Ion beam generated modes in the lower hybrid frequency range in a laboratory magnetoplasma

The interaction of a fast ion beam with a low $\beta$ plasma has been studied in the laboratory. Experiments were performed at the LArge Plasma Device (LAPD) at UCLA. The experiments were done in a Helium plasma ($n \simeq 10^{12} \ \mbox{cm}^{-3}$, $B_0$ = 1000 G - 1800 G, $f_{pe}/f_{ce} \simeq 1 - 5$, $T_e = 0.25\ \mbox{eV}$, $v_{te} \ll v_A$). The ion beam is a Helium beam with energies ranging from 5 keV to 18 keV. The fast ion velocity is on the order of the Alfv\'en velocity. The beam is injected from the end of the machine, and spirals down the linear device. Waves were observed below $f_{ci}$ in the shear Alfv\'en wave regime, and in a broad spectrum above $f_{ci}$ in the lower hybrid frequency range, the focus of this paper. The wave spectra have distinct peaks close to ion cyclotron harmonics, extending out to the 100th harmonic in some cases. The wave generation was studied for various plasma parameters, as well as for different beam energies and pitch angles. The waves were measured with 3-axis electric and magnetic probes. Detailed measurements of the perpendicular mode structure will be shown. Langmuir probes were used to measure density and temperature evolution due to the beam-plasma interaction. Retarding field energy analyzers captured the ion beam profiles.