Posted on: 4th June 2012
A recently refurbished X-ray detection system, KX1, enables JET-scientists to probe further into the 150 million degree core of the plasma than they ever have before.
What they are looking for is contamination of the plasma by tungsten from the ITER-Like wall. While the light metal beryllium was used for most of the wall tiles, tungsten was chosen for where JET’s plasma touches the walls, as it is the metal with the highest melting point (3422 degrees Celsius). However tungsten also has the unfortunate characteristic of having many electrons, which emit radiation out of the plasma as they bounce around between levels, thereby sapping the fusion experiment’s energy. Hydrogen isotopes – fusion fuel – make up the bulk of the plasma, but they do not cause energy leakage via this process. This is because their single electron is stripped off by the heat of the plasma. Similarly beryllium does not contribute much to energy loss either, as it has only four electrons, which are readily removed in the hot environment.
However stripping off all 74 electrons from a tungsten atom is a bigger task – it has 74 protons in the nucleus binding them. Even in JET’s plasmas of over 100 million degrees, more than a third of tungsten’s electrons are typically still attached, jumping around absorbing energy and re-emitting it. It is the energy coming from these inner-shell transitions in tungsten that the new system can see, in the X-ray region of the spectrum. In particular the system is precisely tuned to detect the X-rays from tungsten that has lost 46 of its electrons, a species which occurs in the core of the plasma.
Before reaching the detectors the X-rays travel along a 20 metre pipe and are then dispersed by a crystal. The X-rays create ionisation in a small cell containing a mixture of argon and carbon dioxide, which generates a tiny electrical signal which is amplified and fed into a computer system. The system is extremely fast, sampling every 10 nanoseconds, so it generates one hundred spectra in each second, each composed of a million data points. This amounts to around one quarter of a terabyte of data generated every second. As this volume of data is be impractical to store, a major part of the recent refubrishment has been developing a sophisticated electronic processing system (part of which is pictured above). The system is based on field programmable gate arrays and can filter and analyse the raw data in real time. One of the system’s its features is that it provides additional frequency analysis, on top of the crystal’s dispersion, which allows the separation of tungsten spectral line from the background radiation.
The final tests of the system are being performed by a team from the Polish association IPPLM who developed it. The information yielded by KX1 will reveal the spread of tungsten throughout the core of the plasma, vital information for evaluating the performance of JET’s new ITER-Like Wall.
IPPLM is the Polish signatory to the European Fusion Development Agreement.