Electron-positron Storage Ring Research Articles

Measurement and modeling of electron-cloud-induced betatron tune shifts at the Cornell Electron-Positron Storage Ring test accelerator

We report on extensive measurements at the Cornell Electron-Positron Storage Ring of electron-cloud-induced betatron tune shifts for trains of positron bunches at 2.1 and 5.3 GeV with bunch populations ranging between $0.64\ifmmode\times\else\texttimes\fi{}{10}^{10}$ and $9.6\ifmmode\times\else\texttimes\fi{}{10}^{10}$. Measurements using a witness bunch with variable distance from the end of the train and variable bunch population provide information on cloud decay and cloud pinching during the bunch passage. We employ Monte Carlo simulations of the reflection and absorption of synchrotron radiation photons to determine the pattern of absorption sites around the circumference of the storage ring. The Geant4 simulation toolkit is used to model the interactions of the photons with the beampipe wall and determine the production energy and location distributions of the photoelectrons which seed the electron cloud. An electron cloud buildup model based on fitted ring-averaged secondary-yield properties of the vacuum chamber predicts tune shifts in good agreement with the measurements.

Local impedance measurements from closed orbit distortion due to wake fields at CESR

Abstract Short range transverse wake fields are excited by charged particles passing through a vacuum chamber. The wake fields can act back on a bunch of particles and alter its trajectory. We have measured the distortion of the closed orbit due to transverse wake fields in the Cornell Electron–positron Storage Ring. A transverse monopole wake field is induced by an asymmetric impedance associated with the insertion of a single vertical scraper. A transverse dipole wake field is generated by displacing the bunch from the symmetry axis of the chamber. The orbit is offset vertically through the gap of a pair of symmetrically inserted scrapers and also in a narrow-gap undulator chamber. A fit to the distorted orbits yields the average kick of the transverse monopole wake of the asymmetric scrapers and the kick factor of the symmetric scrapers; the scraper results are in reasonable agreement with the values from numerical simulation of the wake fields. The kick factor extracted from the closed orbit distortion of the dipole wake induced by the narrow-gap undulator chamber is significantly greater than that predicted by the numerical simulation. Possible sources of the discrepancy are discussed.

Measurement of a double spin asymmetry in the photoproduction of $\pi^{-}$ π - -mesons on deuterons

The paper presents results of a double polarization experiment to extract an asymmetry of the $\pi^{-}$ -meson photoproduction on a tensor-polarized deuteron by a linearly polarized photon. The energy of photons lies in the range (300-700) MeV. The measurements were performed on the internal tensor-polarized deuterium target in the VEPP-3 electron-positron storage ring for the electron beam energy equal to 2 GeV. In the experiment, two protons and a scattered electron were detected in coincidence. The results of the measurement are compared with theoretical predictions made in the framework of impulse approximation, taking into account the $\pi N$ and NN-rescattering in the final state.

Determination of RF resonator axis inclination to beam axis in electron-positron storage ring

We proposed and tested a method of experimental determination of the upper limit of RF resonator axis inclination to the beam axis. The electric field transverse component by such a perturbation deflects the particle trajectory. In the horizontal plane, a distortion of the closed orbit leads to a difference in the energies and spin precession frequencies of electrons and positrons in a storage collider with a single ring.

Direct Observation of Incoherent Cherenkov Diffraction Radiation in the Visible Range.

We report on the observation of incoherent Cherenkov radiation emitted by a 5.3GeV positron beam circulating in the Cornell electron-positron storage ring as the beam passes in the close vicinity of the surface of a fused silica radiator (i.e., at a distance larger than 0.8mm). The shape of the radiator was designed in order to send the Cherenkov photons towards the detector, consisting of a compact optical system equipped with an intensified camera. The optical system allows both the measurements of 2D images and angular distribution including polarization study. The corresponding light intensity has been measured as a function of the distance between the beam and the surface of the radiator and has shown a good agreement with theoretical predictions. For highly relativistic particles, a large amount of incoherent radiation is produced in a wide spectral range. A light yield of 0.8×10^{-3} photon per particle per turn has been measured at a wavelength of 600±10 nm in a 2cm long radiator and for an impact parameter of 1mm. This will find applications in accelerators as noninvasive beam diagnostics for both leptons and hadrons.

