Energetic ions in the MeV range are ubiquitous in high performance tokamak plasmas. On one hand they can arise from the use of auxiliary heating systems, for example radiofrequency waves that transfer their power to a minority bulk species and are resonant with the ion cyclotron frequency or its harmonics. On the other hand, they can be born as products of the fusion reactions such as, for example, alpha particles in plasmas with deuterium and tritium. In both cases, their role is essential, as fast ion confinement must be ensured for a sufficiently long time to transfer energy to the bulk plasma and thus sustain the fusion burn.
From a theoretical point of view, a thorough understanding of the suprathermal ion behavior would require direct access to their velocity distribution function but, experimentally, only indirect information can be gathered, mostly by means of spectroscopy measurements of the radiation arising from fast ion driven nuclear reactions. Examples are the suprathermal component of the neutron emission from the main fusion processes, or photons in the gamma-ray energy band from spontaneous interactions between the MeV ions and plasma impurities.
In order to build a bridge between experiments and theory, a new formalism, which makes use of so called “weight functions” in the velocity space, is emerging and is here presented in its application to studies of energetic ions in the MeV range. The weight function method can reveal which part of the whole velocity space contributes to events in a given channel of an arbitrary spectral measurement. The details of the weight functions depend on the diagnostic process, as well as on the angle of observation with respect to the magnetic field. In this way, fundamentally different diagnostic systems can be compared on an even ground, based on their velocity space interrogation capabilities. In their simplest application, when an “a priori” model of the fast ion distribution function is available, weight functions reveal how this is effectively weighted by the different systems, for example by highlighting principle differences between supposedly similar diagnostic methods. In its most advanced application, the formalism combines multiple diagnostic data to solve a tomographic problem in the velocity space. This is used to unravel the energy and pitch-angle resolved fast ion distribution function when the latter is not known beforehand. An illustration of both methods with applications to nuclear data from present day high performance tokamaks is here presented.