Simulated stabilisation of tearing modes by electron Bernstein wave current drive in MAST-U

The efficient operation of a tokamak fusion device relies on maintaining high temperatures and pressures in its core plasma. However, these conditions may be compromised by the neoclassical tearing mode (NTM) instability, which manifests as ‘magnetic islands’ in the plasma. These islands are modifications to magnetic field line topology that degrade confinement and may even prematurely terminate the plasma, hence their rapid stabilisation is necessary for optimal fusion performance. The most common method of NTM stabilisation involves targeting the island with electron cyclotron current drive (ECCD), however ECCD cannot penetrate ‘overdense’ plasmas associated with many spherical tokamak designs, and so an alternative means of stabilising current drive is required for devices such as MAST-U.

Electron Bernstein wave current drive (EBWCD) presents a means of driving current in such plasmas while being theoretically more efficient [1], but requires a more complex and sensitive launch process. As such, NTM stabilisation by EBWCD has yet to be demonstrated either experimentally or in simulation. By combining the EBW ray tracing code ‘Crayon’ with the NTM simulating code ‘co-mre’, we seek to investigate whether EBW current drive may plausibly stabilise simulated NTMs found in MAST-U discharges. The results of this investigation should inform NTM control schemes in future fusion devices such as STEP.

[1] HP Laqua, H Maassberg, NB Marushchenko, F Volpe, A Weller, (W7-AS Team), W Kasparek, and (ECRH-Group). Electron-bernstein-wave current drive in an overdense plasma at the Wendelstein 7-AS stellarator. Physical review letters, 90(7):075003, 2003.