Cherenkov radiation of Alfv$\'e$n Waves by an Intense Proton Beam in a Large Magnetized Plasma

Cherenkov radiation of waves by charged particles has a bearing on a number of astrophysical and terrestrial problems [1, 2]. Spontaneous radiation of traveling Alfv$\'e$n waves by a short-burst of protons (5-20 keV, 2-12 A, pulse-width $\approx$ 8 $\mu$s, pitch-angle $\approx$ 0$^\circ}$) has been investigated on the Large Plasma Device [3]. In these experiments, the proton beam was injected into a magnetized plasma (n $\approx$ 10$^{12}$ cm$^{-3}$, Te = 0.1 - 5.0 eV, B = 0.25 - 1.50 kG, H$^+$ ions, 19 m long, 0.6 m diameter). The beam energy was varied to explore sub- and super-Alfv$\'e$nic regimes of the beam propagation (beam-speed/Alfv$\'e$n-speed = 0.5 - 4.0) for inertial and kinetic Alfv$\'e$n waves. The interaction of the beam with the plasma was diagnosed using a retarding-field energy analyzer, three-axis magnetic-loop, and Langmuir probes. Cherenkov radiation of Alfv$\'e$n waves is observed when the beam-speed exceeds the Alfv$\'e$n speed. In this regime, the wave forms a conical pattern and lags behind the super-Alfv$\'e$nic proton-burst. References: (1) Krechetov, Geomagnetism and Aeronomy, 35(5), 688 (1995); (2) Van Compernolle et. al., Phys. Plasmas 15, 082101 (2008); (3) Tripathi et. al., Phys. Rev. E 91, 013109 (2015)