Beam Induced Gas Desorption in the CERN Intersecting Storage Rings

The maximum beam intensity achieved in the ISR has been limited up to now by the beam induced pressure rise which builds up with the stacked proton current. This pressure increase can be explained by ion induced gas desorption from the vacuum chamber. The pressure P, as a function of the stacked proton current I, can be described in good approximation by P = Po/[1-(k¿/S)I], where Po is the pressure without beam, S the pumping speed, ¿ the net gas desorption coefficient in molecules per incident gas ion and ¿ is a constant. This concept shows the existence of a critical current at which the pressure goes to infinity. The desorption coefficient depends on the primary ion energy, the type of ions and above all, on the surface conditions. Surface treatments yielding low and even negative values of ¿ are discussed together with experimental results obtained.