Electron cooling: Theory, experiment, application

The use of positrons in laboratory plasma physics experiments is considered. Recent progress in this area is discussed, including the creation of a single‐component positron plasma in the laboratory. Specific applications of such antimatter plasmas are also discussed, with emphasis on areas where existing plasma physics technology and that currently under development are likely to produce results in the next few years.

Use of the positron as a plasma particle

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

We have performed for the first time precision spectroscopy on a coasting fast7Li+ ion beam in a storage ring. The ion beam moving with 6.4% speed of light was first electron cooled and then merged with two counterpropagating laser beams acting on two different hyperfine transitions sharing a common upper level (λ-system). One laser was frequency locked to thea
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127J2 hfs frequency component established as a secondary frequency standard at 514 nm. The second laser was tuned over theλ-resonance, which was recorded relative to127J2 hfs components. This experiment is sensitive to the time dilation in fast moving frames and will lead to new limits for the verification of special relatively. The present status of the experiment and perspectives in accuracy are discussed.

Measurement of the transverse Doppler shift using a stored relativistic 7 Li + ion beam

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

First results are presented from an experiment scattering laser light from a relativistic electron beam. The 5 cm diameter continuous electron beam of 28 keV kinetic energy and 2.6 A current presents an electron gas of a density of 8×107 cm−3, from which 20 ns pulses of laser light (490 nm) were scattered at a repetition rate of 15 Hz and an average power of 20 mJ per pulse. The Doppler-shifted wavelength of photons backscattered under 180° was analysed with a Fabry-Perot interferometer. This technique provides, for the first time, a non-destructive measurement of the velocity distribution in an electron beam radially resolved in space. The results presented here comprise the direct measurement of the absolute electron energy and the degree of space-charge compensation in the electron beam. The determination of an upper bound of 10−2 for the ratio of longitudinal to transverse electron temperature implies the first direct measurement of a flattened velocity distribution.

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Measurement of Laser Light Thomson-Scattered from a Cooling Electron Beam

We propose a scheme to stack and accumulate positrons, emitted randomly from a radioactive source. The positrons are moderated and accumulated at low energy.

B. Seligmann

Papers

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

Measurement of Laser Light Thomson-Scattered from a Cooling Electron Beam

Positron accumulation and storage for antihydrogen production