The Extra Low Energy Antiproton ring (ELENA) at CERN — part 1
I have been a project leader of the powering work package for the new Extra Low ENergy Antiproton (ELENA) ring at CERN since 2014. Being part of such an innovative and complex project has been a great managerial experience as well as an opportunity to participate in the design, installation and commissioning of a new complex machine based on cutting-edge technologies. In “Part 1” I explain the motivation behind the ELENA project and present technical details of the future decelerator machine in the Antimatter Factory at CERN.
The Antimatter Factory
Physicists believe that the Big Bang created equal amounts of matter and antimatter. At the same time, it is not understood why the observable universe is composed almost entirely of ordinary matter. What are the properties of antimatter and where did it all go? This asymmetry is a mystery for today’s physics. The European Organization for Nuclear Research (CERN) built the Antimatter Factory facility to investigate if any difference between particles and antiparticles would be able to explain the asymmetry.
The Antiproton Decelerator (AD) is a unique machine that provides low energy antiprotons for studies of antimatter. It is a 182.4 metres long ring and the world’s largest source of antimatter. It has been operational since the year 2000. In contrast to other machines at CERN which accelerate particles to high energies, the AD slows them down. The starting point is a beam of protons from the Proton Synchrotron (PS) which is directed into a block of metal. Such collision creates a variety of particles including a significant amount of antiprotons. These antiprotons travel almost at the speed of light and have too much energy to create antiatoms. Moreover, they have different energies and move randomly in all directions. The Antiproton Decelerator’s bending and focusing magnets keep the antiprotons on the same trajectory, while strong electric fields slow them down. Sideways motion and spread in energies are reduced by passing the antiprotons through the clouds of electrons (“electron cooling”). Eventually, antiprotons are slowed down to 10% of the speed of light and the lowest possible beam energy that can be achieved is 5.3 MeV. Afterwards, antiprotons are sent to the experiments, where they are studied or used to produce atoms of antimatter. The slower the antiprotons, the easier it is for the scientists to study or manipulate them.
The Extra Low ENergy Antiproton (ELENA) ring
Most of the experiments researching antimatter need antiprotons with a kinetic energy of 3-5 keV, which is significantly lower what the AD can achieve. Because of that, experiments use sets of degraders to decelerate further down the antiprotons. Unfortunately, such a method is very inefficient and 99% of antiprotons are lost. The purpose of the ELENA project at CERN is to build a small synchrotron to further decelerate antiprotons from the Antiproton Decelerator from 5.3 MeV to 100 keV. The additional deceleration in a machine equipped with an electron cooler will allow the experiments to increase their antiproton capture capabilities by one to two orders of magnitude. Furthermore, the ELENA will provide an ability to direct the beam almost simultaneously to all experiments, which as a result will significantly increase the total beam time for each experiment.
It was agreed, that a hexagonally shaped ring with a circumference of 30.4 metres will be the best match for the ELENA decelerator with one injection and two extraction lines. The machine will be located within the AD ring, just next to the experimental area. The ELENA ring magnet system is going to be built out of many different types of elements such as bending magnets, quadrupoles, skew quadrupoles, sextupoles, two-plane (H + V) correctors, compensation solenoids, septum and kicker magnets. The electron cooler as a separate device will have an independent magnetic system.
The ELENA will provide particles to experiments every 100 seconds using optical elements that influence the beam using an electric field rather than the traditional magnetic field. The total length of the electrostatic transfer lines is going to be 95 meters. It is foreseen, that the efficiency of the ELENA machine will be 60% and it will provide four beam bunches, each consisting of 
antiprotons. Installation of the ELENA ring and all lines required for commissioning is planned during the second half of 2015 and beginning of 2016. The commissioning is foreseen until the end of 2016, possibly split into two parts, without and with the electron cooler. Its main goal is to commission the ring with injection and extractions lines using an external source of particles. After successful commissioning of the ring, phase two will begin. The existing magnetic transfer lines from the AD to the experiments will be removed and the new electrostatic lines from the ELENA installed. The first physics run with 100 keV antiprotons from the ELENA is planned during the second half of 2017.
Summary
In order to stimulate the antimatter research programme at CERN, it has been necessary to increase the antiproton capture capabilities of the experiments in the Antimatter Factory. The future Extra Low ENergy Antiproton (ELENA) ring will enhance the performance of the antiproton source and will allow the new types of experiments.
