Reduction of 400  GeV/c slow extraction beam loss with a wire diffuser at the CERN Super Proton Synchrotron

Abstract

Slow extraction of a quasicontinuous flux of high-energy protons is an important requirement for many high-energy physics experiments. This extraction type is associated with an unavoidable beam loss due to scattering on the thin septum element. The energy deposition of scattering products and resulting activation place performance limits on existing and planned high-power, high-energy fixed-target proton facilities. In the 400 GeV=c Super Proton Synchrotron (SPS) at CERN, a diffuser (or prescatterer), comprising an array of dense wires or ribbon located upstream of the electrostatic septum, has been designed to reduce absolute losses on the septum wires. As part of a concerted effort to investigate loss reduction techniques in the SPS in view of new physics experiments, the diffuser concept was explored in numerical simulation and analytically. A prototype device has been designed, built, installed, and tested in the SPS to prove the feasibility and quantify the performance reach. In this paper, the diffuser concept is briefly recalled and design considerations for the SPS use case are presented, with the analytical considerations and simulation studies for the optimization of the material and geometry. The device design is described, and the experimental results with a beam are presented and analyzed. The results are discussed, and an outlook is given for the operational feasibility and maximum obtainable performance gain. Conclusions are drawn on the implications for the application of the concept.

Septum shadowing by means of a bent crystal to reduce slow extraction beam loss

“…16) is essentially a profile of the ES. The thickness estimated in this way is 500 μm, which is in line with what has been obtained from the analysis, combined with simulations, of the diffuser data [20].…”

“…Using the simulation tools described in the previous section, tracking simulations were performed to specify the crystal characteristics needed to achieve loss reduction at the ES. In the simulation studies presented in this paper an effective ES thickness of 500 μm was assumed based on the results of empirical linear scans of the passive (wire-array) diffuser installed and tested with beam earlier in 2018 [20]. The parameters of the crystal used for the SPS tests were specified in [21].…”

“…The prototype crystal diffuser tank developed by the UA9 collaboration for this application [19] was installed in the same location as a prototype passive (wire-array) diffuser [20], which was swapped out during the second Injectors Technical Stop in September 2018. The longitudinal space available on the beam line was limited to 180 mm.…”

Resonant slow extraction with constant optics for improved separatrix control at the extraction septum

“…Note that the expected loss reduction is in the order of a few percent only for an ES equivalent thickness above 300 μm. From measurements in 2018 with both passive and active diffusers the effective thickness was estimated to be in the order of 500 μm [3,4]. Hence, no significant loss reduction should be expected from the reduction of the angular spread alone at the ES.…”

“…Methods of reducing these losses and mitigating their effects are therefore an active study topic for CERN's Physics Beyond Colliders program [2]. Various promising techniques to reduce slowextraction losses by means of, for example, bent silicon crystals [3], diffusers [4] or phase space folding with multipoles [5] have been proposed. All these benefit in efficiency from a constant phase space representation, i.e., constant separatrix angle, throughout the spill or constant optics in the case of exploiting multipoles for loss reduction.…”