Pellet injection advances to next stage in the US

Researchers at the Oak Ridge National Laboratory (ORNL) have developed a continuous extruder for fusion fuel and are advancing state-of-the-art fuelling and plasma control for ITER. Reliable, high-speed continuous fuelling is essential for ITER to meet its goal of operating at 500 MW for several minutes at a time.

The latest pellet injection experiments using US ITER prototype designs were performed during the week of 22 July at the DIII-D Tokamak operated by General Atomics in San Diego, California. The conceptual design review for the ITER pellet injection system was completed earlier this year, and preparations are now underway for full-scale prototype testing.

The task of the pellet injection system is to provide plasma fuelling, while also lessening the impact of plasma instabilities due to large transient heat loads. The ITER pellet injectors must operate continuously, which is very different from most existing tokamak pellet injectors. The ITER machine also requires a higher rate of pellet fuelling throughput.

According to Dave Rasmussen, team leader for the US ITER pellet injection and disruption mitigation systems, „The ITER pellet injectors will require an increase in the deuterium-tritium mass flow and duration by a factor of 1,000 compared to present systems.”

To produce the pellets, researchers developed a twin-screw extruder which shapes a continuous ice stream of deuterium-tritium fuel into specific diameters and lengths.

„There are existing extruders used on tokamaks today, but they cannot meet the requirements of ITER. On most current installations, extruders have only needed to supply a few seconds of fuel pellets at a time, but the ITER Tokamak will require almost an hour of a continuous ice stream for pellet injection. The ORNL twin-screw extruder is designed to meet the requirements of ITER,” notes Mark Lyttle, a project engineer for the US ITER pellet injection and disruption mitigation systems.

Multiple pellet injectors will be installed on the ITER Tokamak, with up to two injectors at each of three locations on the machine. Some locations will be used more for fuelling while others will be deployed for lessening the impact of plasma instabilities known as edge localized modes (ELMs) by a technique called pellet ELM pacing. The pellet injector can also insert impurity pellets made of argon, neon, or nitrogen into the Tokamak for plasma impurity studies. The pellet injectors must also be able to handle tritium, a radioactive isotope of hydrogen with a half-life of about 12 years, safely.

„There is a 30-year technology development history at ORNL behind the ITER pellet injection design,” says Lyttle.

Under testing now is a 1:5 scale pellet twin-screw extruder. „We do plan to build a full-scale prototype and test it at the Spallation Neutron Source cryogenic facility at ORNL, where we have access to a supply of supercritical helium. Supercritical helium at only 5 degrees above absolute zero is used as the coolant to form the pellet ice and we are lucky to have one of the few facilities in the world that can supply our needs here at ORNL,” says Lyttle.

Other key upcoming activities for researchers and engineers are tests of a propellant gas recirculation loop for the pellet injection system using a tritium-compatible vacuum pump. The recirculation loop supplies the pressurized propellant gas and assures that the gas used to accelerate the pellets is not injected into the vacuum chamber of the ITER Tokamak during the fuelling process.

„Initial tests on the pumping speed look promising,” observes Lyttle. This pump has been tested with helium gas and soon will be tested with hydrogen gas. Ultimately, the pump and loop will undergo a multi-year „lifetime” test to assure its readiness for the ITER pellet injection system, where 99.9% availability is required.

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