The three-year, $1-million award will support the construction of two new devices at PPPL, including collaboration with French scientists and deployment of a post-doctoral researcher to test the installed devices at CAE Laboratories in Cadarache, France, the home of the WEST facility.
“We are extremely proud that our proposal was chosen considering that there was strong competition from our community,” said PPPL physicist Luis F. Delgado-Aparicio. “Developing innovative X-ray diagnostics will allow us to push technology ahead and give us a great opportunity to be part of an incredible team of French scientists and engineers at WEST.”
Other researchers on the team include PPPL diagnostics division head Brentley Stratton and principal research physicist Ken Hill, according to a press release.
What is WEST?
WEST is the upgrade to the Tora Supra, a French Tokamak situated at the nuclear research center of Cadarache, Bouches-du-Rhône in Provence, one of the sites of the Commissariat à l’Énergie Atomique. The WEST facility has plasma-facing components that use carbon, like those in the National Spherical Torus Experiment-Upgrade (NSTX-U) at PPPL.
WEST has been designed to test prototype components and accelerate their development for Iter, which will be by far the world’s most powerful fusion reactor when it starts up. Refitting of the Sora Tupra was begun in 2013. Upgrades included installing new poloidal field magnetic coils in the vacuum vessel, and a new cooling system, including active cooling for the divertor.
Researchers also replaced the carbon components with those made of tungsten – a material that can withstand the superhot temperatures of fusion plasmas without absorbing gas from the plasma, otherwise, the gas can be released and degrade plasma performance.
The Multi-Energy Hard X-ray (ME-HXR) Camera
One PPPL diagnostic is called the “Multi-Energy Hard X-ray (ME-HXR) Camera.” Its primary function will be to measure X-ray emissions over a broad energy range from the plasma that fuels fusion reactions. Measuring the soft, or low energy X-Ray emissions will allow scientists to determine the plasma’s temperature and electric charge.
Just as important, 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 to be useful in a number of experiments.
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). Electrons carry the current in the WEST, and it’s important to know where these fast electrons absorb RF energy.
Another place the camera will be utilized is in probing 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. In this case, this information is critical.
The Compact X-ray Imaging Crystal Spectrometer (cXICS)
The other diagnostic developed at PPPL is called a “Compact X-ray Imaging Crystal Spectrometer (cXICS).” This device is actually a variation of a device Hill and senior physicist Manfred Bitter invented for PPPL’s National Spherical Torus Experiment (NSTX) and the Alcator C-Mod tokamak at the Massachusetts Institute of Technology (MIT).
Basically, being a spectrophotometer, this device will create a low-resolution, two-dimensional cross-section image of the plasma, showing where impurities, like argon, molybdenum, xenon, and tungsten are located.
“These are two different but complementary instruments,” Delgado-Aparicio said. “They will provide vital information about the plasmas in WEST — which can inform future fusion devices.”
The planned design and delivery of the two new instruments “builds on long-term expertise on X-ray diagnostic development at PPPL,” Stratton said. “This laboratory is known quite favorably for its X-ray diagnostics research, and we want to continue that.”
