High resolution Thomson scattering on the COMPASS tokamak—extending edge plasma view and increasing repetition rate

Abstract

The Thomson scattering (TS) diagnostic system on the COMPASS tokamak consists of separate collection optics for core and edge plasmas. Till now, the adverse orientation of the viewport available for the edge TS limited the investigated plasma scenarios to those, where H-mode plasma pedestal was possible to be observed. The vacuum vessel port has been recently modified and directed more to the plasma edge. To bridge the time for designing and manufacturing a new lens, the so far used edge TS collection lens was tilted and the fibre bundles rearranged to comply with the new geometry. However, the full potential of the new port is utilized only by the newly designed and manufactured lens. Comparison of performance of the collection lenses before and after the viewport modification is presented. Apart from the edge collection system upgrade, the laser system was extended by two lasers with parameters similar to the original two, thus increasing the repetition rate of the TS system to 120 Hz or allowing to obtain four profiles with tight timing in order to measure fast transient events in the plasma.

Experimentally corroborated model of pressure relaxation limit cycle oscillations in the vicinity of the transition to high confinement in tokamaks

“…Additional discharges representing H and He discharges were added using integrated data analysis [30]. The dataset from COMPASS (R = 0.56 m, C wall) uses T e , n e from the Thomson scattering system [31] and q from the magnetic reconstruction evaluated at ψ N ≈ 0.95. The dataset is based on discharges either previously reported in [10] or similar ones.…”

On the transport of edge localized mode filaments in the tokamak scrape-off layer

“…The divertor probe measurements are compared to the pedestal electron temperature, T e,ped , obtained by a high-resolution Thomson scattering system [22] in the last 30% of the previous ELM cycle (corresponding to fully developed H-mode pedestal). The value T e,ped is then determined by the two-line technique [23].…”

Temporal characteristics of ELMs on the COMPASS divertor

“…The value of τ || is given by τ || = L || /c s with the sound speed c s calculated for equal electron and ion pedestal temperatures, T e ped = T i ped . The values of T e ped were obtained from T e profiles measured by high resolution Thomson scattering (HRTS) system [33] using the so-called two-line fitting technique [24,34]. Note that the measurements of the HRTS system with a constant repetition frequency allowed us to get reasonable measurements of the T e ped in the last 30% of the previous ELM cycle (corresponding to fully developed Hmode pedestal) only in a few cases.…”