Negative magnetic shear modes of operation in the Alcator C-Mod tokamak near the beta limit

The stability properties of non-inductively driven, reserved-shear type current profiles that may be achieved for Alcator C-Mod tokamak-like parameters (aspect ratio R/a = 3) are examined. It is found that without a conducting wall, the best stability results (βN up to 3.7) are achieved for highly triangular plasmas (δ = 0.7) at qmin 2.2, for relatively broad pressure profiles (ρ(0)/Pavg = 2.8) and large values of rmin/a ( = 0.75). For elliptical or circular cross-sectional plasmas with little triangularity, the stability limits are significantly lower (βNmax = 2.2). Finally, more-peaked pressure profiles (ρ(0)/Pavg 4.5) have lower beta limits than broader profiles. (Author)