Description
Plasmas in the Mega Ampere Spherical Tokamak Upgrade (MAST-U) are performance limited by MHD instabilities. The q=m/n=2/1 tearing mode (TM) has been identified as one of the most deleterious modes, occurring in the majority of MAST-U shots and reducing plasma beta by ~25%[1]. Electron Cyclotron Current Drive (ECCD) is considered the primary method for actively stabilising TMs in conventional tokamaks[2], but this is not possible in MAST-U due to being overdense. Electron Bernstein Wave Current Drive (EBWCD) -- not yet active in MAST-U -- may be an alternative active scheme, but work is needed to demonstrate its efficacy. It is thus imperative that alternative TM mitigation techniques are found to achieve record performance.
This work explores the effects of toroidal field ramping and plasma shaping on the stability of the 2/1 mode using the reduced MHD version of JOREK[3], a non-linear MHD code. We show that negative toroidal field ramping has a partially stabilising effect on the 2/1 mode, reducing the island saturation width by ~50% in a typical case. This is found to be due to the radial movement of the rational surface leading to a reduction in classical drive and an increase in local shaping, as shown by theoretical modelling using the modified Rutherford equation[4,5]. Qualitative agreement is found between these results and experiments performed in the MU04 MAST-U experimental campaign.
[1] J. R. Harrison et al., Nuclear Fusion. 64, 112017 (2024).
[2] O. Sauter, et al., Plasma Physics and Controlled Fusion, 52, 25002 (2010).
[3] M. Hoelzl et al., Nuclear Fusion. 61, 65001 (2021).
[4] H. Lütjens et al., Physics of Plasmas. 8, 4267–4270 (2001).
[5] T. O’Gorman et al., Nuclear Fusion. 54, 82002 (2014).