STUDIES OF ELECTRON TRANSPORT AND CURRENT DIFFUSION IN SWITCHED ECCD EXPERIMENTS IN TCV

1. 33rd EPS Conference on Plasma Physics and Controlled Fusion, 19-23 June 2006, Rome, Italy STUDIES OF ELECTRON TRANSPORT AND CURRENT DIFFUSION IN SWITCHED ECCD EXPERIMENTS IN TCV C. Zucca, S. Alberti, R. Behn, S. Cirant 1, E. Fable, F. Gandini 1, T. P. Goodman, O. Sauter Centre de Recherches en Physique des Plasmas, Association EURATOM – Confédération Suisse, CRPP-EPFL, Station 13, CH – 1015, Lausanne, Switzerland 1 Istituto di Fisica del Plasma, EURATOM – ENEA – CNR Association, Milano, Italy • Aims: • To provide a better insight on the magnetic shear profile modification in Switched Electron Cyclotron Current Drive (SECCD) experiments [1]. • To study the plasma response in the presence of SECCD in order to understand the differences in time scales and transient behaviour between the various models employed and under different experimental conditions such as modulation period and deposition location. Numerical Results: • Some parameters are fixed during all simulations: k, d, PECH(r), ne(r),Ti(r), Ip. • IECCD and in certain cases Te are the only actuators for transport properties modifications. Fixed experimental Te profile (co-phase): • Switched-ECCD experiments at TCV: • Modulation of ECCD at constant total input power. • Target L-mode plasmas: a = 0.25 m, R0 = 0.8 m,  = 1.6, Ip = 150 kA, PECH = 500kW, ne0 = 1-2 1019 m-3, B0 = 1.42 T. • HOW ARE SECCD EXPERIMENTS REALIZED? • During a discharge, co- and cnt-ECCD are alternatively injected inside the plasma at constant modulation frequency and same input power. • Symmetric aiming of the beams and constant power deposition profilePECH(r). • UNDERLYING MOTIVATION OF SECCD EXPERIMENTS: • To decouple the contributions of heating from those of the current density tailoring. • This way, any modification in the transport properties of the plasma is to be ascribed only to the shear profile modulation realized by the ECCD switching. • For each SECCD discharge, a preliminaryECH shot is performed, with alternated on/off phases of the two beam clusters, to check that the total plasma energy stays constant and thus adjust PECH. Varying experimental Te profile (alternated co-/cnt-phases): RLW model for ce: RLW*s2 model for ce: Experimental Te data are averaged over each phase. • Color code: co-SECCD in blue, cnt-SECCD in red. • Time traces of TCV shot #24867, see Ref. [1]. • Modelling: • To account for shear modulation, modelling of jp is necessary, there being no direct measurement available at TCV. • ASTRA [2] code for transport analysis coupled with CLQ3D Fokker-Planck quasi-linear code providing the ECCD profiles (required due to the effect of fast particle transport [3]). • ASTRA employed in bothpredictive and interpretative mode,solving1D flux-averaged diffusive equations for the Te and y • Equilibrium reconstruction by a 2D fixed-boundary Grad-Shafranov solver to calculate and update the flux surfacesat every time step • 3 different cases for electron energy transport: • a) interpretative mode, providing measured Te(r, t) as input to ASTRA. • b) predictive mode, Rebut-Lallia-Watkins (RLW) semi-empirical local transport model [4]; • Critical value of Te is essentially negligible outside • the deposition region in case of ECH heated plasmas. • c) predictive mode, a modified RLW model with linear shear dependence: (ce, RLW- ce, neo)* s2. • Motivated by the fact that in the RLW model the reduction of transport is related to an increase on the local shear, ce  |1/s|, which is adequate for TCV discharges in case of negative or large positive shear. • The two free parameters Ce,an and Cgcrthave been determined to adequately reproduce the measured Te for the corresponding ECH discharges. Off-axis Shear modulation: Icd = 17 kA Te = Te (co) Icd = 10 kA Te = Te (co) Icd = 10 kA RLW model Icd = 17 kA RLW model • MHD activity in newer discharges: • A more recent series of SECCD discharges with double ECH power (i.e. 2+2 gyrotrons) and feedback control on the plasma elongation was realized to create a large database featuring different values of the plasma current, radial location and width of the deposited power. • Unfortunately these discharges exhibit constant MHD activity, which complicates the correct interpretation of the observed electron temperature modulation. • The mode activity is very intense during all co-SECCD phases and fades out when switching to cnt-SECCD, revealing that the q profile is indeed modified locally. • This also explains why in these new discharges no significant effect on Te is observed. • Nevertheless, the identification of the toroidal and poloidal mode numbers for these modes should allow a possible validation of the ASTRA modelling by comparison with the simulated rational q surfaces. • Such work could also be used to aid in avoiding MHD modes and thus in designing future SECCD experiments. cocnt f [Hz] t [s] • Conclusions: • Similar shear variation as obtained with a previous modelling based on electrodynamics calculations [1]. • Location and extent of the shear modulation is essentially independent of the transport model employed, confirming the robustness of the shear modelling (main contribution from jECCD). • Experimental Te shows that Te(co) > Te(cnt), consistent with decrease of confinement properties if the shear increases in the radial region where s < 1, as predicted by gyrokinetic simulations [5]. • With a reasonableIECCD = 10 kA at rdep = 0.5, s spans from 1.4 to 1.65  possible to experimentally investigate the predicted inverse behaviour of transport properties at s > 1-1.5. • The SECCD experiments should be complemented by a comparison with steady-state conditions to provide more information about the link between electron transport and magnetic shear. References: [1] S. Cirant et al., Nucl. Fusion 46, 500-511 (2006) [2] G.V. Pereverzev et al., Max Planck – IPP Report, IPP 5/42 (1991) [3] P. Nikkolaet al., Nucl. Fusion 43, 1343-1352 (2003) [4] P. H. Rebut et al., Proc. 12th Int. Conf. On Plasma Phys. And Controlled Nucl. Fus. Research, Nice 1988, IAEA VIENNA 1989, Vol. 2, p. 191 [5] A. Bottino et al., Plasma Phys. Control. Fusion 48, 215-233 (2006) This work was partly supported by the Swiss National Science Foundation. Email: costanza.zucca@epfl.ch