Posted on: 1st April 2011

Vacuum conditions inside the JET torus are critical for proper operation of the machine. Several types of vacuum pumps are used to ensure that the required ultra-high vacuum is achieved. Full conditioning takes several weeks and there are several key stages in the pumpdown process. The initial rough vacuum is produced by pumps in a separate building from the torus through two large pumping lines.

When the pressure is low enough (typically of the order of 1 millibar, or one thousandth of normal atmospheric pressure) a different type of pump will come into operation. These pumps are called ‘turbomolecular pumps’ (or turbo pumps for short) and they reduce the torus pressure by a further factor of 1,000,000.

During this shutdown the turbo pumps have been exchanged for a new type in which the rapidly spinning internal rotor is magnetically levitated on special bearings. This type of pump has been used in industry for many years, but not on JET where the magnetic fields around the torus could affect operation of the bearings. A careful study and some tests have given us confidence that the new pumps will perform well and they have been installed.

This week they have been connected to their control cubicles and all the pumps started up as expected. They can not be run to full speed until the vacuum conditions are correct but another critical system is now commissioned and ready to go.