Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Wanted: the right wall material for ITER

12.10.2007
ASDEX Upgrade at Max Planck Institute of Plasma Physics (IPP) in Garching, Germany, recently became the world's first and only device allowing experiments with a wall completely clad with metal, viz. tungsten. The results are highly promising: Tungsten as wall material could also afford an attractive solution for the ITER international fusion experiment.

The objective of IPP is to develop a power plant that, like the sun, derives energy from fusion of atomic nuclei. Its feasibility is to be demonstrated by the ITER (Latin for "the way") international experimental reactor with a fusion power of 500 megawatts. Construction will start next year at Cadarache in France. The aim here is to confine the fuel – an ionised low-density hydrogen gas, a "plasma" – in a magnetic field cage without letting it touch the wall and then heat it to ignition temperatures of over 100 million degrees. One of the major challenges involved is to achieve tolerable interaction between the hot plasma and the wall of the enclosing vessel.

The problem
The problem here is that high-energy plasma particles can dislodge atoms from the wall, which then penetrate the plasma and contaminate it. Unlike the light hydrogen, the heavy atoms from the wall are not completely ionised, even at the high fusion temperatures needed. The more electrons that are still bound to the atomic nuclei, the more energy they extract from plasma, emitting it then as ultraviolet or X-ray light. In this way they cool the plasma and rarefy it, thus reducing the fusion yield. Whereas light impurities in concentrations of a few per cent are still tolerable, the limit for heavy impurities such as iron or chromium are much lower. Present-day devices therefore all use light materials for the wall, such as beryllium or carbon. These two are also envisaged for the wall of the ITER test reactor.

For ITER, however, beryllium and carbon are no longer without problems: Sputtering of these when bombarded with hydrogen is relatively high. The high hydrogen fluxes from the large ITER plasma would therefore cause severe material erosion. Furthermore, hydrogen particles readily accumulate in carbon, and in ITER so also does its radioactive version, tritium, this being highly undesirable for safety reasons. A wall completely clad with tungsten would obviate these problems with light elements: Tungsten affords advantageous thermal properties, low sputtering caused by hydrogen, and no long-term accumulation of tritium. This leaves the critical question how many of the heavy tungsten particles can penetrate to the core of the plasma. Recent estimates for ITER indicate that it may not be more than a few hundred thousandths.

Tungsten experiments at IPP
The pioneer in testing tungsten as wall material is the ASDEX Upgrade experiment at Garching.

Despite bad experience in other laboratories, IPP started in 1996 to apply tungsten to special areas of the wall otherwise completely clad with carbon tiles. This relied on the differently chosen, ITER-like, i.e. cold plasma edge of ASDEX Upgrade. The positive result prompted a further reduction of carbon. The aim was to check how this affected the plasma and its interaction with the tungsten components. In order not to jeopardise other research objectives, the tungsten surface was only successively enlarged. Reliably determining the particular tungsten concentration amassing in the plasma is not a simple matter, the less so when the emission losses are attributable not only to a single impurity. Once the necessary measuring methods were developed it was found, however, that even an extensive tungsten surface does not unduly affect the plasma of ASDEX Upgrade.

It remains to prove that even complete metal cladding of the vessel is compatible with the favourable plasma states wanted for ITER – such as the high-confinement regime developed at IPP. After the last carbon tiles had been replaced and all surfaces carefully cleaned, experiments were recently resumed with a purely tungsten wall. To ensure clean experimental conditions no resort was made to the otherwise standard pretreatment of the vessel with boron. In order to reduce losses due to impurity radiation, this process serves to coat the wall surfaces with a thin layer of boron by means of a glow discharge in a boron hydrogen gas. However, in ITER or a subsequent power plant this will no longer be possible.

ASDEX Upgrade has therefore also started without boronisation – and was successful: The tungsten concentration is below the critical threshhold and the desired favourable plasma states can be achieved with only slight loss of quality. Further investigation will aim at exactly checking the tungsten compatibility in ITER-relevant plasma states. The decisive issue will be whether permanently "good" high-confinement plasmas can be achieved without boronisation. For this work IPP has about two years – before the decision on the interior wall of ITER is taken.

Isabella Milch | alfa
Further information:
http://www.ipp.mpg.de/eng/index.html

More articles from Physics and Astronomy:

nachricht Temperature-controlled fiber-optic light source with liquid core
20.06.2018 | Leibniz-Institut für Photonische Technologien e. V.

nachricht New material for splitting water
19.06.2018 | American Institute of Physics

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Temperature-controlled fiber-optic light source with liquid core

In a recent publication in the renowned journal Optica, scientists of Leibniz-Institute of Photonic Technology (Leibniz IPHT) in Jena showed that they can accurately control the optical properties of liquid-core fiber lasers and therefore their spectral band width by temperature and pressure tuning.

Already last year, the researchers provided experimental proof of a new dynamic of hybrid solitons– temporally and spectrally stationary light waves resulting...

Im Focus: Overdosing on Calcium

Nano crystals impact stem cell fate during bone formation

Scientists from the University of Freiburg and the University of Basel identified a master regulator for bone regeneration. Prasad Shastri, Professor of...

Im Focus: AchemAsia 2019 will take place in Shanghai

Moving into its fourth decade, AchemAsia is setting out for new horizons: The International Expo and Innovation Forum for Sustainable Chemical Production will take place from 21-23 May 2019 in Shanghai, China. With an updated event profile, the eleventh edition focusses on topics that are especially relevant for the Chinese process industry, putting a strong emphasis on sustainability and innovation.

Founded in 1989 as a spin-off of ACHEMA to cater to the needs of China’s then developing industry, AchemAsia has since grown into a platform where the latest...

Im Focus: First real-time test of Li-Fi utilization for the industrial Internet of Things

The BMBF-funded OWICELLS project was successfully completed with a final presentation at the BMW plant in Munich. The presentation demonstrated a Li-Fi communication with a mobile robot, while the robot carried out usual production processes (welding, moving and testing parts) in a 5x5m² production cell. The robust, optical wireless transmission is based on spatial diversity; in other words, data is sent and received simultaneously by several LEDs and several photodiodes. The system can transmit data at more than 100 Mbit/s and five milliseconds latency.

Modern production technologies in the automobile industry must become more flexible in order to fulfil individual customer requirements.

Im Focus: Sharp images with flexible fibers

An international team of scientists has discovered a new way to transfer image information through multimodal fibers with almost no distortion - even if the fiber is bent. The results of the study, to which scientist from the Leibniz-Institute of Photonic Technology Jena (Leibniz IPHT) contributed, were published on 6thJune in the highly-cited journal Physical Review Letters.

Endoscopes allow doctors to see into a patient’s body like through a keyhole. Typically, the images are transmitted via a bundle of several hundreds of optical...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

Munich conference on asteroid detection, tracking and defense

13.06.2018 | Event News

2nd International Baltic Earth Conference in Denmark: “The Baltic Sea region in Transition”

08.06.2018 | Event News

ISEKI_Food 2018: Conference with Holistic View of Food Production

05.06.2018 | Event News

 
Latest News

Creating a new composite fuel for new-generation fast reactors

20.06.2018 | Materials Sciences

Game-changing finding pushes 3D-printing to the molecular limit

20.06.2018 | Materials Sciences

Could this material enable autonomous vehicles to come to market sooner?

20.06.2018 | Materials Sciences

VideoLinks
Science & Research
Overview of more VideoLinks >>>