Description
We report on poloidal fuelling variation in double null, neutral beam heated H-mode plasmas on MAST Upgrade and the first results exploiting the high-speed video (HSV) diagnostic, which has been absolutely calibrated to infer neutral species quantities from $\mathrm{D_\alpha}$ emission [1]. We see that the low-field side (LFS) pedestal electron density, $n_e$, is resilient to the poloidal fuelling location and remains largely unaffected, as does the neutral density, $n_0$, at the LFS separatrix, despite differences in neutral pressure measured at the wall. Fuelling from the LFS was seen to increase the peak ionisation source rate, $S_0$, and decrease the LFS pedestal electron temperature, $T_e$, which increases edge collisionality, $\nu^*$, but increases core $n_e$, allowing for increased plasma fuelling. This is supported by an increase in inwards main ion species flux, $\Gamma$, and a negative convective transport coefficient, $v$, at the pedestal top, consistent with an inward particle pinch. These flux profile inferences are constrained with $S_0$ from HSV applied to a 1D radial transport model, which in turn are used in a simple diffusive-convective ansatz to estimate the transport coefficients [2].
In the most recent physics campaign on MAST-U, additional poloidal fuelling experiments were conducted with approximately double the beam power, allowing for lower collisionalities. In addition to HSV, we also incorporate neutral inferences from other $\mathrm{D}_\alpha$ emission diagnostics (i.e., charge-exchange spectrometer and linear $\mathrm{D}_\alpha$ camera) to constrain the poloidal distribution of neutrals. Work is ongoing to incorporate the results from these shots into our understanding of plasma fuelling in spherical tokamaks (STs), which plays an important role in power plant prototype machines, such as STEP.
Acknowledgements
Work supported by DOE Awards DE-SC0023289, DE-SC0023372, and DE-AC05-00OR22725, and by the Engineering and Physical Sciences Research Council [grant number EP/W006839/1].
References
[1] S. Thomas et al. (In progress) The poloidal fuelling location and its effect on H-mode plasma performance in MAST Upgrade Plasma Phys. Control. Fusion
[2] A. Rosenthal et al. 2023 Nucl. Fusion 13 056103