Ion cyclotron emission by a spiraling proton beam on the Large Plasma Device

Resonant interaction between energetic-ions and plasma waves is a fundamental topic of importance in space and laboratory plasma physics. We report results on the ion cyclotron emission (ICE) by a spiraling proton beam (5-15 keV, 10 A) in a linear magnetized plasma (n ≈ 10¹⁰ – 10¹³ cm^-3, T_e ≈ 5.0-15.0 eV, B = 0.6–1.8 kG, He⁺ and H⁺ ions, 19 m long, 0.6 m diameter) on the Large Plasma Device (LAPD). The ICE appears as a sequence of narrow frequency peaks extending up to 150^th harmonics of the beam gyro-frequency. These peaks are accompanied by a broad continuum around the lower-hybrid frequency. In this experiment, the proton beam forms a helical orbit (pitch-angle ≈ 7°–55°) and propagates with an Alfvénic speed (beam-speed/Alfvén-speed = 0.2–3.0). The role of resonant processes in destabilizing these waves were examined by recording the mode-structure of these waves and relevant parameters of plasma using a variety of diagnostic tools (retarding-field energy analyzer, three-axis magnetic-loop, Dipole, and Langmuir probes). Our results suggest that ICE on LAPD are electrostatic-beam modes associated with long parallel wavelengths and destabilized by Doppler-shifted cyclotron resonance with the beam.