Gude, A.

Leuterer, F.

Manini, A.

Neu, R.

Pütterich, T.

Sips, A.

Stober, J.

Wagner, D.

Zohm, H.

English

Zohm, H. 

MPG.PuRe

First experiments with the extended ECRH systemon ASDEX UpgradeJ. Stober, A. Gude, F. Leuterer, A. Manini, R. Neu, Th. Pütterich, A.C.C. Sips, D. Wagner,H. Zohm and the ASDEX Upgrade teamMax-Planck-Institut für Plasmaphysik, EURATOM Association, Boltzmanstrasse 2, 85748,Garching, GermanyIntroductionExperiments on ASDEX Upgrade in the ongoing campaign are focused on the operation of thetokamak as a full tungsten device [1]. In the previous opening W-covered tiles have also beeninstalled in the divertor and previously installed W-covered tiles in the main chamber have beencleaned. The machine has been started-up without using boronisation. The ECRH has provento be essential to operate low density high performance plasmas. Some of these plasmas showthe typical features of the improved H-mode scenario. These aspects of low-density tungstenoperation are discussed in the next section.The ECRH system of ASDEX Upgrade consists of the first system with four units deliveringeach 0.4 MW to the plasma for 2 s and the new second system actually under construction. Inits final stage it will consist of 4 multi-frequency units of up to 1 MW for 10 seconds each. Thefirst unit is now in operation at 105 GHz and 140 GHz and, at 140 GHz, delivers more than800 kW to the plasma [2]. The total amount of ECRH power available for heating of ASDEXUpgrade is currently 2.5 MW. The launchers of the new system are steerable poloidally alsoduring discharges allowing to vary the location of power deposition during the discharge. Thefinal goal is to manipulate MHD instabilities such as neoclassical tearing modes (NTMs) andsawteeth by feedback-control of the ECRH deposition in analogy to the functionality of theECRH-system foreseen for ITER. First feed-forward mirror movements have been performedand effects on MHD and impurity control have been observed as reported in the last section.Low density H-mode operation in a full tungsten deviceH-mode operation with low or no additional gas puff at low values of   has become increas-ingly difficult at ASDEX Upgrade as the first-wall W-coverage increased during the last yearssuch that low  , high 	 edge conditions lead to increased W-influx. The general observationfor discharges heated with NBI only is a central accumulation of heavy impurities eventuallyleading to a reduction of the core temperature due to radiation. For some years it is known34th EPS Conference on Plasma Phys. Warsaw, 2 - 6 July 2007 ECA Vol.31F, P-5.138  (2007)that central heating with ICRH or ECRH can counteract such accumulations [3]. Especially thepowerful (  5 MW) multi-frequency ICRH system, capable to cope with type-I ELMs due tothe use of 3dB-couplers, was used for this purpose at several values of  . As the limiters closeto the ICRH were covered with tungsten it turned out that ICRH operation released additionalW from the limiters, especially in the presence of light impurities [4]. Therefore, H-modeoperation with small gas puff was preferentially performed at a few operational days after aboronisation. ECRH was proven to be very effective in reducing central accumulation, but waslimited in length and power, especially with one of the four gyrotrons of the old system out ofoperation since 20051.In the new campaign with the fully tungsten covered plasma facing components no boroni-sation was used so far. Operation started with high gas-puff, high   plasmas. As expectedfrom previous experience, operation at low gas puff shows central accumulation when usingNBI only. ICRH can still be used, but only with strong gas puff [1]. Experiments with lowgas puff have therefore been done with ECRH. Fig. 1 shows time traces of a pair of dischargeswith moderate gas puff with and without ECRH. With ECRH a quasi steady-state situation isachieved whereas without ECRH central accumulation occurs accompanied by a reduction ofconfinement. Due to the reduced confinement the accumulated impurities can escape and theplasma consequently recovers, leading to an oscillatory behaviour. Encouraged by these resultsthe gas puff was switched-off completely. H-factors IPB98(y,2) of 1.1 were reached transiently,but (3,2)- or (2,1)-NTMs set in reducing the confinement. The NTMs could be prevented bydelaying the third NBI beam as shown in figure 2. This is typical for the improved H-modescenario in ASDEX Upgrade, which allows to reach high  -values only if the current profile isgiven time to equilibrate after having entered the type-I H-mode [5]. During the ECRH phasefishbone-MHD-modes are observed, as they occur in the best performing improved H-modes[6]. In the discharge shown in figure 2 a maximum value of  =2.2 was obtained (H-factor1.2). An increase of the NBI heating is foreseen in the ongoing campaign. Figure 2 also showsthat