Low energy ion induced desorption on technical surfaces at room temperature

The ion-induced pressure instability is a hard limitation for the maximum intensity, and hence the ultimate luminosity achievable in a proton accelerator. This instability is due to the interaction of high intensity proton beams with the residual gas generating positive ions. These ions, accelerated by the beam space charge, impact on the vaccuum chamber wall and lead to the desorption of gaseous species like $H_{2}, CH_{4}, C_{2}H_{4}, C_{2}H_{6}, CO$ and $CO_{2}$. These gases can in turn be ionized by the circulating beam, and initiate a pressure run-away process causing the loss of the stored beam. This phenomenon was first registered right at the beginning of operation of the Intersecting Storage Rings (ISR) at CERN in 1970. Later on, a long term evolution of the pressure was recorded for a stable stored beam current where a change of the residual gas composition was measured. In order to adapt the pumping speed and the surface treatments to the desired circulating beam currents, mathematical tools (e.g. VASCO code) exist to calculate pressure profiles in accelerator vacuum systems. A key input for these programs is the desorption yield of the surface, i.e. the number of molecules released by incoming ions and one of the limitations of these programs results from the lack of data concerning the ion-induced desorption yields dependence on the nature and mass of the incident ions. To improve our knowledge of these desorption yields, the ion-induced desorption of Oxygen-Free High Conductivity (OFHC) copper samples has been studied at room temperature for various primary ions: noble gas ions and ions produced by the ionization of the common gases encountered in accelerator vacuum systems, i.e. $H^{+}_{2}, CH^{+}_{4}, CO^{+}$ and $CO^{+}_{2}$. The measured dependence of the desorption yields on the mass, energy and nature of the incident ions is presented and discussed. In this context, the decrease of the ion-induced desorption yield as a function of the incident ion dose (so called "beam cleaning") has been studied for OFHC-copper and other materials. From these measurements, desorption cross-sections have been calculated and are compared with results from corresponding measurements found in the literature. A model, which relates the sputter yield with the nuclear and electronic energy loss of ions in matter, has been applied to the ion-induced desorption: Conclusions concerning the influence of the mass and energy on the mechanism of desorption are presented.