Knowing the ion temperature in the scrape-off layer (SOL) of tokamaks is of great importance for understanding the heat flux to plasma facing components. Few measurements are available for SOL ion temperatures compared with electron temperatures due to the relative complexity of the measurement. Two retarding field energy analysers (RFEAs) have been used in the mega amp spherical tokamak (MAST) to measure the ion temperature at both the midplane and the divertor in ohmic L-mode plasmas with a range of densities. Midplane SOL T i was found to be higher than T e by a factor of about 2 or greater. Divertor T i measurements at the target showed T i ≈ T e having considered the effect of plasma flows on the divertor RFEA measurements.
A Fast Ion Deuterium Alpha (FIDA) spectrometer was installed on MAST to measure radially resolved information about the fast ion density and its distribution in energy and pitch angle.Toroidally and vertically-directed collection lenses are employed, to detect both passing and trapped particle dynamics, and reference views are installed to subtract the background. This background is found to contain a substantial amount of passive FIDA emission driven by edge neutrals, and to depend delicately on viewing geometry. Results are compared with theoretical expectations based on the codes NUBEAM (for fast ion distributions) and FIDASIM. Calibrating via the measured beam emission peaks, the toroidal FIDA signal profile agrees with classical simulations in MHD quiescent discharges where the neutron rate is also classical. Long-lived modes (LLM) and chirping modes decrease the core FIDA signal significantly, and the profile can be matched closely to simulations using anomalous diffusive transport; a spatially uniform diffusion coefficient is sufficient for chirping modes, while a core localized diffusion is better for a LLM. Analysis of a discharge with chirping mode activity shows a dramatic drop in the core FIDA signal and rapid increase in the edge passive signal at the onset of the burst indicating a very rapid redistribution towards the edge. Vertical viewing measurements show a discrepancy with simulations at higher Doppler shifts when the neutron rate is classical, which, combined with the fact that the toroidal signals agree, means that the difference must be occurring for pitch angles near the trapped-passing boundary.Further evidence of an anomalous transport mechanism for these particles is provided by the fact that an increase of beam power does not increase the higher energy vertical FIDA signals, while the toroidal signals do increase.
A new beam emission spectroscopy turbulence imaging system has recently been installed onto the MAST spherical tokamak. The system utilises a high-throughput, direct coupled imaging optics, and a single large interference filter for collection of the Doppler shifted D(α) emission from the ~2 MW heating beam of ~70 keV injection energy. The collected light is imaged onto a 2D array detector with 8 × 4 avalanche photodiode sensors which is incorporated into a custom camera unit to perform simultaneous 14-bit digitization at 2 MHz of all 32 channels. The array is imaged at the beam to achieve a spatial resolution of ~2 cm in the radial (horizontal) and poloidal (vertical) directions, which is sufficient for detection of the ion-scale plasma turbulence. At the typical photon fluxes of ~10(11) s(-1) the achieved signal-to-noise ratio of ~300 at the 0.5 MHz analogue bandwidth is sufficient for detection of relative density fluctuations at the level of a few 0.1%. The system is to be utilised for the study of the characteristics of the broadband, ion-scale turbulence, in particular its interaction with flow shear, as well as coherent fluctuations due to various types of MHD activity.
This paper reports on experiments undertaken to compare the radiation resistance of two types of ceramics, boron nitride (BN) and pure alumina (Al 2 O 3 ), which are used in a TAEA antenna coil installed in the MAST spherical tokamak. Samples of the investigated materials (bulk BN and a 20 µm film of Al 2 O 3 on Al substrate) were exposed on the axis of the plasma-focus PF-1000 device, which can emit intense streams of hot plasma (v ≈ 10 7 cm s −1 and N pl ≈ 10 18 cm −3 ) and fast deuteron beams (E i ≈ 100 keV). The most powerful plasma-ion pulse lasted 0.2-1.0 µs and its intensity decayed in about 100 µs. The irradiation process was diagnosed using fast optical cameras, laser interferometry and optical spectrometry. Experiments were performed at power flux densities equal to 10 9 -10 10 W cm −2 or 10 8 -10 9 W cm −2 during the most powerful stage of the interaction process. The irradiated specimens were investigated by means of optical microscopy and x-ray structure analysis (XRSA). It was shown that at 10 10 W cm −2 pulses the Al 2 O 3 coating was completely evaporated, whereas a surface of the BN sample became smoother than in the virgin one. A direct comparison of both samples after the action of 10 8 W cm −2 pulses demonstrated a wave-like structure (more distinct on Al 2 O 3 ). Weighing of these samples showed, however, that the evaporation of BN was about two times stronger than that of Al 2 O 3 in spite of the lower irradiation flux; the XRSA showed no evidence of cracking of Al 2 O 3 after these pulses. The insulation properties of Al 2 O 3 did not decline, and the Al 2 O 3 coating may be potentially more beneficial, provided that it is kept below its melting point. Characteristic features of damages of a material based on the carbon-fiber composite with additions of silicium carbide (SiC; 8-40% volumetric) were also investigated. It was found that at q = 10 9 W cm −2 , the surface erosion is associated with sputtering and evaporation. The degree of this erosion depends on the fibers' orientation in relation to the direction of the plasma-ion streams, and on the percentage of the SiC admixture.
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