A. Marinoni

JET (3He)-D scenarios relying on RF heating: survey of selected recent experiments

Recent JET experiments have been devoted to the study of (He-3)-D plasmas involving radio frequency (RF) heating. This paper starts by discussing the RF heating efficiency theoretically expected in such plasmas, covering both relevant aspects of wave and of particle dynamics. Then it gives a concise summary of the main conclusions drawn from recent experiments that were either focusing on studying RF heating physics aspects or that were adopting RF heating as a tool to study plasma behavior. Depending on the minority concentration chosen, different physical phenomena are observed. At very low concentration (X[He-3] > 10% electron mode conversion damping becomes dominant. Evidence for the Fuchs et al standing wave effect (Fuchs et al 1995 Phys. Plasmas 2 1637-47) on the absorption is presented. RF induced deuterium tails were observed in mode conversion experiments with large X[He-3] (approximate to 18%). As tentative modeling shows, the formation of these tails can be explained as a consequence of wave power absorption by neutral beam particles that efficiently interact with the waves well away from the cold D cyclotron resonance position as a result of their substantial Doppler shift. As both ion and electron RF power deposition profiles in (He-3)-D plasmas are fairly narrow-giving rise to localized heat sources-the RF heating method is an ideal tool for performing transport studies. Various of the experiments discussed here were done in plasmas with internal transport barriers (ITBs). ITBs are identified as regions with locally reduced diffusivity, where poloidal spinning up of the plasma is observed. The present know-how on the role of RF heating for impurity transport is also briefly summarized.

Safety factor profile requirements for electron ITB formation in TCV

On the TokamakConfiguration Variable (TCV), electron internal transport barriers (eITBs) can be formed during a gradual evolution from a centrally peaked to a hollow current profile while all external actuators are held constant. The formation occurs rapidly (<τeE) and locally and, according to ASTRA modelling, is consistent with the appearance of a local minimum in the safety factor (q) profile. The eITB is sustained by non-inductively driven currents (including the off-axis bootstrap current) for many current redistribution times while the current in the tokamak transformer is held constant. The maximum duration is limited by the pulse length of the gyrotrons. The transformer coil can be used as a counter (or co-) current source with negligible accompanying input power. In established eITBs the performance can be enhanced (degraded) by altering solely the central current or q-profile. New experiments show that the same stationary eITB performance can be reached starting from discharges with centrally peaked current. A fine scan in surface voltage shows a smooth increase in performance and no sudden improvement with voltage despite the fact that qmin must pass through several low-order rational values. The appearance, in

Investigating profile stiffness and critical gradients in shaped TCV discharges using local gyrokinetic simulations of turbulent transport

The experimental observation made on the TCV tokamak of a significant ă confinement improvement in plasmas with negative triangularity (delta < ă 0) compared to those with standard positive triangularity has been ă interpreted in terms of different degrees of profile stiffness (Sauter ă et al 2014 Phys. Plasmas 21 055906) and/or different critical gradients. ă Employing the Eulerian gyrokinetic code GENE (Jenko et al 2000 Phys. ă Plasmas 7 1904), profile stiffness and critical gradients are studied ă under TCV relevant conditions. For the considered experimental ă discharges, trapped electron modes (TEMs) and electron temperature ă gradient (ETG) modes are the dominant microinstabilities, with the ă latter providing a significant contribution to the non-linear electron ă heat fluxes near the plasma edge. Two series of simulations with ă different levels of realism are performed, addressing the question of ă profile stiffness at various radial locations. Retaining finite ă collisionality, impurities and electromagnetic effects, as well as the ă physical electron-to-ion mass ratio are all necessary in order to ă approach the experimental flux measurements. However, flux-tube ă simulations are unable to fully reproduce the TCV results, pointing ă towards the need to carry out radially nonlocal (global) simulations, ă i.e. retaining finite machine size effects, in a future study. Some ă conclusions about the effect of triangularity can nevertheless be drawn ă based on the flux-tube results. In particular, the importance of ă considering the sensitivity to both temperature and density gradient is ă shown. The flux tube results show an increase of the critical gradients ă towards the edge, further enhanced when d < 0, and they also appear to ă indicate a reduction of profile stiffness towards plasma edge.

Critical temperature gradient length signatures in heat wave propagation across internal transport barriers in the Joint European Torus

New results on electron heat wave propagation using ion cyclotron resonance heating power modulation in the Joint European Torus (JET) [P. H. Rebut et al., Nucl. Fusion 25, 1011 (1985)] plasmas characterized by internal transport barriers (ITBs) are presented. The heat wave generated outside the ITB, and traveling across it, always experiences a strong damping in the ITB layer, demonstrating a low level of transport and loss of stiffness. In some cases, however, the heat wave is strongly inflated in the region just outside the ITB, showing features of convective-like behavior. In other cases, a second maximum in the perturbation amplitude is generated close to the ITB foot. Such peculiar types of behavior can be explained on the basis of the existence of a critical temperature gradient length for the onset of turbulent transport. Convective-like features appear close to the threshold (i.e., just outside the ITB foot) when the value of the threshold is sufficiently high, with a good match with the theoreti...