Electron cooling: Theory, experiment, application

Anti-hydrogen physics

Advances in ion-source, accelerator and beam-cooling technology have made it possible to produce high-quality beams of atomic ions in arbitrary charged states as well as molecular and cluster ions that are internally cold Ion beams of low emittance and narrow momentum spread are obtained in a new generation of ion storage-cooler rings dedicated to atomic and molecular physics The long storage times ( approximately 5<or approximately= tau <or approximately=days) allow the study of very slow processes occurring in charged (positive and negative) atoms, molecules and clusters Interactions of ions with electrons and/or photons can be studied by merging the stored ion beam with electron and laser beams The physics of storage rings spans particles having a charge-to-mass ratio ranging from <0002(C60+ and C70+) to 04-10 (H+, D+, He2+, , U92+) and collision processes ranging from <1 MeV to approximately 70 GeV It incorporates, in addition to atomic and molecular physics, tests of fundamental physics theories and atomic physics bordering on nuclear and chemical physics This exciting development concerning ion storage rings has taken place within the last five to six years

https://iopscience.iop.org/article/10.1088/0034-4885/58/10/003/pdf

Atomic and molecular physics with ion storage rings

M. Benedikt, A. Blas, J. Borburgh, R. Cappi, M. Chanel, V. Chohan, G. Daems, A. Fowler, R. Garoby, J. Gonzalez, D. Grier, J. Gruber, S. Hancock, C.E. Hill, A. Jansson, E. Jensen, A. Krusche, P. Maesen, K.D. Metzmacher, R. Losito, J. Olsfors, M. Paoluzzi, J. Pedersen, U. Raich, J.P. Riunaud, J.P. Royer, M. Sassowsky, K. Schindl, H. Schönauer, L. Sermeus, M. Thivent, H. Ullrich, W. Van Cauter, F. Völker, M. Vretenar.

The PS Complex as Proton Pre-Injector for the LHC - Design and Implementation Report

At the Large Hadron Collider (LHC), the world’s largest and highest energy particle accelerator, ion bunches circulate in two counter-rotating beams and are brought into collision. Each bunch is confined within a bucket by the longitudinal focusing effect of the radio frequency (RF) cavities. The RF period is 2.5 ns, while the minimum bunch spacing is 25 ns. Thus, 9 out of every 10 buckets should be empty, as well as additional gaps to allow for the rise-time of injection and dump kickers. In practice, however, small numbers of particles can occupy these supposedly empty buckets, causing problems for machine protection and for the absolute calibration of the LHC’s luminosity. The Longitudinal Density Monitor (LDM) is a new monitor, designed to measure the longitudinal distribution of particles in the LHC with a sufficiently high dynamic range to quantify the relative particle population in the supposedly empty buckets. A non-interceptive measurement is made possible by the use of synchrotron radiation (SR). Single photon counting with an avalanche photo-diode operating in Geiger mode allows a very high dynamic range to be achieved despite the low levels of light available. The imperfect response of the avalanche photo-diode is compensated using a specially designed correction algorithm which reduces noise and distortion to a minimum. This work presents the design, implementation and operation of the LDM. Signal correction methods are discussed with reference to the deadtime and afterpulsing of the avalanche photodiode, and the analysis of the LDM data for use in LHC luminosity calibration is explained. Experimental results with both proton and heavy ion beams are shown illustrating the LDM‘s exceptional performance, combining a high dynamic range of 105 with a 90 ps time resolution. Finally, a novel scheme to extend the dynamic range by several more orders of magnitude is presented.

https://link.aps.org/pdf/10.1103/PhysRevSTAB.15.032803

Longitudinal density monitor for the LHC

A sizeable part of the LHC physics programme foresees lead-lead collisions with a design luminosity of 10 cms. This will be achieved after an upgrade of the ion injector chain comprising Linac3, LEIR, PS and SPS machines [1,2]. Each LHC ring will be filled in 10 min by almost 600 bunches, each of 7×10 lead ions. Central to the scheme is the Low Energy Ion Ring (LEIR) [3,4], which transforms long pulses from Linac3 into highbrilliance bunches by means of multi-turn injection, electron cooling and accumulation. Major limitations along the chain, including space charge, intrabeam scattering, vacuum issues and emittance preservation are highlighted. The conversion from LEAR (Low Energy Antiproton Ring) to LEIR involves new magnets and power converters, high-current electron cooling, broadband RF cavities, and a UHV vacuum system with getter (NEG) coatings to achieve a few 10 mbar. Major hardware changes in Linac3 and the PS are also covered. An early ion scheme with fewer bunches (but each at nominal intensity) reduces the work required for early LHC ion operation in spring 2008.

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The LHC Lead Injector Chain

First electron cooling experiments were performed with 10 7 to 2×10 9 stored antiprotons of 50, 21 and 6 MeV at the Low Energy Antiproton Ring (LEAR) at CERN. Most effort was put into the study of the longitudinal cooling. Schottky pick-up signals were used to measure the equilibrium momentum spread and the longitudinal cooling time. From the equilibrium between stochastic heating and electron cooling the longitudinal friction force in the low 10 3 m/s relative velocity range could be deduced. This method was used also to increase the cooling force by improving the alignment between the antiproton and the electron beam. Some of the experimental data are compared with results of a simulation program for electron cooling (SPEC).

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Further results and evaluation of electron cooling experiments at LEAR

The first results are presented of electron cooling experiments in the Low-Energy Antiproton Ring (LEAR) at CERN, performed with a proton beam of about 50 and 21 MeV. The number of stored protons ranged from 107 to 3 × 109. Cooling times of the order 1 s and proton drag rates of up to 0.7 MeV/s were obtained. The capture of cooling electrons by protons producing hydrogen atoms was used to derive an effective electron temperature (0.25 eV). From the angular profile of the neutral hydrogen beam an upper limit of 3π mm.mrad could be deduced for the horizontal equilibrium proton-beam emittance. The lowest equilibrium momentum spread was 2 × 105 (FWHM), as derived from the analysis of the longitudinal Schottky signal. This Schottky signal exhibited an unusual behaviour with beam intensity and under certain conditions showed a doublepeak structure which was associated with collective beam noise. For very cold beams transverse instabilities were observed, which resulted in a rapid spill-off of protons and a stabilization at lower intensities. The threshold of these instabilities was raised by heating the proton or the electron beam. The cooling of a bunched proton beam was investigated. The reduction of the proton momentum spread led to bunch lengths of about 2 m, containing 3 × 108 protons.

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First results of electron cooling experiments at LEAR

Recent work on the electron cooling device at the CERN Low Energy Antiproton Ring (LEAR) is summarized. This included an improvement of the collector performance and of the vacuum. Relations between the different cooler parameters were measured with a view to simplifying the control system. Results of vacuum tests performed before the installation in LEAR are presented, and a short description of the status of the cooling device after its installation is given.

The LEAR Electron Cooler: Recent Improvements and Tests

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C.E. Hill

Papers

The LHC Lead Injector Chain

Further results and evaluation of electron cooling experiments at LEAR

First results of electron cooling experiments at LEAR

The LEAR Electron Cooler: Recent Improvements and Tests

Performance of the CERN Linac 2 with a high intensity proton RFQ