UCLA

Ion cyclotron resonance heating (ICRH) systems are critical components of current and future tokamak experiments aimed at producing nuclear fusion energy. During ICRH a host of deleterious effects occur, including increased heat flux to plasma facing components and modification of launched wave power. A suspected root cause of these effects is the radio frequency (RF) rectification of the plasma potential. Interest in the antenna scrape-off layer (SOL) region has drawn increasing interest, as it is recognized that mitigating these effects is necessary to achieving fusion power. This dissertation investigates the RF rectification of the plasma potential and the resulting cross-field flows that form due to an active RF antenna. The experiment is performed in the Large Plasma Device (LAPD) utilizing a fast wave antenna and RF amplifier system developed for these studies. The RF system is capable of 150 kW output power for a 1 ms pulse that is repeated at the 1 Hz repetition rate of the LAPD plasma discharge.

Upon application of the RF pulse to the antenna, the DC plasma potential, measured with an emissive probe, dramatically increases in certain spatial locations by a factor greater than 10 Te. The largest plasma potentials are observed at locations magnetically connected to the top and bottom of the antenna, and they exist only in the private SOL created between the antenna and a limiter placed 3.6 m away along the LAPD axis. The DC rectified potentials scale linearly with the antenna current over a factor of 12x in the applied current. These DC potentials increase plasma materials interactions (PMI), resulting in the sputtering of antenna materials whose presence is detected in the bulk plasma by the coatings that develop on probe diagnostics. The DC rectified potentials persist in the plasma long after the RF current in the antenna has rung down on the same time scales as the change in the density.

At the top and bottom of the antenna are circular flows, often called convective cells. These ?�?? flows arise due to the spatial variation of the RF rectified potentials across the background magnetic field. The maximum strength of the electric field causing these flows was

found to scale quadratically with antenna current, giving rise to drift velocities that are a substantial fraction of the local sound speed, v_drift/c_s ≈ 0.8. These flows have a dramatic effect on the density, which increases in the SOL and develops poloidal asymmetries in the plasma region magnetically connected to the front face of the antenna. The convective cells cause a density depletion at the antenna midplane that increases with antenna current until a threshold current. The 2-D density modification is physically consistent with the calculated ?�?? flows.

These results show a plethora of problems that must be solved for successful ICRH operation, even at low antenna powers. These deleterious effects may be mitigated by antenna designs that reduce rectified potentials and utilize PMI-resilient materials. The saturation in the density depletion at the antenna midplane suggests methods for targeted density injection may be successful in improving antenna wave coupling.