Compact ECEI system with in-vessel reflective optics for WEST

An electron cyclotron emission imaging (ECEI) diagnostic system for WEST (W Environment for Steady state Tokamak) is under development to study the MHD instabilities affected by tungsten impurities. The system will provide 2-D T_e fluctuation images (width × height = ∼18 cm × ∼ 34 cm at low field side and ∼13 cm × ∼ 39 cm at high field side) from a poloidal cross section with high spatial (≤1.7 cm) and temporal (≤2 μs) resolutions. While the key concept and electronic structure are similar to that of prior ECEI systems on other tokamak devices such as KSTAR, DIII-D, or ASDEX-U, part of the imaging optics have to be placed inside the vacuum vessel in order to resolve issues on limited installation space and longer beam path to the detector position. The in-vessel optics consisting of two large curvature-radius mirrors are expected to withstand the extreme heating on long-pulse operation scenario (∼1000 s). The out-vessel optical housing is constructed as compact as possible to remove easily from the installation site in case of necessity. Commissioning of the system is scheduled on the second experimental WEST campaign end of 2017.

Design of the reflective optics for Tore Supra ECEI system

A 2D electron cyclotron emission (ECE) imaging system for Tore Supra is under design for studying the MHD physics of the magnetically confined plasma such as sawteeth, tearing modes, and turbulent fluctuations. Complex beam path due to the tight access in Tore Supra led to the design of reflective optics made of 6 or more large cylindrical∕flat mirrors. The total path length of the ECE beam is about 11 m, including almost 4 m inside the vacuum vessel. The imaging property of the optics has been estimated using the Gaussian beam simulation and ray transfer analysis. The possible setups for the optical alignment of the diagnostic and the operation scenarios with single- or dual-array measurement system are discussed.

Redistribution of suprathermal electrons due to fishbone frequency jumps

MHD instabilities driven by fast electrons identified as fishbonelike modes have been detected on Tore Supra during lower hybrid current drive discharges. Direct experimental evidence is reported of a novel feature: the regular redistribution of suprathermal electrons toward external tokamak regions which are correlated to periodic mode frequency jumps. Sharp drops of the electron temperature time trace are factually linked to the cyclical deterioration of the fast electron confinement.