A study of resistive peeling-ballooning modes across spherical tokamaks

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Abstract
We investigate how non-ideal-MHD effects, in particular plasma resistivity, impact the peeling-ballooning stability thresholds in spherical tokamaks. This analysis follows the discovery of resistive kink-peeling modes in ELMing NSTX discharges. In the present study we extend this modeling to ELMing pulses in MAST and MAST-U, where we find a clear resistive scaling for peeling-ballooning modes. While in NSTX ideal-MHD predicts stability for ELMing discharges, in MAST-U we find that the plasma is slightly unstable to peeling-ballooning modes, but is fully stabilized once diamagnetic effects are considered in terms of a growth rate normalization. A resistive power law scaling is calculated for these modes on MAST-U, which lies in between that of tearing modes and resistive interchange modes. A comparison between M3D-C1 and NIMROD shows reasonable agreement for this scaling. Resistivity destabilizes the modes and the peeling-ballooning unstable domain is considerably expanded in both, MAST and MAST-U. In addition to the MAST/-U pulses we also analyze resistive PB stability in a NSTX-similarity discharge on DIII-D. While having a different aspect ratio from NSTX, this discharge uses NSTX-like shaping parameters, toroidal field and plasma current. By considering these discharges alongside NSTX cases, we identify conditions influencing the onset of resistive peeling-ballooning modes. Our findings indicate that magnetic shear in the pedestal region is closely linked to the emergence of resistive edge modes.
Funding Information
  • Office of Science (DE-AC02-05CH11231)
  • Fusion Energy Sciences (DE-AC02-09CH11466)
  • Fusion Energy Sciences (DE-FC02-04ER54698)

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