A hollow current profile is observed to form in the early stages of the Canberra tokamak LT-3. A helical perturbation appears, of the mode predicted by resistive MHD theory (m = 4, n = 1) for the measured q-profile. Subsequently, the plasma relaxes rapidly to a state of approximately uniform current. Runaway electrons diffuse very rapidly during this period with step sizes round the torus of up to 1 cm. These are indicative of a disruption of the magnetic surfaces, probably as a result of the growth of the helical perturbation.

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A study of runaway electrons has been made during the current rise stage of discharges in tokamak LT-4. The bremsstrahlung from a small target just inside the plasma boundary was measured as a function of q (target). The correlation between X-ray bursts and integer q was established over a large range of q-values. It is suggested that lower-energy runaway electrons are carried with the expanding integer-q regions and that an upper limit could be placed on the energy of electrons surviving within the plasma.

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.