Electron Bernstein Emission Diagnostic of Electron Temperature Profiles at W7-AS Stellarator

Abstract

Electron temperature profiles at densities above the ECE cutoff aremeasured atthe W7-AS stellarator by a novel diagnostic based on black body emissionandmode conversion of electron Bernstein waves (EBWs).The EBWs, otherwise confined within the upper hybrid layers, reachthe out-side of the plasma after Bernstein-extraordinary(X)-ordinary(O) modeconver-sion. Such O-mode polarised output is detected along a special obliqueline ofsight by an antenna with gaussian optic. This has been optimised bymeans ofEBWs ray tracing calculations in full stellarator geometry, in orderto maximisethe conversion efficiency and to minimise the Doppler broadening dueto theoblique nature of the line of sight. The obliquely detected O-modepolarizationis elliptical, but by λ/4 phase shift and proper orientationof optical axes it ischanged into a linear polarization by an elliptical waveguide. Thesignal is thenspectrum-analysed in a heterodyne radiometer. Finally temperatureprofiles arederived from the spectra by means of the 3D ray tracing code mentionedbefore.The diagnostic has been applied to time- and space-resolved measurementsofedge-localized modes, low-to-high confinement transitions and radiativecollapsesat densities up to ne = 3.8x10^20 m-3 .Moreover, for the first time the heat wave propagation method forthe determi-nation of local heat transport coefficients has been extended beyondECE cutoffdensity, combining EBW emission measurements at the first harmonic(f =66-78GHz) with modulated EBW heating at the second harmonic (140GHz).