Characterisation of detached plasmas on the MAST tokamak

Abstract

The mitigation of the steady-state heat loading to plasma-facing components in the divertor structures is an essential prerequisite for the operation of next-generation conventional and tight aspect-ratio Tokamaks. Currently, the most attractive means of reducing this heat loading is the so-called "detached" regime, which is the primary focus of this study.This thesis describes work that was carried out on the MAST tokamak to accurately characterise the plasma conditions within divertor plasmas, and to test the validity of those measurements. To assist in this study, an interpretive code, OSM [1], has been upgraded with additional numerical schemes to ascertain the dominant mechanisms governing parallel scrape-off layer transport including flux expansion, dynamic viscosity and cross-field drift motion in attached conditions. It has been found that the invocation of flux expansion, parallel viscosity and diamagnetic drift motion assist the OSM numerical scheme to converge in the presence of (static+dynamic) parallel pressure gradients often observed between the outboard midplane and lower outer divertor leg.It has also been found that the ion pressure, which is currently unknown, could resolve the observed pressure discrepancy.Line-of-sight spectroscopy of high-n Balmer emission lines has been used to ascertain line averaged T e and line integrated n e during the detached phase of MAST discharges with high spatial resolution (≈ 7mm) to determine electron static pressure profiles along flux contours. Furthermore, narrow-bandwidth imaging spectroscopy of D α , D γ , CII and CIII emission during the onset and sustainment of detachment has been carried out with high temporal (5kHz) and spatial (≈ 3mm) resolution. These data have been combined with ion flux measurements to the divertor target plates using embedded Langmuir probes (6-9mm radial resolution) and upstream Thomson scattering measurements of n e , T e . The data was input into the OSM code, coupled with the kinetic neutral transport code EIRENE [2], to check the experimental data set for internal consistency. The experimental data collected has been used to reconstruct the plasma conditions within the detached divertor leg, heavily constrained by experimental data. EIRENE calculations with re-constructed plasma conditions are able to reproduce experimentally observed D γ /D α line emission ratios to within a factor iii iv of 2 within the recombining region. AcknowledgmentsThe work presented in this thesis is the culmination of the work of many people, who I gratefully acknowledge here. Foremost, I thank Dr. Steven Lisgo for his guidance, enthusiasm and assistance over the last 4 years. It is only on reflection that I appreciate how he kept this work on track, whilst encouraging me to explore my own ideas and how doing so has enhanced the quality of the work presented here. Drs. Andrew Kirk and Geoff Fishpool also deserve special thanks for their guidance, and above all, their understanding in the final months of this work. I also owe great thanks to...

Characterisation of the L-mode scrape off layer in MAST: decay lengths

“…All the discharges were performed with double null diverted configurations. It is important to remark that, due to the particular divertor and wall geometry of MAST, detachment is particularly difficult to achieve [17] and none of the results presented here was in that regime.…”

A new experimental classification of divertor detachment in ASDEX Upgrade

“…The electron density in the divertor volume, n e,V , is determined via a spectroscopic measurement of the Stark broadened D ǫ line. This method is commonly used on other devices, such as JET, MAST or C-Mod [21,22,8]. Here, synthetic Stark broadened line shapes, calculated and published by Stehlé [23] based on the model microfield method, are convoluted with the instrument function and the Doppler broadening of a fixed neutral temperature of T n = 5 eV.…”

Effect of reflections on 2D tomographic reconstructions of filtered cameras and on interpreting spectroscopic measurements in the JET ITER-like wall divertor

“…Tomographic reconstructions of tangential camera views are an established method for resolving the 2D distribution of divertor emission in various tokamak devices, [15][16][17][18][19][20][21][22][23] including the spectroscopic divertor camera system in JET. 24,25 The reconstructed 2D emission distributions assist in localizing the line-integrated divertor spectroscopy measurements, allow inferring, e.g., the spatial evolution of ionization and recombination fronts, and provide valuable reference data for divertor plasma modeling. However, especially in the case of all-metallic wall configurations, such as the ITER-like wall of JET, 26 camera measurements can be affected by reflections from the wall surfaces.…”