Current profile modification experiments in EXTRAP T2R

Abstract

Pulsed poloidal current drive (PPCD) experiments have been conducted in the resistive shell EXTRAP T2R reversed-field pinch experiment. During the current profile modification phase, the fluctuation level of the m = 1 internally resonant tearing modes decreases, and the velocity of these modes increases. The m = 0 modes are not affected during PPCD, although termination occurs with a burst in the m = 0 amplitude. The PPCD phase is characterized by an increase in the central electron temperature (up to 380 eV) and in the soft x-ray signal. Spectroscopic observations confirm an increase in the central electron temperature. During PPCD, the plasma poloidal beta increases to 14%, and the estimated energy confinement time doubles, reaching 380 µs. The reduction in the fluctuation level and the corresponding increase in the energy confinement time are qualitatively consistent with a reduction in parallel transport along stochastic magnetic field lines.

Enhanced Confinement and Quasi-Single-Helicity Regimes Induced by Poloidal Current Drive

“…Experimentally, only quasi-singlehelicity (QSH) states are observed [3][4][5], i.e., regimes where the magnetic fluctuation is dominated by a single mode, but also other harmonics are present, so that the improvement involves only a limited volume of the plasma and the overall level of chaos is not significantly reduced. However, an effective technique was devised at the MST experiment to reduce in a pulsed fashion the global level of fluctuations (the pulsed poloidal current drive, PPCD [6]) and successfully applied in all the RFP experiments [7][8][9][10]. The mechanism consists of varying the toroidal field at the edge so that a current profile modification is transiently induced in the plasma, concentrating the toroidal flux in the core [11].…”

High-β, improved confinement reversed-field pinch plasmas at high density

“…4,5 This technique has also been successfully applied to other RFP plasmas. [6][7][8] The largest improvements in energy confinement and beta have been limited to relatively low density, n e ഛ 10 19 m −3 . 2, 4,5,[7][8][9][10][11] Above this density in the MST, m = 0 instability is triggered, and energy confinement degrades.…”

Numerical studies of active current profile control in the reversed-field pinch

“…Numerical computations indicate that flattening of the current profile in the core plasma reduces the tearing modes [5]. Many different CPC schemes have been suggested, and various schemes have been tested both experimentally [6][7][8][9][10][11] and numerically [12].…”