Externally applied magnetic fields have been used to induce major disruptions in the Tokoloshe tokamak. The investigations have included 'resonant' and 'non-resonant' external field configurations, using any one of three different stellarator-like windings with ℓ = 1, 2 and 3. In cylindrical geometry, these coils produce magnetic islands with dominant modes m/n = 1/1, 2/1 and 3/1, respectively. In the case where the Mirnov oscillations in the plasma are weak, i.e. where the existing tearing mode islands are much smaller than the islands of the same helicity produced by external coils, a major disruption of the plasma occurs when the coil current exceeds a certain critical value. The time to disruption τd decreases with increasing coil current, consistent with a simple model in which a critical perturbing radial field must be exceeded for each coil to induce a disruption. The characteristic time to reach this critical field is similar for each of the three coils and is approximately the time needed for resistive field penetration to the q = 2 surface. No clear precursor is seen until a rapidly growing locked m = 2 mode becomes significant just before disruption. Simultaneous measurements of θ and r show that this is a tearing-like mode, with the locking orientation dependent on coil current direction. The growth time, τg = 200 μs, is essentially independent of τd, with τg << τd, typically. Comparisons with magnetic field geometry calculations show that the observed disruption limits are consistent with tearing mode destabilization precipitated by, and phase locked with, overlapping or nearly overlapping 2/1 and 3/1 coil islands, where the latter island also overlaps the limiter. This holds for each of the coils and for both 'resonant' and 'non-resonant' cases. The experimental results are also compared with the case of strong Mirnov oscillations. Here the Mirnov signal can be suppressed when the coil current is sufficiently high. This is clearly due to mode locking (∂/∂t → 0) rather than to mode suppression (θ → 0).
A tokamak with nearly uniform coil stress based on the virial theorem