,Boltzmannstr. 2 D-85748 Garching, GermanyInstitut für Plasmaforschung, Universität Stuttgart, Pfaffenwaldring 31, D-70569 Stuttgart,GermanyCEA Cadarache, F-13108 Saint-Paul-les-Durance, France, EURATOM AssociationIntroduction In tokamak plasmas with low collisionalities , neoclassical tearing modes(NTMs) are the most severe limitation to achieve highvalues. This type of mode is excited bythe lack of helical bootstrap current caused by the flattening of the pressure profile over an initialsmall seed island in a plasma otherwise stable against tearing modes. The local replacement ofthis lack of helical bootstrap current is experimentally realized by current drive with electroncyclotron heating (ECCD), which is known to allow a very localized current drive. Results ofthese experiments are compared with theoretical calculations.Current drive in the islands O-point The most effective stabilisation can be expected by alocal co-current drive (parallel to the plasma current) within the O-point of the island [1]. Thecurrent has to be localised with respect to the minor plasma radius 	 and to the relative phaseof the rotating island. A radial scan has been done on a discharge by discharge based scan ofthe toroidal magentic field  . The correct phase of the ECRH has been chosen by a feedbacksystem between the ECRH and a combination of Mirnov signals giving the phase of the (3/2)mode. A partial reduction of the island size and a recovery of 20% of the mode induced loss incould be achieved with ! #"$ MW with 2 gyrotrons [2].Theoretical calculation [3] as well as experimental results [2] suggest that DC current driveat or near the resonant surface is almost as efficient as phased ECCD. The further experimentswere therefore done with a technically much easier DC ECCD scheme.Experimental setup for DC current drive The usual plasma parameters for stabilisationexperiments for phased ECCD as well as for DC ECCD were discharges with %'&)(* #"$ MA,+,.- T ( /10324(65 ) and with  87 (:9; =<>9;-#"? MW of neutral beam heating power. The densitywas typically of the order of@  A?BC9; D   0 m D  , and before the (3/2) NTM onset  (.-"E-F<G-#"$could be reached. The island is detected by a set of Mirnov coils and ECE measurements, whichgive the exact radial location and the width H of the island, as well as the location and widthIof the ECRH deposition. Typical values for the island width are H (.$J<69K cm and for thedeposition widthI (65L<M? cm.The DC ECCD with 3 gyrotrons at 140 GHz, each depositing 0.4 MW RF power, has beenapplied to the plasma for 2 s after the mode has reached its stationary amplitude, leading toa loss of confinement. Typically 20% ofFis lost compared to the value at the onset of themodes. The toroidal angle of the mirrors launching the RF wave have been set to NG9K?PO withrespect to the magnetic field for counter- and co- current drive respectively. Additional to thecurrent drive there is of course always a local heating effect at the resonant surface, leading tohigher temperatures and hence reduced resistivity. On the one hand a subsequent increase ofthe current within the island stabilises the mode. On the other hand a modification of the localcurrent gradient changes also QSR and hence the mode stability.27th EPS CCFPF 2000;   M. Maraschek et al.: Stabilisation of Neoclassical Tearing Modes in ASDEX Upgrade with ... 30During the ECCD phase a slow magnetic field ramp (5 % of the nominal field in 1.5 s) of thetoroidal magnetic field shifts the radial location of the ECCD resonance with respect to the islandlocation. The ECRH is launched from the low field side of the torus, whereas the resonance islying on the high field side. During the scan the position of the ECCD resonance moves over theisland from a smaller to a larger minor radius. The ECCD resonance makes a shift in the orderof 8 cm, whereas the shift of the minor radius of the mode is in the order of 2 cm [4]. The scanwas necessary because from discharge to discharge there is jitter in the exact island position andshould