Beam Quality Characterisation and the Optimisation of Next Generation Antimatter Facilities

Abstract

The Extra Low Energy Antiproton (ELENA) ring is a new upgrade to the antimatter facility at CERN. By further decelerating beams from the Antiproton Decelerator from energies of 5.3 MeV down to 100 keV, it will allow for increased antiproton trapping efficiencies by a factor of 10--100 for experiments. In order to guarantee the best possible beam quality from ELENA and for other next generation ultra-low energy antiproton and ion facilities, unique diagnostic solutions must be developed. Two new algorithms have been developed for use with a scraper system to determine the transverse beam emittance within ELENA and machines facing similar diagnostic related challenges. These new methods improve the state of the art of beam scraping techniques for low energy ion and antiproton facilities. The algorithms are capable of accurately reconstructing the emittance in a region of non-zero dispersion. Additionally, an algorithm which combines scraping results from opposing directions is capable of the same task for non-Gaussian beams, which are expected due to more efficient electron cooling towards the core of the beam. The new scraping algorithms have been tested through simulations and error tolerances have been established for a range of effects. They also have been shown to be capable of accurately estimating other beam quantities, such as the momentum-dependant closed orbit, and a quantity which indicates the magnitude of a correlation between the emittance of particles and their momentum offset. Using the two scan algorithm, analysis of data taken during ELENA commissioning showed decreases of 28±2% and 81±10% in vertical and horizontal emittances respectively, during 6.7 s seconds of electron cooling along the intermediate energy plateau at 650 keV. At the extraction plateau of beam energy 100 keV, the vertical and horizontal emittances were reduced by 79±2% and 78±10%, respectively, when comparing with and without 3.9 seconds of electron cooling after deceleration. In both cases, non-Gaussian beam profiles were observed. The emittance-momentum offset correlation coefficient showed a significant change towards a positive correlation during electron cooling. To further determine and optimise beam quality at the experiments, a realistic 3D simulation of the electrostatic transport line from ELENA to the ALPHA experiment has been developed in the GEANT4 based beam transport code G4Beamline. Employing the use of realistic elements which include fringe fields and field maps from finite methods simulations, the transport lines have been optically tuned to the experimental handover point. Realistic beam distributions based on scraper measurements and profile measurements taken along an existing transport line from ELENA have been tracked to the target and error tolerances have been established. The development of the simulations has resulted in a comprehensive toolkit for simulating beam transport with electrostatic elements and laid the groundwork for further optimisation using machine learning methods.