Tracer pellets – fusion research with watchmaker's tools

Experiments on impurity transport in fusion plasmas / Japanese-European collaboration

March 03, 2021

Tiny hand-made pellets may show the way to tracking how unwanted impurities migrate throughout fusion plasmas.

Researchers at the fusion institutes NIFS in Japan, CIEMAT in Spain and Max Planck Institute for Plasma Physics (IPP) in Greifswald have joined forces to learn how performance-reducing impurities diffuse through the hot plasma in their fusion devices. By shooting tiny pellets with impurities into the plasma at predetermined locations, they hope to study how these substances migrate and learn whether the turbulent plasma can be made to push them out towards its edge.

A sub-millimeter sized pellet used to inject trace amounts of impurities – gold in this photo – into the LHD stellarator at NIFS, Japan.

Originally, the Tracer Encapsulated Solid Pellets or TESPELs were produced by NIFS for use in their stellarator LHD in Toki, Japan. Over the past years, NIFS researchers have transferred their know-how to colleagues at the Spanish fusion institute CIEMAT for use in the European stellarators TJ-II in Madrid and Wendelstein 7-X in Greifswald. Ten such 0.7 millimeter pellets crafted in Madrid have now been successfully tested in LHD. Each contained only micrograms of impurities such as titanium, iron, nickel, copper and tungsten, enough to make up a few percent of the total plasma mass.

At the Wendelstein 7-X stellarator in Greifswald, fusion researcher René Bussiahn is glad about the pellet production at CIEMAT and their successful tests at NIFS. "During the last Wendelstein 7-X campaign, TESPEL injections have shown to be a very reliable way to deposit impurities right where we want them: deep in the core plasma. They're an excellent tool to study the migration and build-up of these impurities." Together with a laser blow-off system that generates impurities in the outer edge of the Wendelstein plasma, it is now possible to observe impurity transport throughout the plasma.

"Making these pellets takes a steady hand", explains Ireland-born physicist Kieran McCarthy at CIEMAT. When NIFS researcher Naoki Tamura contacted CIEMAT to install a TESPEL injector on the local TJ-II stellarator, McCarthy took up watchmaker's tools: "It's all done by hand under a microscope", says McCarthy: "from forming the polystyrene balls in a mold half a millimeter wide, to drilling out an inner cavity with a watchmaker's drill, packing in the contaminant powder with a toothpick and sealing the pellet with a 0.3 mm lid."

Handcrafted polystyrene pellets containing trace impurities. Left to right: a 0.7 millimeter pellet ready to be hollowed out, a pellet filled with titanium and a pellet with a lid on.

The researchers at NIFS, CIEMAT and IPP believe that the successful injection of CIEMAT-made pellets at LHD is a crucial step in bolstering this inter-continental collaboration. As a next logical step, CIEMAT will focus on production for the German stellarator Wendelstein 7-X, the biggest fusion device of its kind. "It takes about an hour and quite a bit of craftsmanship to create one single pellet. So with the new TESPEL fabrication lab at CIEMAT we can significantly increase production", adds René Bussiahn.

For the upcoming Wendelstein 7-X campaign, 100 to 200 TESPELs are foreseen to be injected. In the future, the tiny tracer pellets produced at both CIEMAT and NIFS might even be exported to other fusion experiments worldwide.

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