Large Lenses Enable Fusion Research

The largest lenses ever produced for a fusion research facility are being used to measure plasma density and temperatures up to 23 million ºC inside the MegaAmp Spherical Tokamak (MAST) experiment at the Culham Centre for Fusion Energy (CCFE). Supplied by Spanoptic Ltd. of Glenrothes, Scotland, the six lenses collect light scattered from laser beams fired into the tokamak, a technique known as Thomson scattering.

Measuring plasma properties such as temperature and density is one of the most challenging aspects of fusion research. The Thomson scattering diagnostic is based on measuring electron velocity distribution from the Doppler shift of light scattered by the electrons. This scattering process has a very small cross section, so, to get enough scattered photons, a very intense, short-pulse laser light source and large lenses are needed.

Collection lens on MegaAmp Spherical Tokamak (MAST) supplied by Spanoptic Ltd. (Copyright: Monty Rakusen)

Completed in September 2009, MAST’s Thomson scattering laser diagnostic is the world’s leading system of its type, generating 30,000 measurements during each plasma pulse, to give UK physicists unprecedented insights into the behavior of fusion plasmas. The upgrade was partly funded by the University of York and the Northern Way collaboration of regional development agencies.

Manufacturing the six collection lenses presented significant technical challenges, not least the size – at up to 450 mm in diameter. Producing durable, radiation-hardened optics that will retain their performance in the extreme conditions close to the tokamak was another exacting requirement.

“The specifications for the lenses were extremely demanding, as high-precision optics are essential to gather the data we’re looking for,” said Martin Dunstan of CCFE. “The new system is working well and we’re continually amazed by the results we’ve been getting so far.”

“The lenses are some of the largest optics manufactured by Spanoptic; the manufacturing process involved a combination of modern CNC machining, traditional manufacturing and inspection techniques,” said Richard Green of Spanoptic.

For more information, visit: www.ccfe.ac.uk