Non-linear modelling of performance limiting MHD and disruptions in spherical tokamaks (Plasma Strand Project)

Supervisor: Prof. Jonathan P. Graves (University of York)

External Supervisor: Dr. C. Ham (CCFE)

Future energy producing fusion reactors such as EU-DEMO and STEP need to avoid unmitigated disruptions of the plasma. Identifying reliable disruption avoidance and mitigation schemes is probably the number one priority for the European magnetic fusion programme. Occurring over tens of microseconds, the disruption event places massive forces on the vessel, potentially damaging critical components such as superconducting coils, and cooling systems. Disruption events are common during plasma operation of present-day tokamaks such as the tight aspect ratio spherical tokamak MAST-U. Usually these events do not cause critical damage, but for MAST-U to reach high performance, long pulse operation and contribute to the design of STEP, it is essential to understand, avoid, mitigate or control the magnetohydrodynamic (MHD) instabilities responsible for disruptions in spherical tokamaks, or otherwise lead to degraded fusion performance.

The project will start by analysing data from MAST-U high-performance experiments to identify important instabilities that lead up to disruptions (such as neoclassical tearing modes which may or may not be coupled to ideal kink modes). Where resistive instabilities are found to be important, the project will extend the so called Delta-prime code T7 [C. J. Ham, Plasma Phys. Control. Fusion 54, 105014 (2012)] to account for general tight aspect ratio geometry. Such an approach provides a convenient platform for advanced kinetic modelling in the region of magnetic islands. For cases that are deeply non-linear, the extended MHD codes JOREK or XTOR [A. Kleiner, J. P. Graves, Nucl. Fusion 56, 092007 (2016)] will be deployed and compared against experiments. The next step will be to investigate the thermal quench process of the disruption, and in particular to explore leading theories such as the recent one by A. Boozer [A. Boozer, Phys. Plasmas 26, 042104 (2019)]. It will be important to identify the extent to which such fast reconnection can be captured in nonlinear extended MHD codes.

The work will naturally inform strategies to avoid or mitigate disruptions in future MAST-U operations, JT-60-SA and ITER, with a potential aim for aiding the design of STEP.

After the studentship programme at York (YPI) during the first year, the main working location will be split between UKAEA and YPI. There will be regular contact between the student, UKAEA supervisor and the university supervisor, through emails and Teams/Zoom calls, and visits of student and supervisors to UKAEA and YPI. Additional collaboration with the NSTX-U team would be highly desirable, and training on MHD codes will be given by UKAEA experts, including through participation in code annual general meetings.

The project is offered by the University of York. For further information please contact Prof. Jonathan Graves: Dr Chris Ham:

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

Picture above shows a disruption in MAST-U triggered at 30 micro seconds caused by a rapidly growing magnetohydrodynamic instability visible at 10 and 20 micro seconds.