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 fo = c/λo, 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 fD = u⊥k⊥/2π = u⊥ 2sinθt/λo. The incident angle θt 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 fD of up to 5 MHz. The measured perpendicular velocity is u⊥ = vE × B + vphase, 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 Er profile.