SAMI-2: two-dimensional Doppler imaging of tokamak plasmas

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.

The ultimate goal of fusion is to generate substantial quantities of safe, commercially-competitive, carbon-free electricity onto the grid without producing long-lived radioactive waste. Achieving this goal will benefit everyone on the planet. Great things are rightfully expected of ITER, the next generation tokamak, at $15bn the most expensive terrestrial science experiment, currently under construction in the South of France and expected to become operational in the mid-2020s. Some members of the magnetic fusion community are also interested in smaller, higher field devices that make particular use of high temperature superconductors. Whatever the pathway to commercial fusion, it will require predicting the performance of future devices on the basis of extrapolation from existing machines. It is known that the current density in the edge of fusion plasma fuel is a key factor in the stability and performance of tokamaks, yet this quantity is not currently directly measured. The SAMI-2 diagnostic proposed here will achieve this for the first time, and can therefore play a crucial role in guiding the operational expectations of future machines and ultimately the success of the global fusion programme.
Microwave diagnostics are particular well-suited to reactors since microwave antennas can be built of similar materials to the vessel wall and therefore withstand high heat and neutron loads. However, there are currently only few trained microwave fusion diagnosticians in the UK. The project proposed here will contribute to increasing the UK's capability in this important area.
SAMI-2 could be described as a phased-array wideband radar system. However, unlike a conventional radar, SAMI-2 looks in all directions at once! Instead of the radar beam being steered either mechanically or electronically, SAMI-2 is steered in software and consequently can simultaneously look in an arbitrary number of directions (limited only by the associated computational expense). Attendance at the European Microwave Conference in 2019 will provide an opportunity to advertise SAMI-2's capabilities to communities outside fusion research. Possible applications include higher performance weather radar in aeroplanes (reducing the number of moving parts required) and security scanning (using microwaves to penetrate the walls of soft-sided lorries).
If the transmitting antenna is switched off, them SAMI-2 images the microwave radiation in its surroundings. The most likely disruptive application for this capability is medical imaging. The microwave emission from human/animal tissue is a measure of the tissue temperature, which in turn is a proxy for tissue health or viability. The advantage of imaging microwaves rather than visible light is that the microwaves come from tissue up to 2cm below the skin's surface. This information can be used either as a diagnostic tool or to aid in surgery. This idea has already led to a patent published jointly by the SAMI project leader and Sylatech Ltd (patent no. PCT/GB2016/050648) and a Knowledge Transfer Partnership funded by InnovateUK to translate our fusion diagnostic techology into a medically-relevant device. Our objective is to commercialise an inexpensive portable medical imaging system that could be used in GP surgeries as well as hospitals to improve patient diagnosis and treatment.