ensure that the island and and the ECCD position match.Complete stabilisation by co ECCD at resonant surface With the scheme described above,stabilisation of (3/2) NTMs could be reliably achieved. The ECRH power of 1.2 MW representsonly 10% of the heating power of TUWVYX ZK[ MW from the neutral beam injection (see Fig.1). After the mode has reached its saturated size the ECCD is switched on together with thepreprogrammed magnetic field ramp. The launch angle of \]Z;^#_ has been used for co currentdrive. After the mode has been completely stabilised the ` recovers to the same value whichcould be reached in a discharge without mode, but with otherwise identical conditions.Figure 1: Complete stabilisation of a (3/2) NTM by co ECCD. Additionally to 10 MW of NI injection1.2 MW co-ECCD is applied (first trace) together with a magentic field ramp (last trace). During thescan the saturated NTM could be completely stabilised (third trace). The loss in abU completely recoverscompared to a case without an NTM but otherwise identical conditions.During the ECCD phase an additional loss of confinement is observed. This is partly due toa loss in density which is then controlled by the gas feedback system increasing the gas puffingrate and leading to a loss of confinement [5]. The details of this density pump-out are not yetfully understood.A further increase of the heating power from 10 MW to 12.5 MW leads to an increase of `,Uwhich reexcites the mode at slightly higher values (see Fig. 2a). This may happen as the ECRHresonance has again moved out of the resonant surface. An attempt to increase ` U after themode stabilisation to higher values compared to the onset value has been done. The field ramphas been stopped at the value of optimum stabilisation (maximal decay of mode amplitude).Steady state ` U Xdce^ with 1.2 MW ECRH and a stabilised mode could be achieved in thisoptimized case (see Fig. 2b). This has to be compared to `fUhgic with an unstabilized mode.A 10% increase in heating power gives 25% increase in `fU and hence a 50% increase in fusionpower ( Tjlk'monqpsrSt!`=u ). After the ECCD is switched off, the mode instantaneously reappears inthe plasma. This reexcitation has been only observed in this optimised case, where `,U recovers27th EPS CCFPF 2000;   M. Maraschek et al.: Stabilisation of Neoclassical Tearing Modes in ASDEX Upgrade with ... 31to higher values and ECCD clearly supresses the mode.n=2 amplitude ⇒ (3/2) modeASDEX Upgrade #13195Heating Power (NBI) ECRH Power * 10Beta Normalised1.6 2 2.4 2.8 3.2 3.6 4 time  (s)Toroidal FieldShift of EC resonancen=2 amplitude ⇒ (3/2) mode -1015E+061E+07WHeating Power (NBI) ECRH Power * 10ASDEX Upgrade #13631Beta Normalised 22.51.6 2 2.4 2.8 3.2 3.6 4 time  (s)Toroidal Field-2.2-2.14T48cmShift of EC resonance(a) (b)Figure 2: (a) A further increase of the NI power from 10 MW to 12.5 MW reexcites the mode at onlyslightly higher vw . (b) A stop of the xzy ramp at the optimal value for stabilisation leads to higher vw|{}~values. At the end of the ECCD phase the mode gets excited again instantaneously pointing to aperfect resonance condition.Co versus counter ECCD at resonant surface In order clarify wether the current drive orthe heating is the main stabilising effect, a control experiment with counter-ECCD has beendone. The launch angle of the ECRH has been changed to ]K . In the case of counter-ECCDthe local heating effect and the increased current within the island remains the same, but theexternally driven current should have a destabilising effect, as it has the same direction as themissing bootstrap current.ASDEX Upgrade #12257, MOD:EvenN, f=2kHz, smooth=803.0 3.2 3.4 3.6 time [s]01234amplitude [a.u.]co-injection⇒ complete stabilisation2.5 3.0 3.5 time [s]01234amplitude [a.u.]ASDEX Upgrade #12538, MOD:EvenN, f=2kHz, smooth=80counter-injection⇒ 45 % reductionNI offNumerical modelling(a)(b)Figure 3: (a) Mode amplitude for co- and counter-ECCD determined from magnetic measurements dur-ing the field scan. In case of co-ECCD a