A low energy proton source for ASACUSA's matter studies

The ASACUSA experiment aims to measure the ground-state hyperfine structure of antihydrogen in order to test CPT and possibly discover physics beyond the Standard Model. However, due to the currently ongoing Long Shutdown 2 at CERN no slow anti-protons, necessary for antihydrogen formation, can be produced. During this time the 'mixing scheme' used to produce antihydrogen by mixing e$^+$ and $\overline{p}$, can be improved by exchanging antiparticles with the corresponding particles in order to form hydrogen. Subsequently, a low energy proton source has been designed and developed. It consists of three modules, an electron gun, a gas cell–penning trap and an extraction module. Protons are produced inside the gas cell trap by electron impact ionisation of H$_2$-gas. The protons can then be radially confined by a magnetic field produced by rod magnets and axially confined by a quadrupolar electric field generated with electrodes. The extraction module is used to focus and steer the proton beam into the hydrogen mixing trap. The source was tested and characterised with a MCP-delay-line detector, which will later be used for both Rydberg-hydrogen and proton detection. The characterisation measurements showed that it produces a beam of H, H$_2$ and H$_3$ ions, which can be moved and focussed by the extraction module. As expected from the partial ionisation cross-sections of H$_2$-gas, without the application of cooling pulses, the number of generated protons is low in comparison to the number of H$_2^+$ and H$_3^+$ produced by the source. However, it was also found that, by applying rotating wall pulses with a frequency of $0.47$ MHz on the gas cell trap, the number of protons can be increased while both H$_2$ and H$_3$ ions are driven out of the trap. It is estimated that in this mode, the source can generate approximately two million protons and half a million H$_2^+$ and H$_3^+$ each, per second.