The current GSI accelerators will serve as injectors for the FAIR accelerator facility, thus having to match the highly demanding FAIR specifications. To this end, an upgrade program for the GSI machines is ongoing. In the course of machine experiments at the beginning of the accelerator chain (after the high current injector), where the beam has an energy of 1.4 MeV/u, a new record intensity for uranium beams has been achieved.
For the charge state 28+, which is standard for acceleration in the UNILAC Alvarez section, a maximum intensity of 7.8 mA was measured at very good beam quality. This substantially exceeds the old record of about 5 mA, achieved in 2007. This became possible thanks to modifications in the gas stripper section. There, the U4+-ions delivered by the MeVVa ion source are stripped of electrons in collisions with a gas target, thus increasing their charge state. So far, a nitrogen gas-jet target was used. In the course of the upgrade program, a pulsed stripper was developed, which allows creating targets of highest density for the duration of the beam passage.
In the current experiments, short gas pulses were injected into the stripper section synchronously to the 0.1 ms long uranium pulses, and the performance of different gases was measured. Hydrogen gas proved to most efficiently ionize uranium to the 28+ charge state. This is in agreement with expectations based on theoretical work, e.g., of Prof. Viatcheslav Shevelko of the P.N. Lebedev Physical Institute at the Russian Academy of Science in Moscow, who spent two months at GSI as a visiting scientist through the guest scientist program of the Helmholtz Institute Mainz.
The experiments were jointly performed by the SHE Chemistry department, which was put in charge of the gas stripper upgrade by the project head of FAIR@GSI, and the project division LINAC&Operations, especially the departments Ion Source and Linac.
Text: Christoph Düllmann, Winfried Barth
for the SHE chemistry department and the project division LINAC&Operations