The MAST experiment, located at Culham Centre for Fusion Energy (CCFE), now has the world’s most advanced system for recording the plasma temperature and density profiles.

Figure 1: The MAST Thomson scattering diagnostics system. Eight Nd:YAG lasers follow a beam path through the plasma (yellow).”. A large lens (orange) collects light coming from the laser. Depending on their exact location along the laser beam, the light beams reaching the lens are directed into individual optical fibre. That way data from 130 separate locations at a resolution of only 10 mm can be recorded. The system employs an additional lens (green) to measure at the plasma edge. (Picture: CCFE)

Thomson scattering is used to obtain local measurements of electron temperature and density inside the hot plasma – which can reach over 20 million °C in MAST – by measuring the scattering of light from laser beams fired into the plasma. This is done by imaging the laser beam in the plasma into optical fibres via a large collection lens. (Figure 1). The fibres transfer the light from each spatial location to a respective spectrometer. While the previous system imaged 30 – 50 mm of laser light into one spectrometer, the large lens of the upgraded system collects enough light to image 10 mm of laser light per spectrometer, leading to a higher resolution. With this new diagnostic, MAST MAST’s Thomson scattering diagnostics has been upgraded from four to eight lasers (Photo: CCFE) measures at 130 spatial points using 130 spectrometers. The diagnostic can now perform high resolution measurements over the whole radius of the plasma, while previously this resolution could only be obtained at the outer edge of MAST.

The new collection lens at MAST’s Thomson scattering diagnostics weighs 100 kg and is the largest of its type in any fusion experiment (Photo: CCFE)

One of the system’s primary goals is to measure the size and structure of magnetic islands that affect the confinement of the plasma and reduce the fusion energy output. This can now be achieved at high resolution and thus detailed profiles of the evolution of many plasma phenomena have already been recorded. The upgrade has also improved the temporal resolution by increasing the number of lasers used from four to eight, effectively doubling the number of time points in a measurement burst during a MAST plasma pulse. A ‘smart’ triggering device can now synchronise the lasers to the exact time of specific ‘events’ during the pulse, such as the formation of the plasma or the injection of fuel pellets. Researchers from the University of York’s Plasma Physics and Fusion Group, in collaboration with CCFE, will exploit the upgraded system to confirm the theoretical principles of plasma behaviour. They are now able to run experiments on MAST directly from York, using a new remote control room recently installed at the university. A better understanding of the processes occuring in plasmas will help to improve the performance of future fusion devices such as ITER. Dr Mike Walsh, who was the CCFE project leader and who has now moved to the ITER Organization to become Head of the Diagnostics Division, explained how the MAST Thomson scattering diagnostic will give researchers an extremely detailed view of the evolution of the plasma:

We expect the system to throw up new physics and allow us to observe effects we have never been able to see in plasmas before.

The £2 million upgrade, jointly funded by the UK Engineering and Physical Sciences Research Council, University of York and the Northern Way collaboration of Regional Development Agencies, was completed in September 2009 and the diagnostic is already providing data that exceeds its design specifications.

Jennifer Hay and Rory Scannell,CCFE