Erckmann, V.

Maassberg, H.

Marushchenko, N.

Turkin, Y.

English

MPG.PuRe

Analysis of ECCD scenarios for different configurationsof the W7-X StellaratorN.B. Marushchenko, V. Erckmann, H.P. Laqua, H. Maaßberg, Yu. TurkinMax-Plank-Institut für Plasmaphysik, EURATOM-Association, Greifswald, GermanyIn theW7-X stellarator (under construction in Greifswald, Germany), the toroidal mirror ratioBmax Bminon axis can be varied significantly, from 1.004 to 1.22 for the “low-mirror” and the“high-mirror” configurations, respectively (for the “standard” configuration Bmax Bmin  1  09).The corresponding trapped-particle fractions on axis are varied due to ripples from f tr  0  02 for“low-mirror” up to ftr  0  45 for ‘high-mirror”, while ftr  0  3 for the “standard” configuration.Through its dependence on the toroidal mirror ratio value, mono-energetic bootstrap currentcoefficients are largest for the “low-mirror" configuration. In particular, the bootstrap currentwas minimized for the “high-mirror” configuration, whereas the neoclassical confinement isoptimum in the “standard” configuration. The total plasma current affects the edge value ofthe rotational transform, which may fall outside the required range for proper island divertoroperation without external field compensation. Due to the absence of an ohmic transformer,electron cyclotron current drive (ECCD) will be used for compensating the bootstrap current.Since the ECCD efficiency is quite sensitive not only to the plasma parameters, but also tothe magnetic configuration, it is an important task to estimate properly the range of ECCDvalues taking into account all features of the magnetic configuration, and to check its ability tocounteract the residual bootstrap current. It is necessary to take special care for high densitieswhere the ECCD efficiency is reduced.The numerical tools for calculating the ECCD efficiency developed to date do not cover com-pletely the range of collisional regimes for the electrons involved in the current drive, especiallyin stellarators. Nevertheless, the collisionless limit (with trapped particles taken into account) isquite reasonable, especially for the regimes with moderate densities and high temperatures. Inthis work we analyze the requisite ECCD scenarios for operating the different magnetic config-urations with help of the ray-tracing code TRAVIS [1]. The (adjoint) Green’s function appliedfor the ECCD calculations is formulated with momentum conservation taken into account [2];this is especially important and even critical for those scenarios, where mainly bulk electronsare responsible for absorption of the RF power (e.g. when obliqueness of launch is not high).The ECRH system in W7-X is designed for continuous operation with a total injected powerup to 10 MW at 140 GHz [3] (the resonance magnetic field is B0  2  5 T). In the density rangene  1  2  1020 m  3 the X2 scenario (extra-ordinary mode at the 2nd harmonic) is applicable,34th EPS Conference on Plasma Phys. Warsaw, 2 - 6 July 2007 ECA Vol.31F, P-5.129  (2007)and for higher densities, up to 2  4  1020 m 	 3, the O2 scenario will be applied. Due to thehigh optical thickness of the plasma for the X2-mode, there are no strict launch conditionsfor this scenario, and this freedom can be used for tailoring a desired deposition and currentdrive profile. The O2 scenario is much more limited because of low optical thickness of theplasma for the expected range of parameters. The most attractive way for the O2 scenario is topreheat plasma by the X2-mode, to change then the polarization from X- to O-mode by meansof rotation of the corrugated mirrors, and to increase the density to the requested value [4,5].In order to prevent an overheating of the wall by the shine-through power and to control itsreflection, mirrors at the inner wall are installed, and all beams planned for O2 operation mustbe directed to these mirrors. In the calculations, three passes are taken into account for the O2scenario. Because of symmetrical location of the mirrors about the launch ports, only five beamsout of ten can be launched in the same direction, and operation with maximal counter-ECCD ispossible only with a total power up to 5 MW.The calculations were done for five different beams launched from the ports E10 and A51with 1 MW power for each beam. Both X2 and O2 scenarios are modelled for the same fixedFigure 1: Summarized ECCD profiles for bothX2 (red) and O2 (blue) scenarios for the standardmagnetic configuration with ne  0  8  1020 m 	 3.launch conditions for the three differentmagnetic configurations. If after two passesthrough the plasma less than 90% of thepower was absorbed, this scenario was ex-cluded from consideration. The plasma pro-files and bootstrap current were calculatedby the 1D transport code [4] (for details, seealso [5]) coupled self-consistently with theray-tracing code TRAVIS [1]. The transportmodelling is based on the DKES databaseof monoenergetic transport coefficients, andthermal transport coefficients are obtained by energy convolution with Maxwellians. The DKEScode [6] uses as collisional term only the pitch-angle Lorentz operator without momentum con-servation, leading to an overestimate of the bootstrap current with dependence on Zeff(in presentcalculations, Zeff  1  5 is used).In order to increase single-pass absorption of the O2-mode in optically “gray” plasmas, theresonance is shifted in the low-field-side