Wave coupling and propagation from the helicon antenna in LAPD

Low-power (∼100 W) helicon wave (fast waves in the lower hybrid range of frequencies) experiments were conducted on the LArge Plasma Device (LAPD) at UCLA to study wave propagation and coupling properties with a 10-element comb-line traveling wave antenna. Helicon waves are launched unidirectionally along the long axis of the machine from the inductively coupled antenna modules. Varying the launch frequency within the passband of the antenna selects the principal normalized parallel wavenumber n∥. The accessible diagnostic suite and simplified geometry of the LAPD make it well equipped to conduct this systematic study of the comb-line antenna coupling and wave propagation in the plasma. With well resolved spatial and temporal measurements of density, fields, and antenna power, wave power in the plasma was measured across a range of tunable parameters. These parameters included the background magnetic field (B0=0.7-2 kG), plasma density (∼1010 - 1012 cm-3), wave launch direction (sign of n∥), launcher angle (0 to 45 degrees with respect to B0), antenna power (∼10-3 -102 W), and launch frequency (461-491 MHz, thereby changing n∥ in the range 2<|n∥ |<4). Across the range of densities there are times during the discharge where both the fast (helicon) and slow ('lower hybrid wave') branches may simultaneously propagate in the plasma core. An extensive data set was obtained to characterize the wave coupling and propagation.