Gamma ray spectroscopy for fast ion studies in tokamaks: results and nuclear data needs

Gamma-ray spectroscopy is a proven method for measuring MeV-range ions in tokamaks [1,2], typically relying on spontaneous nuclear reactions between fast ions and impurities. These reactions create heavy nuclei that emit gamma-rays at discrete energies. In some cases, 10–20 MeV gamma-rays result from rare branches of fusion reactions (probability ~10^-5) and can be used to assess fusion rates [3].

This work presents examples of gamma-ray spectroscopy in fast ion studies and outlines nuclear data needs. Data are drawn from JET, the largest tokamak to operate until 2023, where ⁹Be was the main impurity. In experiments using radio-frequency heating, gamma-ray measurements revealed spectral features from MeV-range D, ³He, and ⁴He ions. These were interpreted using nuclear data on cross sections, level structures, and decay probabilities. Detailed nuclear data were crucial, as no direct link exists between gamma-ray spectral features and ion energies.

With future tokamaks moving to tungsten walls, ⁹Be will be replaced by injected boron for diagnostics, especially for alpha particle studies. A new effort explores alpha generation in D–³He plasmas and boron-based gamma-ray diagnostics. We present early results and related nuclear data needs.

[1] M Nocente et al 2020 Plasma Phys. Control. Fusion 62 014015

[2] V G Kiptily et al 2006 Plasma Phys. Control. Fusion 48 R59

[3] A. Dal Molin et al., “Fusion Meets Nuclear Physics: Quantifying Fusion Power from the Radiative Branches of the DT Reaction”, this conference