The Antiproton Decelerator of CERN began operation in 1999 to serve experiments for studies of CPT invariance by precision laser and microwave spectroscopy of antihydrogen ($\bar{\rm H}$) and antiprotonic helium ($\bar{p}{\rm He}^+$). The first 12 years of operation saw cold $\bar{\rm H}$ synthesized by overlapping clouds of positrons ($e^+$) and antiprotons ($\bar{p}$) confined in magnetic Penning traps. Cold $\bar{\rm H}$ was also produced in collisions between Rydberg positronium atoms and $\bar{p}$. Ground-state $\bar{\rm H}$ was later trapped for up to $\sim 1000$ s in a magnetic bottle trap, and microwave transitions excited between its hyperfine levels. In the $\bar{p}{\rm He}^+$ atom, UV transitions were measured to a precision of (2.3-5) $\times$ $10^{-9}$ by sub-Doppler two-photon laser spectroscopy. From this the antiproton-to-electron mass ratio was determined as $M_{\bar{p}}/m_e=$1836.1526736(23), which agrees with the p value. Microwave spectroscopy of $\bar{p}{\rm He}^+$ yielded a measurement of the $\bar{p}$ magnetic moment with a precision of 0.3%. More recently the magnetic moment of a single $\bar{p}$ confined in a Penning trap was measured with a higher precision, as $\mu_{\bar{p}}=-2.792845(12)$$\mu_{\rm nucl}$ in nuclear magnetons. Other measurements include the energy loss of 1-100 keV $\bar{p}$ traversing conductor and insulator targets; the cross sections of <10 keV $\bar{p}$ ionizing gas targets; and the cross sections of 5-MeV $\bar{p}$ annihilating on target foils via nuclear collisions. The biological effectiveness of $\bar{p}$ beams destroying cancer cells was measured as a possible method for radiological therapy. New experiments under preparation attempt to measure the gravitational acceleration of $\bar{\rm H}$ or synthesize $\obar{\rm H}^+$.

Physics at CERN's Antiproton Decelerator

At the last LEAP conference in Vancouver 2011 the authors stated that a project ”ELENA”, as an abbreviation for Extra Low ENergy Antiproton ring and as first discussed in 1982 for LEAR by H. Herr et al., was freshly proposed with a substantial new design and revised layout and that it was under consideration to be built at CERN. ELENA is an upgrade of the Anti-proton Decelerator (AD) at CERN and is devoted to special experiments with physics using low energy anti-protons. The main topics are the anti-hydrogen production and consecutive studies of the features of this anti-matter atom as well as the anti-proton nucleon interaction by testing the QED to high precision. During the last years the project underwent several steps in presentations at different committees at CERN and was finally approved such that the construction has started. ELENA will increase the number of useful anti-protons by about two orders of magnitude and will allow to serve up to four experiments simultaneously. Very first and convincing results from the experiments at the AD have been published recently. For high precision physics, however, it appears to be cumbersome, time consuming and ineffective when collecting the needed large numbers and high densities of anti-proton clouds with the present AD. Both the effectiveness and the availability for additional experiments at this unique facility will drastically increase, when the anti-proton beam of presently 5 MeV kinetic energy is reduced by the additional decelerator ELENA to 100 keV.

ELENA: the extra low energy anti-proton facility at CERN

This paper gives an overview of the evolution of stochastic beam cooling from the very beginning (in 1968) until the present-day (2011). The early history, the main achievements and the growing number of the worldwide applications are outlined.

/pdf/history-of-stochastic-beam-cooling-and-its-application-in-4bbyaqcjiw.pdf

History of stochastic beam cooling and its application in many different projects

The Extra Low Energy Antiproton ring (ELENA) is a CERN project aiming at constructing a small 30 m circumference synchrotron to further decelerate antiprotons from the Antiproton Decelerator AD from 5.3 MeV to 100 keV. Controlled deceleration in a synchrotron equipped with an electron cooler to reduce emittances in all three planes will allow the existing AD experiments to increase substantially their antiproton capture efficiencies and render new experiments possible. The ELENA design is now well advanced and the project is moving to the implementation phase. Component design and construction are taking place at present for installation foreseen during the second half of 2015 and beginning of 2016 followed by ring commissioning until the end of 2016. New electrostatic transfer lines to the experiments will be installed and commissioned during the first half of 2017 followed by the first physics operation with ELENA. Basic limitations like Intra Beam Scattering limiting the emittances obtained under electron cooling and direct space charge effects are reviewed and the status of the project is reported.

https://cds.cern.ch/record/1742277/files/CERN-ACC-2014-0144.pdf

Extra Low ENergy Antiproton ring ELENA: From the Conception to the Implementation Phase

In 2002, the ATHENA experiment was the first to produce large amounts of antihydrogen atoms at the CERN Antiproton Decelerator (AD) In this review article, we collect and discuss all the relevant results of the experiment: antiproton and positron cooling and their recombination dynamics in the nested Penning trap, the methods used to unambiguously identify the antiatoms as well as the protonium background, the dependence of the antihydrogen formation on mixing time and temperature An attempt to interpret the results in terms of the two-body and three-body formation reactions, taking into account the complicated nested-trap dynamics, is also made The relevance of the ATHENA results on future experiments is discussed, together with a short overview of the current antimatter physics at the AD

The ATHENA experiment for the study of antihydrogen

To produce dense antiproton beams at very low energies (100 keV), it has been proposed to install a small decelerator ring between the existing AD ring and the experimental area. Phase-space blowup during deceleration is compensated by electron cooling such that the final emittances are comparable to the 5MeV beam presently delivered by the AD. An immediate consequence is a significant increase in the number of trapped antiprotons at the experiments as outlined in the proposal CERN/SPSC-2009-026; SPSC-P-338. This report describes the machine parameters and layout of the proposed ELENA (Extra Low ENergy Antiproton) ring and also gives an approximate estimate of cost and manpower needs. Since the initial estimate, published in 2007 (CERN-AB-2007-079), the ELENA design has evolved considerably. This is due to a new location in the AD hall to accommodate for the possibility of another experimental zone, as suggested by the SPSC, and also due to improvements in the ring optics and layout. The cost estimate that is presented is based on the initial document and has been updated according to the improvements in the design and layout. Where applicable, inflation has also been taken into account. The SPSC has recognized the substantial potential impact of ELENA on the AD experimental program and “strongly supports the ELENA proposal” (minutes of the meeting 29.09.2009) and the RB endorsed the conclusion from the SPSC concerning the strong scientific case for ELENA (minutes of the meeting 02.12.2009).

/pdf/elena-an-updated-cost-and-feasibility-study-36nr5tcm2v.pdf

ELENA – An Updated Cost and Feasibility Study

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ELENA – An Updated Cost and Feasibility Study