Wendelstein 7-X Stellarator Restarting
Written by Tim Lash, Focus Fusion Society Contributor.
The Wendelstein 7-X stellarator reactor at Greifswald is restarting for a second round of experiments. These tests are slated for higher temperatures now that reactor modifications are complete. The reactor changes will also allow for longer plasma confinement times. An important goal of this round of experimentation centers on the proper functioning of ten plasma divertors.
After an initial round of shakedown tests, the Wendelstein 7-X went offline last year. This downtime allowed for more construction to take place. Upgrades to instrumentation occurred. The metal walls of the reaction chamber were lined with over 8,000 graphite tiles. These tiles protect the metal walls from the extreme plasma temperatures. The tiles will allow for generation of higher temperatures. Experiments in this round hope to achieve confinement times of ten seconds.
The 2016 testing occurred with heating energy of four mega-joules of microwave power. This year’s experiments will utilize 80 mega-joules. Plasma densities will also be doubled. The increased heating energy will raise ion temperatures from 10 million degrees to 70 million degrees.
The Wendelstein 7-X reactor is designed with 10 plasma divertors. A divertor is a device that allows removal of waste material from the plasma while the reactor is still operating. This allows control over the buildup of fusion products in the fuel, and removes impurities in the plasma that have entered into it from the vessel lining. Keeping the plasma fuel clean is critical for maintaining high temperatures. Heavier ions in the fuel mixture can absorb a larger proportion of energy compared to the fuels thereby dropping the fuel temperature.
Further stages of modification are being planned following these tests. For example, the newly installed graphite tiles are to be replaced in a few years by carbon-fiber-reinforced carbon elements that are additionally water-cooled. This will allow plasma discharges lasting up to 30 minutes.