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• W2245854414 abstract "The origin of Edge Localised Modes (ELMs), especially the large type I variety has been the subject of intense study since their discovery. According to the peeling-ballooning mode theory these instabilities are triggered when the combined effect of the pedestal pressure gradient and the edge current density makes an edge localised ideal MHD mode unstable. Although this mode is expected to grow on a fast timescale several experiments revealed the presence of slowly growing or saturated precursors before this ELM type. A recent study of type I ELMs on MAST [1], using line integrated density measurements, pointed to the existence of a precursor with about 10 kHz frequency without the capability of localising the mode. A recently installed trial Beam Emission Spectroscopy (BES) diagnostic[2] has a highfrequency local density measurement capability therefore an attempt was made to study the location and temporal structure of these precursors and their relation to ELMs. The measurements presented in this paper were made in a strongly off-axis 1.5 MW NBI heated lower single null divertor configuration with Ip=600 kA, q95≈3. In the L-mode phase these discharges exhibited sawteeth. In H-mode phase large ELMs appear, the appearance of which seems to be related to MHD events in the core plasma. Diagnostics The present BES diagnostic uses an existing CXRS optics observing the heating Deuterium beam in a direction approximately aligned to the magnetic field lines. The Doppler shifted Dα light is filtered by a single interference filter and imaged onto 8 avalanche photodiodes. The signals are amplified with 1 MHz bandwidth and sampled at 2 MHz, the resulting Signal to Noise ratio is 5-10. The 8 channels have about 4 cm radial separation in the plasma and they cover the range from the SOL to about r/a≈0.5. The radial resolution of typically 3-4 cm is defined by the misalignment of the lines of sight and the local magnetic field across the heating beam width. At the edge these signals are proportional to the local plasma density, while deeper in the plasma they drop to half amplitude over a length of 20-30 cm due to beam attenuation. The signals are somewhat contaminated by radiation from a Carbon line from the plasma edge which could not be separated by the single interference filter. Besides the BES other diagnostics were also used in this study: a 30-channel horizontally viewing and a 16 channel tangential soft X-ray array with 12 �m Be filter foil, single-channel Dα detectors looking into the divertor region, the inner wall and tangentially at the plasma edge, an array of Mirnov coils and a 100 kHz frame rate fast camera looking at a fraction of the poloidal cross-section at the outer top of the plasma." @default.
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• W2245854414 date "2008-12-01" @default.
• W2245854414 modified "2023-10-02" @default.
• W2245854414 title "Observations of ELM pre-cursor structures using beam emission spectroscopy in MAST" @default.
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