A review of the use of gamma ray spectrometry as a diagnostic of nuclear reaction rates and nuclear reaction product densities in high temperature fusion plasmas is presented. In this review we will discuss the historic genesis of the concept, a brief overview of the relevant nuclear physics, the experimental techniques utilized in the measurements and some of the analytical techniques required to extract the diagnostic information from the basic measurements. Of particular interest is the ability to measure the population of confined fast alpha particles in future burning plasma experiments.

Gamma-ray images of fast D- and 4He-ions accelerated with third-harmonic ion-cyclotron-resonance heating of 4He-beam were simultaneously recorded for the first time in JET tokamak experiments dedicated to the investigation of burning plasmas with 3.5 MeV fusion alpha (α) particles. Gamma (γ) rays, born as a result of nuclear reactions, 9Be(4He, nγ)12C and 12C(D, pγ)13C, between the fast ions and the main plasma impurities, are measured using a two-dimensional multicollimator spectrometer array, which distinguishes the γ-rays from accelerated D- and 4He-ions. Tomographic reconstruction of the γ-ray emission profiles gives images of the fast-ion population in the poloidal cross-section. The potential of this technique to visualize several energetic ion species and to determine their behaviour in different plasma scenarios is demonstrated.

The spectral broadening of characteristic γ-ray emission peaks from the reaction (12)C((3)He,pγ)(14)N was measured in D((3)He) plasmas of the JET tokamak with ion cyclotron resonance heating tuned to the fundamental harmonic of (3)He. Intensities and detailed spectral shapes of γ-ray emission peaks were 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 (14)N excitation levels yielding the observed γ-ray emission. The results provide a paradigm, which leverages knowledge from areas of physics outside traditional plasma physics, for the development of nuclear radiation based methods for understanding and controlling fusion burning plasmas.