Plasma ejection phenomenon during non-inductive spherical tokamak start-up by ECH/ECCD on LATE

The plasma ejection phenomenon that repeatedly occurs during non-inductive spherical tokamak start-up by electron cyclotron resonance heating and current drive (ECH/ECCD) on Low Aspect ratio Torus Experiment is reported. Measurements by interferometers, Langmuir probes, AXUV and fast visible CCD cameras show that, in a single event, the bulk plasma contained from the core to the edge within the last closed flux surface is ejected out and lost towards the vessel wall along the field line. The density degradation accompanied by such an event is as large as 40% within around a hundred microseconds. Magnetic probe measurements indicate that current-carrying electrons are also lost to the wall during the event. Operational parameters are scanned to investigate the parameter dependence on the ejection phenomenon. The interval and crash times are around 0.3–1 ms and 100–200 µs, respectively, and become shorter as the plasma current increases. The dependence on plasma elongation shows that less elongated plasma bears almost no visible ejection but does not contribute to density increase. This is a new phenomenon found in a high-q, low aspect ratio tokamak with anisotropic pressure, produced and sustained only by ECH/ECCD.