New innerwall of fusion reactor JET requires tighter control
Fusion reactor JET in Culham, UK, is acting like a totally new device, courtesy of its brand new inner wall made out of tungsten and beryllium. For instance, fast disruptions of the plasma now leak more heat to the wall, but paradoxically these disruptions cause far fewer problems when restarting the plasma in subsequent experiments. Plasma physicist Dr. Peter de Vries from FOM-DIFFER, together with an international team, described the results of a year's experimenting at JET in the 22 November issue of Plasma Physics and Controlled Fusion. The experiments are a step to transforming JET into a test bed for the advanced fusion reactor ITER.
The village of Culham, nearby Oxford, houses the current world record holder in fusion power, the reactor JET (Joint European Torus). In JET, physicists heat a hydrogen plasma to a temperature of 150 million degrees, hot enough for the atomic nuclei to melt together and form helium. In an 18 month upgrade campaign, JET was given a brand new inner wall made of beryllium and tungsten, the same materials that JET's successor ITER will use. "The new wall has an enormous influence on JET's behavior", says plasma physicist Peter de Vries of the FOM Institute DIFFER. De Vries is permanently stationed at JET and together with some of his co-authors in the paper leads the research groups at the fusion reactor.
Puffing
In Plasma Physics and Controlled Fusion, De Vries and his colleagues describe the effect of the new inner wall on so-called disruptions, during which the plasma containment suddenly collapses. The beryllium/tungsten wall allows for less radiating away of the plasma energy, which instead is directly guided onto the wall. This would lead to undesirably high heat loads, but quickly injecting gas (puffing) allows the plasma to radiate some of its heat, spreading the load more evenly. This way, the JET team can prevent any damage to the reactor wall.
The new inner wall also has a positive effect on JET operations. Previously, the inner wall was contaminated after a disruption, necessitating careful preparations before the next plasma discharge. The new wall materials tungsten and beryllium do not suffer from this effect, which means start-up after a disruption has become easier.
Test bed for ITER
During its one and a half year upgrade JET was fitted with a new inner wall, improved heating systems and better diagnostics: the reactor is once again state of the art. In 1997, JET achieved the current world record in fusion power, nearly performing break-even. In 2015, the researchers want to improve that record.
With all the upgrades, JET is now an ideal test bed for the advanced fusion reactor ITER, currently under construction in the south of France. ITER has been designed to be the first fusion reactor ever to produce more power from fusion than the machine uses itself. The ITER power of 500 MW is comparable to a small coal-fired power plant, and operating the reactor will consume use 50 MW.
Contact
Gieljan de Vries (030) 609 69 02 (FOM Institute DIFFER)