the central tungsten concentration remains at an acceptable level, although in a small zoneof a few cm width (probably inside the ECRH deposition) the concentration reaches the 10 level. These results indicate that sufficient ECRH allows improved H-mode operation in a fullytungsten covered machine. Still we note that   is still high. As long as the ECR power1The broken gyrotron has been replaced by a similar gyrotron from W7-AS. In total 2 gyrotrons from W7-ASare now in use as replacements in the old ASDEX Upgrade system.34th EPS 2007; J.Stober et al. : First experiments with the extended ECRH system on ASDEX Upgrade 2 of 4is mainly available at 140 GHz this can be achieved only by increasing the plasma current.Experiments at 1.0 MA have just started and will allow to compare to previous data for the im-proved H-mode scenario. On a longer time scale we can make use of the three multi-frequencygyrotrons to be installed from late 2007 to 2009. These gyrotrons will have 2 intermediate fre-quencies at 117 GHz and 127 GHz allowing to reduce  and thereby   . In summary we notethat the extended ECRH system helps to extend the operational range of the full tungsten devicetowards low density and high  , i.e. high edge-temperature.First experiments with the steerable launcherAs mentioned above the last mirror of the new launcher can be moved around one axis duringthe discharge. Conceptually this movement corresponds to a poloidal movement of the beam,whereas the other axis (usually generating toroidal movements) has to be set on a shot-by-shotbasis. A first feed-forward programmed movement has been successfully performed, such thatthe beam crosses the   surface towards the plasma centre. A negative toroidal angle of6 degrees is used such that some co-current drive (ECCD) is expected. Figure 3 shows theequilibrium, five lines-of-sight of the soft X-ray diagnostic and the position of the ECR heatingat different time points. Figure 4 shows time traces of the discharge. The central radiationstrongly increases as the NBI heating is doubled. The accumulation of radiating impurities onlystops 0.6 sec later. We ascribe this to the location of the ECRH deposition, since we knowthat central ECRH leads to an immediate effect (  100 ms) on central impurity accumulation(see also figure 2). As can be seen in figure 4, a mode sets in close to the maximum of thecentral radiation. It shows up in channels 50 to 58 of the respective SXR-camera (figure 3).Analysis shows that this is a (1,1)-mode. We therefore assume that the lines-of-sight of channel50 and 58 are about tangential to the  !" -surface. The mode disappears approximately 60msafter the ECRH is switched off. In summary these data indicate that the region to be heatedwith the ECRH for impurity control is highly localised in the core. Additionally we are able toaffect MHD modes with our movable launcher. Although the simultaneous occurrence of the(1,1)-mode and the reduction of the radiation peaking is tempting we do not speculate aboutany correlation. In fact, most discharges with central ECRH do not show a (1,1)-mode andwe suspect that its occurrence is related to the applied ECCD. Obviously, further experimentsmoving the mirror back and forth are planed to broaden these initial results.References[1] Neu, R. et al., Plasma Phys. Control. Fusion, (this conference, invited paper I1.006)[2] Wagner, D. et al., Fusion Technology, in press34th EPS 2007; J.Stober et al. : First experiments with the extended ECRH system on ASDEX Upgrade 3 of 4[3] Dux, R. et al., Plasma Phys. Control. Fusion, 45, 1815, (2003)[4] Dux, R. et al., J. Nucl. Materials, 363-365, 112, (2007)[5] Sips, A.C.C. et al., Nucl. Fusion, submitted[6] Stober, J. et al., Nucl. Fusion, in pressFigure 1: Comparison of discharges with and with-out ECRH at a moderate gas puff rate of #%$'&)(+*-,/.0 .1  =2.4 T, 2 346574 , average triangularity at theseparatix is 0.2.Figure 2: Similar discharge as #21851 in figure1, but with no gas puff and the third NBI sourcedelayed. No NTMs are observed.ll0.80 1.16 1.52 1.88 2.24 2.60-1.40-0.84-0.280.280.841.40#21761 1.6 sSoft X-Ray+ mag. axisl1.0 s1.6 s2.0 sECRH Dep5051545758Figure 3: Geometrical setup of 21761:separatrix shape, magnetic axis, ECRHdeposition (varying due to mirror move-ment) and selected SXR lines-of-sightused in figure 4.Figure 4: Evolution of the central radiation, tungsten con-centration and MHD activity during an ECRH mirror move-ment as shown in figure 3. The ECRH is injected with atoroidal angle of -6 degree. The plasma current is 1 MA.34th EPS 2007; J.Stober et al. : First experiments with the extended ECRH system on ASDEX Upgrade 4 of 4

First experiments with the extended ECRH system on ASDEX Upgrade

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First experiments with the extended ECRH system on ASDEX Upgrade

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