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Disruption mitigation shattered pellet injector gets ready for JET experiments

Shattered pellet injection components on test stand at ORNL

Shattered pellet injection components on a test stand at Oak Ridge National Laboratory before installation on JET at the Culham Centre for Fusion Energy in the UK. Photo: US ITER/ORNL

April 29, 2019

A shattered pellet injector (SPI) using the same technology as planned for the ITER disruption mitigation system has been installed on the JET tokamak at the Culham Centre for Fusion Energy in the UK and will be used in upcoming experiments. Technical commissioning of the components is already underway.

“We are very excited to start testing the new shattered pellet injector on JET – it is a core part of EUROfusion’s upcoming programme. The dedication shown by the project team to get the SPI installed and commissioned has been vital, and I’m sure they will feel immensely proud when the first shattered pellets are injected and the first results are published,” said Joe Milnes, JET Operating Contract Senior Manager for the UK Atomic Energy Authority.

SPI installed on JET
The shattered pellet injection components after installation on the JET tokamak. Photo: Eurofusion

In order to achieve ITER’s mission to produce a self-heated, burning plasma, a disruption mitigation system is essential for reliable machine operations. Plasma disruptions can produce large heat loads, electromagnetic forces, and runaway electron beams. ITER partners have investigated designs for mitigation systems and concluded in a 2017 international workshop that shattered pellet injection of frozen pellets of deuterium, neon, and/or argon will be the baseline method for the ITER system. Experiments on the DIII-D tokamak in San Diego, California produced findings that SPI leads to more effective thermal mitigation than massive gas injection, and it provides deeper penetration of a fragment spray.

 “The extrapolation of SPI performance to ITER is greatly enhanced by employing an SPI on JET to see how the mitigation metrics scale with plasma size and energy. This will give higher confidence on the predicted mitigation outcome on ITER,” said Larry Baylor, a distinguished scientist at Oak Ridge National Laboratory’s Fusion Energy Division. “A unique feature of JET is that it has an ITER-like wall of beryllium and tungsten, which influences disruption behavior.”

The SPI installed on JET is similar to the injectors planned for use on ITER but scaled to JET plasma parameters. The SPI will utilize three distinct pellets, sized from 4.5 mm to 12.5 mm, depending on the experiment.

“The experiments will help answer questions about whether shattered pellet injection will remove energy from the plasma fast enough and uniformly enough to effectively mitigate disruptions in a large tokamak,” said Baylor. “We’ll also learn how the physics of SPI disruption mitigation scales to larger, more energetic plasmas.”

Planning is already underway for the next SPI experiments on yet another tokamak, KSTAR in Daejeon, Korea. In the KSTAR experiments, two identical 3-barrel SPI systems will be deployed to mimic the planned multi-injector ITER installation. The overall goal is to extend the disruption mitigation experimental and modeling database, in order to extrapolate to the ITER configuration and larger plasma scale.

Shattered pellet injection involves cryogenically freezing pellets of deuterium, neon, argon, or some combination in a specially designed cryogenic “pipe gun.” The pellet is injected into the plasma at speeds of 500–1800 km per hour when a disruption is detected. By shattering the pellets in a curved tube before the material enters the vacuum vessel, it is possible to form collimated sprays of pellet material that penetrate deeply and rapidly into the plasma. For ITER, a sufficient quantity of material must be delivered to the plasma when a disruption is detected, as the ITER plasma volume is ten times greater than JET’s. This quantity will be achieved with multiple shattered pellets and multiple injectors.

Source: ITER Newsline

Last Updated: May 3, 2019 - 11:07 am