First experience with carbon stripping foils for the 160 MeV H− injection into the CERN PSB

Abstract

160 MeV H − beam will be delivered from the new CERN linear accelerator (Linac4) to the Proton Synchrotron Booster (PSB), using a H − charge-exchange injection system. A 200 µg/cm 2 carbon stripping foil will convert H − into protons by stripping off the electrons. The H − charge-exchange injection principle will be used for the first time in the CERN accelerator complex and involves many challenges. In order to gain experience with the foil changing mechanism and the very fragile foils, in 2016, prior to the installation in the PSB, a stripping foil test stand has been installed in the Linac4 transfer line. In addition, parts of the future PSB injection equipment are also temporarily installed in the Linac4 transfer line for tests with a 160 MeV H − commissioning proton beam. This paper describes the foil changing mechanism and control system, summarizes the practical experience of gluing and handling these foils and reports on the first results with beam.

Measurements with the stripping foil test stand in the Linac4 Transfer Line

“…The design of the stripping foil handling and exchange mechanism, so-called TKSTR, of which an illustration is given in Figure 1, was extensively presented in [8,9]. Based on commercially available components [10], the unit for the L4 test stand is identical as the mechanism intended to be installed in the PSB.…”

“…The required foil thickness for PSB injection is 200 µg/cm 2 (∼1 µm) to ensure a theoretical stripping efficiency >99% while keeping the emittance increase below ∼0.1 mm mrad for the LHC beam (required ∼2 mm mrad rms normalised emittance at injection) and the uncontrolled beam losses below the 10 −4 level [12]. The 32 mm wide foils are glued, on identical 58 mm vertical beam aperture stainless steel frames, with a mixture of 50% demineralised water and 50% Aquadag R 18% solution, of which a detailed description is presented in [9]. An illustration of the stripping foil holder attached to the rotating belt is given in Figure 2.…”

“…Two beam current transformers (BCT), located upstream (L4T) and downstream (L4Z) of the foil, are used to measure the stripping efficiency by looking at their relative current decrease. Following the issues of a 20% higher signal at the downstream monitor, reported in [9], a cross-calibration of these BCTs at the percent level was done. The observed preamplifier saturation effects, due to a strong 352 MHz RF component well outside the presumed passband of the transformer, was corrected by the addition of appropriate low pass filters to have a reliable evaluation of the number of protons with respect to the original injected H − [18].…”

“…Correct position of the foil, with respect to the beam, is important for optimizing the stripping efficiency and for this reason a retractable optical beam observation system (BTV), consisting of a 1 mm thick Chromox (Al 2 O 3 doped with CrO 2 ) scintillating screen, can be placed 6 mm in front of the foil [19]. The image of the screen, or of the stripping foil, is recorded by a radiation-hard camera, allowing checking either the beam position or the integrity of the foil [9]. Before measuring the stripping efficiency of a foil, a single beam pulse is shot on the BTV screen, to determine its position.…”

“…During the first beam tests, that took place in October 2016, two foils (XCF-200) got broken while continues beam pulses were sent on the BTV during ∼1 minute periods. After removing the screen, the foil reappeared damaged [9]. The mechanism that caused this foil disruption was thought to be the beam induced charging of the screen, and probably also the foil, during beam impacts causing rupture when the screen is moved.…”