Momentum Firewall Effect on Runaway Acceleration by External Whistler Waves*

Mitigation or prevention of runaway electron (RE) beam in the disruption phase is necessary for the safe operation of tokamaks. One of the strategies for RE mitigation is the resonant wave injection [1-3], limiting the parallel momentum of the RE. Single-particle analyses [2, 3] reveal that electron motion in an optimal coherent whistler wave is sensitive to the initial condition, and an instantaneous scattering on the order of nanoseconds is expected, i.e., this wave can serve as a "firewall" in momentum space. Here, we reproduce this result by self-consistent two-dimensional particle-in-cell (PIC) simulations. The firewall effect by external whistler wave is tested for runaway electron acceleration by the bump-on-tail instability during the self-interaction with the plasma waves. As a result, we verified that the optimal whistler wave prevents further parallel acceleration of the electrons by prompt pitch angle scattering. We also discuss the required power to maintain the wave perturbation in KSTAR and ITER conditions.

References

[1] Z. Guo et al., Phys. Plasmas 25, 032504 (2018)

[2] P. M. Bellan, Phys. Plasmas 20, 042117 (2013)

[3] Y. D. Yoon et al., Phys. Plasmas 28, 060702 (2021)