Magneto-hydro-dynamic limits in spherical tokamaks

The operational limits observed in spherical tokamaks, notably the small tight aspect ratio tokamak (START) device [A. Sykes et al., Nucl. Fusion 32, 694 (1992)], are consistent with those found in conventional aspect ratio tokamaks. In particular the highest beta achieved (similar to 40%) is consistent with an ideal magneto-hydro-dynamic (MHD) Troyon type limit, the upper limit on density is well described by the Greenwald density (pi a(2)(n) over bar(e)/I-p similar to 1) and the normalized current (I-p/aB(t)) is limited such that q(95)greater than or similar to 2. Stability calculations indicate scope for increasing both normalized beta and normalized current beyond the values so far achieved, although wall stabilization is generally needed for low-n modes. In double null configurations current terminating disruptions occur at each of the operational boundaries, though the current quench tends to be slow at the density limit and disruptions at high b may be due to the low q. In early limiter START discharges, before the divertor coils were installed, disruptions rarely occurred. Instead internal reconnection events which have all the characteristics of a disruption except the current quench occurred. These various disruptive behaviors are explained in terms of a model in which helicity is conserved during the disruption. Due to the low toroidal field beam ions in START, and alpha particles in a ST power plant, are super-Alfvenic. This gives the possibility for toroidal Alfven eigenmodes (TAEs) to occur and such modes are frequently observed in START neutral beam injection (NBI) discharges, but seem to be benign. The features of these observed TAEs are shown to be in agreement with MHD calculations. (C) 1999 American Institute of Physics. [S1070-664X(99)95005-X].