Metastable beta limit in DIII-D
The long-pulse, slowly evolving single-null divertor (SND) discharges in DIII-D with H-mode, ELMs, and sawteeth are found to be limited significantly below (factor of 2) the predicted ideal limit beta N = 4l(i) by the onset of tearing modes. The tearing modes are metastable in that they are explained by the neoclassical bootstrap current (high beta theta) destabilization of a seed island which occurs even if Delta prime < 0, (ie otherwise stable). For sufficiently high beta theta, there is a region of the modified Rutherford equation such that dw/dt > 0 for omega larger than a threshold value; the plasma is metastable, awaiting the critical perturbation which is then amplified to the much larger saturated island. Experimental results from a large number of tokamaks indicate that the high beta operational envelope of the tokamak is well defined by ideal magnetohydrodynamic (MHD) theory.
Bibliographic Reference: Article: Proceedings of 24th EPS Conference on Controlled Fusion and Plasma Physics, Berchtesgaden 9-13 June 1997
Record Number: 199810034 / Last updated on: 1998-02-03
Original language: en
Available languages: en