Wave Coupling and Propagation from the Helicon Antenna in LAPD

Low-power (~100 W) ‘helicon’ (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. The antenna was constructed using modules from the DIII-D low-power Helicon antenna. The parameter space explored included: background magnetic field (0.7 – 2 kG), plasma density (~10¹⁰ – 10¹² cm^-3), imposed n_|| (2 – 4), and launcher angle with respect to B₀ (0 – 45 degrees). 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. Propagating waves are observed in locations and times during the discharge that agree with the expected wave dispersion relation. These experiments verified important properties of the antenna design. Unidirectional launch along the long axis of the machine and control of the parallel wavelength of the waves propagating in the plasma are demonstrated.