Firewall Effect on Electron Acceleration by External Whistler Waves

In magnetized plasmas, charged particles and waves exchange energy through mechanisms like resonance, damping, and scattering, which are crucial for understanding various plasma phenomena. A recent single-particle analysis revealed that electrons in an optimally tuned coherent R-wave or whistler wave undergo instantaneous pitch angle scattering [1,2]. We have extended the analysis by including steady-state electric field parallel to the steady-state magnetic field and applied to the runaway electron (RE) situation in tokamak plasma [3-5], predicting a firewall effect of R-waves to suppress electron acceleration. We then validate the firewall effect using self-consistent particle-in-cell (PIC) simulations, finding the condition of the externally applied whistler waves to inhibit the growth of bump-on-tail features in the electron distribution. The timescale of pitch angle scattering is compared to predictions from quasilinear diffusion theory. The total power required to achieve the firewall effect is estimated, suggesting that wave injection offers a promising strategy for mitigating energetic electron in fusion devices.