Generation of shear Alfv\'{e}n waves due to resonant interactions with a spiraling ion beam on the Large Plasma Device

The role of Landau and Doppler-shifted ion-cyclotron resonances (DICR) in extracting the free-energy from an ion-beam and destabilizing Alfv\'{e}n waves was explored. The experiment was conducted on the Large Plasma Device (LAPD) in a dual-species magnetized plasma ($n \approx 10^{10}$--10$^{12}$ cm$^{-3}$, T$_e \approx$ 5.0 eV, B = 1.0--1.8 kG, 92$\%$ He$^+$ and 8$\%$ H$^+$ ions, 19 m long, 0.6 m diam). A hydrogen ion beam (15 kV, 10 A) was obliquely injected into the plasma. The interaction of the beam with the plasma was diagnosed using a retarding-field energy analyzer, three-axis magnetic-loop, and Langmuir probes. Measurements of the beam profiles at multiple axial locations evinced a spiraling ion-beam (J $\approx$ 50-140 mA/cm$^2$, pitch-angle $\approx$ 53$^{\circ}$) that traveled at Alfv\'{e}nic speed (beam-speed/Alfv\'{e}n-speed = 0.2--1.2). Although, a variety of waves were generated by the beam, this presentation will focus on shear Alfv\'{e}n waves. Parameters of the ion-beam and ambient plasma were varied to examine the resonance conditions under a variety of scenarios. The experimental results demonstrate that the DICR process is particularly effective in exciting left-handed polarized shear Alfv\'{e}n waves that propagate in the direction opposite to the ion beam.