Fuel ion densities and distributions in fusion plasmas Modeling and analysis for neutron emission spectrometry

Abstract: Neutrons produced in fusion reactions in a magnetically confined plasma carry information about the distributions and densities of the fuel ions in the plasma. This thesis presents work where various theoretical models of different fuel ion distributions in the plasma are used to calculate modeled components of the neutron energy spectrum. The calculated components can then be compared with measured data, either to benchmark and validate the model or to derive various plasma parameters from the experimental data. Neutron spectra measured with the spectrometers TOFOR and the MPR, which are both installed at the JET tokamak in England, are used for this purpose. The thesis is based on three papers.The first paper presents the analysis of TOFOR data from plasmas heated with neutral beams and radio frequency waves tuned to the third harmonic of the deuterium cyclotron frequency, which creates fast (supra thermal) ions in the MeV range. It is found that effects of the finite Larmor radii of the fast ions need to be included in the modeling in order to understand the data. These effects are important for fast ion measurements if there is a gradient in the fast ion distribution function with a scale length that is comparable to - or smaller than - the width of the field of view of the measuring instrument, and if this scale length is comparable to - or smaller than - the Larmor radii of the fast ions.The second paper presents calculations of the neutron energy spectrum from the T(t,n)4He reaction, for JET relevant fuel ion distributions. This is to to form a starting point for the investigation of the possibility to obtain fast ion information from the t-t neutron spectrum, in a possible future deuterium-tritium campaign at JET. The t-t spectrum is more challenging to analyze than the d-d and d-t cases, since this reaction has three (rather than two) particles in the final state, which results in a broad continuum of neutron energies rather than a peak. However, the presence of various final state interactions - in particular between the neutron and the 4He - might still allow for spectrometry analysis.Finally, in Paper III, a method to derive the fuel ion ratio, nt/nd, is presented and applied to MPR data from the JET d-t campaign in 1997. The trend in the results are consistent with Penning trap measurements of the fuel ion ratio at the plasma edge, but the absolute numbers are not the same. Measuring the fuel ion ratio in the core plasma is an important task for fusion research, and also a very complicated one. Future work should aim at measuring this quantity in several independent ways, which should then be cross checked against each other.