ION-TEMPERATURE-GRADIENT MODES IN NONCIRCULAR TOKAMAK GEOMETRY

The work of Xu and Rosenbluth [Phys. Fluids B 3, 627 (1991)] on ion-temperature-gradient modes in toroidal geometry is extended to noncircular cross sections to examine the scaling with the elongation kappa. The growth rate, wave numbers, and stability thresholds are calculated for a flat density profile characteristic of H-mode operation, and a mixing-length estimate of the ion thermal diffusivity chi(i) is calculated. It is found that in the plasma core, growth rates decrease as the central elongation kappa increases, due to finite-Larmor-radius effects. The calculated chi(i) is approximately of the form chi(i) = DT(i)3/2[(L(Ti))c - L(Ti)] where D and (L(Ti))c depend on tau = (T(e)/T(i)) and geometry, and L(Ti) = \T(i)/del T(i)\. Preliminary evidence indicates fair agreement with experimental data near the plasma edge, where L(Ti) much less than (L(Ti))c, but the fit is poorer in the core where L(Ti) is similar to (L(Ti))c and the theoretical chi(i) is very sensitive to the accuracy in calculating (L(Ti))c.