Energetic particle instability analysis for high density regime stellarator reactors*

Energetic particle (EP) driven instabilities and the associated EP transport will be an important issue for fusion reactor-regime stellarator projects such as the Stellarator Fusion Pilot Plant (FPP). Loss of EP confinement poses a risk both to maintaining self-sustained ignition and damaging plasma-facing components. Since the phase space structure of intense alpha particle populations in 3D systems cannot be created in non-DT experiments, this risk cannot be fully validated prior to operation of reactor-scale systems; thus, simulation is critical. We have initiated new simulations of reactor regime DT stellarators using gyrokinetic-based models and have studied the effects of the high plasma density regimes that are unique to stellarators. Alfvénic instabilities driven by alpha particles are analyzed along constant fusion power operational contours. Although initial results indicate that high density operation cannot completely suppress these instabilities, their drive can be significantly reduced if the plasma density is above around 3 x 10²⁰ m^-3. Further mitigation methods involving profile and shape optimization are under development that can provide additional control techniques.