Experimental Investigation of Impact-Induced Molecular Desorption by 4.2 MeV/u Pb ions

In preparation for the heavy ion program of the LHC [1], accumulation and cooling tests with lead ion beams have been performed in the LEAR storage ring [2]. These tests have revealed that due to the unexpected, large outgassing of the vacuum system, the dynamic pressure of the ring could not be maintained low enough to reach the required beam intensities. To determine the actions necessary to lower the dynamic pressure rise, an experimental program has been initiated for measuring the molecular desorption yields of stainless steel vacuum chambers by the impact of 4.2 MeV/u lead ions with the charge states +27 and +53. The test chambers were exposed either at grazing or at per-pendicular incidence. Different surface treatments are re-ported in terms of the molecular desorption yields for H 2, CH4, CO and CO2. Unexpected large values of molecular yields per incident ion up to 1.5×104 molecules/ion have been observed. The implications of these results for the vacuum system of the future ion accumulator ring (LEIR) and possible remedies to reduce the vacuum degradation will be discussed.