Toroidal ion temperature gradient mode stability at large temperature gradients

It is well known that the ion temperature gradient (ITG) or ηi mode (ηi = [(1/Ti)(dTi/dr)]/[(1/n0)(dn0/dr) ] = Ln/LT, Ln = density scale length, LT = temperature scale length) has an instability threshold (ηilow) around one for density scale lengths of the order of the magnetic field curvature radius LB. In this work the existence of an upper ηi stability threshold (ηiup) in the range ηiup ∼ 10 or larger is shown using a kinetic ion description. The lowest ηiup is found to occur at wavelengths corresponding to k perpendicularto2ρi2 ∼ 1 (k perpendicularto = perpendicular wavenumber, ρi = ion Larmor radius). It is also shown that the density gradient (small Ln) is stabilizing - lowering the growth rate and the upper ηi stability threshold. (orig.)