Overview of results from the MST reversed field pinch experiment

Abstract

An overview of recent results from the MST programme on physics important for the advancement of the reversed field pinch (RFP) as well as for improved understanding of toroidal magnetic confinement more generally is reported. Evidence for the classical confinement of ions in the RFP is provided by analysis of impurity ions and energetic ions created by 1 MW neutral beam injection (NBI). The first appearance of energetic-particle-driven modes by NBI in a RFP plasma is described. MST plasmas robustly access the quasi-single-helicity state that has commonalities to the stellarator and 'snake' formation in tokamaks. In MST the dominant mode grows to 8% of the axisymmetric field strength, while the remaining modes are reduced. Predictive capability for tearing mode behaviour has been improved through nonlinear, 3D, resistive magnetohydrodynamic computation using the measured resistivity profile and Lundquist number, which reproduces the sawtooth cycle dynamics. Experimental evidence and computational analysis indicates two-fluid effects, e.g., Hall physics and gyro-viscosity, are needed to understand the coupling of parallel momentum transport and current profile relaxation. Large Reynolds and Maxwell stresses, plus separately measured kinetic stress, indicate an intricate momentum balance and a possible origin for MST's intrinsic plasma rotation. Gyrokinetic analysis indicates that micro-tearing modes can be unstable at high beta, with a critical gradient for the electron temperature that is larger than for tokamak plasmas by roughly the aspect ratio.

Energetic-particle-driven instabilities and induced fast-ion transport in a reversed field pinch

“…Furthermore, s ¼ 6:2 ms is close to the fast-ion confinement time measured from lowbeam concentration (no excitation of instabilities) experiments using the beam blip method. 29 As in Figure 2(b), the classically predicted core fast ion density saturates at $2:5 Â 10 18 m À3 after $18 ms injection (40 A), which is $25% of the core electron density (n e0 $ 1 Â 10 19 m À3 ). In reality, however, the core fast ion density is expected to be below the classical prediction, as instabilities occur $2:5 ms after beam injection (Sec.…”

Ion Heat and Parallel Momentum Transport by Stochastic Magnetic Fields and Turbulence

“…We note in passing that the kinetic stress has been linked directly to plasma rotation by studies on the Madison Symmetric Torus reversal field pinch [33,34]. By a combination of probes and polarimetry, Ding et al [33] demonstrated a clear correlation between the divergence of the measured kinetic stress and the mean u profile (see Figure 2.…”

Helical magnetic self-organization of plasmas in toroidal pinches with transport barriers formation

“…The paper is focused on the reversed-field pinch (RFP) configuration [1]. Numerical solution of such visco-resistive MHD model applied to the RFP predicted that the magnetic field can acquire a helical state in a self-organized manner [2][3][4][5][6], a feature later observed in high-current plasmas in various RFPs devices [7][8][9]. More recently, the MHD model has improved its descriptive capability of the typical experimental intermittency [10], predicted new ways for channeling helical self-organization towards a selected state highlighting the role of small edge helical magnetic field [11], a set of results then validated by experiments [12] in RFX-mod in Italy.…”