NEGATIVE MAGNETIC SHEAR MODES OF OPERATION IN THE ALCATOR-C-MOD-TOKAMAK NEAR THE BETA-LIMIT

The stability properties of non-inductively driven, reversed-shear typ e current profiles that may be achieved for Alcator C-Mod tokamak-like parameters (aspect ratio R/a = 3) are examined. It is found that with out a conducting wall, the best stability results (beta(N) up to 3.7) are achieved for highly triangular plasmas (delta = 0.7) at q(min) = 2 .2, for relatively broad pressure profiles (p(0)/p(avg) = 2.8) and lar ge values of r(min)/a (= 0.75). For elliptical or circular cross-secti onal plasmas with little triangularity, the stability limits are signi ficantly lower (beta(Nmax) = 2.2). Finally, more-peaked pressure profi les (p(0)/p(avg) = 4.5) have lower beta limits than broader profiles.