(EX/P3-7) Particle and Impurity Transport in Electron-Heated Discharges in TCV

H. Weisen1), A. Zabolotsky1), M. Bernard1), A. Bortolon1), B.P. Duval1), E. Fable1), A. Karpushov1), M. Maslov1), O. Sauter1), V. Piffl2), Ch. Schlatter1), G. Veres3)
 
1) CRPP, Ecole Polytechnique Federale de Lausanne, Lausanne, Switzerland
2)Institute of Plasma Physics, Czech Academy of Sciences, Prague, Czech Republic
3)KFKI-Research Institute for Particle and Nuclear Physics, Budapest, Hungary

Abstract.  The behaviour of particle and impurity transport in electron-heated TCV discharges in view of developing physics understanding for a predictive capability for alpha-heated, ignited reactor conditions. A key question with respect to ITER is whether peaked density profiles persist in electron heated plasmas at reactor relevant β. In TCV the addition of core ECH leads to partial flattening of the density profile (often dubbed ‘pumpout’). This flattening effect saturates for a total power exceeding the Ohmic power in the target discharge by a factor of 3 or more, leaving a moderately peaked density profile, both in L-mode and H-mode plasmas independently of the ECH power. Although the core flattening by ECH is in qualitative agreement with drift wave turbulence theory, the saturation is not predicted by theory. Stationary H-modes with ne(0)/⟨ne⟩∼1.6 heated with 1.5 MW of ECH to βN≈2 have been obtained both in type I ELMy and in stationary ELM-free regimes with low particle confinement. These observations suggest that even in the presence of strong alpha heating in a reactor, density profiles may remain peaked enough for significantly improving the fusion power output. Steady-state electron ITB’s are obtained in TCV by fully sustained off-axis ECCD, which leads to shear reversal. Unlike L-modes, density gradients in the barrier do not experience flattening when central ECH is applied. Instead of a shear dependence, as observed in L-mode, the barrier region is characterized by a relation between electron temperature and density gradient lengths, which, for the strongest barriers, is expressed as Lne∼2.2 LTe. Another issue is the behaviour of impurities. The presence of convective transport is clearly identified for intrinsic impurities and can differ significantly from convective electron transport. Experimental carbon density profiles from CXRS in stationary Ohmic and ECR heated L-mode discharges are always peaked. Both electron and carbon density peaking factors scale with the peaking of the current profile and with the minor radius of ECH deposition. For q95 > 4, the carbon profiles are significantly more peaked than the electron density profiles. Since these discharges are dominated by anomalous transport, the convective effects must be interpreted as being of anomalous origin.

Full paper available (PDF)