A diaphragm is installed in TCV and delivers promising first results!

© 2019 EPFL

© 2019 EPFL

A diaphragm made of carbon baffles was installed in the TCV tokamak. The diaphragm separates the hot plasma from the region where particles escaping from the plasma interact with the vessel wall, creating a sort of an exhaust chamber. Initial results show the advantages of the new structure.

In a fusion reactor, the fuel of heavy hydrogen isotopes must be heated to very high temperatures (100MºC), at which it will be in the plasma state. The plasma will be kept away from material walls by magnetic fields. However, a fraction of the plasma will manage to escape the magnetic cage and follow open field lines until they impact, typically, the bottom of the vessel. One of the remaining challenges in fusion research is to reduce the number and temperature of the particles that reach the wall to prevent excessive erosion.

A promising method is to increase the density of the neutral gas in the lower part of the machine. Such a neutral cushion removes energy and momentum from the escaping particles before they hit the floor. This gas unfortunately has the propensity to penetrate into the hot plasma, where it degrades the performance.

The physicists and engineers of the Swiss Plasma Center then imagined installing a diaphragm in the machine that separates the lower region from the hot plasma and impedes the free circulation of neutrals.

Today it is done! Under the leadership of Dr (MER) Holger Reimerdes, Carbon baffles were designed, manufactured, equipped with measuring instruments and installed inside the machine, thus forming this diaphragm to isolate the two regions from one another. The neutral leakage past the diaphragm is ultimately controlled by the position of the plasma itself.

The first experiments show that it is indeed possible to increase the neutral pressure in the lower region without disturbing the hot plasma in the upper region. The increased neutral pressure is furthermore seen to reduce the particle impact on the bottom of the vessel as intended.

First results will be presented at the annual meeting of the Division of Plasma Physics of the American Physical Society in October.