direction by use B  2  53 T on axis. In Fig.1, the ECCDprofiles for both scenarios are shown (the corresponding deposition profiles have a very similarshape). These profiles summarize the contributions from all beams (with three passes taken34th EPS 2007; N.B.Marushchenko et al. : Analysis of ECCD scenarios for different configurations of the W7-X... 2 of 4into account for O2-mode), which produce the counter-ECCD, i.e. ECCD is directed againstthe bootstrap current. Note, that the ECCD (as well the deposition) profiles for X2 and O2scenarios are quite different. Due to very high optical thickness, the location of the depositionFigure 2: Density scan for different magnetic con-figurations. Both Icd(solid) and Ibc(dashed) areshown. X2 and O2 scenarios are labeled by redcircles and by blue triangles, respectively.(as well ECCD) profile for X2-mode is al-most completely defined by the resonancelocation, and mainly a Doppler broaden-ing is responsible for the width. The shapeof the ECCD profile for X2 scenario is al-most independent of the density, and onlyits value scales roughly as 1  ne (see Fig.2).A temperature dependence is not stronglypronounced due to an absence of its strongvariation: for ne increased from 0  4  1020m  3 to 1  0  1020 m  3, Te is varied from7.3 keV to 5.0 keV, respectively. For theO2 scenario, the situation is much more so-phisticated. First of all, the plasma is op-tically “gray” and a significant part (up to20%) of the power is absorbed during asecond pass (a third pass is of minor im-portance). The absorption of the O2-mode,being proportional to T 2e , is very sensitiveto the shape of Te profile (ne profile is al-most flat). On the other hand, the Te profileis defined mainly by the deposition profile.Due to this feedback, the resulting deposi-tion (as well as ECCD) profiles for the sce-narios with the resonance shifted to the low-field-side are rather independent of ne, andits shape is quite similar to those shown inFig.1. Again, the O2 driven current scalesroughly as 1  ne.In Fig.2, the results of the density scan for the three magnetic configurations are shown. Asexpected, the current driven by X2-mode is larger than that of the O2-mode. The bootstrap cur-34th EPS 2007; N.B.Marushchenko et al. : Analysis of ECCD scenarios for different configurations of the W7-X... 3 of 4rent can be compensated by X2-ECCD for the “low-mirror” and “high-mirror” configurations.For the “standard” configuration (apart from the lower densities), this problem requires addi-tional study. Full current control at high density (in operation with O2-mode) is only obtainedfor the “high-mirror” configuration.In high-density operation at low ECRH power, the bootstrap current might exceed the maxi-mum ECCD both for the X2- and the O2-scenarios. Only the "high-mirror" configuration wasshown to have rather small bootstrap currents thus confirming the corresponding W7-X opti-misation criterion. For the "standard" configuration with improved neoclassical confinement,however, ECCD control of the bootstrap current is only possible in X2-mode at lower density.With full current control by ECCD, only a few skin-times (i.e. less than 10 sec) are necessaryto obtain stationary conditions for optimum divertor operation.High density scenarios are also important for the W7-X island divertor operation since fairlyhigh separatrix densities (about 0  4  1020 m  3) are required. Consequently, the O2-scenariosare expected to be very important for longer discharges. For these conditions, however, onlythe "high mirror" configuration allows for bootstrap current control by ECCD. In particular forthe "standard" configuration, another discharge scenario must be chosen to obtain stationaryconditions. Here, the edge value of the rotational transform in the vacuum configuration mustbe reduced by the amount which is generated by the bootstrap current in the final steady state.The discharge is operated at low density and rather low heating power with strong co-ECCD (inX2-mode) up to roughly stationary plasma current, i.e. the desired island divertor configurationis reached. In this scenario, however, the evolution of the plasma current scales on the L  R-time(i.e. several 10 sec). Then, the density as well as the heating power can be ramped up and theco-ECCD is reduced with the increasing bootstrap current, i.e. the total current is controlled.The internal current densities and the ῑ-profile become stationary again on the skin time. Such adischarge scenario, however, is much more complex compared to the case of full ECCD controlof the bootstrap current.References[1] N.B. Marushchenko et al., Proc. of the 16th Toki Conference, Dec. 5 - 8, 2006, Toki, Japan,http://itc.nifs.ac.jp/index.html[2] M. Romé et al., Plasma Phys. Control. Fusion 40 (1998) 511[3] V. Erckmann et al., will be published in Fusion Sci. and Technol. 52 (2007)[4] Yu. Turkin et al., Fusion Sci. and Technol. 50 (2007) 387[5] Yu. Turkin et al., this conference, P1-148[6] W.I. van Rij and S.P. Hirshman, Phys. Fluids B1 (1989) 56334th EPS 2007; N.B.Marushchenko et al. : Analysis of ECCD scenarios for different configurations of the W7-X... 4 of 4

Analysis of ECCD scenarios for different configurations of the W7-X Stellarator

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Analysis of ECCD scenarios for different configurations of the W7-X Stellarator

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