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Fusion energy research is working toward the goal of using fusion as a source of clean energy. Since energetic particles are the main source of heating in current and future devices, understanding fast ion transport and loss mechanisms is vital. Experimental research in this thesis uses two main fast ion diagnostics, the fast ion loss detector and the imaging neutral particle analyzer, to discern how energetic particles are transported in the presence of various magnetic perturbations. First, fast ion losses in the presence of externally applied, 3D magnetic fields are analyzed. Second, transport of confined particles and simultaneous losses are observed and discussed in the presence of a neoclassical tearing mode. Lastly, work on intermittent fast ion losses in the presence of multiple Alfven Eigenmodes is observed.

Diagnosis of the fast ion population, which is necessary for heating but can drive dangerous instabilities, is crucial in achieving and maintaining a burning plasma regime. Present fast ion diagnostic methods tend to rely on delicate components unlikely to survive in a reactor environment, motivating exploration into alternative diagnostic methods. One candidate, the subject of this thesis, entails measurement of collective ion cyclotron emission (ICE) spectra via robust magnetic pickup loops integrated with the first wall, making it potentially compatible with ITER and other reactor-relevant devices [116, 69]. ICE, distinct from thermal electron cyclotron emission, manifests as narrowband peaks at harmonics of the ion cyclotron frequency $f_{ci}$ and is generally localized near the core or the edge of the plasma. These modes have been observed to be sensitive to the fast ion population, be they introduced by auxiliary heating systems or fusion products. However, the connection between observed ICE spectra and the fast ion population has not been sufficiently established, meriting diagnostic upgrades and dedicated experiments.This work aims to characterize ICE mode structure and fine-scale phenomena in L- and H-mode plasmas on DIII-D using the ICE diagnostic, which was recently upgraded to include additional channels and consequently new measurement capabilities. In addition to basic frequency and amplitude information, measurements of mode polarization at the plasma edge, amplitude as measured at the centerpost and outer wall of the machine, and toroidal mode number are now possible and will be demonstrated through this work. Additionally, mode sensitivity to the vacuum region, plasma shape, and particularly the fast ion distribution is explored. The detailed mode structure measurements and phenomenology presented in this work can be used to test and validate theoretical models en route to developing a more complete physical understanding of ICE, so that it can be

This thesis presents advances in computational modelling, analysis and techniques that can be used to study mission critical topics in fusion. Energetic particles is an important field of research since information about the plasma state can be encoded in the fast-ion distribution function. Energetic ions can resonate with plasma waves in a fusion device, degrade plasma performance or confinement, and damage the inner walls of the vessel. There is a need in the community to better understand the fast-ion distribution function, improve plasma performance, and mitigate unwanted impacts from wave-particle interactions.In chapter 2, the weight function for 3-MeV protons produced in d(d,p)t fusion reactions between a fast ion and a thermal deuteron is developed. The weight function W(X) is a diagnostic sensitivity to phase-space variables X that relates the measured signal C to the distribution function F(X) through the equation C = integral[ W(X) F(X) dX ]. The algorithm developed here accounts for the complications associated with the curved "sightline" trajectories of the escaping protons. Time-reversed orbits are initially calculated to get the effective solid angles and sightlines for the range of incident proton velocity vectors. Synthetic diagnostic code FIDASIM [Plasma Phys. Cont. Fusion 62 (2020) 105008] is upgraded to accept these inputs, then calculate the reactivity averaged over the thermal distribution of the "target" deuterons and the probability that a fast ion of specified energy and pitch has a gyroangle that is consistent with the kinematic equations along each of the sightlines. The outputs of FIDADSIM are verified using independent calculations on the Mega Amp Spherical Tokamak.In chapter 3, a conceptual design to diagnose the lifetime of spin polarized fusion experiments using existing port geometries is developed at DIII-D. The cross sections for the D-T and D-3He fusion reactions are increased by as much as 50% if the fuel remains spin polarized p

Fast ion research is necessary to understand confinement of high energy particles in fusion devices like DIII-D. Measuring the fast ion distribution is necessary to understand how well it is confined. The fast ion D-alpha (FIDA) diagnostic and neutral particle analyzer (NPA) are two diagnostics that measure part of the distribution. Upgrades to the FIDA diagnostic are necessary to resolve scattering issues from the unshifted D-alpha emission that contaminates other parts of the measured spectra. Effective filter performance is possible using the reflection of a narrow bandpass filter. The imaging neutral particle analyzer (INPA) was developed on DIII-D to provide greater spatial and energy resolution compared to traditional NPAs. Measured images are validated using simulations. Robust passive signals are also observed and found to come from fast ion populations traversing the edge. Fast ions can also interact with plasma waves or modes to transfer energy which can lead to losses. The energetic particle-induced geodesic acoustic mode (EGAM) is one plasma mode that is excited by fast ions. Various phenomena associated with EGAMs are characterized to assess the impact it has on fast ions.