Mitigation of Heavy Ion-Stimulated Gas Desorption from Accelerator Components

Ion-stimulated desorption is a serious problem regarding intensity limitation for heavy ion circular accelerators. Lost beam ions colliding with the wall of the beam tube trigger the release of huge amounts of gas. This leads to local pressure increases up to three orders of magnitude, which in turn enhances further beam losses. Future accelerators, such as the Facility for Antiproton and Ion Research (FAIR), are designed to deliver ion beams of highest intensities. For stable beam conditions, this requires low desorbing surfaces for beam exposed accelerator components and excellent vacuum conditions with pressures in the range of 10⁻¹² mbar. The objective of this thesis was to develop surfaces with minimal ion-stimulated desorption by testing a large variety of pretreatment methods for copper, tungsten and aluminum samples including extended ex-situ thermal annealing and different combinations of lapping, polishing, etching and milling as well as sputter cleaning with keV argon ions. In addition, coatings with TiZrV, titanium nitride and carbon were tested. Ion-stimulated desorption was determined by irradiation experiments with calcium or gold ions with 4.8 MeV/u specific energy at the UNILAC beamline M1 at the GSI Helmholtz Center for Heavy Ion Research. In addition, ion-stimulated desorption measurements with highly charged ions (Au⁷⁸⁺) of different specific energies (2, 3 and 4.8 MeV/u) were conducted at CRYRING. For all experiments, desorption was quantified by recording the pressure increase with a total pressure gauge and a quadrupole mass spectrometer. For ion-stimulated desorption, the pressure increase during irradiation is converted into a number of desorbed molecules per impacting ion, the so-called desorption yield, while in complementary thermal desorption experiments outgassing is quantified by the amount of desorbed gas per surface area (mbar∙l/cm²). As surface treatments, diamond milling and polishing turned out to be appropriate solutions. Sputter cleaning with keV argon ions also result in a clear reduction of the desorption yield. Ex-situ annealing at 400 °C for about 4 h under ultra-high vacuum conditions was identified as excellent pretreatment method to reduce ion-stimulated desorption. The low desorption is preserved even after storage in atmosphere for at least 11 months. Storage in argon is recommendable, because these samples show lower desorption compared to atmosphere storage. The fact that the low desorption persists under storage in atmosphere is a strong indication that ion-stimulated desorption is not limited to the surface, but that the bulk also plays a significant role. To prepare accelerator components with minimal outgassing, both surface and bulk properties have to be optimized