Reconstruction of MHD instabilities using Soft X-ray emission on MAST Upgrade (plasma strand project)

Supervisor/s – M. Cecconello (Durham University)

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.

Modelling of the interaction between fast ions and MHD instabilities relies on the accurate description of the plasma equilibrium perturbations both in terms of their spatial structure and time evolution. Numerical codes are used to compute the perturbation eigenfunctions but their experimental verification is limited to magnetic field flux measurements outside of the plasma region using pick-up coils. No direct measurement of the perturbation spatial profile and amplitude is available. Soft-X rays emission can be potentially used to infer the structure of these perturbation on a very fast time scale (sub millisecond) which is comparable with their time evolution. MAST Upgrade is equipped with an array of SXR detectors which provide a good coverage of the whole plasma region. This, combined with MAST Upgrade flexible NBI heating and low magnetic field will allow to study a wide range of fast ion physics scenarios in ITER and fusion reactor relevant conditions.

This project is focused on the forward modelling of the SXR emission of MAST Upgrade and its validation against experimental measurements with the aim to constrain the spatial profile and amplitude of the plasma perturbation affecting the confinement of fast ions in a wide range of operating scenarios. Inversion methods (such as tomography) will be also investigated to determine the structure of the perturbations. The perturbation so calculated will be used in reduced transport modelling codes for the evaluation of the level of fast ion redistribution and losses using TRANSP/NUBEAM and HALO codes and then compared with measurements from a wide range of fast ions diagnostics (FILD, FIDA, compact NPA and charged fusion product detector). This is a modelling project for which good numerical computation skills are required. The research will be carried out mainly at Durham University with collaborations with CCFE. International travel and participation to international conference and workshops are foreseen.

Image above: Spectrogram of a core SXR channel for MAST plasma 29976 indicating the presence of localized Toroidal Alfvén Eigenmodes, Fishbones and Long lived modes.

Feature image, a simulation of the local SXR perturbation due to a n = 1, m = 6 TAE.

The project will be mainly based at Durham with short stays at CCFE.

This project is offered by Durham University. For further information please contact: M. Cecconello (marco.cecconello@physics.uu.se)

This project may be compatible with part time study, please contact the project supervisors if you are interested in exploring this.