Paul Scharrer

Upgrade preparation for the 1.4 MeV/u gas stripper system for FAIR

A key projectile for the FAIR facility will be U. In routine operation of the GSI UNILAC, U is generated by a MEVVA ion source that delivers ions with comparably low charge states (4), which are accelerated to 1.4 MeV/u in the high current injector (HSI). The 1.4 MeV/u beam passes a region of high gas density, in which the charge is increased to 28 by stripping of electrons. [1] Generally, higher intensities at charge states, preferably above 28, are desirable. This would allow to operate the accelerator more reliable and efficiently. To optimize the stripping efficiency and potentially increase the achieved ion charge states a program to upgrade the gas stripper has started. The modified stripper setup is depicted in Fig. 1. As a first modification, switching from the continuously fed supersonic N2-jet to a pulsed gas injection, synchronized with the beam timing structure, has been implemented. This allows to increase the gas pressure inside the stripper chamber during a beam pulse, while still reducing the total gas flow. The gas injection was positioned inside a T-fitting, which was installed in the main stripper chamber to match the beamline. In a first test the pressure in the main stripping section as function of the opening time of the valve and the pressure along the beamline adjacent to the gas stripper section were measured.