Tokamak Energy Ltd, UK, is developing spherical tokamaks using high temperature superconductor magnets as a possible route to fusion power using relatively small devices. We present an overview of the development programme including details of the enabling technologies, the key modelling methods and results, and the remaining challenges on the path to compact fusion.

Results are presented from thin (resistive) shell experiments on HBTX and compared with theoretical (linear and non-linear) studies of the plasma stability. Current pulses of 3--5 ms are obtained, compared with the shell time constant for vertical field penetration of 0.5 ms. Theoretically predicted thin shell modes, phase locked to the wall, are prominent experimentally.

Arrays of edge magnetic coils and statistical analysis techniques have been used to investigate the magnetic fluctuation structure in the HBTX-1A reversed field pinch. The superficially random fluctuations can in fact be attributed almost entirely to global modes with poloidal mode number m = 0 and 1, provided account is taken of toroidal distortion of the modes. A toroidal array of coils discloses a broad spectrum of toroidal mode numbers with peak at Inl ~ 10 and significant variation with time and frequency. Cross-correlation establishes that | n | ~ 10 corresponds to m = 1, a helical mode resonant inside the reversal surface, and also shows the presence of m = 0, n ~ 0. The time-scales of the fluctuation indicate that the instabilities are probably resistive in character, and the mode amplitudes are such that island overlap and magnetic field ergodization should occur. The energy confinement time due to stochastic transport, estimated from the measured fluctuations, is consistent with that observed experimentally.

The temperature and strain response of fibre Bragg grating sensors in a cryogenic environment has been evaluated. The fibre Bragg grating temperature response was found to be nonlinear, decreasing to approximately zero for temperatures less than 100 K. The response to strain was found to be temperature independent.