High-beta injection experiments on the ISX-B tokamak

Neutral-beam injection of up to 2.5 MW into plasmas in the ISX-B tokamak (R0=0.93 m, a=0.27 m, Bsub(T)=0.9-1.5 T, Isub(p)=70-210 kA, anti nsub(e)=2.5-10x1013 cm-3) has created plasmas with volume-averaged beta of up to approximately 2.5%, peak beta values of up to approximately 9%, and root-mean-square beta values of up to approximately 3.5%. Energy confinement time is observed to decrease by about a factor of two as beam power goes from 0 to 2.5 MW; the decrease is caused predominantly by the electron confinement time falling below the predictions of 'Alcator scaling' by a factor of 3-4 at high beam power. An empirical relationship of the form βsub(p)Isub(p)sup(1/2)=f(Psub(b)) fits our measurements over a wide range of plasma parameters. The function f(Psub(b)), where Psub(b) is the beam power, is linear for Psub(b)<=1.2 MW but tends to saturate for 1.2 MW<=Psub(b)<=2.5 MW. Although the equilibria attained in ISX-B are predicted to be above the threshold for the ideal magnetohydrodynamic (MHD) ballooning instability, no evidence of these modes is observed. (author)