Reduced fast ion transport calculations of infernal-like Fishbone instabilities in MAST-U

Fast ion transport induced by infernal-like Fishbone (FB) instabilities in MAST-U is estimated using the ORBIT-Kick reduced model [1]. This guiding-center-based approach uses prior simulation and measurement data to simplify computations and significantly reduce computational costs. Measurements in MAST-U are used as inputs for ORBIT-Kick to analyze fast ion transport. Unlike the FBs in the internal kink regime (with the existence of q = 1 surface), the mode structures and frequencies of infernal-like FBs (where qmin > 1) cannot be fully resolved by internal kink model, preventing their direct use as inputs for FB transport calculations. Instead, measurements of mode amplitude, 2D spatial structure, and frequency chirping obtained from diagnostics such as beam emission spectroscopy (BES), guide the construction of analytical mode structures for ORBIT-Kick. Calculations based on these analytical poloidal harmonics indicate that fast ion transport and losses scale with the mode amplitude. Furthermore, while frequency chirping does not markedly increase particle loss, it affects particle energy and momentum more significantly compared to fixed-frequency cases. This behavior can be due to the orbits perturbed by frequency changes remain distant from the plasma edge, preventing losses beyond the last closed flux surface (LCFS).

Preliminary TRANSP results show that the relative neutron rate variations predicted by the FB-induced fast ion transport are comparable to experimental measurements. The kick probability matrices derived from ORBIT- Kick can be incorporated into TRANSP [2] to predict changes in various one-dimensional profiles due to the targeted modes. A series of TRANSP simulations, with FB mode amplitudes spanning the uncertainties extrapolated from BES measurements, estimates the drop in neutron rate attributable to the FB instability. However, the measured neutron rate drop occurs approximately 1 ms later than both the peak amplitude and the simulated neutron emissions. Future work will focus on understanding the underlying causes of this delay.

[1] M Podestà et al 2014 PPCF 56 055003

[2] R.J. Hawryluk 1980 PoPCTC vol 1, pp 19–46