in 1752, Scottish physicist Thomas Melvill passed some light through a prism and saw something quite unexpected. He was using light emitted by different substances in flames (e.g. salt) and saw, not the familiar rainbow colours of the spectrum, but thin bands of colour, with dark gaps between them. We now know that these patterns of colours are caused by electrons jumping between energy levels, and are unique to each element. The study of these light patterns, spectroscopy, is an extremely useful tool for identifying even tiny traces of elements. At JET there are dozens of different measurement systems set up to analyse the spectra given off by the plasma, some looking through the middle of the plasma, others at the edge; some at the visible wavelengths, others in the ultraviolet; some making measurements 100 000 times per second, while others collect light for longer to examine fainter emission.
The picture shows the spectrum of tungsten, rendered in knitwear. Although at JET this format is not used, this spectrum has suddenly become very important at JET, with the installation of the ITER-Like Wall. As part of this project tiles in the lower section of the vessel, known as the divertor, have had their carbon-fibre tiles entirely replaced with tungsten tiles. The plasma actually touches the divertor tiles, which leads to tungsten contaminating the plasma, but hopefully in a far less detrimental way than carbon, which was found to bind with the deuterium and tritium fuel all too readily.
Tungsten’s extremely high melting point (3410 degrees Celsius) is its attraction, however it has its drawbacks too. In plasma the more electrons an element has, the more it radiates light, in the process sapping energy from the plasma core – tungsten, with its 74 electrons, is a lot more of a problem than carbon, with only six.
“Tungsten has a very complex spectrum, because of all its electrons” says Dr Costanza Maggi, a physicist working on the spectroscopic systems at JET. ”We are learning how to connect the radiation from tungsten to the power being radiated from the core of the plasma. We also need to study the mechanisms that transport the tungsten away from the divertor, so that we can prevent tungsten contamination of the hot, core plasma.”
Many new spectroscopic systems were installed in the 2010 – 2011 shutdown, including systems that can operate in the extreme ultraviolet wavelengths, where tungsten emits strongly (not shown on scarf). This investment is vital to the future of ITER; the savings for ITER, should the tungsten prove a suitable material, is estimated at 400 million euros.
Source: EFDA
