Knowledge of the ion temperature (Ti) is important for tokamaks when determining plasma parameters from diagnostics which require knowledge of the ion and electron temperature (Te) and also because ions of high energy could damage plasma facing components by sputtering on high power devices. In the tokamak edge there are limited data for Ti, however the few existing measurements show that, beyond the confined plasma, Ti > Te, unlike in the confined plasma where ions and electrons are thermal coupled. A measurement technique using retarding field energy analyser(RFEA) probes to measure Ti has been implemented on the Mega Amp Spherical Tokamak (MAST) to allow measurements at the midplane and also at the divertor target which are two key areas in the scrape-off layer (SOL) plasma. Measurements in ohmic low confinement (L-mode) plasma discharges have shown that Ti ≈ 2Te at the midplane, while Ti ≈ Te at the target. Additionally heated L-mode plasmas have shown that Ti can be greater than Te at the divertor target. In inter-ELM high confinement (H-mode) plasmas, Ti is often found to be higher than Te, with extremes in high power plasmas of Ti/Te ≈ 3 at the target. Initial measurements of the temperature of ions released by an average ELM show that Ti reduces with distance from the strike point at the target. The measured upper limit on Ti in an average ELM reduces from 60 eV to 30 eV over ∼ 4 cm radially at the target. Measurements made by the target RFEA during ELMs show peak Ti ∼ 35 eV compared to peak Te measurements by Langmuir probes of ∼ 25 eV. These initial measurements of Ti agree with modelling of the ELM evolution in the SOL which show hot electrons arriving at the target before the remaining particles released by the ELM. Measurements by RFEA probes are subject to a correction in the presence of parallel flows in the SOL. It has been found from modelling of the MAST SOL that measurements made by the target RFEA should be multiplied by a correction factor between 0.65 and 0.7. Although this reduces the ratios measured of Ti/Te, measurements in inter-ELM H-mode and additionally heated L-mode plasmas would still show Ti > Te at the target. In ohmic L-mode plasmas, however, the reduction in Ti would mean Ti < Te.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:592870 |
| Date | January 2013 |
| Creators | Elmore, Sarah |
| Contributors | Bradley, James; Walsh, James; Kirk, Andrew |
| Publisher | University of Liverpool |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://livrepository.liverpool.ac.uk/14513/ |
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