PBX-M research progress. Approach to second stability

Research on the PBX-M tokamak has focussed on pursuing access to the second stability regime by developing tools to control the plasma shape as well as the plasma pressure and current profiles. Theory indicates that a broad current profile provides enhanced shaping capability and plasma stability properties necessary for accessing second stability at values of q(0) near unity. In PBX-M, the plasma surface was closely fitted to a conducting wall by broadening the current profile utilizing a current ramp technique, resulting in the achievement of high-〈β_t〉 (6.8% at high values of I/aB = 2.3 MA/m-T), high confinement (T_i(0) = 5.5 keV, τ_E ≈ 1.5τ_E^{Kaye-Goldston}) discharges. Although these discharges terminated in disruptions, a clear influence of the close fitting conducting shell in PBX-M was evidenced by reduced growth rates of the n = 1 disruption precursors relative to those in the predecessor PBX device (without passive plates) and to those in PBX-M plasmas with large plasma-plate separations. Discharges near the threshold of second stability were achieved at medium values of I/aB ≈ 0.9 MA/m-T, with 〈β_t〉/(I/aB) reaching values as high as 4.5 %-m-T/MA in this regime. These plasmas exhibited H-transitions, with resulting H-mode confinement times of order 2τ_E^{Kaye-Goldston} and 2.5τ_E^ITER-89P. The β-limiting processes in these plasmas may be related to the onset of ideal instabilities. In both the medium-and high-I/aB regimes, the maximum 〈β_t〉 values were 30 to 50% greater than those achieved in PBX, indicating an overall increase in the stability and confinement of PBX-M plasmas.