The experimental program of the future facility for Antiproton and Ion Research (FAIR) project requires a high number of cooled anti-protons per hour [1]. The FAIR proton injector linac has to deliver a 70 MeV, 35 mA pulsed proton beam at a repetition rate of 4 Hz. During recent machine investigations at GSI a high current proton beam was achieved in the Universal Linear Accelerator (UNILAC) [2]. In preparation for this the ion source was equipped with a newly developed 7-hole extraction system and optimized for single charged hydrocarbon beam (isobutane gas) operation. This beam was accelerated to 1.4 MeV/u and cracked in a new pulsed gas stripper into protons and charged carbon ions. The new stripper setup injects high density gas pulses synchronous with the transit of the beam pulse close to the beam trajectory. With this setup a proton (up to 4.3 mA) as well a carbon beam (up to 9.5 mA) intensity record at beam energy of 1.4 MeV was achieved. The proton beam was accelerated up to 3.6 MeV/u inside the first Alvarez-section with full transmission. The paper will present beam measurement in comparison to the former beam investigations using a 2 mA proton beam in the entire UNILAC.

The GSI UNILAC will serve as part of an injector system for the future FAIR facility, currently under construction in Darmstadt, Germany. For this, it has to deliver short-pulsed, high-current, heavy-ion beams with highest beam quality. An upgrade for the 1.4 MeV/u gas stripper is ongoing to increase the yield of uranium ions in the desired charge state. The new setup features a pulsed gas injection synchronized with the beam pulse transit to increase the effective density of the stripper target while keeping the gas load for the differential pumping system low. Systematic measurements of charge state distributions and energy-loss were conducted with U-ion beams and different stripper gases, including H2 and He. By using H2 as a stripper gas, the yield into the most populated charge state was increased by over 50 %, compared to the current stripper. Furthermore, the high gas density, enabled by the pulsed injection, results in increased mean charge states.

The GSI UNILAC will serve as an injector system for the FAIR facility. Therefore it has to meet high demands in terms of beam brilliance. A key projectile for FAIR will be 238 U [1]. In current routine operation U 4+ -ions from a MEVVA ion source are accelerated to 1.4 MeV/u by the High Current Injector (HSI). Inside the adjacent gas stripper, the charge state of the ions is increased to raise the efficiency of further acceleration. Behind the stripper, a system of dipole magnets allows the selection of ions with the desired charge state (U 28+ ) [2]. To increase the beam intensity after the gas stripper, an upgrade program has started to increase the stripping efficiency into the desired charge state. The current gas stripper is based on a supersonic N2-jet, created through a laval nozzle at 0.4 MPa back-pressure. The continuous gas flow limits the usable gas pressure due to the high gas load for the differential pumping system. This also prevents the optimal use of other promising stripper gases, as a saturated charge distribution cannot be reached [3]. To overcome this limit, a modified gas stripper setup was developed [4]. The flange with the laval nozzle on top of the main stripper chamber was replaced by a new flange, featuring a pulsed gas valve designed for a back-pressure of up to 12 MPa and an opening time down to a few microseconds. The new flange is shown in Fig. 1. To prevent the gas from instantaneous removal, an extension was added to the flange with a T-fitting at the end to match the beam line. This creates a high-pressure interaction zone for the stripping process. The valve is located in the extension, facing down towards the beam line. The pulsed gas injection is triggered by a timing signal of the central accelerator control unit. The valve is opened only when a beam pulse passes the stripper and closed immediately afterwards, decreasing the gas load for the pumping system and lowering the gas consumption by a factor of up to 200. During two measurement campaigns in 2014 the new stripper setup was tested with a U 4+ -beam (1 Hz, 100 µs pulse length). An opening time of 0.5 ms was used for the pulsed gas valve. The opening time was chosen based on pressure measurements near the gas inlet as well as beam current measurements behind the stripper. Besides N2, which allowed comparing the setup to the current gas-jet stripper, the charge spectra were measured for various other gases (H2, He, O2, Ne, Ar and CO2). For all used gases except H2, a saturation of the charge state distribution was observed with an increase of the back-pressure. The beam emittance and the energy loss were measured using the determined settings for a saturated charge state distribution. Additionally, the stripping

As part of an injector system for FAIR, the GSI UNILAC has to meet high demands in terms of beam brilliance at a low duty factor. To accomplish this goal an extensive upgrade program has started. To increase the beam intensity behind the UNILAC, it is aimed to increase the efficiency of the 1.4 MeV/u gas stripper. A modification of the stripper setup was developed to replace the N2-jet with a pulsed gas injection, synchronized with the transit of the beam pulse. The pulsed gas injection lowers the gas load for the differential pumping system, rendering possible the use of other promising gas targets. In recent measurements the performance of the modified setup was tested using an 238 U-beam with various stripper media, including H2, He, and N2. The data provide a systematic basis for an improved understanding of slow heavy ions passing through gaseous media. The stripping performance of the current N2-jet was excelled by using H2 at increased gas densities, enabled by the new pulsed gas cell.