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  • A world in itself

    From a height of some 50 metres, you have the entire ITER worksite at your feet. The long rectangle of the Diagnostics Building stands out in the centre, with [...]

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  • US completes toroidal field deliveries for ITER

    The US Domestic Agency achieved a major milestone in February by completing the delivery of all US-supplied toroidal field conductor to the European toroidal fi [...]

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  • Thin diagnostic coils to be fitted into giant magnets

    Last week was marked by the first delivery of diagnostic components—Continuous External Rogowski (CER) coils—from the European Domestic Agency to the ITER Organ [...]

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  • Addressing the challenge of plasma disruptions

    Plasma disruptions are fast events in tokamak plasmas that lead to the complete loss of the thermal and magnetic energy stored in the plasma. The plasma control [...]

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  • Blending (almost) seamlessly into the landscape

    Located in the foothills of the French Pre-Alps, the ITER installation blends almost seamlessly into the landscape. The architects' choice ofmirror-like steel c [...]

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Of Interest

See archived articles

An eye on the plasma edge

-Robert Arnoux

In terms of temperature and physical phenomena, the outer region of the plasma is ''very similar to the Sun's corona,'' says Robin Barnsley. © Soho-Nasa/Esa (Click to view larger version...)
In terms of temperature and physical phenomena, the outer region of the plasma is ''very similar to the Sun's corona,'' says Robin Barnsley. © Soho-Nasa/Esa
Impurities, in the form of particles detached from the machine components, will always find their way into a burning plasma. "They are unavoidable," explains Robin Barnsley, ITER Diagnostic Division Responsible Officer for spectroscopy. "It is important however, to detect and monitor them. Beyond certain limits, impurities would dilute the fusion fuel and degrade the reaction."

Impurities have different origins. Some are "expected," like the beryllium, tungsten, iron or carbon particles that come from the plasma-facing components of the machine. Others, like oxygen, copper or other metal particles, can originate from leaks or damage affecting other in-vessel components.

Because of the extremely high temperatures of the plasma, confined particles radiate light at specific wavelengths. They all have a "signature" that is both specific to their nature—beryllium will send a signal that is different from tungsten or copper—and to the temperature they are submitted to.

Identifying these signatures, and hence the nature and temperature of the particles, is the role of spectroscopy.

In ITER, several spectrometers will monitor different regions of the plasma in order to measure radiation and identify all relevant impurities. "Because of the very high temperature gradient in the plasma," explains Robin Barnsley, "light is emitted over the whole spectrum, from infrared,visible to ultra-violet and x-ray. No single spectrometer could survey the whole plasma in all those different wavelengths."

The Vacuum Ultra-Violet (VUV) Edge Imaging Spectrometer whose Procurement Arrangement (PA) was recently signed by ITER Director-General and the Head of the Korean Domestic Agency, is one of subsystems that, together, will monitor the ITER core, edge and divertor regions of the plasma. It is the first PA to be signed for spectrometry diagnostic equipment.

The VUV Edge Imaging Spectrometer will be looking at a region located at the upper edge of the D-shaped plasma which is typical of the outer 10% of its total volume. (Click to view larger version...)
The VUV Edge Imaging Spectrometer will be looking at a region located at the upper edge of the D-shaped plasma which is typical of the outer 10% of its total volume.
The VUV Edge Imaging Spectrometer , whose prototype is already under development and will be ready in two years, will be looking at a region located at the upper edge of the D-shaped plasma which is typical of the outer 10 percent of its total volume.

This is not the hottest part of the plasma: in terms of temperature (around one million degrees Celsius) and physical phenomena, this region is "very similar to the Sun's corona," says Robin Barnsley, who did his PhD in an X-ray astronomy group. "The physics and hence the instrumentation used for observation are closely related."

Data gathered in real-time by the VUV Edge Imaging Spectrometer, along with that of other spectrometry devices observing other regions of the plasma, will enable the machine operators to adjust the plasma's parameters for optimal performance.


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