Measurement of the Asymmetry of Photoproduction of π– Mesons on Linearly Polarized Deuterons by Linearly Polarized Photons

The first results of a double polarization experiment to extract the asymmetry of the reaction of photoproduction of a π- meson by a linearly polarized photon on a tensor-polarized deuteron in the energy range of the virtual photon (300–700 MeV) are presented. The measurements were performed on an internal tensor-polarized deuterium target in the VEPP-3 electron-positron storage ring for the electron beam energy equal to 2 GeV. The experiment employed the method of recording two protons and the scattered electron in coincidence. The obtained measurement results are compared with the theoretical predictions obtained in the momentum approximation with allowance for πN and NN rescattering in the final state.

Measurement techniques for low emittance tuning and beam dynamics at CESR

After operating as a High Energy Physics electron-positron collider, the Cornell Electron-positron Storage Ring (CESR) has been converted to become a dedicated synchrotron light source for the Cornell High Energy Synchrotron Source (CHESS). Over the course of several years CESR was adapted for accelerator physics research as a test accelerator, capable of studying topics relevant to future damping rings, colliders and light sources. Initially some specific topics were targeted for accelerator physic research with the storage ring in this mode, labeled CesrTA. These topics included 1) tuning techniques to produce low emittance beams, 2) the study of electron cloud (EC) development in a storage ring and 3) intra-beam scattering effects. The complete conversion of CESR to CesrTA occurred over a several year period, described elsewhere [1–3]. A number of specific instruments were developed for CesrTA. Much of the pre-existing instrumentation was modified to accommodate the scope of these studies and these are described in a companion paper [4]. To complete this research, a number of procedures were developed or modified, often requiring coordinated measurements among different instruments [5]. This paper provides an overview of types of measurements employed for the study of beam dynamics during the operation of CesrTA.

Vertical-longitudinal coupling induced by wakefields at the Cornell Electron-Positron Storage Ring Test Accelerator

Transverse vertical wakefields can increase vertical emittance and distort the phase space of a bunch in a storage ring. Here we report recent measurements and simulations of these effects from wakefields from movable scrapers at the Cornell electron-positron storage ring test accelerator. Charge-dependent vertical beam size growth was observed with a single scraper inserted through the top of the storage ring vacuum chamber. No change in the beam size was observed with top and bottom scrapers inserted symmetrically. The apparent growth in the vertical beam size was due in large part to the $yz$ coupling (vertical crabbing) induced by the transverse monopole wake of the asymmetric scraper configuration. We explored this $yz$ coupling and thus the orientation at the observation point by varying the vertical betatron phase advance between the vertical beam size monitor and the scrapers. In addition, we found that existing residual, current-independent $yz$ coupling, perhaps due to nonzero vertical dispersion in the rf cavities, could be compensated by the scraper wake. Predictions from tracking simulations are in good agreement with the measurements. Moreover, the vertical beam size as a function of vertical displacement at narrow-gap chambers (closed scrapers and undulator chamber) was measured. We found the transverse wakefield produced by the off-axis beam could also introduce a $yz$ tilt as well as dilute the beam emittance.

Impedance computations and beam-based measurements: A problem of discrepancy

High intensity of particle beams is crucial for high-performance operation of modern electron–positron storage rings, both colliders and light sources. The beam intensity is limited by the interaction of the beam with self-induced electromagnetic fields (wake fields) proportional to the vacuum chamber impedance. For a new accelerator project, the total broadband impedance is computed by element-wise wake-field simulations using computer codes. For a machine in operation, the impedance can be measured experimentally using beam-based techniques. In this article, a comparative analysis of impedance computations and beam-based measurements is presented for 15 electron–positron storage rings. The measured data and the predictions based on the computed impedance budgets show a significant discrepancy. Three possible reasons for the discrepancy are discussed: interference of the wake fields excited by a beam in adjacent components of the vacuum chamber, effect of computation mesh size, and effect of insufficient bandwidth of the computed impedance.

