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

Abstract

Magnetically confined plasmas do not necessarily fulfill the accessibility criteria for electromagnetic waves in the electron cyclotron (EC) frequency range, polarized in the ordinary (O) or extraordinary (X) mode. This may hold especially in high density stellarator plasmas, not affected by the Greenwald limit, and in low magnetic field spherical tokamaks. In such cases the electron Bernstein (B) mode, the third electron cyclotron mode in a hot magnetized plasma is the “natural” candidate for EC heating, current-drive and diagnostic, since it propagates without upper density limit. On the other hand, it is an electrostatic mode unable to propagate in vacuum, which has to be excited and detected within the plasma, e.g. by mode conversion or direct coupling ⁵. Overdense plasma heating by electron Bernstein waves (EBWs) was demonstrated at W7-AS ¹ taking advantage of the O-X-B mode conversion mechanism⁴. The coupling of O and slow X-mode occurs at the cutoff layer for the O-mode: if it is approached with a suitable inclination defined by⁴

$$ k_y = 0,k_z = \omega \omega _c /(\omega + \omega _c )c $$

((1))

in a frame with the z axis parallel to the magnetic field B and the x axis along the density gradient, the two modes coalesce, locally. In the phase space, then, one branch is the prolongation of the other without crossing a region of evanescence (k _x ² < 0). In the physical space this means that 100% of input O-mode energy flows into X-mode and (being unaffected by the O-mode cutoff) goes ahead. Then it turns back to the upper hybrid layer and converts completely into the B-mode. The reversed scheme, BXO, was used at W7-AS for the first spectral measurements of electron Bernstein emission (EBE) ². Since emission reachs blackbody levels, more recently was possible even to measure the first temperature profile ³. This paper describes the first routinely operating dedicated diagnostic and its results, with the focus on the features distinguishing EBE from electron cyclotron emission (ECE) of underdense plasmas. The article is organized as follows. The Sec.2 is devoted to the ray tracing code for mode-converted electron Bernstein waves (EBWs), developed to design a dedicated antenna and to interpret the experimental results. The experimental setup is described in Sec.3 with special regard to the broadband phase shifter allowing for pure O-mode detection. In Sec.4 we present a typical profile, reconstructed from the spectral measurements by means of the above mentioned ray tracing code. Heat waves measurements combining EBW emission diagnostic with modulated EBW heating at very high density are presented in Sec.5. Finally Sec.6 shows applications of the good EBE time resolution compared to Thomson scattering, which also access high density but cannot resolve, for example, ELMs or profile oscillations observed by EBE before radiative collapse.