In July 2010, the Hungarian Association KFKI-RMKI and CCFE completed the installation of the Beam Emission Spectroscopy (BES) diagnostics in the MAST tokamak. The BES diagnostics system measures plasma turbulence by detecting light emitted by the neutral heating beams. Thanks to a new, highly sensitive detector as well as extensive optical systems both inside and outside the vessel, the system provides an extraordinary data quality. It is the first 2-dimensiona turbulence BES diagnostics built in Europe. The project received preferential EFDA support as it is expected to provide important data which will enable the improved modelling of heat and particle loss during turbulence. The diagnostics will also enhance the understanding of the internal transport barrier and other turbulence phenomena in the spherical tokamak MAST.

Neutral beam heating systems inject fast atoms into the plasma, extending deep into its core. At MAST, these beams measure 10-20 centimetres in diameter. The atoms of the beam are excited – primarily by collisions with the plasma ions – and emit photons with a characteristic frequency. This light is registered by the BES diagnostics. The variations in intensity reflect the local plasma density fluctuations, thus enabling the plasma turbulence to be measured. The challenge lies in the very low light levels that require extremely sensitive detectors as well as relatively big optical elements. A trial BES diagnostics system was installed on MAST in 2006 by KFKI RMKI and CCFE. It has successfully demonstrated the detector technology, but its light level was not sufficient to resolve the small amplitude turbulences in the core plasma. The system has been in operation since 2006 and will, in future, be used alongside the new installation.

The new BES diagnostics is expected to collect around 100 times more light and thus dramatically enhance the data quality. It features a high efficiency detector unit, developed and built by Adimtech, a spin-off company of RMKI. The low noise, high-frequency camera is a standalone unit and incorporates all of the necessary features, including temperature stabilisation via an integrated cooler, internal calibration, signal digitising, high voltage generators and an optical Ethernet interface. The detector has also proved to be an attractive proposition for other applications that require highfrequency measurements at low light levels. The size of the optical elements, the first objective lens in the vacuum chamber measures 14 centimetres in diameter, posed a great challenge in terms of mechanical design, due to the limited space available inside the vessel. Another issue was choosing an observation point that permitted the monitoring of turbulent eddies. These are a couple of centimetres in diameter and form metre-long flute-like structures that follow the magnetic field lines. In order to be able to observe them, you need to look along field lines and the first optical elements should be both large in size and close to the beam. The optics were designed by CCFE and placed in the MAST vacuum chamber close to the plasma edge. They needed to be mounted, protected against the harsh environment, but at the same time they also needed to enable the rotation of the first mirror in order to take measurements along the beam in various radii. These requirements were solved by a complex but sound mechanical system which was designed, built and tested at the Hungarian Association in close collaboration with CCFE. An important element of the mechanism is a helmet formed rotatable shutter that protects the optics when not in use. Changing the observation direction – the 2D diagnostics measures in 4*8 spatial positions in the radial poloidal plane – requires the adjustment of various other optical elements, thus the four moving elements of the resulting system are controlled by computer. KFKI-RMKI has already installed a BES diagnostics in TEXTOR and JET and is currently constructing one for COMPASS. A similar detector unit will be installed at JET at the beginning of 2011.

Happy faces all round after the successful installation of the BES external optical hardware at MAST (in the background). From left to right: Tibor Krizsanóczi, Sándor Zoletnik, István Gábor Kiss, Anthony Field, Dániel Dunai, all from KFKI-RKMI, Anthony Field, from CCFE. (Photo: KFKI-RKMI)

Thanks to István Gábor Kiss at KFKI-RMKI for his input