MHD behaviour during RF current profile control experiments in PBX-M

Control of the plasma current profile is a key issue in determining the ultimate attractiveness of the tokamak as a reactor concept. Recent experiments on the Princeton Beta Experiment-Modification (PBX-M) tokamak have produced initial results on localized current profile control with Lower Hybrid Current Drive (LHCD) and Ion Bernstein Wave Heating (IBWH). Localized modification of the current profile has been achieved and sustained with LHCD in neutral beam heated plasmas with βp approx. 1. The modification of plasma equilibria was demonstrated by the changes in their MHD stability and by measurements of q(r) with the motional Stark effect (MSE) diagnostic. Current profile modification has also been achieved with IBWH, where high βp plasmas (βp approx. 1.8, βn = 2.5, qedge approx. 5) with peaked density profiles were created with IBWH in the presence of high power neutral beam injection (NBI), producing a high core confinement mode (CH-mode) by creating a core transport barrier near q approx. 1.2. This core barrier greatly reduced the ELM-driven thermal losses at this location. The IBW-induced pressure profile created ballooning-like high frequency modes with m-numbers from 5 to 14 and n=m-1, which were localized at the position of the core barrier, near q approx. 1.1 - 1.5. These modes probably caused the β-limit. Code calculations predict ballooning modes to be marginally unstable in that region, with the kink modes stabilized by the conducting shell. Results suggest that the detailed tailoring of current profiles required to access advanced tokamak regimes is possible with RF current drive techniques, but the stability of the instantaneous profiles intermediate to this goal must be maintained to avoid loss of control of the profiles. (author). 13 refs, 4 figs