Life and Work of Rolf Wideröe by © Pedro Waloschek,     => Contents

Box 10:  Vacuum in Storage Rings

Gaede's vacuum pumps of Wideröe's Karlsruhe time could at best maintain a residual gas pressure of 10-6 millibar in a well sealed container. This is just about enough to run betatrons, cyclotrons, smaller synchrotrons and linacs. The acceleration process in each of these machines is completed within a fraction of a second.

The situation changes completely when particles are to be stored in a ring for longer periods. This necessitates an improvement of the vacuum of at least a factor 100. It quickly became apparent that such machines could only be built in places where particularly good vacuum experts were available.

The situation changes again when 1010 high energy electrons or positrons are to be stored, as is the case in some rings. These particles create so much electromagnetic (`synchrotron') radiation, that the temperature of the vacuum chamber increases dramatically. Therefore the gases which are enclosed at the surface, escape. The heat may be drawn off by water cooling, but the gases have to be pumped out. It may take several weeks before the vacuum is sufficient to run a storage ring, that is, before the vacuum attains 10-8 millibar when the beam is circulating.

Synchrotron radiation of protons is negligible (at the energies which can be reached today), and causes practically no rise in the temperature of the vacuum chamber and therefore no vacuum problems. In the HERA storage ring at DESY [Wa91] most of the 6.3 km long vacuum chamber for the proton beam is even kept at less than -2680C. This has a similar effect to the so-called `cryo-pumps': any remaining gases condense on the surface. The vacuum becomes so good that it can no longer be measured. This corresponds to about 10-11 millibar or better. The mean life of the proton beam then reaches several hundred hours.

Many technical and industrial innovations were necessary to achieve such progress in vacuum technology and thus to make possible the construction of modern storage rings. Almost all the parts used nowadays for these constructions are made of metal. Plastics, oil and mercury all belong to the past. Vacuum-tight welded and braced joints and flanges are generally used. The search for leaks in so called `ultra high vacuum' systems is today a highly skilled profession.