Spontaneous excitation of waves by an intense ion beam on the Large Plasma Device*

A hydrogen ion beam (15 keV, 10 A) has been injected into a large magnetized plasma ($n \approx 10^{10}$--$10^{13}$ 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) for performing fast-ion studies on the Large Plasma Device (LAPD). The beam forms a helical orbit (pitch-angle $\approx 7^\circ$--$55^\circ$), propagates with an Alfv\'{e}nic speed (beam-speed/Alfv\'{e}n-speed = $0.2$--$3.0$), and significantly enhances the electron temperature and density when injected during the plasma afterglow. We report results on spontaneous generation of Alfv\'{e}n waves and electrostatic waves in the lower-hybrid range of frequencies by the beam. Roles of normal and anomalous Doppler-shifted ion-cyclotron resonances in destabilizing the Alfv\'{e}n waves were examined by measuring the phase-speed of waves and relevant parameters of the plasma using a variety of diagnostic tools (retarding-field energy analyzer, three-axis magnetic-loop, Dipole, and Langmuir probes). Conditions for the maximum growth of these waves were determined by varying the parameters of the beam and ambient plasma and examining the mode-structures in the fluctuation-spectra. Reference: Tripathi et al., Phys. Rev. E 91, 013109 (2015)