Investigation of Active Feedback Control of Turbulent Transport in a Magnetized Laboratory Plasma

Abstract

Summary form only given. Many toroidal fusion devices now routinely generate edge and/or core transport barriers, where heat and particle transport are reduced far below Bohm diffusion levels. However, minimal particle transport is not necessarily desirable, since it can lead to core impurity accumulation, or alpha particle buildup. Ideally, active, stable control over the transport, rather than simple minimization, could be obtained. To this effect, research is now underway to investigate active control of particle transport. Turbulence and transport dynamics are, of course, strongly nonlinear, and apparently not deterministic. However, modern nonlinear control methods now exist, such as chaotic control and fuzzy control, which do not rely on a model of the system dynamics to affect stable control. Experiments are being conducted in the HELCAT (HELicon-CAThode) linear device at UNM. HELCAT is a 4 m long device, with B < 0.22 T, and cathode-produced densities, n ~ 1-5x10nland12 cmnland-3. Sheared ExB flows, generated via biased concentric rings, are utilized to modify the transport. Fluctuations and flux are monitored with probe arrays. Parameters, such as RF power, gas pressure and magnetic field, are investigated for their effects on plasma behavior (turbulence, blobs, profile, shear layer, fluctuations, etc.). Open loop experiments have demonstrated that drift fluctuations can be fully suppressed by simple biasing. Additionally, a bias regime between drift instability and full suppression exits where fluctuations are chaotic. Experimental results and analysis will be presented.