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H-mode transitions in TEXT-U limiter plasmas have been observed at and kA ( kW) with at least 300 kW of central electron-cyclotron heating (ECH). These are dithering transitions which are induced by sawtooth crashes and display the typical signatures of H-modes ( drop, spontaneous density increase, evidence of a transport barrier). However, they show only a slight improvement over L-mode energy confinement. The vessel walls are boronized and conditioned prior to experiments to achieve low-impurity influx and particle recycling. Discharges which undergo transitions are fuelled almost entirely on residual recycling. Transitions are observed when limited on a toroidally localized top or bottom limiter and, more often, when the limiter surface is `fresh', which is achieved by alternating between top and bottom limiters on successive shots. No strong dependence upon the distance from the low-field-side limiter has been found. Transitions are not yet observed when limited on the high-field-side wall tiles or in the case of TEXT-U diverted configurations. Preliminary measurements with the 2 MeV heavy-ion beam probe (HIBP) (in the core) and Langmuir probes (in the edge) indicate that the plasma potential drops outside the q = 1 radius while only small changes are observed in the density fluctuation level.

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 is driven by the local edge potential fluctuations. The edge potential fluctuations are modified in a broad frequency band. Moreover, the potential fluctuations can be reduced (≤ 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. The local plasma parameters at the edge and the estimated fluctuation-induced radial particle flux are somewhat affected by the edge feedback.

Experiments in modifying plasma edge turbulence were carried out on the Texas Experimental Tokamak (TEXT) by launching waves using two electrostatic probes in the shadow of the limiter. Measurements indicate that the wave, launched with a typical frequency range of 15 to 50 kHz from the edge of the machine top, is received by sensing probes located halfway around the torus. The detected signal strength depends weakly on the frequency of the wave and on the phasing of the applied AC signal between the launching probes. Modifications to the spectrum of the edge potential fluctuations are observed at the launching frequency. These experiments have been extended to the control of the edge plasma fluctuation level using feedback. The launcher is driven by the floating potential of the fluctuating plasma at the location of the launching probes. Edge fluctuations are reduced or excited, depending on the phasing between the launching probes, both locally and at the downstream sensing probes. The fluctuation induced particle flux as well as the local plasma parameters are affected by feedback phasing

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.

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 flexibility and nonintrusive nature of spectroscopic measurements can complement results from Langmuir probes by extending fluctuation measurements to otherwise inaccessible regions of the plasma. On TEXT, studies of fluctuations in Hα light using a photodiode array (PDA) detector [Hurwitz et al., Rev. Sci. Instrum. 63, 4614 (1992)] have yielded estimates for fluctuation characteristics that are in qualitative agreement with those from probes. The most obvious limitation of this spectroscopic diagnostic is its extended radial viewing volume which overlaps the naturally occurring velocity shear layer just inside the last closed flux surface (LCFS). We present here a three sample volume estimate for the spectral density function S(k,ω) which models the PDA signal as consisting of two modes in k space corresponding to the contributions from inside and outside of the shear layer. This method aids in the radial localization of the PDA measurement and may be applied to other diagnostics as well.

We developed an imaging detector to measure high‐frequency fluctuations in visible and near‐UV emission from tokamaks. The detector is intended for the study of plasma turbulence, mhd phenomena, and edge‐localized modes. Particularly in the first two applications, it will complement existing techniques by providing higher spatial resolution as well as measurement capability in otherwise inaccessible regions of the plasma. The device consists of an optical system, a linear array of 32 photodiodes, and an amplifier for each photodiode. The amplifiers have a transimpedance gain of 105–106 and the frequency response is flat to 100 kHz. Experience with this device has shown that optical imaging systems can be easily designed and tailored to a specific measurement because of the small size and close spacing of the individual light‐sensitive elements. The device has been successfully tested on TEXT‐U in measurements of Hα fluctuations.

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.