Plasma rotation profile measurements using Doppler reflectometry

Abstract

High spatial resolution radial profiles of the perpendicular plasma rotation velocity u ⊥ using a dual channel 50-75 GHz Doppler reflectometer system on the ASDEX Upgrade tokamak are presented for a variety of discharge scenarios, including Ohmic, L-mode, H-mode, etc with forward and reversed magnetic field and co-and counter neutral beam injection. The reflectometers have steppable launch frequencies f o = c/λ o , with selectable O-or X-mode polarization, giving tokamak edge to mid-radius coverage. Low-field-side antennae (hog-horn antenna pairs) with deliberate tilting (primarily poloidally) produce a Doppler shifted spectrum directly proportional to the perpendicular velocity f D = u ⊥ k ⊥ /2π = u ⊥ 2 sin θ t /λ o . The incident angle θ t between the beam and cut-off layer normal varies with plasma shape, cut-off layer position and refraction. However, typical angles range from 5˚to 27˚giving a probed turbulence wavenumber, k ⊥ , range of 1.8-14.3 cm −1 , with resulting Doppler shifts f D of up to 5 MHz. The measured perpendicular velocity is u ⊥ = v E×B + v phase , which for a typical H-mode is slightly positive in the tokamak scrape-off-layer with a deep negative well across the H-mode steep pressure gradient pedestal region and then following the perpendicularly projected toroidal fluid velocity in the core, should be dominated by the E × B velocity, as the intrinsic phase velocity is predicted to be small, which may allow u ⊥ to be interpreted directly as the radial electric field E r profile.

Discriminating the trapped electron modes contribution in density fluctuation spectra

“…Contrary to spectra of Doppler backscattered signals [35], conventional reflectometry spectra are expected to be rather symmetric. However, a small asymmetry is always observed between positive and negative frequencies.…”

The role of plasma elongation on the linear damping of zonal flows

“…Experimental evidence of the dependence of the GAM frequency on plasma elongation has been provided by Doppler reflectometry in ASDEX-Upgrade. 13 The goal of this paper is to establish a simple relationship between plasma elongation and zonal flow oscillation properties. A fluid model is used to derive an expression for the GAM frequency where the plasma elongation appears as an explicit parameter.…”

E × B flow velocity deduced from the poloidal motion of fluctuation patterns in neutral beam injected L-mode plasmas on KSTAR

“…is the intrinsic phase velocity of the turbulence in the E Â B flow frame. [7][8][9][13][14][15][16] This equation originates from a spectral shift model (x ? ¼ k ?…”

“…[13][14][15] Although the equation was originally introduced to explain the discrepancy in the mode frequency (observed versus calculated), it has also been utilized to determine the E Â B flow velocities from turbulence rotation measurements with Doppler reflectometry. [7][8][9]16 If the E Â B flow velocity dominates, the measured turbulence rotation velocity can be a good approximation of the E Â B flow velocity. 7,8,16 Direct measurement of the intrinsic phase velocity is not trivial since the E Â B flow velocity is nonzero in most plasmas.…”

“…This method provides the radial electric field profile over a wide radial range so that one can evaluate the E Â B flow shear. The second method is based on the measurement of the rotation velocity of density fluctuations in the direction perpendicular to the magnetic field with Doppler reflectometry, [7][8][9] and then, the intrinsic turbulence phase velocity estimated from simulations is subtracted from the measured rotation velocity. When the measured rotation velocity of fluctuation is sufficiently faster than the intrinsic turbulence phase velocity, it is close to the E Â B flow velocity.…”