Observations of 2D Doppler backscattering on MAST

TL;DR

This paper demonstrates the feasibility of 2D Doppler backscattering on MAST using the SAMI diagnostic, enabling detailed plasma edge measurements, including turbulence velocity, magnetic pitch angle, and phenomena like neutral beam injection effects.

Contribution

It introduces a novel 2D DBS technique with a steerable phased array, providing new capabilities for plasma edge diagnostics on MAST.

Findings

Initial measurements of turbulence and momentum injection are consistent with conventional DBS.

The 2D imaging allows measurement of magnetic pitch angle.

Limitations of beam steering techniques are discussed.

Abstract

The Synthetic Aperture Microwave Imaging (SAMI) diagnostic has conducted proof-of-principle 2D Doppler backscattering (DBS) experiments on MAST. SAMI actively probes the plasma edge using a wide (+-40 degrees vertical and horizontal) and tuneable (10-35.5 GHz) beam. The Doppler backscattered signal is digitised in vector form using an array of eight Vivaldi PCB antennas. This allows the receiving array to be focused in any direction within the field of view simultaneously to an angular range of 6-24 degrees FWHM at 10-34.5 GHz. This capability is unique to SAMI and is an entirely novel way of conducting DBS experiments. In this paper the feasibility of conducting 2D DBS experiments is explored. Initial measurements of phenomena observed on conventional DBS experiments are presented; such as momentum injection from neutral beams and an abrupt change in power and turbulence velocity coinciding with the onset of H-mode. In addition, being able to carry out 2D DBS imaging allows a measurement of magnetic pitch angle to be made; preliminary results are presented. Capabilities gained through steering a beam using a phased array and the limitations of this technique are discussed.