Fast ion physics studies on MAST Upgrade with a neutron diagnostic

In fusion plasmas, fast ions have energies much higher than the thermal plasma background. Fast ions are generated by external auxiliary heating such as Neutral Beam Injection (NBI) and Ion Cyclotron Resonance Heating (ICRH) or by the fusion reactions themselves. In the former cases, fast ions are hydrogen isotopes with energies in the range from tens of keVs up to a few MeVs. Fusion reactions produce, in addition to hydrogen isotopes, alpha particles with energies in the MeV range.

Fast ions play an important role in heating the plasma, maintaining the high temperatures necessary to sustain the fusion reactions and crucial in achieving a burning plasma. NBI heating is also important for current drive, that is for long pulse operation of tokamaks beyond the inductive regime and therefore for the realization of a fusion reactor.

Confining fast ions in the plasma for time long enough so that they can transfer their energy to the background plasma is therefore crucial for achieving the goal of a power plant based on thermonuclear fusion reactions. However, fast ion confinement is degraded by plasma instabilities some of which are triggered by the fast ion themselves. In this case, energy exchange between the fast ions and the instabilities result in the redistribution and loss of fast ions, ultimately reducing the performances of fusion reactors.

Furthermore, the loss of fast ions in the plasma can result in the damage of the reactor first wall, an issue particularly for the very energetic alpha particles that will be produced in ITER and DEMO.

MAST Upgrade unique capabilities provide the opportunity to study the interplay between fast ions and plasma instabilities in a wide range of plasma scenarios that are not achievable in other present day conventional tokamaks. The combination of broader NBI power deposition profiles and low magnetic field allows the study of the behavior and confinement properties of super-Alfvénic fast ions.

MAST Upgrade is equipped with a wide range of diagnostics dedicated to the study of fast ions: an upgraded neutron camera, a fast ion loss detector, a fast ion D-alpha monitor, a compact neutral particle analyzer and a charged fusion product detector. Combined, these diagnostic will provide the most exhaustive description of fast ion confinement in spherical tokamak to date and will enable the benchmarking and development of the simulations codes that are used to design fusion reactor operating scenarios extrapolating from present day devices.

The project will focus on the analysis and modelling of the recently installed 6-channels neutron measurements in the presence of a wide range of instabilities such as sawteeth, fishbones, toroidal Alfvén eigenmodes, long-lived modes and edge localized modes, as well as assessing the role of on-axis and off-axis NBI heating in suppressing the drive of energetic particle modes and in driving plasma current non-inductively. This is mainly an experimental project which includes participation to experiments at MAST Upgrade at CCFE and data analysis and modelling at Durham University.

Modelling will be carried out using the codes TRANSP and NUBEAM which, in combination with synthetic diagnostics simulation codes, are used to compare experimental and simulations observations.