Finite-Larmor-radius stabilization of the m = 2 instability in the Isar T1-B high-beta stellarator

The m = 2 mode stability is investigated in the ISAR T1-B toroidal high-beta stellarator (major diameter 2.7 m, capacitor bank energy 0.5 or 1.5 MJ) for a great variety of plasma parameters using deuterium filling gas (0.5 < ω L · τ ii < 200; 0.1 < r L /a < 0.35; ω L = ion Larmor frequency; r L = ion Larmor radius; a = plasma radius; τ ii = ion self-collision time). The ion mass dependence was checked by a small number of discharges with hydrogen. For r L /a < η crit , η crit = 0.14, fast-growing m = 2 modes were found, the limit η crit being essentially independent of the collision frequency within the accessible range (2 ≲ ω L · τ ii ≲ 20). For r L /a > 0.14 no fast-growing m = 2 modes were observed, the plasma lifetime being limited by a m = 1, k ≈ 0 mode. The normalized growth rate, = γ/(V i /a) = 0.15 ± 0.05, (V i = ion thermal speed) was about half the sharp boundary value and slightly higher than that calculated for a diffuse pressure profile. The experimentally determined minimum value of r L /a, for which the m = 2 mode is stabilized, is by about a factor of three smaller than Freidberg's estimation using a sharp-boundary Vlasov model. Advanced calculations in a sharp boundary model by Lewis and Turner deviate in the critical limit only by a factor of about 1.5 and recent results using a diffuse pressure profile by Herrnegger and Schneider and by Freidberg and Hewett agree quite well with the experimental data.