Characterization of Quasi-Single-Helicity States in a Low-Aspect-Ratio RFP

Abstract

Characteristics of quasi-single-helicity (QSH) states have been studied in a low-aspect-ratio reversed field pinch (RFP) machine RELAX mainly by magnetic diagnostics. Internal profiles of the fluctuating radial, poloidal and toroidal magnetic fields have shown good agreement with eigenfunctions of a single helical mode. The edge magnetic fluctuation spectra are somewhat broader than what are expected from internal magnetic field profiles. In spite of these slight discrepancies, the usual measure for the QSH, the spectral index N S lower than 2, still provides a reasonable measure for QSH states in RELAX. The QSH persistence has been improved in RELAX by the reduction of the poloidal resistance of flanges at poloidal gaps, mainly due to the improved axisymmetry of the toroidal magnetic field. QSH persistence more than 30% of the flat-topped current phase has been realized with current density lower than in other RFP, and probability of spontaneous QSH is 12.8 ± 7.3% which is higher than high-aspect-ratio RFP. It suggests the advantage of low-aspect-ratio configuration in attaining to the QSH. Comparison with recent experiments using active resonant perturbation may suggest the importance of further reduction of field errors to improve the quality of QSH.

The reversed field pinch

“…The end of the QSH phase came with a distortion of the m = 0 magnetic island chain, and the occurrence of a chaotic region, expelling energy at the location of the LM [97]. In RELAX, QSH states lasting above 30% of the flat-top duration can be obtained [98].…”

Relaxation models for single helical reversed field pinch plasmas at low aspect ratio

“…F<0 region. The n=3 becomes dominant only when F is slightly positive [23], maybe suggesting a different nature for this mode (see the discussion at the end of the paper).…”

“…Signals from the radial array of magnetic probes, which measured three components at 10 radial locations, were time-integrated, and a numerical band-pass filter (2 kHz < f < 50 kHz) was used to extract only the fluctuating components. The ordinate is the fluctuating magnetic field amplitude normalized to the edge poloidal field [23]. It should also be noted that the theoretical curves are multiplied by an arbitrary normalization factor and shifted radially of 0.2 of the normalized radius, to take into account the relatively big toroidal Shafranov shift present in the experiment (5 over 25 cm).…”

“…Signals from the radial array of magnetic probes, which measured three components at 10 radial locations, were time-integrated, and a numerical band-pass filter (2 kHz < f < 50 kHz) was used to extract only the fluctuating components. The ordinate is the fluctuating magnetic field amplitude normalized to the edge poloidal field [23].…”

Initial Experimental Results of Three-Dimensional Plasma Structure Reconstructed from Seven Independent X-Ray Images on RELAX

“…3 (b). These images reflect the axisymmetric component of SXR, as plasmas are believed to rotate in the toroidal direction [4]. The resultant image obtained by subtracting (b) from (a) at each time is shown in Fig.…”