complete stabilisation, in case of counter-ECCD a stabilisationto 55% of the initial amplitude could be achieved. (b) Mode amplitude during a shift of the ECCD reso-nance for co- and counter-ECCD from modelling. For co-ECCD a complete stabilisation, for counter-ECCD only a partial stabilisation is predicted in agreement with the experiment.The experiments have shown that also in the counter-ECCD case there is a small overall sta-bilising effect. This reveals that the current drive effect and the heating effect are of the sameorder (see Fig. 3a for the mode amplitudes taken from magnetics). Instead of a complete sta-bilisation the mode amplitude could only be reduced to 55 % of its initial amplitude and  wremains significantly lower.27th EPS CCFPF 2000;   M. Maraschek et al.: Stabilisation of Neoclassical Tearing Modes in ASDEX Upgrade with ... 32Island evolution and comparison with theory The evolution of the island width taken frommagnetic measurements for the co- and the counter-ECCD cases are shown in Fig. 3a. Theisland width has been determined from the magnetic perturbation field under the assumption   C. The measured behaviour has been compared with theoretical modelling (seeFig. 3b).The model takes self consistenly the bootstrap current and the ECRH heating and current driveeffect into account [3,6]. The island width is plotted as a function of the distance between theECRH deposition layer and the location of the resonant surface of the mode. This scan has beenperformed in the experiment, besides the fact that the field can not be linearly translated to thisresonance position. It can be clearly seen that the optimum position for co- and counter-ECCD isdifferent in accordance with the experiment. Only for co-ECCD a complete stabilisation occurs,whereas in counter-ECCD case only a partial stabilisation is predicted.0.0 0.5 1.00.00.51.0relative island width w/atime [s]only (0,0) component(3,2), (6,4) and (0,0) componentshift of deposition radius at 3 cm/sFigure 4: Fourier analysis of the ECRH deposition for the stabilising effect of the different harmonics.Only the (0/0) component ( L ) and the higher (3/2) and (6/4) components together can stabilise themode.An additional information about the stabilising effects can be taken out of a spatial Fourieranalysis of the deposition of the ECRH. The (0/0) component contributing mainly through thechange in the local current gradient and hence   alone can not stabilise the mode in the model.Only a reduction of 10-20% of the amplitude of the mode can be reached. It is required to usehigher harmonics, especially the (3/2) and the (6/4) component to get a complete stabilisation(see Fig. 4). This shows clearly that the helical current in the island is required for the modestabilisation.Summary Reproducibly a complete stabilisation of (3/2) neoclassical tearing modes withECRH current drive and heating could be achieved. The ECRH delivers only 10 % of the heat-ing power and a local current of 1-2% of the plasma current is driven. For the future a moreautomatically working system for NTM stabilisation is planed to prove the practical use over awider range of discharge conditions.References[1] A. W. Morris, R. Fitzpatrick, T. C. Hender, and M. R. O’Brien, Controlled Fusion andPlasma Physics, Proceedings of the 19th European Conference, Innsbruck I, 423 (1992).[2] H. Zohm et al., Nucl. Fusion 39, 577 (1999).[3] Q. Yu, S. Günter, G. Giruzzi, K. Lackner, and M. Zabiego, Phys. Plasmas 7, 312 (2000).[4] J. P. Meskat et al., Europhysics Conference Abstracts (CD-ROM), Proc. of the 26th EPSConference on Controlled Fusion and Plasma Physics, Maastricht, 1999 23J, 1377 (1999).[5] A. Kallenbach et al., Plasma Phys. Controlled Fusion 41, B177 (1999).[6] G. Giruzzi et al., Nucl. Fusion 39, 107 (1999).

Stabilisation of Neoclassical Tearing Modes in ASDEX Upgrade with ECRH

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Stabilisation of Neoclassical Tearing Modes in ASDEX Upgrade with ECRH

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