STRIPPING FOIL ISSUES FOR H- INJECTION INTO THE CERN PSB AT 160 MEV

Beam physics considerations for the stripping foil of the 160 MeV PSB H injection system are described, including the arguments for the foil type, thickness, geometry and positioning. The foil performance considerations are described, including expected stripping efficiency, emittance growth, energy straggling, temperature and lifetime. The different beam loss mechanisms are quantified in the context of the aperture limits, operational considerations and collimation requirements. INTRODUCTION Linac4 [1] is an H linear accelerator which will replace Linac2 as injector to the PS Booster (PSB). A new chargeexchange H injection system is required for the PSB [2], which will allow transverse phase space painting to control the transverse emittances. The momentum range for the injected beam is large, due to the need for longitudinal painting [3]. Injection can either be made with the incoming beam dispersion matched to the PSB ring, which will minimize the emittance growth from dispersion mismatch but will require a larger foil; or the dispersion for the injected beam can be zero, which produces some dispersion mismatch but minimizes the foil size and number of foil hits. For the matched dispersion case the average number of foil hits per proton is about 25, and with zero dispersion the average number of proton hits is about 10. Since the emittance growth and uncontrolled beam loss both increase with increasing foil hits, the zero dispersion arrangement is presently the baseline. FOIL MATERIAL, THICKNESS AND SCATTERING EFFECTS For thermal stability, high sublimation temperature, radiation and mechanical resistance the foil material will be carbon, either amorphous or possibly diamond [4], with an assumed density in the range 1.7-2.0 g/cm. A thicker foil gives better stripping efficiency and is more stable mechanically, but will give higher beam loss and emittance growth through scattering. Inelastic scattering and large-angle elastic scattering are assumed to result directly in beam loss, while multiple Coulomb scattering will give a small angle increase and will blow up the circulating beam emittance. The optimum foil thickness is a compromise between these effects. Stripping efficiency From Figure 1, showing charge fraction as a function of foil thickness, one concludes that for a theoretical stripping efficiency of >99 %, the foil thickness should be greater than 150 μg/cm. The cross-sections for the relevant charge exchange processes, Table 1, have been extrapolated from data measured [8] at 200 MeV. Table 1: Measured and scaled charge exchange crosssections for a carbon stripping foil