Ion power balance in neutral beam heated discharges on the National Spherical Torus Experiment (NSTX)

In this dissertation, ion power balance is studied in the National Spherical Torus Experiment (NSTX). In order to fully understand all the terms in the power balance equation, an experiment was conducted in which the input power of the NSTX discharges was modulated using the neutral beam input power. Individual neutral beam sources were turned on or off to increase or decrease the input power. The modulations were designed to create a localized perturbation in the fast ion phase space distribution. Measurements were taken with diagnostics on NSTX including the neutral particle analyzer (NPA), the NSTX Thomson scattering diagnostic (MPTS) and the charge exchange diagnostic (CHERS), which measure the ion and electron density and temperature.;Reconstructions of the plasma equilibrium were made using the LRDFIT code and power balance was calculated using the TRANSP transport code. The TRANSP code inputs the electron and ion temperatures and densities, and calculates the various power balance terms.;Excess ion heating is shown to exist for some neutral beam heated discharges on NSTX. When all three neutral beam sources are on, the total power outflow through the various loss terms exceeds the input power calculated through classical heating of the ions by the slowing down of the neutral beams. The excess heating is approximately 0.5 MW of power, which represents 8% of the total power put into the plasma from the neutral beams.;This excess heating occurs at the same time as an increase in the amplitude of high frequency compressional Alfven eigenmodes (CAEs) with frequencies of 0.5--1.4 MHz. The CAE amplitudes coincide both temporally and spatially with the onset of the excess heating. The high-k microwave scattering diagnostic and magnetic pickup coils were used to estimate the amplitudes of these modes. Full orbit calculations were then performed using these modes, which demonstrate that thermal ions can be heated by the CAEs.;The Edge Neutral Density Diagnostic (ENDD) was developed to aid in the understanding of ion power balance. The ENDD uses a 2-D charge-coupled device camera to measure neutral hydrogen line emissions from the plasma, which are input into a collisional-radiative model to determine the edge neutral density profile. The data from the ENDD was used as an input into the TRANSP calculations.;The NPA was used in an attempt to study the fast ion population. However, limitations in the NPA design prevented it from measuring the full fast-ion distribution. This dissertation discusses the limitations of the NPA and suggests future improvements in diagnostics for ion power balance measurements.;This work was supported by the Department of Energy, contract DE-AC02-09CH11466.