TOROIDAL ION TEMPERATURE-GRADIENT MODE-STABILITY AT LARGE TEMPERATURE-GRADIENTS

It is well known that the ion temperature gradient (ITG) or eta(i) mod e (eta(i) = [(1/T-i)(dT(i)/dr)]/[(1/n(o))(dn(o)/dr)] = L-n/L-T, L-n = density scale length, L-T = temperature scale length) has an instabili ty threshold (eta(flow)) around one for density scale lengths of the o rder of the magnetic field curvature radius L-B. In this work the exis tence of an upper eta(i) stability threshold (eta(iup)) in the range e ta(iup) approximate to 10 or larger is shown using a kinetic ion descr iption. The lowest eta(iup) is found to occur at wavelengths correspon ding to k(perpendicular to)(2) rho(i)(2) approximate to 1 (k(perpendic ular to) = perpendicular wavenumber, rho(i) = ion Larmor radius). It i s also shown that the density gradient (small L-n is stabilizing - low ering the growth rate and the upper eta(i) stability threshold.