Damping of radial electric field fluctuations in the TJ-II stellarator

Abstract

The drift kinetic equation is solved for low density TJ-II plasmas employing slowly varying, time-dependent profiles. This allows to simulate density ramp-up experiments and to describe from first principles the formation and physics of the radial electric field shear, which is associated to the transition from electron to ion root. We show that the range of frequencies of plasma potential fluctuations in which zonal flows are experimentally observed is neoclassically undamped in a neighborhood of the transition. This makes the electron root regime of stellarators, close to the transition to ion root, a propitious regime for the study of zonal-flow evolution. We present simulations of collisionless relaxation of zonal flows, in the sense of the Rosenbluth and Hinton test, that show an oscillatory behaviour in qualitative agreement with the experiment close to the transition.

Self-consistent determination of the ion thermalization rate at the plasma boundary of TJ-II by the supersonic He beam diagnostic

“…These values are quite insensitive to the assumption about the T i edge profile, and are in good agreement with previous reports of poloidal rotation velocities in TJ-II when extrapolated to the radial location explored here [23,24], although only carbon ion (CV and CVI lines) rotation data are available. They also imply electric fields (1 kV m −1 ∼ 1 km s −1 ), which are in reasonable agreement with neoclassical calculations [25] and Doppler reflectometry measurements in TJ-II [26]. Unfortunately, only absolute velocity values are deduced here since, in the configuration here described, the diagnostic is unable to distinguish between the two possible rotation directions.…”

Moderation of neoclassical impurity accumulation in high temperature plasmas of helical devices

“…Nevertheless, the contribution to transport of ions of higher en- ergy in the Maxwellian distribution, which are in the 1/ν regime (for E r = 0) or √ ν regime, is not negligible, as can be seen from the non-linearity of Γ i (E r ) e.g. in [41]. Furthermore, the electron collisionality is lower than that of the ions.…”

Compressible impurity flow in the TJ-II stellarator

“…In this work we restrict ourselves to the discussion of flow deviations from incompressibility as an indirect measure of the C 6+ density inhomogeneity, since several instrumental uncertainties of the TJ-II CXRS system prevent the interpretation of signal intensities as relative density measurements. On the other hand, parallel and perpendicular impurity flows and temperatures are routinely provided by the CXRS system and have been shown to be in reasonable agreement with other diagnostics and/or NC theory predictions [4,15]. This paper is organized as follows: in section 2, the diagnostic set-up and geometry are presented together with the methodology used to relate the flow fields to the CXRS velocity measurements through the appropriate geometric quantities.…”