Lachin, Tracey; (1998) The investigation of opacity in the JET tokomak divertor region. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The purpose of this thesis is to investigate Lyman line absorption by deuterium atoms in the divertor region of the JET tokamak for four high density, low temperature, detached plasma pulses. A collisional radiative model of deuterium level populations has been used to estimate the extent of Lyβ radiative absorption in the divertor along the same line of sight as a VUV spectrometer. This uses a first order escape probability method to evaluate the line escape probabilities and gives a self consistent model of the level populations and radiation field. These results are compared with experimental measurements of the branching ratio of Lyβ to Dα from the VUV spectrometer and various visible diagnostics. Both the theoretical and experimental results agree that opacity reduces the level of Lyβ emission from the divertor plasma. The effects of opacity on the ionisation and power balance of the plasma are examined for various conditions. The results of this investigation are compared with other theoretical work in the field. It is shown that the levels of opacity are not great enough to significantly alter the ionisation and power balance of the plasma for the conditions presently being created within the JET tokamak. The population code requires information about the background plasma. This can be provided by either a fluid code or an 'onion-skin' plasma simulation. Both models are used in this investigation and their levels of accuracy are compared. Finally, a brief investigation into the level of opacity in a future tokamak, ITER, is carried out using predicted plasma profiles. It is shown that opacity levels in the divertor region of the ITER tokamak could match those of JET and by creating highly detached plasmas could easily exceed these levels.

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