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LS2 Report: waiting for antiprotons...

In the AD hall, a beam of H- ions has reached the GBAR and ALPHA experiments, marking the end of the installation of new transfer lines from the ELENA deceleration ring


ELENA Experiment's Connexion with GBar
Transfer line connecting the ELENA ring (behind the wall on the right) to the GBAR experiment (left) (Image: CERN)

“The Antiproton Decelerator (AD) is like CERN in miniature: it has two synchrotrons circulating particles and five experiments at the end of transfer lines, all located in a semi-buried hall. It’s small, but it’s a great place to work!” François Butin, Technical Coordinator for the AD (also known as the antimatter factory), might have added that it is a CERN in reverse: in a laboratory that works to accelerate particles, the AD and its new colleague ELENA (Extra Low Energy Antiproton deceleration ring) decelerate particles so that they reach very low energies. Three years after the first antiprotons were circulated in ELENA, the new ring has been able to inject H- ions (used in place of antimatter until the restart of the Proton Synchrotron) and deliver them to the GBAR and ALPHA experiments for the first time. This is possible thanks to the new transfer lines that have been installed during Long Shutdown 2 (LS2).

Despite its modest 30-metre circumference, ELENA is the new keystone of the antimatter factory. ELENA receives antiprotons from the AD with an energy of 5.3 MeV and decelerates them to 0.1 MeV, which avoids the need for deceleration systems that are responsible for particle loss.

The new GBAR experiment has been connected to ELENA since it was installed in 2017, but the other experiments remained connected to the AD. Since November 2018, LS2 has largely been spent dismantling and then replacing the old transfer lines between ELENA and ALPHA, AEgIS, ASACUSA, BASE and other potential future experiments.

Far from being exact copies of their predecessors, the new transfer lines have introduced innovative technology to the antimatter factory: the old electromagnets have been replaced by dipole and quadrupole electrostatic plates, which are enough to guide the particles that have been slowed down to such an extent. By not using magnet systems, the antimatter factory has opted for a solution that is economical and frees up space to increase the density of the quadrupoles, thereby further stabilising the beam. There have also been innovations in instrumentation: the previous GEM (Gas Electron Multiplier) beam control systems have made way for Secondary Emission Monitors (SEM), which allow increased and non-destructive control of the beam along the entire transfer lines.

“The experiments and operations team will have access to a denser, more stable and better controlled beam of antiprotons that have been slowed down by ELENA”, explains Christian Carli, ELENA project leader. The experiments will be able to benefit from these advantages once the Proton Synchrotron (PS) and the AD target (also the subject of major renovation) start up again.

The first diagnostics of the H- ion beams travelling through the new lines to the experiments are very positive: “The optics are good, as is the size of the beams. After two years working towards this goal, it’s so rewarding to see the particles injected into the lines!”, says Laurette Ponce, who is responsible for AD and ELENA operations. For Laurette and the operations team, it’s the beginning of several months of work to characterise the ion beam, before the AD can supply antiprotons to the new transfer lines and the experiments linked to ELENA.