The largest lenses ever produced for a fusion research facility are being used to measure plasma density and temperatures up to 23 million degrees C at 130 different locations inside the MegaAmp Spherical Tokomak (MAST) experiment at the Culham Centre for Fusion Energy (CCFE) in Oxfordshire. Supplied by Fife-based Spanoptic, the six giant 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.
Manufacturing the six collection lenses presented significant technical challenges, not least the size – at up to 450mm in diameter, producing durable, radiation-hardened optics that will retain their performance in the extreme conditions close to the tokamak was another exacting requirement.
'Finding a supplier who could deliver the job was not easy!' says Martin Dunstan of Culham Centre for Fusion Energy. 'The specifications for the lenses were extremely demanding, as high-precision optics are essential to gather the data we’re looking for. The new system is working well and we're continually amazed by the results we've been getting so far.'
Richard Green of Spanoptic added: '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. Our continued investment in the most modern optical processing equipment and a highly skilled workforce allows us to support the most demanding optical projects.'