Optimizing the Exhaust Performance via Magnetic Geometry in Advanced Divertors – plasma strand project

Supervisor/s: Dr Kirsty McKay (University of Liverpool), Dr Lingyan Xiang (UKAEA) & Dr David Moulton (UKAEA).

Power exhaust at the plasma boundary remains a challenging issue foreseen in fusion reactors, such as the UK prototype reactor STEP. Novel magnetic geometries could potentially offer a solution to this issue as the experiments on MAST Upgrade tokamak have shown since its restart of operation since 2021 with tightly baffled advanced outer divertor [1]. The experiment has explored various magnetic geometries including open divertor (OD), conventional divertor (CD), extended conventional divertor (ECD), extended divertor (ED) and super-X divertor (SXD), and has shown evidence of advantages in facilitating detachment (via improved power exhaust in the plasma boundary) in ED and SXD, with respect to CD. In the coming years, MAST Upgrade will operate with a boundary plasma condition even closer to reactor scale. To help to understand the physics behind the experimental observations which have many non-measurable processes at play and to extrapolate from current tokamak to reactors, numeric modelling is absolutely needed.

This project aims to improve our understanding of exhaust performances in various magnetic geometries via rigorous interpretative simulations of MAST Upgrade plasma using the sophisticated SOLPS-ITER code. It includes the following objectives:

(1) Select well diagnosed, high power, attached to partially detached MAST Upgrade plasma in OD, CD, ECD, ED and SXD geometries with similar core plasma conditions in double null configuration and reproduce as closely as possible the experimental results in SOLPS-ITER simulations. Investigate what are the main driving factors in these geometries for the improved/degraded power exhaust performance. Explore the potential to develop a reduced theoretical model with the insights gained in the simulation and experiment for extrapolating to reactors like STEP.

(2) Study how the asymmetries between the inner divertor and the outer divertor evolve in CD, ECD, and SXD geometries in single null configuration on MAST Upgrade. This will offer valuable knowledge to STEP where the single configuration is yet to be explored numerically.

(3) Since the code applies ad-hoc diffusive radial transport model, and some of the A&M rates used are not up-to-date, to minimize the effects of uncertainties in the transport coefficients and A&M rates used in the code, the candidate will integrate a Bayesian optimization tool [2] into the simulation.

Furthermore, TCV tokamak equipped with tight baffles and capability to run in very flexible magnetic geometry will operate in Hydrogen plasma in 2023. Different from MAST Upgrade, the strike points in various divertor magnetic geometries are always well below the baffles on TCV, which reduces the complexity of different degrees of neutral baffling in different geometries on MAST Upgrade. Interpretative simulations of such TCV plasma with tight baffle will enlarge the database for better studying the physics regimes of power exhaust improvement in various magnetic geometries. This also offers the opportunity to eliminate one essential uncertainty present in SOLPS-ITER simulations of deuterium plasma, i.e., the scaling of A&M rates from hydrogen to deuterium. In this project, we will collaborate with TCV to reproduce partially detached hydrogen plasma in different divertor magnetic geometries (the strike points placed at different locations on the target floor).

References: [1] K. Verhaegh et al 2023 Nucl. Fusion 63 016014 [2] C Bowman et al 2020 Plasma Phys. Control. Fusion 62 045014

This project offers many opportunities to learn a host of new skills particularly around coding, and experimental data analysis. There will be opportunities to enhance presentation skills and scientific/technical writing.

The project will be mainly based at UKAEAs Culham site, but there will be opportunities for travel to conferences etc.

This project is offered by University of Liverpool. For further information please contact: k.mckay@liverpool.ac.uk, lingyan.xiang@ukaea.uk

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