Keep the plasma clean

• External Links
• Chromatography
• Further interest
• Focus On: Power Supplies

In order to keep plasmas as clean as possible, the vacuum system pumps the JET vessel continuously, even during the plasma discharge. The continuous pumping has negligible influence on plasma fuelling (i.e. on supplying the working gas), because at very low densities the fuel gas expands immediately to the whole vessel. The gas influx is electrically neutral, therefore not guided by the magnetic field. Plasma exhaust, to the contrary, is guided by magnetic fields towards the bottom of the JET vessel, to the divertor, where it is continuously collected by dedicated cryopumps.

Turbomolecular pump turbine rotor

JET is unique in the world as a fusion research experiment able to work with tritium, and, as a consequence, it has to be operated with all precautions required for active isotope handling. All the gases that are pumped from the vessel must go through a dedicated pipeline to the Active Gas Handling System. In this system, chromatography and cryodistillation processes allow for safe separation and storage of the different isotopes from the pumped gases – namely of tritium (active), deuterium and helium (stable). This procedure is required at all times, even when JET is not operating with tritium, as traces of tritium continuously desorb from the vessel structure into the main pumped volume.

JET can achieve a very good level of vacuum, up to a millionth of a millionth of the density of air (in technical terms, the final pressure of impurities can achieve up to 10-9 mbar, that is 10-7 Pa). The procedure required to achieve and maintain that good vacuum is actually quite complicated, and several techniques must be employed.