Optimizing plasma heating for Quasi-Isodynamic Stellarator Reactor

As optimization techniques for stellarators have improved over the years, it is now possible to achieve quasi-isodynamic configurations, such as SQuID, for future fusion reactors. One of the key challenges in designing such a reactor is developing a plasma heating scenario that is not part of stellarator optimization. Current stellarators use reactor-relevant electron cyclotron resonance heating (ECRH) employing highly absorbing single-pass O1- and X2-modes and a multipass O2-mode for high-density scenarios, as stellarators are shielded from the current-related Greenwald density limit, aiming at high confinement via high densities.

However, at high magnetic fields (> 5 T), the standard ECRH scenarios are challenging to realize in a reactor, as the second harmonic frequency is not feasible with the current state of gyrotron technology, due to a decline in power efficiency with increased frequency. First harmonic ECRH solutions are restricted to low densities ≤ 2 × 10^20 m^-3 due to a density cutoff.

This work presents the first results from ray-tracing and radiation transport calculations for ECRH using the TRAVIS code for SQuID. First simulation results indicate that in SQuId, due to the high mirror ratio, the line of sight is available with a saddle point in the magnetic field gradient, which could be utilized for low-field-side ECRH injection and a reduced gyrotron frequency. An additional complex multi-mode heating scenario is also presented to go from plasma startup to ignition conditions.