Cherenkov radiation of shear Alfvén waves

Abstract

A calculation is presented of the radiation pattern of shear Alfvén waves excited by a burst of charged particles propagating along the confinement magnetic field. The characteristic wake patterns are obtained for the inertial and kinetic regimes of wave propagation. In the inertial regime, the waves are only excited by particles moving slower than the Alfvén speed. The radiated wake exhibits an inverted V-shape due to the backward-wave nature of the modes. In the kinetic regime, particles moving faster as well as slower than the Alfvén speed can radiate propagating modes. The super Alfvénic particles, however, excite modes with relatively short transverse scales. The motorboat type of wake of the kinetic modes is more typical of the Cherenkov process obtained in scalar dielectrics. The predictions are in agreement with experimental observations [B. Van Compernolle et al., Phys. Plasmas 13, 09112 (2006)] and computer simulations [F. S. Tsung et al., Phys. Plasmas 14, 042101 (2007)] in which a burst of fast electrons generated by resonant absorption in a magnetized plasma excites a pulse of large-amplitude Alfvén waves.