COMPRESSIBILITY EFFECTS ON IDEAL AND RESISTIVE BALLOONING STABILITY IN THE TJ-II HELIAC DEVICE

The magnetohydrodynamic (MHD) ballooning stability of the TJ-II heliac standard configuration is examined using a criterion derived for thre e dimensional (3-D) general configurations. From the ideal limit of th is criterion, a critical (beta) similar or equal to 1.3% is obtained. This critical value is very close to that obtained from the Mercier cr iterion and is almost unaffected when those terms associated with comp ressibility are retained. The inclusion of resistivity is shown to hav e almost no effect on the attainable critical (beta) even though unsta ble modes persist for lower (beta) values due to their small growth ra te. For parameters relevant to this configuration, the resistive press ure convection limit predicts unstable modes, scaling as gamma similar to m(2) eta for all (beta) below the ideal limit, which are shown to disappear when compressibility terms are retained in the calculations. Unstable resistive ballooning modes with gamma tau(A) similar to 10(- 3) are then found for (beta) values down to 90% of the ideal critical (beta). The growth rate rapidly falls if the pressure gradient is furt her decreased owing to the stabilizing effect of compressibility for g amma tau(A) much less than 1.