Vacuum specifications for the CLIC Main LINAC

The maximum tolerable pressure in the vacuum of the CLIC electron Main Linac is determined by the threshold above which the fast ion instability sets in over a bunch train. With micro-metric beam sizes, the macroscopic electric field of the bunches reaches values above the field ionization threshold, thus producing more ions than the classical scattering ionization. In this paper we first discuss the extent of the transverse areas that gets fully ionized along the ML. Then, we show the results of the instability simulations made with the FASTION code using the new model, and consequently review the pressure requirement in the ML.