Helicon Wave Experiments on the LArge Plasma Device (LAPD)

Helicon waves, also known as fast waves in the lower hybrid range of frequencies or whistler waves, are a proposed means for non-inductive current drive in reactor-grade fusion devices. Work at DIII-D to evaluate the prospect of helicon current drive has been done in the low power (<0.5 kW) regime [R.I. Pinsker et al 2018 Nucl. Fusion 58 106007], and high power (~0.5-1 MW) experiments are currently ongoing [B. Van Compernolle et al 2021 Nucl. Fusion 61 116034]. Through the use of two antenna structures, a low-power traveling wave antenna and a four strap antenna, the LAPD can be configured to be representative of the DIII-D scrape-off-layer that these waves must couple and propagate through. Experiments have been conducted to characterize the wave coupling and propagation from the traveling wave antenna over a large range of plasma parameters. Progress of current experiments that are investigating the role that edge turbulence plays in wave propagation in the helicon regime will be reported. Here we will look specifically at how density filaments can cause wave scattering [A.K. Ram 2016 Phys. Plasmas 23, 022504] and ‘stimulated mode-conversion’ [P.L Andrews 1985 Phys. Rev. Lett. 54] leading to wave power in undesirable locations.