A new pellet extrusion concept for nowadays tokamak plasma fuelling and control system

CEA Grenoble /DSBT proposes a new pellet production method for plasma fuelling or ELM control systems for nowadays tokamaks. This method is based on CEA developments of solid hydrogen ribbon systems for high-energy laser-matter interactions [1]. A demonstration test bench was set up at CEA for batch operation mode and consists of a 250 cm 3 extrusion cell filled with solid hydrogen, for instance. The cell is pressurized at around 60 bar by precooled hydrogen gas to extrude a hydrogen ice rod at around 10 K through a Ø2 mm die. The target solid throughput is around 350 mm 3 /s for typically two minutes duration. Post treatments of the extruded rod videos give access to the extrusion speed. The die outlet pressure, measured in the pumping duct where the ice rod sublimates, gives a second extrusion speed measurement, thanks to pumping speed calibration of the pumping system. The solid production process uses either hydrogen, deuterium, or a mixture of these gases. Numerical simulations were conducted to predict the operating pressure and temperature conditions at the die level and at the top of the extrusion cell. During the extrusion, the solid hydrogen behaves as a non-Newtonian fluid and exhibits a highly non-linear viscosity. This new pellet source is proposed as an alternative solution to the existing mechanical screw; or piston-driven extruders in the current pellet injection systems. The lack of mobile mechanical parts shall increase the extrusion source reliability used on the pellet injection system, especially for tritium applications. This paper presents the experimental setup for the extrusion demonstration, the first results of our model, the experimental campaigns obtained at CEA, and future applications.