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High energy ions used in the beams of particle accelerators may collide with residual gas molecules in the beam vacuum chamber, capture or lose an electron and, due to changed charge state, leave the beam at a bending magnet and finally collide with the vacuum chamber wall. There the ions desorb gas out of the chamber wall. Particle accelerators have, during operation with heavy ion beams, shown a significant pressure increase due to ion induced desorption, which could reduce dramatically the beam lifetime. The aim of the experiment was to study desorption yields, η, (released molecules per incident ion) from different materials at the ion impact energies in the range of 5 to 17.7 MeV/u. The experiment was performed in a new UHV compatible beamline at The Svedberg Laboratory in Uppsala, Sweden. We used Ar8+ 5 MeV/u, Ar9+ 9.6 MeV/u and Ar12+ 17.7 MeV/u. A 2 m long thin-film NEG coated tube was used for differential pumping. The desorption was measured by the throughput method, where a known conductance between the measurement chamber and pump chamber provides an accurate pumping speed. Both chambers were equipped with residual gas analyzers and extractor gauges.
One aspect to investigate was the impact by secondary particles (most likely electrons and/or backscattered ions) on the desorption process in the test vacuum chamber. For this purpose the ion induced desorption was measured from flat and tubular samples of 316LN stainless steel, Ta, etched and non-etched OF Cu, respectively. The main result is that the ion induced desorption in accelerator vacuum chambers at ion energies 5 to 17.7 MeV/u consists of two parts, when using a low Z beam compared to the target components: desorption due to direct ion bombardment and due to secondary effects like projectile backscattering, electron and ion emission. In the case of flat sample measurements, the desorption is a sum of primary ion induced desorption from the sample and secondary particle induced desorption from the test chamber. The tubular samples allow avoiding this uncertainty, because the secondary particles desorb mainly on the same material as the primary ion beam. But still only the total desorption is measured, since secondary particles and backscattering rates were not studied here.
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