γ-ray diagnostics of energetic ions in JET

This paper reports recent progress in the field of gamma-ray diagnosis of fast ions in the JET tokamak. The gamma-rays, born in nuclear reactions between fast ions and main plasma impurities and/or plasma fuel ions, are analysed with a new modelling tool (the GAMMOD code) that has been developed for a quantitative analysis of the measured gamma-ray energy spectra. The analysis of the gamma-ray energy spectra identifies the different fast ions giving rise to the gamma-ray emission and assesses the effective tail temperatures and relative concentrations of these fast ions. This assessment is possible, since the excitation functions for the different nuclear reactions are well established and exhibit a threshold or/and a resonant nature. The capabilities of the gamma-ray spectral analysis are illustrated with the examples from the recent gamma-ray diagnostic measurements of He-4, He-3, deuterium and hydrogen ions accelerated by ion-cyclotron resonance frequency heating in JET. Simultaneous measurements of several fast ion species, including highly energetic gamma-particles, are demonstrated. In addition to the gamma-spectroscopy, tomographic reconstructions of the radial profile of the gamma-ray emission are performed using the JET neutron profile monitor, thus providing direct measurements of the radial profiles of fast ions in JET.