Advances in global MHD mode stabilization research on NSTX

Abstract

Stabilizing modes that limit plasma beta and reducing their deleterious effects on plasma rotation are key goals for efficient operation of a fusion reactor. Passive stabilization and active control of global kink/ballooning modes and resistive wall modes (RWM) have been demonstrated on NSTX and research now advances to understanding the stabilization physics and reliably maintaining the high beta plasma for confident extrapolation to ITER and CTF. Active n = 1 control experiments with an expanded sensor set, combined with low levels of n = 3 field phased to reduce error fields, reduced resonant field amplification and maintained plasma rotation, exceeded normalized beta = 6, and produced record discharge durations limited by magnet system constraints. Details of RWM active control show the mode being converted to a rotating kink that decays, or saturates leading to tearing modes. Discharges with rotation reduced by n = 3 magnetic braking suffer beta collapse at normalized beta = 4.2 approaching the no-wall limit, while normalized beta greater than 5.5 has been reached in these plasmas with n = 1 active control, in agreement with single-mode RWM theory. Advanced state-space control algorithms proposed for RWM control in ITER theoretically yield significant stabilization improvements. Values of relative phase between the measured n = 1 mode and the applied correction field that experimentally produce stability/instability agree with theory. Experimental mode destabilization occurs over a large range of plasma rotation, challenging the notion of a simple scalar critical rotation speed defining marginal stability. Stability calculations including kinetic modifications to ideal theory are applied to marginally stable experimental equilibria. Plasma rotation and collisionality variations are examined in the calculations. Intermediate rotation levels are less stable, consistent with experimental observations. Trapped ion resonances play a key role in this result. Recent experiments have demonstrated magnetic braking by non-resonant n = 2 fields. The observed rotation damping profile is broader than found for n = 3 fields. Increased ion temperature in the region of maximum braking torque increases the observed rate of rotation damping, consistent with theory.

Detection of Disruptions in the High-β Spherical Torus NSTX

“…Rather, they are indicative of bursts of fast ion loss [111]. Shown in red in the same figure is the n=1 B P perturbation [112], indicative of the growth of n=1 locked modes [47] and resistive wall modes (RWMs) [47,108,[112][113][114][115]. There is strong growth in this signal starting just before t 2 , as the plasma develops a strong 3D distortion.…”

Toroidal precession as a geometric phase

“…These two features may make this approach an especially attractive one to implement in stability codes such as MISK 26 or MARS-K 27 . In these codes, functions of the precession frequency are integrated over the constants of motion space.…”

Exploration of the Equilibrium Operating Space For NSTX-Upgrade

“…Indeed, NSTX has observed and documented many features of the RWM stability in a spherical torus [39,121,[126][127][128][129][130][131][132]. Calculations of the resistive wall mode stability is not within the scope of the paper.…”

“…Calculations of the resistive wall mode stability is not within the scope of the paper. However, we note that when sustaining the rotation with error-field correction [39,40] and avoiding the RWM with fast n=1 feedback [128,131,132], reliable operation in the wall-stabilized regime has been achieved.…”