Experimental investigation of active feedback control of turbulent transport in a magnetized laboratory plasma

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 new HELCAT (HELicon-CAThode) linear device at UNM. HELCAT is a 4 m long device, with B\u3c0.22 T, and cathode-produced densities, n~1-5times1012 cm-3. Sheared EtimesB flows, generated via biased concentric rings, will be utilized to modify the transport. Fluctuations and flux will be monitored with probe arrays. Additionally, a 1D transport code is being utilized to model the system in order to investigate possible control methods numerically. Initial experimental and modeling results will be presented

Minority and mode conversion heating in (3He)-H JET plasma

Radio frequency (RF) heating experiments have recently been conducted in JET (He-3)-H plasmas. This type of plasmas will be used in ITER's non-activated operation phase. Whereas a companion paper in this same PPCF issue will discuss the RF heating scenario's at half the nominal magnetic field, this paper documents the heating performance in (He-3)-H plasmas at full field, with fundamental cyclotron heating of He-3 as the only possible ion heating scheme in view of the foreseen ITER antenna frequency bandwidth. Dominant electron heating with global heating efficiencies between 30% and 70% depending on the He-3 concentration were observed and mode conversion (MC) heating proved to be as efficient as He-3 minority heating. The unwanted presence of both He-4 and D in the discharges gave rise to 2 MC layers rather than a single one. This together with the fact that the location of the high-field side fast wave (FW) cutoff is a sensitive function of the parallel wave number and that one of the locations of the wave confluences critically depends on the He-3 concentration made the interpretation of the results, although more complex, very interesting: three regimes could be distinguished as a function of X[He-3]: (i) a regime at low concentration (X[He-3] < 1.8%) at which ion cyclotron resonance frequency (ICRF) heating is efficient, (ii) a regime at intermediate concentrations (1.8 < X[He-3] < 5%) in which the RF performance is degrading and ultimately becoming very poor, and finally (iii) a good heating regime at He-3 concentrations beyond 6%. In this latter regime, the heating efficiency did not critically depend on the actual concentration while at lower concentrations (X[He-3] < 4%) a bigger excursion in heating efficiency is observed and the estimates differ somewhat from shot to shot, also depending on whether local or global signals are chosen for the analysis. The different dynamics at the various concentrations can be traced back to the presence of 2 MC layers and their associated FW cutoffs residing inside the plasma at low He-3 concentration. One of these layers is approaching and crossing the low-field side plasma edge when 1.8 < X[He-3] < 5%. Adopting a minimization procedure to correlate the MC positions with the plasma composition reveals that the different behaviors observed are due to contamination of the plasma. Wave modeling not only supports this interpretation but also shows that moderate concentrations of D-like species significantly alter the overall wave behavior in He-3-H plasmas. Whereas numerical modeling yields quantitative information on the heating efficiency, analytical work gives a good description of the dominant underlying wave interaction physics

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Experimental investigation of active feedback control of turbulent transport in a magnetized laboratory plasma

Digital Object Identifier: 10.1109/PLASMA.2006.1707109Summary 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 new HELCAT (HELicon-CAThode) linear device at UNM. HELCAT is a 4 m long device, with B<0.22 T, and cathode-produced densities, n~1-5times1012 cm-3. Sheared EtimesB flows, generated via biased concentric rings, will be utilized to modify the transport. Fluctuations and flux will be monitored with probe arrays. Additionally, a 1D transport code is being utilized to model the system in order to investigate possible control methods numerically. Initial experimental and modeling results will be presented.IEE