Wave-filament Interaction Experiments on the LArge Plasma Device (LAPD)

Experiments have been conducted on the LArge Plasma Device (LAPD) to investigate the role that filamentary density structures play on the propagation of fast waves in the lower hybrid range of frequencies (LHRF). In these experiments we launch waves at 0.8f_LH<f_rf <15f_LH; at the lower frequencies only the fast branch propagates in the plasma, at higher frequencies both branches can simultaneously propagate at a given density. It is well understood that density fluctuations may cause scattering and mode-conversion in the LHRF. Additionally, density filaments can provide a means for wave power to flow along the background magnetic field lines through the evanescent slow branch as a surface mode. We investigate these two mechanisms on the LAPD by setting up a quiescent background plasma with a sharp density peak in the core to represent a filament. Using a new carbon iris masking system, we can adjust the size of the filament over the course of the experiments. Results from these experiments have been compared to full wave cold plasma models in both 2D and 3D geometries. Here we will discuss the experiment results and these models as well as the implications to other experiments such as heating and current drive efforts on tokamaks.