Neutron and gamma-ray emission spectroscopy as fast ion diagnostics in fusion plasmas

This thesis work addresses neutron and γ-ray emission spectroscopy as fast ion diagnostics for fusion plasmas. Two main topics are considered. The first one is the determination of the fast ion energy distribution from measured neutron and γ-ray emission spectra. Neutron spectroscopy has been used at the JET tokamak since 1984. Advanced spectrometers have been built and several features of the neutron emission spectrum have been measured and interpreted in terms of the reaction kinematics. This thesis adds to this knowledge base by inspecting the role of nuclear elastic scattering in producing high energy components in the spectrum from fusion neutrons. The analysis focuses on the possibility to determine the energy distribution tail temperature of radio- frequency heated 3He ions in deuterium plasmas with an admixture of 3He. The RF generation of fast 3He ions is described and the knock-on components are determined with the help of newly derived 3He +d scattering cross sections. Results are presented on the neutron emission spectrum and its contributions from different deuteron velocity components. It is shown that knock-on leaves an observable feature in the spectrum with a clear dependence on absorbed RF power. The investigation is then generalized to D, (3He)D, DT and (3He)DT plasmas, where the relative magnitude of nuclear elastic scattering from different fusion products is determined. The resulting signatures in the neutron emission spectrum are calculated and their relevance for fast ions diagnosis in a burning plasma experiment is discussed. Gamma-ray spectroscopy is a relatively new technique compared to neutron spectroscopy. The Doppler broadening of characteristic γ-ray emission peaks from 12C(3He, pγ)14N reactions in fusion plasmas was measured for the first time in 2008 at the JET tokamak thanks to the installation of a High Purity Germanium detector. In this thesis, intensities and detailed spectral shapes of γ-ray emission peaks are successfully reproduced using a physics model combining the kinetics of the reacting ions with a detailed description of the nuclear reaction differential cross sections for populating the L1-L8 14N excitation levels yielding the observed γ-ray emission. A Monte Carlo code, named GENESIS, was written for the purpose of interpreting γ-ray emission from fusion plasmas and is used here to determine the tail temperature of fast 3He ions from the observed peak shapes. Experiments performed in 4He plasmas of the JET tokamak are also presented. 4He ions were accelerated to the MeV range by coupling third harmonic radio frequency heating to an injected 4He beam. For the first time, Doppler broadening of γ-ray peaks from the 12C(d, pγ)13 C and 9Be(α, nγ)12 C reactions was observed and is here interpreted with the GENESIS code. Acceleration of 4He particles at energies as high as 6 MeV is demonstrated; implications of these results for α particle observations through γ-ray emission spectroscopy in next step deuterium-tritium plasmas are discussed. A second topic addressed by this work is the study of fast ion driven instabilities through γ-ray emission spectroscopy, with emphasis on the development of instrumentation. A high efficiency, high resolution, fast γ-ray spectrometer based on the LaBr3 scintillator and designed for measurements in the MHz range was developed and is here presented. An algorithm based on pulse shape fitting was written to reconstruct γ-ray spectra from digitized data and is shown to provide an energy resolution equivalent to a traditional analog spectrometry chain at low counting rates. The same system was used to perform γ-ray spectroscopy in the MHz range, as demonstrated in experiments performed at nuclear accelerators. γ-ray emission spectra at rates as high as 4 MHz from p + 27Al reactions collected at the Tandem Van der Graaf accelerator of the Nuclear Institute “Horia Hulubei“ (Magurele, RO) are presented and show little or no degradation of the energy resolution. The developed system was also employed to study instabilities driven by fast protons in the ASDEX Upgrade tokamak. The observed γ-ray emission level induced by energetic protons is used to determine an effective tail temperature of the proton distribution function that can be compared with Neutral Particle Analyzer measurements. More generally the measured emission rate is used to assess the confinement of protons with energies less than 400 keV in discharges affected by Toroidal Alfven Eigenmode instabilities. The derived information on confined ions is combined with observations made with the ASDEX Upgrade Fast Ion Loss Detector. The results presented in this thesis represent a step forward in the development of nuclear radiation based methods for burning plasma diagnostics. In particular, they demonstrate that quantitative information on the energy distribution of fast ions and their interaction with plasma instabilities can be inferred from neutron and γ-ray mea- surements by taking into account in detail the reaction processes contributing to the emission of nuclear radiation from the plasma.