Beta-limiting MHD Instabilities in Improved-performance NSTX Spherical Torus Plasmas

Abstract

Abstract. Global magnetohydrodynamic stability limits in the National Spherical Torus Experiment (NSTX) have increased significantly recently due to a combination of device and operational improvements. First, more routine H-mode operation with broadened pressure profiles allows access to higher normalized beta and lower internal inductance. Second, the correction of a poloidal field coil induced error-field has largely eliminated locked tearing modes during normal operation and increased the maximum achievable beta. As a result of these improvements, peak beta values have reached (not simultaneously) β t = 35%, β N = 6.4, β N = 4.5, β N /l i = 10, and β P = 1.4. High β P operation with reduced tearing activity has allowed a doubling of discharge pulse-length to just over 1 second with sustained periods of β N ≈ 6 above the ideal no-wall limit and near the with-wall limit. Details of the β limit scalings and β-limiting instabilities in various operating regimes are described.

Nonlinear simulation studies of tokamaks and STs

“…The upper regions are flattened due to the existence of a large m = 1 magnetic island there. Such flattening of both pressure and toroidal flow have been observed in experiments in the presence of a large magnetic island [11]. At the complete reconnection, both pressure and toroidal flow patterns are flattened.…”

Next-Step Spherical Torus Experiment and Spherical Torus Strategy in the Fusion Energy Development Path

“…In the area of plasma confinement, the observed global plasma energy confinement time in the NBI heated NSTX and MAST plasmas exceed the conventional confinement scalings both in H-mode and L-mode, diverted as well as inboard-limited plasmas [8,12]. In Moreover, due to the low loop voltage (~ 0.1 V compared to 0.5 V for typical NBI heated discharges), the high poloidal beta regime was maintained for over t-skin or about 5 t E [9].…”

“…Indeed, the progress in the ST plasma research being conducted worldwide has been quite rapid and encouraging [7,8]. In the area of high beta operations, the high average toroidal beta values of <b T > ~ 35% were achieved at high plasma current of Ip ~ 1.2 MA with E T ~ 200 kJ of stored energy on NSTX [9], which is a significant progress from the previous START high beta results obtained with Ip ~ 0.2 MA with E T ~ 20 kJ [10]. In another experiment, the high normalized beta value of b N £ 6.5 (b N £ 10 li) was achieved where the so-called no-wall beta limit was significantly exceeded by about 35% [11].…”

Lithium Pellet Injector Development for NSTX

“…The National Spherical Torus Experiment (NSTX), located at the Princeton Plasma Physics Laboratory, is evaluating the physics principles of a spherical torus (ST) geometry and has been extensively described elsewhere [1]. The NSTX research plan calls for the capability to inject small solid or micro pellet ensembles of lithium, and other low-Z impurities for operational and diagnostic applications.…”