Investigation of turbulence properties via spectral broadening of Doppler reflectometry signals in ASDEX Upgrade

摘要

1.Introduction Doppler reflectometry (backscatter, BS) is an established microwave diagnostic technique for measuring plasma flows and turbulence in magnetically confined plasmas with sub-millisecond temporal and mm spatial (radial) resolution. Fig. 1 shows a typical signal spectrum from an ASDEX Upgrade (AUG) L-mode edge plasma (limiter, Bt = -2.2 T, {formula}) using an X-mode, 50 - 75 GHz stepped frequency reflectometer [1]. From the Doppler shift f_D (obtained by Gaussian fit to the asymmetric spectral component [1]) the turbulence velocity {formula} is extracted. {formula} is the turbulence wavenumber at the beam turning point, obtained from beam-tracing (torbeam). Further, the peak power Ad, or rather the integrated peak S(k⊥) ∝ A_D . wd, gives a measure of the turbulence level |δn|~2 at the probed k⊥. Here w_D is the 1/e power spectral half-width, usually assumed to be determined by the diagnostic wavenumber resolution δk⊥, set by the probing beam width/divergence and curvature effects [2]. However, in AUG, wd is much wider than expected from the intrinsic δk⊥ alone. In fact, peak broadening may arise from several factors: turbulent and coherent flow oscillations, such as GAMs: flow shear; non-linear turb. interactions; as well as beam spreading from low k forward-scattering (FS). These effects are investigated with an eye to the turbulence properties.