Ionic instability thresholds in tokamaks

Linear ionic instability thresholds are numerically calculated for tokamaks. Proof is given that the stability domain is determined by trapped-ion modes for positive values of ηi=d(log Ti) /d(log ni). The boundary is characterized by ηi≥1 and a threshold LTi/R of order 0.2/(1+Ti/Te). This value is significantly smaller than previously predicted. Experimental temperature profiles in additionally heated discharges are usually marginal with respect to this criterion. It is also shown that marginal eigenmodes are low-frequency, low-wave-number modes, which may produce a very large transport once the ion temperature gradient threshold is reached.