"Energetic particle transport by drift waves and Alfvén eigenmodes, from beams to fusion alphas”

Abstract:

 

Energetic particle (EP) heating of plasmas, from modern neutral beam injection (NBI) schemes to future burning plasma scenarios heated by energetic fusion-produced alpha particles, requires sufficient confinement of EPs to allow collisional energy transfer to the thermal plasma. Unfortunately, spontaneous fluctuations can drive EP cross-field transport well above the slow collisional level. These instabilities fall into two main categories: microturbulence driven at the thermal ion Larmor scale by thermal species (electron and ion) pressure gradients and low-n Alfvén eigenmodes driven unstable by a pressure gradient in the EPs themselves. Recent years have seen significant progress in the gyrokinetic simulation of these phenomena as they pertain to EP transport. The continuum gyrokinetic code GYRO [1] is one of several gyrokinetic codes to be applied to the EP transport problem. Models agree that microturbulent EP transport diminishes in importance with increasing EP energy, but it remains non-negligible in many cases. An effort to include microturbulent transport in existing NBI profile calculations is currently underway. Alfvén eigenmodes represent the larger potential threat to fusion performance. While these modes are fundamentally described by MHD, recent studies in GYRO (among others) have demonstrated many non-trivial kinetic corrections to mode structure and frequency. Ultimate transport depends in large part on the dominant mechanism of nonlinear saturation. A regime of soft transport mediated by cross-scale coupling between microturbulence and Alfvén eigenmodes has been identified in GYRO [2].  The extent to which Alfvén eigenmodes will drive strong transport in ITER, and whether such transport will be continuous or occur in bursts, is still an open question.

 

[1] J. Candy and R.E. Waltz, Phys. Rev. Lett. 91, 045001 (2003).

[2] E.M. Bass and R.E. Waltz, Phys. Plasmas 17, 112319 (2010).