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Fusion looks to optics for plasma diagnosis

24 Jan 2007

In 10 years, ITER will be a major experimental facility studying the feasibility of fusion power. Jacqueline Hewett visits JET – Europe’s flagship fusion experiment – to discover the crucial role that optical technologies are playing in the development of the new facility.

Plasma physics may be unfamiliar territory for the photonics community but as work on the international fusion facility ITER steps up a gear, unique opportunities are emerging. Optical diagnostics will play a crucial role in monitoring the plasma, and the list of required components and technologies is extensive.

The aim of the ITER experiment is to demonstrate the feasibility of fusion energy, and it is a stepping stone to the first electricity-generating power station based on the magnetic confinement of high-temperature plasma. The organization that oversees ITER was officially established last year, when an agreement was signed by ministers from the seven countries involved, on 21 November – a landmark day for ITER.

ITER will be constructed in Cadarache, France, at an estimated cost of €5 bn. The project’s timeline anticipates that the assembly of the central tokamak structure will begin in 2012, with the first plasma being produced in 2016. Although on paper it looks like a long timescale, the specialist technologies required for ITER are still being researched by experts at existing fusion facilities to be ready for installation in 2016.

Where does optics fit in?

The UKAEA’s Culham Science Centre, UK, houses what is currently the world’s largest fusion experiment – the Joint European Torus (JET). UKAEA operates JET for experiments by teams of scientists from all over Europe. At JET, a gas containing deuterium and tritium is heated to temperatures of around 150 million °C (about 10 times hotter that the centre of the Sun) and becomes a plasma. At these temperatures, the isotopes are fully ionized and are contained in a torus-shaped vacuum vessel, known as a tokamak, using a magnetic confinement system.

Fusion relies on collisions of energetic deuterium and tritium ions producing neutrons (which are absorbed outside the plasma, producing heat for electricity generators in the case of a power plant) and alpha particles, which are trapped and slowed down in the plasma, keeping it hot enough for the deuterons and tritons to continue to fuse.

Measuring plasma properties, such as temperature and density, is one of the most challenging aspects of fusion research and this is where optical technologies come into their own.

“The alpha particles heat the electrons in the plasma primarily, and they in turn heat the deuterons and tritons, so it is vital to know the electron temperature,” explained William Morris, manager of UKAEA’s experiments department, which is responsible for diagnostic measurements on JET. “The rate of the reaction depends on the density of the deuterons and tritons. In a clean plasma, this density is equal to the electron density, so must be measured and controlled.”

In the JET plasma, the temperature of the electrons can range from 2 million °C near the edge to over 100 million °C in the centre. Diagnostic systems based on Thomson scattering and LIDAR principles measure the electron temperature and density profiles and their time-evolution.

“The basic principle is to measure the electron velocity distribution from the Doppler shift of light scattered by the electrons,” said Morris. “This scattering process has a very small cross-section, so to get enough scattered photons a very intense light source, such as a short-pulse laser, is generally required to maximize the signal-to-noise.”

Morris explains that the short pulse length also leads to the use of fast detectors or fast image-intensifiers with a high quantum efficiency that allows the background light to be gated out. He adds that the measurement of the scattered spectrum also includes relativistic corrections.

JET uses two diagnostic systems based on Thomson scattering and LIDAR to determine the plasma electron temperature and density: the core system looks at the bulk of the plasma, while the divertor system studies the edge of the plasma.

The diagnostic systems

The core system uses 0.3 ns pulses from a 1 J ruby laser operating at 694 nm and a repetition rate of 4 Hz, which are fired across the plasma diameter. The detection system consists of six microchannel plate photomultipliers (rise time 0.3 ns), each connected to a fast storage oscilloscope (1 GHz bandwidth). The scattered spectrum is dispersed into the six detection channels using a set of dielectric edge filters producing a high-throughput spectrometer.

The detection system captures the changes in the back-scattered spectrum from the fast-moving plasma electrons. By analysing these changes and knowing the time-of-flight of the pulse, the temperature and density variations across the whole diameter of the plasma are obtained. The divertor system operates on the same principle but has a 3 J laser with a repetition rate of 1 Hz and uses four photomultipliers and detection channels.

Having demonstrated the capability and convenience of a Thomson scattering/LIDAR system on JET, the researchers involved believe it is an excellent candidate for ITER. There are formidable design challenges, however, in deploying this diagnostic system; not least that the maximum electron temperature expected in ITER may exceed 400 million °C, and so give a very broad scattered spectrum.

UKAEA is leading a group of fusion laboratories designing the LIDAR system for ITER. The team has produced a list (see box) of the technologies that are required for this diagnostic, and for another in which it expects to participate. Any interested parties should contact Dan Mistry, fusion and industry manager, tel. +44 (0) 1235 466607, e-mail dan.mistry@ukaea.org.uk.

For more information see: www.fusion.org.uk, www.jet.efda.org and www.iter.org.

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