Electron cooling: Theory, experiment, application

Mass and lifetime measurements lead to the discovery and understanding of basic properties of matter. The isotopic nature of the chemical elements, nuclear binding, and the location and strength of nuclear shells are the most outstanding examples leading to the development of the first nuclear models. More recent are the discoveries of new structures of nuclides far from the valley of stability. A new generation of direct mass measurements which allows the exploration of extended areas of the nuclear mass surface with high accuracy has been opened up with the combination of the Experimental Storage Ring ESR and the FRragment Separator FRS at GSI Darmstadt. In-flight separated nuclei are stored in the ring. Their masses are directly determined from the revolution frequency. Dependent on the half-life two complementary methods are applied. Schottky Mass Spectrometry SMS relies on the measurement of the revolution frequency of electron cooled stored ions. The cooling time determines the lower half-life limit to the order of seconds. For Isochronous Mass Spectrometry IMS the ring is operated in an isochronous ion-optical mode. The revolution frequency of the individual ions coasting in the ring is measured using a time-of-flight method. Nuclides with lifetimes down to microseconds become accessible. With SMS masses of several hundreds nuclides have been measured simultaneously with an accuracy in the 2 x 10(-7)-range. This high accuracy and the ability to study large areas of the mass surface are ideal tools to discover new nuclear structure properties and to guide improvements for theoretical mass models. In addition, nuclear half-lives of stored bare and highly charged ions have been measured. This new experimental development is a significant progress since nuclear decay characteristics are mostly known for neutral atoms. For bare and highly charged ions new nuclear decay modes become possible, such as bound-state beta decay. Dramatic changes in the nuclear lifetime have been observed in highly charged ions compared to neutral atoms due to blocking of nuclear decay channels caused by the modified atomic interaction. High ionization degrees prevail in hot stellar matter and thus these experiments have great relevance for the understanding of the synthesis of elements in the universe and astrophysical scenarios in general.

Mass and lifetime measurements of exotic nuclei in storage rings

The electron-ion capture rate for low electron energies is calculated for various electron velocity distributions. Capture rates for electron-ion recombination stimulated by irradiation with light are evaluated. The results are applied to electron cooling and to positron-antiproton recombination to form antihydrogen. It is shown that laser-induced capture is a powerful method to study the electron cooling process and to maximize the antihydrogen rate. With this technique a pulsed antihydrogen beam of selectable energy and well collimated with an intensity of a few atoms per second can be anticipated.

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Laser-enhanced electron-ion capture and antihydrogen formation

▪ Abstract The development of heavy-ion storage-cooler rings for atomic physics has made it possible to produce high-quality beams of molecular ions that are internally cold. The stored molecular-ion beam is immersed in a cold electron bath, which gives a beam of low divergence and small cross-sectional area. The electron cooler also serves as a target for electron–molecular ion collision experiments. This allows the study of dissociative recombination of cold molecules with respect to cross sections, branching ratios, and angular distibutions at an unprecedented luminosity.

Dissociative recombination with ion storage rings.

We report on an experimental demonstration of electron cooling of high-energy antiprotons circulating in a storage ring. In our experiments, electron cooling, a well-established method at low energies (< 500 MeV/nucleon), was carried out in a new region of beam parameters, requiring a multi-MeV dc electron beam and an unusual beam transport line. In this letter we present the results of the longitudinal cooling force measurements and compare them with theoretical predictions.

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Experimental demonstration of relativistic electron cooling.

Experimental Studies of electron Cooling

Experiments on electron cooling were carried out on the antiproton storage ring, NAP-M, using 50 MeV proton beams, cooled with 27 keV electrons from a three electrode gun. The proton beam lifetime increased from 900 s to 5,000 s with electron cooling. Diagnostic methods are described, and some possible applications of electron cooling are discussed. (PMA)

Experiments on Electron Cooling

An electron-positron colliding beam facility is described. The facility comprises a 4*7 GeV B-factory with a luminosity of 5*10/sup 33/ cm/sup -2/ s/sup -1/; a phi -factory with a luminosity of over 10/sup 33/ cm/sup -2/ s/sup -1/; an injector consisting of a linear accelerator and a cooling storage ring for providing the factories with intense electron and positron beams. After investigating the relevant problems of accelerator physics and running the first cycle of experiments with the CMD-3 detector, the second stage of the project will come into being. It will be devoted to increasing the luminosity. To attain this, the magnetic structure and the storage ring design provide for a three-bunched beam regime applying electrostatic beam separation at the side interaction points. >

Status of the Novosibirsk phi-factory project

High-luminosity projects imply collisions of intense short bunches with micro-beta at the IP, which entails high variations of the particle energy on the IP passage, due to the opposing bunch electric fields. This effect is shown to limit the attainability of the incoherent and coherent longitudinal motion which enhance the required RF voltage depending on the design beta minimum at the IP. The results of this analytical treatment and computer simulation of the off-axis particle synchrobetatron motion present the limitations, imposed on the space charge parameter by the longitudinal beam-beam effects, and the main parameters of the machine. >

Longitudinal beam-beam effects for an ultra-high luminosity regime

A method of colliding beams is developed to study the fundamental properties of matter in two directions. The first direction is aimed at achieving higher energies allowing the discovery of new particles and fields. The goal of the second one is the study of more delicate fundamental properties of quarks and fields by increasing luminosity in crudely studied energy ranges. The study of CP violation is the most famous example. To study the CP violation in the region of B-quarks, the project of a B-factory as an asymmetrical electron-positron collider with a beam energy of 7/4 GeV and luminosity up to 10/sup 34/ cm/sup -2/ s/sup -1/ was developed at BINP. During the development of this project, it became cleat that the Institute's economical situation, which grew worse in the period of 1990-1993, did not allow us to carry out this project in a reasonable time. Besides, Japan (KEK) and USA (SLAG) began to build the B-factories. Therefore, it is hardly probable to obtain any new results on the third B-factory. On the other hand, a beam energy region of 0.7-2.5 GeV, which still is very interesting, is not studied completely. The interest in this field of research is due to a great body of information obtained on a LEAR facility which is utilized to study proton-antiproton annihilation approximately in this energy region. Studying the cross-section of production of observed resonances in electron-positron annihilation noticeably increases the value and reliability of their interpretation. In 1993, as a result of discussions, the International Program Committee on High-Energy Physics of the Ministry of Science recommended to reorientate the Novosibirsk project VEPP-5 into a Tau-Charm-factory with high luminosity.

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A.N. Skrinsky

Papers

Experimental Studies of electron Cooling

Experiments on Electron Cooling

Status of the Novosibirsk phi-factory project

Longitudinal beam-beam effects for an ultra-high luminosity regime

Novosibirsk Tau-Charm factory design study