Sensitivity of the kink instability to the pressure profile

The stability of the n=1 ideal kink in DIII-D-like [Fusion Technol. 8, 441 (1985)] configurations is found to depend critically on the details of the current density and pressure profiles. The maximum stable normalized β, βN=β/(I/aB) (I in MA, a in meters, and B0 in tesla), is found to increase dramatically as the peak in the pressure gradient is shifted from the central region toward the plasma boundary. Further, for peaked pressure profiles, the β limit is insensitive to the internal inductance, li, whereas for broad pressure profiles, the β limit depends more strongly on li and can increase with li almost up to the point where the q profile becomes hollow. With broad pressure profiles and a conducting wall at 1.5 minor plasma radii, kink-stable βN values of 5.8−significantly above both the Troyon limit and the ballooning limit−have been found.