Scientists at Germany’s Max Planck Institute for Plasma Physics (IPP) on 25 June reported a new record performance at the Wendelstein 7-X stellerator, which began operation in 2015. Earlier experiments saw the plasma in the reactor achieve higher temperatures and densities than ever before, and now the records have been broken again in a new test with upgraded components. Like the tokamak, the stellarator uses large superconducting magnets to suspend hydrogen plasma and heat it to the temperatures and pressures needed to fuse hydrogen into helium. The Wendelstein 7-X has 50 superconducting magnet coils some 3.5 metres high. However, while the tokamak confines plasma in a doughnut shaped torus, the stellarator traps the plasma in a twisting spiral shape, which is designed to cancel out instabilities in the suspended plasma.
The latest tests was Wendelstein 7-X achieve a record stellarator "fusion product" - a measure of the ion temperature, density of the plasma, and energy confinement time. The Wendelstein 7-X reached an ion temperature of about 40 million degrees and a density of 0.8 x 1020 particles per cubic metre, producing a fusion product of 6 x 1026 degrees x second per cubic metre. Thomas Sunn Pederse, director of the Stellarator Edge and Divertor Physics Division at the IPP, said in a press release: "This is an excellent value for a device of this size, achieved, moreover, under realistic conditions, i.e. at a high temperature of the plasma ions."
Upgrades designed to improve efficiency, include a new interior wall of graphite tiles (divertor), which can withstand higher temperatures. The divertor protects the twisting chamber walls and allows technicians to pump more plasma in at higher temperatures, increasing control over the density and purity of the hydrogen plasma as well. "First experience with the new wall elements are highly positive," Sunn Pedersen says.
In previous experiments pulses of plasma lasting about six seconds were achieved. This has now increased to 26 seconds. Heating energies introduced to the system were also increased to 75 megajoules - about 18 times those of earlier experiments. The Wendelstein 7-X team was also able to optimise the reactor based on data from the previous experiments. Plasma experiments will resume with the reactor in July, and in the future, the IPP plans to replace the graphite tiles with water-cooled carbon-reinforced carbon components, allowing plasma pulses of up to 30 seconds.