Abstract
Neutral beam heating has recently been installed on the START tight aspect ratio or 'spherical tokamak' (R/a greater than or equal to 1.2), using equipment loaned by Oak Ridge National Laboratory, USA. This has led to significant improvements in plasma performance - for example, an increased Murakami parameter, n(c)R/B. High central (similar to 48%) and volume averaged (similar to 8.5%) beta-values have been obtained. Calculations show that Troyon scaling still holds at low aspect ratio (with coefficients similar to 4 for ballooning modes, similar to 5 for link modes), indicating that the START device should reach still higher values of beta. It is found that the limiting edge-q rises for aspect ratios less than approximately 1.5, in agreement with theoretical calculations. Energy confinement at low aspect ratio is as good as, or better than, most H-mode scalings from conventional devices (up to twice ITER-93H) - with NBI there is no evidence so far for a power degradation (although P-NBI less than or equal to 1.5P(OH)). In both Ohmic and NBI plasmas, the scrape-off layer (SOL) in double null geometry is narrow, providing a valuable test of SOL models. As reported previously, START is resilient against current-terminating disruptions. It does, however, have more benign internal reconnection events when approaching density and current limits which show the initial features of a disruption (e.g, transient I-p rise and beta(p) fall). A model based on helicity conservation shows that, at tight aspect ratio, smaller falls (and sometimes rises) in edge-q result from falls in beta(p) and I-i, suggesting a possible explanation for the resilience to disruptions of START.