The LEP ring has one of the best vacuum pressures in the world, and can achieve a pressure of about 10-10 Pa. This is about 10 million times lower pressure than in open space in the universe or 1015 times lower than the atmospheric pressure at sea level. The LEP beampipe has a total volume of 270 000 liters and to achieve the vacuum pressure needed, a number of different pumping methods has to be applied.
First the LEP ring is heated up to about 150-300o C, this is called the bake-out and removes all the water vapor. An initial vacuum of about 10-7 Pa is then created by 60 mobile pumping stations. About 2200 stationary ion pumps then take the pressure down to 10-8, and finally 20 km of getter ribbon and titanium sublimation pumps push the pressure down to 10-10 Pa.
When the electrons and positrons are injected to the LEP ring, the vacuum pressure can unfortunately not be contained at the initial level. When the beam is started a radiation pressure is created as synchrotron radiation from the beam hit the beampipe walls, which will then heat up and emit particles. This so called dynamic pressure increases with the radiated power from the beam (Fig. 3.1). Other sources such as small leaks and astray beam particles will contribute to the static pressure build up from beam-gas particles that reside in the vacuum tube.
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During operation with a four mA beam LEP II therefore has an average pressure of about 10-7 Pa. In the proximity of the interaction point the LEP ring is straight and the beampipe is pumped harder, reducing the pressure with about a factor ten (Fig. 3.2). Beam particles that interact with the rest gas in the beampipe vacuum will loose part of their energy. Particles that are off the beam energy will experience directional changes in the bending magnets and will create a beam halo and some beam loss. This is called the off-energy background.
