Ion Temperature Profile Measurements in ITER by Means of Neutron Spectroscopy

In the next generation of magnetic fusion experiments, such as the International Thermonuclear Experimental Reactor (ITER), information on ion temperature profiles will be needed for burn optimization and transport studies. The feasibility of obtaining these profiles for the core plasma (r < 0.75 of minor radius) directly from the width of measured 14-MeV neutron energy spectra is demonstrated for Maxwellian ion distributions. Neutron energy spectra are calculated using the Monte Carlo technique. Reaction kinematics and velocity distribution of the reacting ions are taken into account, which enables the resulting neutron flux and energy distribution entering a defined collimator to be calculated. Energy spectra of neutrons emitted along a line of sight (LOS) are obtained by adding the contributions from a large number of subvolumes. The associated correction factor (peak temperature over LOS measured temperature) depends on the ion temperature and on the shapes of the temperature and density profiles. The resulting accuracy in the evaluated ion temperature profiles is expected to be better than ± 10%. However, this can be improved to ±5% provided that the ion density profile shape is known. The relative accuracy is estimated to be better than ±5%. Features of several spectrometer candidates are briefly described in relation to ITER conditions and measurement requirements. A time-of-flight (TOF) neutron spectrometer is outlined. Experiments with a test device confirm the calculated energy resolution and separation of neutron from gamma events. The spectrometer is shown to be applicable to ITER under both ohmically heated and ignited conditions. A feedback system will be used to control the detector count rate at high neutron flux levels to accommodate the large dynamic neutron flux range from 5 × 10⁶ to 5 × 10¹⁰ n/(cm² · s). An array of five to nine TOF spectrometers provides ion temperature profiles that satisfy ITER measurement requirements, i.e., T_i ≥ 2.5 keV; 10% accuracy; and spatial and temporal resolutions of 30 cm and 100 ms, respectively.