A fast (0.1 ms) and compact 'multi-colour' soft x-ray array has been developed for time and space-resolved electron temperature (T e) measurements in magnetically confined fusion (MCF) plasmas. The electron temperature is obtained by modelling the slope of the continuum radiation from ratios of the available 1D-Abel inverted radial emissivity profiles over different energy ranges, with no a priori assumptions of plasma profiles, magnetic field reconstruction constraints or need of shot-to-shot reproducibility. This technique has been used to perform fast T e measurements in the National Spherical Torus Experiment (NSTX), avoiding the limitations imposed by the well-known multi-point Thompson scattering, electron cyclotron emission and electron Bernstein wave mode conversion diagnostics. The applicability of this 'multi-colour' technique for magnetohydrodynamic (MHD) mode recognition and a variety of perturbative electron and impurity transport studies in MCF plasmas is also discussed. Reconstructed 'multi-colour' emissivity profiles for a variety of NSTX scenarios are presented here for the first time.

Abstract. The spherical tokamak (ST), because of its slender central column, has very limited voltsecond capability relative to a standard aspect ratio tokamak of similar plasma cross-section. Recent experiments on the National Spherical Torus Experiment (NSTX) have begun to quantify and optimize the ohmic current drive efficiency in a MA-class ST device. Sustainable ramp-rates in excess of 5MA/sec during the current rise phase have been achieved on NSTX, while faster ramps generate significant MHD activity. Discharges with I P exceeding 1MA have been achieved in NSTX with nominal parameters: aspect ratio A=1.3-1.4, elongation κ=2-2.2, triangularity δ=0.4, internal inductance l i =0.6, and Ejima coefficient C E =0.35. Flux consumption efficiency results, performance improvements associated with first boronization, and comparisons to neoclassical resistivity are described.

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Abstract.Saturated internal kink modes have been observed in many of the highest toroidal β discharges of the National Spherical Torus Experiment (NSTX). These modes often cause rotation flattening in the plasma core, can degrade energy confinement, and in some cases contribute to the complete loss of plasma angular momentum and stored energy. Characteristics of the modes are measured using soft X-ray, kinetic profile, and magnetic diagnostics. Toroidal flows approaching Alfvénic speeds, island pressure peaking, and enhanced viscous and diamagnetic effects associated with high-β may contribute to mode non-linear stabilization. These saturation mechanisms are investigated for NSTX parameters and compared to experimental data.

Ideal magnetohydrodynamic stability limits of shaped tokamak plasmas with high bootstrap fraction are systematically determined as a function of plasma aspect ratio. For plasmas with and without wall stabilization of external kink modes, the computed limits are well described by distinct and nearly invariant values of a normalized beta parameter utilizing the total magnetic field energy density inside the plasma. Stability limit data from the low aspect ratio National Spherical Torus Experiment is compared to these theoretical limits and indicates that ideal non-rotating plasma no-wall beta limits have been exceeded in regimes with sufficiently high cylindrical safety factor. These results could impact the choice of aspect ratio in future fusion power plants. Introduction -The superconducting advanced tokamak [1, 2] is presently the leading candidate for producing an efficient magnetic fusion reactor. Alternative concepts such as the compact stellarator [3,4] and spherical torus [5][6][7] are also actively being pursued as possible improvements to the advanced tokamak. The advanced tokamak (AT) and spherical torus (ST) reactor concepts have several features in common. In particular, both rely on the neoclassical bootstrap current [8] to sustain nearly all of the plasma current and on stabilization of pressure-driven external kink modes to achieve sufficiently high beta (ratio of plasma kinetic pressure to magnetic pressure) to produce power efficiently. The AT and ST reactor concepts have been independently optimized for various physics and engineering constraints and arrive at notably different plasma aspect ratio and beta. This difference has motivated the present work which seeks to understand how the theoretical ideal magnetohydrodynamic (MHD) stability limits of the AT and ST are linked. More generally, aspect ratio invariants of stability are sought.The first equilibrium regime studied consists of fully self-driven plasmas utilizing a close-fitting conducting wall to stabilize external kink modes. The stability limits of this regime represent the maximum achievable beta for this class of equilibrium at any aspect ratio given the present understanding of the relevant physics. The second regime studied consists of plasmas with a self-driven current fraction of 50% and no conducting wall stabilization of the external kink mode. The stability limits of this regime have largely been experimentally realized in present-day tokamaks [9] but have only recently been realized in relatively new ST experiments. Finally, the no-wall current limit is studied for typical AT and ST configurations. These studies show that the degeneracy