Stability and Additional Heating Properties of Spherical Tokamak Plasmas on Start

Download or read book Stability and Additional Heating Properties of Spherical Tokamak Plasmas on Start written by and published by . This book was released on 1997 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.