In the LT-4 tokamak, the precursor activity for disruption at q(a)<2.5 does not exhibit the oscillatory low order mode MHD behaviour usually seen elsewhere. Instead, each thermal quench is preceded by a fast transient perturbation with dominant Fourier components m/n = 2/1 and 3/2. The corresponding islands grow so rapidly, at rates consistent with the Rutherford non-linear tearing mode theory, that they reach estimated sizes sufficient to cause disruption in less than a typical Mirnov oscillation period.

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The LT -4 is a relatively small (major radius 0·5 m, minor radius 1'10· 1 m) experimental tokamak using a homopolar generator as its main power source to produce a toroidal field of 3 T. A fast currentforcing feedback control system maintains the plasma position constant to 1'1 1 mm. Fairly standard diagnostics are supported by a data acquisition system which handles 200 kbytes of data through 70 CAMAC controlled ADCs per discharge. Illustrative examples of data obtained during .typical operation show magnetohydrodynamic (MHD) activity (magnetic pick-up coils and soft X-ray detectors) as well as relaxation oscillations within the plasma.

An extensive experimental program has been carried out on the MHD activity occurring in the LT-4 plasma during ohmic heating discharges, under.a wide range of reproducible conditions, for 3·6 > q(a) > 2·5. Perturbations are studied by using large arrays of individually monitored magnetic coils, distributed both poloidally and toroidally, and two toroidally separated soft X-ray poloidal arrays. The results show that the behaviour can be broadly classified into four distinct regimes within each of which the observable MHD phenomena are remarkably reproducible, and whose boundaries can be very well described by the single parameter q(a). Although most of the observations conform with those on other tokamaks, some appear inconsistent with the usual interpretation, for example, the assumption of rigid toroidal rotation. Further, while internal m/ n = 1/1 and m = 0 sawteeth activity at the q = 1 surface are clearly defined, the magnetic signals usually ascribed to a 2/1 mode when q( a) .;; 3 are clearly shown to have a predominant 3/1 structure, while activity at the q = 2 surface appears relatively weak.

The paper presents the results of a detailed computational study of possible ℓ = 3 stellarator winding configurations in which the separatrix radius and position are maintained constant. It is shown that toroidal effects on both the rotational transform properties and the mechanical forces can be reduced by suitably modulating the winding pitch, which results in considerable savings in the required current and mechanical support. The existence of significant magnetic wells is demonstrated. The results generally favour the use of shallow rather than steep windings and fairly large aspect ratios.