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Heavy-ion induced desorption of cryogenic surfaces

Introduction Ion-induced desorption is a severe luminosity limitation of low charge state heavy-ion accelerators. Studying the phenomena for almost 10 years using different target materials and various ion beams and energies, provides a rather clear picture of the physical processes involved in ion-induced desorption at room temperature [1]. Because of limited experimental data [2], the situation is less obvious for gas desorption of cryogenic surfaces. Due to the importance of cryogenic surfaces in future heavy ion accelerators, we investigated the desorption behaviour in combination with the surface coverage of cryogenic surfaces at different temperatures. For the irradiation experiment we used two disk-shaped targets, bare copper and gold-coated (7μm) copper with a nickel diffusion barrier (2μm). The targets were mounted on a commercial vacuum cold head including radiation shielding and temperature diagnostics. A resistive heater allowed the control of the target temperature by means of a software power control (PID). The cold head including the target was mounted at the LINAC-3 beamline at CERN [1,2]. Before cooling, the whole setup was UHV-baked to achieve low pressures. This procedure should help to keep the surface gas coverage of the target as low as possible. From single shot measurements the pumping speed of the setup was measured by the decay of the pressure bump from the beam pulse. This information is necessary, since the surface of the target and shielding contribute to the pumping speed. During ion irradiation of the targets, desorption yields were measured by the pressure-rise method with a Bayard-Aplert vacuum gauge and a residual gas analyzer (RGA), both calibrated. The desorption yields were derived by the ideal gas law as follows: