Fast Beam-ion Instabilities in CLIC Main Linac Vacuum Specifications

Specifications for the vacuum pressure in the CLIC electron Main Linac are determined by the onset of the fast beam-ion instability (FBII). When the electron beam is accelerated in the Main Linac, it ionizes the residual gas in the chamber through scattering ionization. If the density of ions around the beam exceeds a certain threshold, a resonant motion between the electron beam and the ions can be excited. A two-stream instability appears and as a result the beam acquires a coherent motion, which can quickly lead to beam quality degradation or even complete loss. Thus, the vacuum pressure must be kept below this threshold to prevent the excitation of FBII. The CLIC Main Linac poses an additional challenge with respect to previous FBII situations, because the gas ionization does not solely occur via scattering. The submicrometric beam sizes lead to extremely high electric fields around the beam and therefore result in field ionization beyond a certain threshold. The residual gas in the corresponding volume around the beam, where the electric field is high enough, becomes fully ionized during the passage of the first bunches of the train. Hence, the impact of the FBII on the tail of the bunch train is strongly increased in comparison with scattering ionization. In this paper, the results of the instability simulations carried out with the FASTION code are presented for the nominal CLIC Main Linac with a center of mass energy of 3 TeV, an energy scaled version of the same lattice at 1 TeV and a different lattice with a center of mass energy of 500 GeV. Three different compositions of the vacuum have been considered: unbaked vacuum, baked vacuum and baked vacuum with NEG pumping. In addition, an analysis of several gas species as well as a parameter scan have been performed and results are presented.