S. Freethy

Lensless passive and active microwave imaging on MAST

A novel microwave imaging system (SAMI) which utilizes an array of phase sensitive antennas to synthesize an optical aperture has been designed and deployed on MAST. The system requires no optical components for focusing and yet is able to image up to half of the plasma surface simultaneously. All image formation is done in post processing and SAMI can be refocused after the fact. SAMI is capable of imaging both passive thermal emission and scattered radiation from an active probing source simultaneously without compromising performance. We have used the diagnostic to observe highly anisotropic thermal emission characteristic of high β plasma devices which are overdense to regular ECE emission. This is an exciting emerging diagnostic field and we present our first observations here.

Two-dimensional studies of electron Bernstein Wave Emission in MAST

Abstract Angular scanning of electron Bernstein wave emission (EBE) has been conducted in MAST. From EBE measurements over a range of viewing angles, the angular position and orientation of the B-X-O mode conversion (MC) window can be estimated, giving the pitch angle of the magnetic field in the MC layer. The radial position of the corresponding MC layer is found from Thomson scattering measurements. Measurements at several frequencies can provide a pitch angle profile. Results of pitch angle profile reconstruction from EBE measurements are presented in comparison with motional Stark effect measurements. Microwave imaging of the B-X-O MC window is proposed as an alternative to angular scanning. The proposed scheme is based on an imaging phased array of antennas allowing the required angular resolution. Image acquisition time is much shorter than magnetohydrodynamic (MHD) time scales so the EBE imaging can be used for pitch angle measurements even in the presence of MHD activity.

Electron Kinetics Inferred from Observations of Microwave Bursts During Edge Localized Modes in the Mega-Amp Spherical Tokamak

Recent measurements of microwave and x-ray emission during edge localized mode (ELM) activity in tokamak plasmas provide a fresh perspective on ELM physics. It is evident that electron kinetics, which are not incorporated in standard (fluid) models for the instability that drives ELMs, play a key role in the new observations. These effects should be included in future models for ELMs and the ELM cycle. The observed radiative effects paradoxically imply acceleration of electrons parallel to the magnetic field combined with rapid acquisition of perpendicular momentum. It is shown that this paradox can be resolved by the action of the anomalous Doppler instability which enables fast collective radiative relaxation, in the perpendicular direction, of electrons accelerated in the parallel direction by inductive electric fields generated by the initial ELM instability.

2D Doppler backscattering using synthetic aperture microwave imaging of MAST edge plasmas

Doppler backscattering (DBS) is already established as a powerful diagnostic; its extension to 2D enables imaging of turbulence characteristics from an extended region of the cut-off surface. The Synthetic Aperture Microwave Imaging (SAMI) diagnostic has conducted proof-of-principle 2D DBS experiments of MAST edge plasma. SAMI actively probes the plasma edge using a wide (±40° vertical and horizontal) and tuneable (10–34.5 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° FWHM at 10–34.5 GHz. This capability is unique to SAMI and is a novel way of conducting DBS experiments. In this paper the feasibility of conducting 2D DBS experiments is explored. Initial observations of phenomena previously measured by 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.

Optimization of Wide Field Interferometric Arrays via Simulated Annealing of a Beam Efficiency Function

Optimization of antenna array configurations specifically designed to reduce side-lobes was carried out by using a simulated annealing algorithm to find a good approximation to the optimum of a modified array beam efficiency function. The optimization was carried out for the wide field synthetic aperture microwave imaging (SAMI) radiometer, which has the ability to switch between two configurations of eight antennas. This approach enables us to find configurations which simultaneously complement each other.

GPU-Based Data Processing for 2-D Microwave Imaging on MAST

Abstract The Synthetic Aperture Microwave Imaging (SAMI) diagnostic is a Mega Amp Spherical Tokamak (MAST) diagnostic based at Culham Centre for Fusion Energy. The acceleration of the SAMI diagnostic data-processing code by a graphics processing unit is presented, demonstrating acceleration of up to 60 times compared to the original IDL (Interactive Data Language) data-processing code. SAMI will now be capable of intershot processing allowing pseudo-real-time control so that adjustments and optimizations can be made between shots. Additionally, for the first time the analysis of many shots will be possible.