An experimental investigation into tokamak edge MHD behaviour

The HELIOS-CELESTE hybrid diagnostic (Carolan et al., 2001) is a multi-chord, multi-time active spectroscopic diagnostic attached to the COMPASS-D (Fielding et al., 1996) tokamak. Line intensity ratios from a neutral helium jet are used to measure ne and Te at the plasma edge, making use of emission rates based on a full collisional-radiative model of neutral helium (Summers, 1994). Doppler spectroscopy of HeII ions is used to measure velocity and temperature of those ions in the same region. Synergy of the two diagnostics produce neutral density and radial electric field measurements.

Edge Localised Modes (ELMs) (Zohm, 1996) are short, quasi-periodic bursts of particles and energy from the plasma edge, during H-mode (Wagner et al, 1982). A review of ELM phenomenology and theories is presented.

A simple model of line intensity of one of the neutral helium lines based on the coronal model is used as a self-consistency check of the HELIOS measurements. Measurements from HELIOS are benchmarked against Thomson scattering measurements. Combining the HELIOS and CELESTE diagnostic is shown to enhance the usefulness of both diagnostics and the synergy is used to lend new insight into Hmode transition behavior, most notably the absence of a radial electric field before the transition into H-mode.

Data from the HELIOS diagnostic shows a fall in the edge electron pressure gradient prior to the onset of type III ELMs on COMPASS-D. Data from Thomson scattering and a linear Dα camera on MAST is used to validate the observation. This result is compared with a leading theory of ELM dynamics and is found to contribute to the development of the theory.