Instrumentation for the study of low emittance tuning and beam dynamics at CESR

The Cornell Electron-positron Storage Ring (CESR) has been converted from a High Energy Physics electron-positron collider to operate as a dedicated synchrotron light source for the Cornell High Energy Synchrotron Source (CHESS) and to conduct accelerator physics research as a test accelerator, capable of studying topics relevant to future damping rings, colliders and light sources. Some of the specific topics that were targeted for the initial phase of operation of the storage ring in this mode for CESR as a Test Accelerator (CesrTA) included 1) tuning techniques to produce low emittance beams, 2) the study of electron cloud development in a storage ring and 3) intra-beam scattering effects. The complete conversion of CESR to CesrTA occurred over a several year period, described elsewhere [1–3]. In addition to instrumentation for the storage ring, which was created for CesrTA, existing instrumentation was modified to facilitate the entire range of investigations to support these studies. Procedures were developed, often requiring coordinated measurements among different instruments [4]. This paper describes the instruments utilized for the study of beam dynamics during the operation of CesrTA. The treatment of these instruments will remain fairly general in this paper as it focusses on an overview of the instruments themselves. Their interaction and inter-relationships during sequences of observations is found in a companion paper describing the associated measurement techniques. More detailed descriptions and detailed operational performance for some of the instrumentation may be found elsewhere and these will be referenced in the related sections of this paper.

Against the rising flood of information

Every two years or so, Physics Today conducts a reader survey to find out what you like and don’t like about the magazine. If you’ve ever completed one of our surveys, thank you! For the past two decades, the responses have been consistent in their near-universal approval for the Search and Discovery department.That finding is gratifying, not least because I ran the department in 2003–10. Having edited several hundred news stories, I know firsthand how diligently the Search team works to identify and explain significant advances in physics and its related sciences. This cartoon ran with a Search story from September 1970. The topic was the competition among particle physicists to get the first crack at using the 500 GeV beam at the National Accelerator Laboratory (as Fermilab was known before 1976). PPT|High resolutionThe Search and Discovery department made its debut on page 105 of the January 1967 issue. Before then, physics news ran in a department called Research Facilities and Programs. When readers turned to the new department, they found reports on the use of physics in archaeology, the discovery of element 102, the construction of a new electron–positron storage ring by the Midwestern Universities Research Association, a classified NASA investigation into building a giant orbiting mirror to illuminate battlefields at night, and the development of UV lasers.The editor-in-chief at the time, R. Hobart Ellis Jr, chose not to devote one of his regular editorials to the new department. But three years later, the magazine underwent a redesign that entailed, among other things, Search and Discovery being printed on colored paper and moving to the front of the book. About the changes, which appeared in the September 1970 issue, Ellis’s successor, Harold L. Davis, wrote The increased emphasis on these fast-reading news columns is our way of trying to help physicists survive one of the major perils facing modern scientists—that of drowning in the rising flood of unfiltered information. Davis’s words from the pre-internet era might seem quaint. Just how much of an information flood was there in 1967, you might ask? A lot. Fifty years ago most US cities had more than one daily newspaper. Advertising in print and on radio, TV, and billboards was just as pervasive as it is today. Two years earlier in “(I Can’t Get No) Satisfaction,” Mick Jagger sang complainingly of useless information supposed to fire his imagination.As the flood of information continues to rise, Search and Discovery has likely become even more valuable to its readers. On the day I wrote this editorial, I searched EurekAlert!, an online clearinghouse run by AAAS, for press releases from the past 30 days that contained the word “physics.” I found 407. Physics Today can’t ever keep you up to date with everything that’s going on in acoustics, astronomy, atmospheric science, and all the other physical sciences. But once a month we’ll report on a handful of papers from the broad spectrum of physics. And we’ll continue to do that so long as you tell us you appreciate it.© 2017 American Institute of Physics.

