H. Y. W. Tsui

A new scheme for Langmuir probe measurement of transport and electron temperature fluctuations

A new scheme to extend the triple Langmuir probe technique for the measurement of electron temperature fluctuations and the fluctuation‐driven transport has been developed. The extension is aimed at reducing the phase delay error introduced by finite probe tip separations in standard triple‐probe method. The modified triple‐probe scheme provides a more reliable measurement of the temperature fluctuations for a proper interpretation of the density and potential fluctuations and the transport measurement from Langmuir probe data. New results on fluctuations have been obtained from Phaedrus‐T and TEXT‐U tokamaks.

A comparison of edge turbulence in tokamaks, stellarators, and reversed-field pinches

Edge equilibrium, turbulence and transport related plasma parameters from the Advanced Toroidal Facility (ATF) [Fusion Technol. 10, 179 (1986)] torsatron, the ZT-40M [Fusion Technol. 8, 1571 (1985)] reversed-field pinch, the Phaedrus-T [Nucl. Fusion 32, 2040 (1992)] tokamak, the Texas Experimental Tokamak (TEXT) [Nucl. Technol. Fusion 1, 479 (1981)], and the Tokamak Fusion Test Reactor (TFTR) [in Plasma Physics and Controlled Nuclear Fusion Research, 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. 1, p. 9] have been obtained using a standardized Langmuir probe array and a consistent set of data analysis packages. Additional data from some other devices have also been furnished via private communications and incorporated from published results. Experimental results over a wide range of parameters are compared and the turbulence contribution to edge transport are assessed. Certain physical properties that are relevant to the modeling of edge turbulence are identified: namely, shear decorrelation of turbulence, the role of resistive dissipation and electron parallel thermal conduction, radial mode structure in sheared magnetic field, and electromagnetic contribution to the parallel Ohm’s law.

Observation of a quasicoherent mode in the Texas Experimental Tokamak

A quasicoherent feature localized near the magnetic flux surface having a safety factor q=3 has been detected by Langmuir probe and heavy ion beam probe (HIBP) measurements in the edge plasma of the Texas Experimental Tokamak (TEXT) [Nucl. Technol. Fusion 1, 479 (1981)]. A mode is identified with relatively low poloidal wave number (m=12) compared to the usual broadband turbulence with m in the range of 50 to 100. In contrast to the turbulent activity, this mode is collisional and does not produce a fluctuation‐driven radial particle flux via the coupling between density and potential fluctuation. A comparison with the expectation of the drift‐tearing activity indicates that the mode can be collisional drift‐tearing unstable.

A comparison of Langmuir probe techniques for measuring temperature fluctuations

Two techniques are applied and the results compared in the edge plasma of TEXT‐U to estimate temperature fluctuations. The first technique fits the measured current fluctuations from a double probe as a function of probe bias with several fitting functions to provide density, and temperature fluctuations but with no spectral information. The second method uses a triple‐probe technique to measure density, temperature, and floating potential as a function of time. The two techniques are in agreement to within estimated error limits and find T/T≊0.4–0.5 n/n both in the scrape‐off layer and in the main body of the plasma.

Modification of tokamak edge turbulence using feedback

Using active feedback, the turbulent fluctuation levels have been reduced by as much as a factor of 2 in the edge of the Texas Experimental Tokamak (TEXT) [K. W. Gentle, Nucl. Fusion Technol. 1, 479 (1981)]. A probe system was used to drive a suppressor wave in the TEXT limiter shadow. A decrease in the local turbulence‐induced particle flux has been seen, but a global change in the particle transport at the present time has not been observed. By changing the phase shift and gain of the feedback network, the amplitude of the turbulence was increased by a factor of 10.

THE TIME-DOMAIN TRIPLE-PROBE METHOD

A new Langmuir‐probe technique based on the triple‐probe method is being developed to provide simultaneous measurement of plasma temperature, potential, and density with the temporal and spatial resolution required to accurately characterize plasma turbulence. When the conventional triple‐probe method is used in an inhomogeneous plasma, local differences in the plasma measured at each probe introduce significant error in the estimation of turbulence parameters. The time‐domain triple‐probe method (TDTP) uses high‐speed switching of Langmuir‐probe potential, rather than spatially separated probes, to gather the triple‐probe information thus avoiding these errors. Analysis indicates that plasma response times and recent electronics technology meet the requirements to implement the TDTP method. Data reduction techniques of TDTP data are to include linear and higher‐order correlation analysis to estimate fluctuation induced particle and thermal transport, and relationships between temperature, density, and potential fluctuations.

Particle balance in diverted plasmas in TEXT-U

Tokamak plasmas in an open divertor configuration with the X-point at the inner equator were compared with plasmas limited by an inner toroidal belt. This paper describes and compares the particle balance in these two types of discharge. The plasma parameters n e (r), Φ(r), T e (r), n e , and Φ were measured in the scrape-off layer and in the plasma periphery at one poloidal location and mapped onto the rest of the plasma by assuming constancy on flux surfaces. Emission from neutral hydrogen was measured throughout the plasma. The particle source and then the global particle confinement were inferred from these measurements using a 3-D neutral Transport simulation. The SOL profiles are significantly steeper in the diverted discharge. Both the source measurements and estimates from the SOL profiles indicate significantly better confinement in the diverted discharge

FEEDBACK CONTROL AND STABILIZATION EXPERIMENTS ON THE TEXAS EXPERIMENTAL TOKAMAK (TEXT)

Plasma edge feedback experiments on the Texas Experimental Tokamak (TEXT) have been successful in controlling the edge plasma potential fluctuation level. The feedback wave-launcher, consisting of electrostatic probes located in the shadow of the limiter, is driven by the local edge potential fluctuations. In general, the edge potential fluctuations are modified in a broad frequency band. Moreover, it is observed that the potential fluctuations can be reduced ({le}100 kHz) without enhancing other modes, or excited (10 to 12 kHz), depending on the phase difference between the driver and the launcher signal, and gain of the system. This turbulence modification is achieved not only locally but also halfway around the torus and has about 2 cm of poloidal extent. Experiments on the characterization of the global plasma parameters with the edge feedback are discussed. Effects of the edge feedback on the estimated fluctuation-induced radial particle flux as well as on the local plasma parameters are presented.

A database for edge turbulence and transport studies

A database consisting of edge equilibrium, turbulence and transport related plasma parameters has been compiled. Initial results related to fluctuation phenomena are presented. In particular the electrostatic and electromagnetic nature of the fluctuations, the scaling of various transport related parameters, comparison of fluctuation driven particle fluxes with estimates of the total particle fluxes and the relative importance of conduction to convection energy loss are discussed.

Edge plasma and fluctuations in TFTR

Edge fluctuations and their associated transport have been measured in TFTR with a fast reciprocating drive shaft fitted with a Langmuir probe array specially designed to withstand high power density and to provide spatial resolution required for fluctuation measurements. Simultaneous measurements of plasma potential, electron temperature and density in the scrape-off layer (SOL) and in the edge region just inside the last-closed flux surface (LCFS) were obtained to characterize the edge fluctuations in ohmic discharges. Significant changes in the properties of the fluctuations in the SOL were observed in both Supershot and H-mode plasmas.