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Details of Grant 

EPSRC Reference: EP/S018867/1
Title: SAMI-2: two-dimensional Doppler imaging of tokamak plasmas
Principal Investigator: Vann, Professor RG
Other Investigators:
Bale, Dr SJ
Researcher Co-Investigators:
Project Partners:
Durham, University of
Department: Physics
Organisation: University of York
Scheme: Standard Research
Starts: 01 December 2018 Ends: 30 November 2019 Value (£): 213,057
EPSRC Research Topic Classifications:
Plasmas - Laser & Fusion RF & Microwave Technology
EPSRC Industrial Sector Classifications:
R&D Energy
Related Grants:
Panel History:
Panel DatePanel NameOutcome
24 Oct 2018 EPSRC Physical Sciences - October 2018 Announced
Summary on Grant Application Form
Fusion is the process that powers our Sun and indeed all stars. If it could be successfully harnessed on Earth, it would provide a safe, plentiful and carbon-neutral supply of electricity. Terrestrial fusion experiments known as tokamaks confine the plasma (ionised gas) fuel using magnetic fields. It is known that the electric current density in the very outer edge of the plasma critically determines the tokamak's performance & stability, yet this quantity cannot yet be routinely measured. This knowledge gap is particularly important when one considers that current experiments are being used to extrapolate to the design and performance of future reactors.

The objective of the research proposed here is to build from scratch a novel microwave diagnostic, to be known as SAMI-2, that can make routine measurements of the electric current density in the edge of a tokamak plasma. This is challenging because the layer in which the current is carried is thin and the plasma is hot (typically 10 million degrees).

SAMI-2 works by illuminating the plasma surface with a wide-angled microwave beam. The plasma surface is corrugated parallel to the magnetic field because plasma travels much faster along magnetic field lines than across them. The illuminating beam, whose wavelength is comparable to the distance between the corrugations (approx. 1cm), is scattered back towards SAMI-2 preferentially perpendicular to the magnetic field according according to a well-understood condition known as Bragg's Law. Because the plasma is rotating, this back-scattered signal is Doppler shifted i.e. the frequency is shifted above or below the frequency of the illuminating beam depending whether the plasma is rotating towards or away from the diagnostic, respectively. Scanning at frequencies spaced by a few gigahertz corresponds to picking back-scattering surfaces that are a few millimetres apart. If we can resolve the origin of the peaks in the back-scattered signal, then we can deduce the orientation of the magnetic field; if we do this at two locations (corresponding to two frequencies of scanning beam), then we can calculate the edge current density from Ampère's Law.

SAMI-2 images the back-scattered microwaves using an array of 32 receiving antennas. The time taken for the signal to reach each antenna depends on the distance between the antenna and the source; by measuring the time difference (technically, phase difference) between the signals at each pair of antennas ("baseline"), we can reconstruct the emission pattern. (This is the same principle by which surround sound films are recorded using multiple microphones positioned around the subject of interest. The time delay to different microphones depends on the distance of the source to each microphone; the sensation of a source at a particular location is recreated when these phase-delayed signals are played back through loudspeakers.)

We demonstrated the feasibility of this imaging methodology for the first time with the Synthetic Aperture Microwave Imager (SAMI), supported 2009-11 by EPSRC. However SAMI was not originally designed for Doppler back-scattering and could not measure the magnetic field direction with sufficient accuracy to derive the current density. In contrast, SAMI-2 is specifically designed for 2-D Doppler backscattering and shares hardly a component in common with the original SAMI. SAMI-2 will use the ingenious sinuous antenna type which can measure both horizontally and vertically polarised microwaves; it has 32 antennas (four times as many as SAMI, increasing number of baselines from 28 to 496); it will image at two frequencies simultaneously. SAMI-2's antenna array and data transmission method are technologically interesting in their own right.

SAMI-2 will be deployed at the UK's MAST-U tokamak at the Culham Centre for Fusion Energy, the UK's national fusion laboratory, in time for MAST-U's first experimental campaign in 2019.
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Organisation Website: http://www.york.ac.uk