Beam position monitoring system at CESR

The Cornell Electron-positron Storage Ring (CESR) has been converted from a High Energy Physics electron-positron collider to operate as a dedicated synchrotron light source for the Cornell High Energy Synchrotron Source (CHESS) and to conduct accelerator physics research as a test accelerator, capable of studying topics relevant to future damping rings, colliders and light sources. Some of the specific topics that were targeted for the initial phase of operation of the storage ring in this mode, labeled CESRTA (CESR as a Test Accelerator), included 1) tuning techniques to produce low emittance beams, 2) the study of electron cloud development in a storage ring and 3) intra-beam scattering effects. The complete conversion of CESR to CESRTA occurred over a several year period and is described elsewhere. As a part of this conversion the CESR beam position monitoring (CBPM) system was completely upgraded to provide the needed instrumental capabilities for these studies. This paper describes the new CBPM system hardware, its function and representative measurements performed by the upgraded system.

Single-shot beam size measurements using visible-light interferometry at CESR

A new primary mirror for a visible-light beam size monitor (vBSM) was designed and installed in the Cornell Electron-Positron Storage Ring (CESR). The vertical angular acceptance of the mirror was doubled to allow double-slit interferometry with large slit separation (>12mm). In addition, the diffraction associated with the first generation mirror has been eliminated. The resolution of the vertical beam size measurements has been dramatically improved but is ultimately limited by the beam motion. Two fast-response detectors, a Photomultiplier Tube (PMT) array and a gated camera, were employed to study the beam motion. The advantages and limitations of both devices are discussed in this paper. The gated camera was also used to measure single-shot beam width and motion of each bunch in a multi-bunch train. We measured significantly more horizontal motion of electron as compared to positron bunch trains in otherwise identical machine condition. This difference may be a signature for the difference between electron cloud build-up for positron bunch trains versus ions effects characteristic of electron bunch trains.

Observation of Electron Cloud Instabilities and Emittance Dilution at the Cornell Electron-Positron Storage Ring Test Accelerator

Electron cloud related emittance dilution and instabilities of bunch trains limit the performance of high intensity circular colliders. One of the key goals of the Cornell electron-positron storage ring Test Accelerator (CesrTA) research program is to improve our understanding of how the electron cloud alters the dynamics of bunches within the train. Single bunch beam diagnotics have been developed to measure the beam spectra, vertical beam size, two important dynamical effects of beams interacting with the electron cloud, for bunch trains on a turn-by-turn basis. Experiments have been performed at CesrTA to probe the interaction of the electron cloud with stored positron bunch trains. The purpose of these experiments was to characterize the dependence of beam-electron cloud interactions on the machine parameters such as bunch spacing, vertical chromaticity, and bunch current. The beam dynamics of the stored beam, in the presence of the electron cloud, was quantified using: 1) a gated beam position monitor (BPM) and spectrum analyzer to measure the bunch-by-bunch frequency spectrum of the bunch trains; 2) an x-ray beam size monitor to record the bunch-by-bunch, turn-by-turn vertical size of each bunch within the trains. In this paper we report on the observations from these experiments and analyze the effects of the electron cloud on the stability of bunches in a train under many different operational conditions.

Memorable Years for Synchrotron Radiation at the Institute of Nuclear Physics in Novosibirsk

In the early 1970s, the Institute of Nuclear Physics (INP) in Novosibirsk was a unique place in the world of accelerator physics. There were three operational electron-positron storage rings at the institution. All together, they covered beam operational energies from 200 MeV up to 2.2 GeV. It was not a big surprise for the developers of these state-of-the-art machines when the first users of synchrotron radiation showed up at the doorsteps of the Institute of Nuclear Physics, eager to take advantage of such unique radiation sources. And how very unique they were! Compared with several already relatively well-established operational synchrotrons around the world, such as DESY in Hamburg, NINA in Darsbury, and three synchrotrons in the Soviet Union—one at the Physical Institute in Pakhra, another at the Tomsk Polytechnical Institute, and a third at the Erevan Physical Institute—the storage ring sources provided much more stable and brighter radiation beams. Several storage rings built at that time in locations such as Japan, the US, and France were also on the verge of becoming available for synchrotron radiation users.

