Located at the Culham Centre for Fusion Energy (CCFE) near Oxford, the Joint European Torus (JET) project is investigating the potential of exploiting nuclear fusion – the process that powers the Sun and all stars – as a safe, clean, and virtually limitless energy source for future generations. JET is a 'tokamak' device – effectively a magnetic bottle in which fusion fuel composed of different types of hydrogen are heated in a plasma. At temperatures of over 100 millionoC, the nuclei of the atoms combine and release large amounts of energy, which would be used to generate electricity in a fusion power station.

"JET is the largest tokamak in the world and is also the only fusion device capable of producing fusion energy from the two optimum fuels – deuterium and tritium," said Neil Conway, Head of the Diagnostics Unit at CCFE.

In collaboration with fusion researchers from more than 40 European laboratories, the team at JET is currently preparing for a new set of deuterium-tritium fusion tests that Conway reveals will act as a 'key technical rehearsal' for the ITER international fusion project currently being constructed in France.

Exterior of the Joint European Torus

ITER will be twice the size of JET and aims to prove the viability of fusion on an industrial scale - 500MW power output – paving the way for the first power stations by 2050. As one of the main test machines for ITER, Conway also points out that the deuterium-tritium (DT) experiments at JET will provide 'crucial risk mitigation' and give researchers a 'wealth of useful data with which to plan the ITER physics programme.'

Diagnostic technology

In carrying out these experiments, CCFE uses a wide range of diagnostic optics technology for a variety of purposes, including what Conway refers to as 'plasma diagnostics' - instrumentation that helps the JET team to measure the properties and behaviour of the plasma in which the fusion fuel is heated.

Infrared camera view of plasma inside JET, indicating heat loads on the wall materials

"Most of our optical plasma diagnostics are spectrally resolved, either by using grating spectrometers or interference filters to select a range of wavelengths. Some operate by measuring the absolute or relative intensity of light, for example to provide a measurement of the plasma purity," he said.

"Others look at the shape of the spectrum of the plasma light to make their measurements. For example there are many spectral lines emitted by ions within the plasmas – by looking at the line shape with high resolution spectrometers we can measure the Doppler shift and broadening of the line caused by motion of the ions and from this deduce the velocity and temperature of the ions in the plasma," he added.

In addition, JET uses diagnostic cameras to take pictures of the plasma. Some of these run at roughly TV rate of 25 frames per second while others are designed to study high-frequency instabilities in the plasma and run at thousands, or even hundreds of thousands, of frames per second.

"In the main these aren't hugely exotic, although in a few cases the optical elements can be relatively large," said Conway.

"Some optical diagnostics can be mounted directly to the machine, while others need to use mirrors or optical fibres to relay light beyond the thick shielding wall around the machine, which is there to prevent neutrons and other radiation from reaching sensitive instrumentation - and of course people," he added.

Reflective optics

In mid-March, Surrey (UK)-based company Optical Surfaces also won a contract to supply CCFE with state-of-the-art relay imaging reflective optics that will form an important element of two viewing diagnostic assemblies used by JET project staff when using the deuterium-tritium fuel mixture. According to the company, the order will comprise of a series of 'ultra-smooth' flat and spherical mirrors with very large diameters up to 500mm, some of which will have a 'very long radius of curvature.'

The technology will play a key role in enabling the JET team to keep a close eye on a variety of physical and operational parameters within the JET fusion vessel as staff operate remote cameras from behind a 3m thick concrete shield wall. This is especially true because such reflective optics-based relay imaging is more resilient to the hostile conditions they are likely to face in the near future.

"JET normally operates with deuterium plasmas, from which the neutron radiation is much lower than when the 50-50 DT mixture, which is optimal for fusion yield, is used. However, DT operation is next planned a few years from now and most of the optical diagnostics within the machine area are not expected to survive for long in the intense neutron flux," said Conway.

Looking ahead, Conway also argues that JET has already proved that the tokamak is a 'viable design for future fusion reactors and continues to be the cornerstone of the European fusion research plan.'

"The work being done at JET will eventually bear fruit in commercial reactors which could account for a major part of the world energy market from the second half of this century onwards," he added.

Written by Andrew Williams, Contributing Editor, Novus Light Technologies Today

Images are courtesy EUROfusion / Culham Centre for Fusion Energy.