Control of plasma shape and performance of the PBX-M tokamak experiment in high-βt /high-βp regimes

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Abstract
The highly indented plasmas of the PBX‐M tokamak experiment [Plasma Physics and Controlled Nuclear Fusion Research (IAEA, Vienna, 1989), Vol. 1, p. 97] have reached plasma regimes of both high volume‐averaged beta (βt), and high‐beta poloidal (βp), and show evidence of the suppression of external surface modes by the passive stabilizing system. Values of βt up to 4.0 I/aB (% MA/m T) with Ti(0)≊4 keV have been obtained. A magnetohydrodynamic analysis of plasmas with βp=2.0 indicates that these plasmas are near the threshold of the second stability regime. A value of βt of 6.8% has been reached with Ti(0)>5 keV and an indentation of 28%. Control of plasma shape is accomplished with a feedback system that uses a moment expansion about a single equilibrium and is augmented by time‐dependent waveforms to redefine plasma shape. Diagnostics to measure the safety factor q have been developed and used to make accurate measurements of q(r) and to verify changes made in q(0).