Bruno Touschek: From Betatrons to Electron–Positron Colliders

Bruno Touschek’s life as a physicist spanned the period from World War II to the 1970s. He was a key figure in the developments of electron–positron colliders and storage rings, and made important contributions to theoretical high energy physics. Storage rings, initially developed for high energy physics, are being widely used in many countries as synchrotron radiation sources and are a tool for research in physics, chemistry, biology, environmental sciences and cultural heritage studies. We describe Touschek’s life in Austria, where he was born, in Germany, where he participated in the construction of a betatron during WWII, and in Italy, where he proposed and led to completion the first electron–positron storage ring in 1960, in Frascati. We highlight how his central European culture influenced his lifestyle and work, and his main contributions to physics, such as the discovery of the Touschek effect and beam instabilities in the larger storage ring ADONE.

Visible-light beam size monitors using synchrotron radiation at CESR

A beam profile monitor utilizing visible synchrotron radiation (SR) from a bending magnet has been designed and installed in Cornell Electron-Positron Storage Ring (CESR). The monitor employs a double-slit interferometer to measure both the horizontal and vertical beam sizes over a wide range of beam currents. By varying the separation of the slits, beam sizes ranging from 50 to 500μm can be measured with a resolution of approximately 5μm. To measure larger beam size (>500μm), direct imaging can be employed by rotating the double slits away from SR beam path. By imaging the π-polarized component of SR, a small vertical beam size (∼70μm) was measured during an undulator test run in CESR, which was consistent with the interferometer measurement. To measure the bunch length, a beam splitter is inserted to direct a fraction of light into a streak camera setup. This beam size monitor measures the transverse and longitudinal beam sizes simultaneously, which is successfully used for intrabeam scattering studies. Detailed error analysis is discussed.

Review of collective beam instabilities in electron-positron storage rings

The review of studies of collective beam instabilities in electron-positron storage rings is presented. The processes of excitation and suppression of the longitudinal microwave instability, transverse mode coupling instability, longitudinal and transverse multi-bunch instabilities, and instabilities induced by an interaction of a beam with ions or electron clouds are discussed. Important equations for estimation of the threshold beam currents and the rise time of instabilities, as well as the references to the major original works are given. The methods for diagnostics and suppression of instabilities are considered using specific examples.

Feedback for suppression of single-bunch transverse instability in electron-positron storage rings

Transverse head-tail instability is a severe limitationof a single-bunch beam current in circular accelerators.Applicability and efficiency of feedbacks for suppression of theinstability is analyzed. Both chromatic and nonlinear effects havebeen taken into account to understand the processes of excitationand damping of the instability. Analytical estimations arecompared with the results of experiments and numericalsimulations. A feedback system has been developed, installed andcommissioned at the VEPP-4M electron-positron collider. Anoriginal scheme of the kicker powering has been developed toprovide the necessary performance with minimal expenses. Real-timedigital data processing performed by a code running in an FPGAmodule provides high efficiency and flexibility of the system.During the system commissioning, a more than threefold increase ofintensity of the VEPP-4M single-bunch beam has beenachieved.

Bruno Touschek: particle physicist and father of the e+e− collider

This article gives a brief outline of the life and works of the Austrian physicist Bruno Touschek, who conceived and, 50 years ago, brought to completion the construction of AdA, the first electron-positron storage ring. The events which led to the approval of the AdA project and the Franco-Italian collaboration which confirmed the feasibility of electron-positron storage rings will be recalled. We shall illustrate Bruno Touschek’s formation both as a theoretical physicist and as an expert in particle accelerators during the period between the time he had to leave the Vienna Staatgymnasium in 1938, because of his Jewish origin from the maternal side, until he arrived in Italy in the early 1950s and, in 1960, proposed to build AdA, in Frascati. The events which led to Touschek’s collaboration with Rolf Widerøe in the construction of the first European betatron will be described. The article will make use of a number of unpublished as well as previously unknown documents, which include an early correspondence with Arnold Sommerfeld and Bruno Touschek’s letters to his family in Vienna from Italy, Germany and Great Britain. The impact of Touschek’s work on students and collaborators from University of Rome will be illustrated through his work on QED infrared radiative corrections to high energy e + e − experiments and the book Meccanica Statistica.