New X-ray Diagnostics for WEST Fusion Device
Written by Tim Lash, Focus Fusion Society Contributor
A team of scientists at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) won a DOE Office of Science award to develop new X-ray diagnostics for WEST, the Tungsten (W) Environment in Steady-state Tokamak, in France.
The three-year, $1-million award will support the construction of two new devices at PPPL. This includes collaboration with French scientists and deployment of a post-doctoral researcher to test the installed devices. First of all, WEST is the upgrade to the Tora Supra, a French Tokamak situated at the nuclear research center of Cadarache, Bouches-du-Rhône in Provence. The WEST facility uses plasma-facing carbon components. Likewise, the National Spherical Torus Experiment-Upgrade (NSTX-U) at PPPL uses similar components.
WEST has been designed to test prototype components and accelerate their development for ITER. Therefore, refitting of the Tora Supra began in 2013. Ongoing upgrades include installing new poloidal field magnetic coils in the vacuum vessel. A new cooling system, including active cooling for the divertor, will also be added. Researchers replaced the carbon components with those made of tungsten. Tungsten can better withstand the high temperatures environments of fusion plasmas.
One PPPL diagnostic called the “Multi-Energy Hard X-ray (ME-HXR) Camera” will measure X-ray emissions over a broad energy range. Measuring the soft, or low energy X-Ray emissions will allow scientists to determine the plasma’s temperature and electric charge. Importantly, the ME-HXR camera will allow scientists to see how densely and where heavy elements that could slow the fusion reactions are located within the plasma. This information could prove useful in a number of experiments.
Additionally, the camera will also measure plasma’s hard, or high-energy, X-ray emissions. These emissions come from collisions of background ions with high-energy electrons accelerated by a radio frequency (RF) system known as the Lower Hybrid Current Drive (LHCD). Because electrons carry the current in the WEST, it’s important to know where these fast electrons absorb RF energy. Finally, the camera will also probe the X-ray emissions from the tungsten metal tiles covering the interior of the Tokamak. This will let researchers know if the extreme heat from the fusion reaction is dislodging tungsten atoms from the tiles and propelling them into the plasma. Consequently, this information is critical.
Lastly, a diagnostic developed at PPPL called “Compact X-ray Imaging Crystal Spectrometer (cXICS)” will be deployed. This device mimics similar installations at PPPL’s NSTX and the Alcator C-Mod tokamak at the Massachusetts Institute of Technology (MIT). Basically, being a spectrophotometer, this device creates low-resolution, two-dimensional cross-section images of the plasma, showing the location of impurities, like argon, molybdenum, xenon, and tungsten.