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Understanding the behaviour of fast ions in a fusion plasma is very important, since the fusion-born alpha particles are expected to be the main source of heating in a fusion power plant. Preferably, the entire fast-ion velocity-space distribution function would be measured. However, no fast-ion diagnostic is capable of measuring the entire distribution function. The velocity space sensitivity of a fast-ion diagnostic is given by so-called velocity-space weight functions. Here, the development of neutron emission spectrometry weight functions is presented. By combining measurements from several fast-ion diagnostic views, it is possible to infer the distribution function using a tomography approach. Several inversion methods for solving this tomography problem in velocity space are implemented and compared. It is found that the best quality it obtained when using inversion methods which penalise steep gradients in the velocity-space distribution function. The different inversion methods are used to study actual fast-ion measurements from the ASDEX Upgrade tokamak. By calculating tomographies of the distribution before and after a sawtooth crash, it is possible to identify the parts of velocity-space where the fast ions are affected the most. Finally, the first ever tomographies calculated using a combination of measurements from different types of fast-ion diagnostics are presented.
|Publisher||Department of Physics, Technical University of Denmark|
|Number of pages||180|
|Publication status||Published - 2015|