LARGE ETA(I) TOROIDAL ITG MODES IN FLUID AND KINETIC DESCRIPTIONS

A study of the toroidal ion temperature gradient (ITG) or eta(i) mode [eta(i) = L-n/L-T, L-j = j\ dj/dr l(-1) (characteristic scale length)] at larger eta(i) values, eta(i) much greater than 1, been performed u sing a two-fluid model. It is shown that the upper eta(i), stability t hreshold (eta(iup)) is in the range eta(iup) approximate to 10 or larg er and that it is sensitive to FLR (finite Larmor radius) effects and to epsilon(n) = 2L(n)/L-B. Comparisons are made with the results of a gyrokinetic model reported earlier. The two-fluid description is found to reproduce the results of the gyrokinetic description with a good a ccuracy in the regimes b(0) = k(2)rho(i)(2) much less than 1 and epsil on(n) much less than 1 (k = perpendicular wavenumber, rho(i) = ion Lar mor radius) whereas for larger b(0) and epsilon(n) the fluid model giv es a lower eta(iup) than the gyrokinetic model. In the low b(0)/low ep silon(n) regime the density gradient independent relation epsilon(T) = L-T/L-B approximate to b(0)(2) tau/8 at the upper eta(i) stability th reshold is derived (tau = T-c/T-i).