New edge profile measurements, including calorimetric measurements of the parallel heat flux, were made in Extrap T2. Test limiters of pure molybdenum and the TZM molybdenum alloy have been exposed in the edge plasma. The surface damage was studied, mainly by microscopy. Tungsten coated graphite probes were also exposed, and the surfaces were studied by microscopy, ion beam analysis and XPS. In this case cracking and mixing of carbon and tungsten at the interface was observed in the most heated areas, whereas carbide formation at the surface was seen in less heated areas. In these tests pure Mo generally fared better than TZM, and thin and cleaner coatings fared better than thicker and less clean.

A solid target boronisation has been performed in the Extrap T2 reversed field pinch. A boron carbide rod was inserted in the edge plasma during hydrogen RFP-discharges. The amount of material released has been measured and the boron carbide film created on the first wall analysed by scanning Auger spectrometry, X-ray photoelectron spectroscopy, and ion beam techniques. The article reports on the effect the boronisation has upon plasma performance, and contains a quantitative discussion on the oxygen reduction due to oxidation of boron. (C) 1997 Elsevier Science Ltd. All rights reserved.

The effect of resistive shell modes (RSMs) on reversed-field pinch (RFP) operation has been studied on the EXTRAP T2 device. The T2 shell has a magnetic penetration time of 1.5ms and pulses are sustained for about ten shell times. Magnetic diagnostics have been used to measure the mode dynamics. Toroidally localised magnetic perturbations that penetrate the shell are observed. They grow and decay on the shell time-scale but have very small rotation frequencies and are essentially stationary toroidally. Fourier analysis of the mode structure shows that the localised perturbations consist of internally resonant modes with poloidal mode number m = 1 and toroidal mode numbers in the range n = 10-16. The perturbations can cause a degradation of the pulse primarily through impurity release due to localised plasma wall interaction. In addition to this stationary RSM, m = 0 perturbations, associated with the dynamo activity, are observed. These modes rotate in the electron diamagnetic drift direction with velocities of 20-30kms(-1). Measurements of the characteristic parameters modelling the parallel current profile indicate that n operates close to the marginal linear stability limit for an ideal shell RFP. The operation parameters are in the same range as those measured for conducting shell RFPs, despite the fact that the configuration has a resistive shell and RSMs are observed.

The Extrap T2 reversed field pinch experiment is operated with the former OHTE vacuum vessel, of dimensions R = 1.24 m and a = 0.18 m and with a complete graphite liner. It is shown that a rudimentary density control can be achieved by means of frequent helium glow discharge conditioning of the wall. The standard He-GDC is well characterized and reproducible. The trapping and release of hydrogen and impurities at the wall surfaces have been studied by mass spectrometry and surface analysis. The shot to shot particle exchange between wall and plasma can be approximately accounted for.

Edge electrostatic fluctuations, in the Extrap T1 reversed-field pinch [Nucl. Fusion 34, 427 (1994)], are observed to be correlated to internal tearing mode activity. Bispectral analysis of the edge electrostatic fluctuations shows the occurrence of nonlinear coupling between the low frequency internal tearing-mode-related activity and the high frequency, external, electrostatic fluctuations. In addition, the fluctuation levels of both the edge electrostatic fluctuations and the internal tearing modes have comparable scaling with plasma current. These results suggest that suppression of the internal tearing mode activity may decrease the edge electrostatic fluctuations and the related particle loss in the reversed-field pinch configuration.

The T1 device is a toroidal machine with Ra = 0.5 m0.06 m and stainless steel wall. It has been used to study high current density reversed field pinch plasmas. The edge plasma has been extensively investigated with double and triple Langmuir probes, heat flux probes and passive probes. Discharges with 40–90 kA plasma current is characterized by an edge density ne(a) ≈ 1019m−3, Te(a) ≈ 10 eV and a high asymmetric parallel heat flux. A drift perpendicular to the magnetic field can be inferred from Langmuir probe measurements and passive probes and appears to be largely diamagnetic. Edge ion temperatures Ti ≥ 100 eV are inferred from the saturation behaviour of hydrogen ions implanted in passive probes, from the high qi∥ and from the inferred sputtering rate of metals.