Reduced transport modelling of fast ions in 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.

The interaction between FIs and MHD instabilities is an active and intense field of research Recent modelling developments include MHD and particle kinetics codes (with realistic description of the instabilities’ amplitude and spatial structure and full orbit calculations) such as HALO (developed at CCFE) and the reduced transport “kick-model” for TRANSP/NUBEAM (developed at PPPL). Of particular importance, especially for MAST Upgrade, is the modelling of the FIs full orbits for the validation of theoretical predictions of the interplay between FIs and MHD instabilities such as sawteeth, fishbones, toroidal Alfvén eigenmodes, long-lived modes and edge localized modes. The spatial structure and temporal evolution of these instabilities, at times non-linearly coupled to the dynamics of the FIs, is crucial for the correct prediction of the confinement of FIs.

The project is aimed at a systematic comparison of full-orbit and guiding-center reduced transport calculations with a set of fast ions experimental measurements on MAST Upgrade in presence of perturbations of the plasma equilibrium due to sawteeth, TEAs and fishbones. A particular focus will be dedicated to the modelling and interpretation of collimated neutron flux measurements using the upgraded neutron camera installed on MAST Upgrade and the comparison with other FI 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 (for HALO), the Princeton Plasma Physics Laboratory (for TRANSP/NUBEAM) and Aalto University (for ASCOT). As such, international travel and participation to international conference and workshops are foreseen.

The outcome of this project is a framework of reduced, rapid fast ion transport models that will be used to compare simulations and measurements probing different regions of the phase space thus providing an integrated understanding of the FIs dynamics. In turn, the outcome of this project will enable the development of operating scenarios where the effect of performance limiting distributions will be suppressed thus allowing improved FI confinement and non-inductive current drive. The main focus of this project is MAST Upgrade thanks to its on-axis/off-axis NB injection flexibility but the developed framework will be applicable to conventional tokamaks and will be of relevance to STEP, ITER and DEMO.

Image above: Orbit boundaries at t = 0.16 s for fast ions with energy of 50 keV and different pitches launched at R = 0.7 m (solid circles) and R = 1.0 m (open circles) compared with the corresponding energy kick probability probability density calculated by ORBIT in presence of a n = 1 and n = 2 TAE perturbations calculated by MISHKA.

The project will be mainly based at Durham but will require 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.