Snowball in hell

Fusion researchers call it the ‘snowball in hell' – a bullet of frozen deuterium fuel fired at high speed into a fusion plasma that is ten times hotter than the Sun. Firing these tiny, frozen fuel pellets is one of the most effective ways of injecting fuel into the plasma, enabling fusion reactions and the release of energy. And a  team of international scientists has greatly improved the fuel system for the under-construction Japanese-European device the Super Advanced tokamak JT-60SA and for future power plants.

This method of fuel injection is set to become increasingly important as future fusion devices grow bigger and plasmas get hotter reaching ‘ignition', the point at which the plasma heats itself without external input – crucial for power-producing reactors. To achieve ignition, the density of the plasma core must be raised and sustained by fuelling it.

While MAST and JET (Joint European Torus) in the United Kingdom are exploring this fuelling method, WEST (France), Frascati Tokamak Upgrade (Italy) and ASDEX Upgrade (Germany) are providing data for the method to predict the performance for ITER. Planning for upcoming machines such as the JT-60SA are taking into account these results. Since JT-60SA will be the next JET-sized machine and aims to qualify steady state plasma regimes of operation for ITER, it needs a state-of-the-art fuelling system. And pellet-injection method seems the way to go.

Researchers Peter Lang (Max Planck Institute for Plasma Physics, IPP, Germany), Bernhard Ploeckl (IPP), Shinji Sakurai (National Institutes for Quantum and Radiological Science and Technology), Tomohide Nakano, (QST Naka Fusion Institute), Luca Garzotti (CCFE) and Bernard Pégourié (CEA Cadarache) have been working together on this aspects and have published the well-received paper ‘A Flexible Pellet Injection System for the Tokamak JT-60SA: The Final Conceptual Design” in the journal Fusion Science and Technology.

Read in more detail in the latest issue of Fusion In Europe.

Why are pellets frozen?

Fuel pellets are frozen because of the shielding effect of a dense gas shell which surrounds the frozen pellet when travelling inside the incredibly hot plasma. This is similar to the ‘Leidenfrost effect', where water droplets levitate above a hot pan due to a gas cushion.