Experimental results on tokamak beta limits from ISX-B

Abstract
It is observed that the expression β = Aβqψ / κ = 18–22% defines one of the operational boundaries for ISX-B and corresponds to empirical beta limit scalings obtained in other tokamaks. Although this expression closely agrees with simple expressions for ideal-MHD instability thresholds, it is not obvious that it constitutes a real tokamak beta limit. Confinement degradation is observed when β is well below this operating boundary, and there is neither experimental evidence nor a satisfactory theoretical model connecting this phenomenon with ideal-MHD activity.