Finally, a divertor! – Wendelstein powers up

Euro Fusion July 12th 2017

The inside of the stellarator Wendelstein 7-X clearly shows the reddish tiles made of copper-chromium-zirconium. Pictures: IPP

The stellarator Wendelstein 7-X has received its first divertor. Just one step closer towards realising plasma pulse lengths of half an hour without breaking the machine

Fusion is different. Even when it comes to ashtrays. For the scientists working at Wendelstein 7-X, there is no such thing as a newly installed divertor, a component generally referred to as “the ashtray for fusion devices”.

“Limited” performance

“The higher heat handling capabilities of the divertor allows us to make longer pulses with higher energy input”, says Arturo Alonso, W7-X‘s task force leader. A divertor takes the energy that is split out from the main plasma. It diverts waste particles directly into the trash with the help of magnetic field lines. For the first operational period of Wendelstein 7-X (from 10th December 2015 to 10th March 2016) a limiter had to do the job of the actual divertor, but its performance was quite “limited”.

Machine works excellently

Last year, the fusion experiment only had to prove that its magnetic field functions excellently. And so it did. As a result, during its premier operation the longest discharge lasted for six seconds only, with the heating limited to 660 kW due to a permitted injected energy of 4MJ. The plasma was just for practice.

Graphite tiles make the change

During shutdown the inside of Wendelstein 7-X has been covered with graphite tiles. Picture: IPP

Now that the divertor is in place and the wall elements made of copper-chromium-zirconium have been covered by graphite tiles, researchers expect extended pulse lengths of about one minute, as the permitted energy input per discharge moves from 4 Megajoule up to 80 Megajoule, a 20 fold increase in terms of energy input!

Improved grip on particle control

You can imagine the excitement amongst fusion researchers when the stellarator is switched back on at the end of August. Now, they can finally start to characterise the plasma.

“With the installation of a divertor, we expect to have a better grip on the control of particles and impurities and thus be able to substantially increase the plasma performance”, explains Arturo.

Another modification to come in 2020

In the end, the overall goal is to reach pulse lengths of about 30 minutes with a heating power of 18,000 Megajoule. W 7-X will be prepared for this only after 2020. . The passively-cooled divertor will then be changed for an actively-cooled one, which should again enormously increase the energy allowance of the most developed stellarator in the world.