Cornell Electron Storage Ring Research Articles

Lattice design and experimental studies of nonlinear resonance at the Cornell Electron Storage Ring

Lattice design and experimental studies of nonlinear resonance at the Cornell Electron Storage Ring

Study of transverse resonance island buckets at CESR

A 6-GeV lattice with the horizontal tune near a 3rd-order resonance line at 3νx =2 is designed for studying the transverse resonance island buckets (TRIBs) at the Cornell Electron Storage Ring (CESR). The distribution of 76 sextupoles powered individually is optimized to maximize the dynamic aperture and achieve the desired amplitude-dependent tune shift αxx and the resonant driving term h 30000, which are necessary conditions to form stable island buckets. The particle tracking simulations are developed to check and confirm the formation of TRIBs at different tunes with clearing kicks in this TRIBs lattice. Finally, the lattice is loaded in CESR and the TRIBs are successfully observed when the horizontal fractional tune is adjusted to 0.665, close to the 3rd-order resonance line. Bunch-by-bunch feedback is also explored to clear the particles in the main bucket and the island buckets, respectively.

Simulation of the transit-time optical stochastic cooling process in the Cornell Electron Storage Ring

In preparation for a demonstration of optical stochastic cooling in the Cornell Electron Storage Ring (CESR) we have developed a particle tracking simulation to study the relevant beam dynamics. Optical radiation emitted in the pickup undulator gives a momentum kick to that same particle in the kicker undulator. The optics of the electron bypass from pickup to kicker couples betatron amplitude and momentum offset to path length so that the momentum kick reduces emittance and momentum spread. Nearby electrons contribute an incoherent noise. Layout of the bypass line is presented that accommodates optics with a range of transverse and longitudinal cooling parameters. The simulation is used to determine cooling rates and their dependence on bunch and lattice parameters for bypass optics with distinct emittance and momentum acceptance.

Quantitative assessment of finite-element models for magnetostatic field calculations

We present quantitative means for assessing the numerical accuracy of static magnetic field calculations in finite-element models. Our calculations use the three-dimensional Opera simulation software suite of Dassault Systèmes. Our need to assess the effects of fringe fields requires such a 3D algorithm. While we do discuss and compare our approach to a method of accuracy estimation used in the Opera post-processor, our methods are generally applicable to any model using relaxation techniques in finite-element systems. For purposes of illustration, we present modeling and analysis of two types of quadrupole electromagnets presently in operation in the south arc of the Cornell Electron Storage Ring (CESR). Calculations of field multipole expansion coefficients and numerical deviations from Maxwell’s equations in source-free regions are discussed, with emphasis on the dependence of their accuracy on changes to the finite-element model. Successive refinement steps in the finite-element model for the non-extraction type of CESR south arc quadrupole achieve a reduction in the RMS value of the longitudinal component of the curl vector on the magnet axis by a factor of nearly 70 from 4.38×10−2 T/m to 6.36×10−4 T/m, which is 0.0021% of the field gradient. An accuracy of 2.9% is achieved for a dodecapole coefficient of 2.4×10−4 of that of the quadrupole in the longitudinal integral of the vertical field gradient at a radius of 1 cm.

Vacuum system design, construction, and operation for the Cornell High Energy Synchrotron Source upgrade

During the summer and fall 2018, the Cornell High Energy Synchrotron Source (CHESS), underwent an upgrade that recast the facility as primarily a low emittance X-ray source. The upgrade replaced one sixth of the Cornell Electron Storage Ring (CESR) with six double-bend achromats, featuring four novel CHESS Compact Undulator (CCU) insertion devices, and switched CESR to single particle beam operation. At the time of this writing, the upgraded storage ring has achieved the CHESS-U design goal of operating with 200 mA of positron current at 6.0 GeV beam energy. Seven out of eleven CHESS-U beamlines are currently in X-ray user operation. The construction and the commissioning of the new section of the CESR accelerator required novel solutions to design, fabrication, and vacuum system operations. Significant design considerations and efforts were made to minimize X-ray ‘dark-time’, returning to operation less than 1 year. This paper describes the important milestones of the process, the solutions implemented, and the present status of CHESS-U.

Optical stochastic cooling with an arc bypass in the Cornell Electron Storage Ring

A proposed experiment to demonstrate Optical Stochastic Cooling (OSC) in the Cornell Electron Storage Ring (CESR) based on an arc-bypass design is presented. This arc-bypass provides significantly longer optical delay than the dog-leg style chicane, opening up the possibility of a multi-pass or staged optical amplifier that can achieve the gains required for effective cooling of hadron or heavy-ions. Beyond introducing the arc-bypass, in this paper we study the stability requirements for the dipoles comprising it and investigate the use of an optical feedback system to relax the dipole and light-path stability tolerances.

Generation of incoherent Cherenkov diffraction radiation in synchrotrons

Incoherent Cherenkov diffraction radiation was recently produced in the Cornell electron storage ring using counterpropagating beams (electrons and positrons) passing in the close vicinity of a dielectric made of fused silica. We present in this paper a collection of the experimental investigations that were performed on Cherenkov diffraction radiation in both the infrared and the visible range. Measurements were performed using an optical system functioning either in imaging conditions or in far field conditions to retrieve the angular distribution of the radiation. Polarization studies were also performed and showed that, when selecting the appropriate polarization, the beam size can be measured accurately. This study opens the path for new applications in noninvasive beam diagnostic for highly relativistic charged particle beams.

Design and fabrication of crotch absorbers with beryllium diffuser for Cornell high energy synchrotron source upgrade

With the CHESS-U upgrade in 2018, the south arc region of Cornell Electron Storage Ring (CESR) has been replaced with new magnets and vacuum chambers, and CESR will operate with higher beam energy and beam current at lower beam emittance. A new crotch absorber has been designed for the upgrade, based on previous CHESS crotch absorbers. The main body of the crotch absorber is a water-cooled hollow cylinder made of oxygen-free high-conductivity copper, and it has a 5-mm thick beryllium band brazed on the outside in the middle section to reduce the power density it absorbs on the surface. In this paper, we describe the mechanical design, present finite element thermal and structural analysis results with a focus on the region with high temperature and high thermal stress, and discuss the fatigue lifetime of the absorber. For typical operation with 6 GeV beam energy and a 200 mA beam current, the highest temperature on the crotch is approximately 300 °C based on the thermal analysis. Correspondingly, the fatigue lifetime is conservatively estimated to be around 4 × 104 cycles. We also show some details in the fabrication of the crotch assembly, especially about the vacuum furnace brazing joint between the beryllium band and the copper main body, including X-ray fluorescence inspection of the brazing joint. In addition, in order to have a relevant comparison in an effort to estimate the fatigue lifetime, we carry out finite element analyses (FEA) for the low cycle fatigue experiment with beryllium tiles using electron-beam heating of Watson et al. [Fusion Eng. Des. 37, 553 (1997)] in 1990s and provide new understanding for some of their experimental observations. These analyses assure the effectiveness of the design.

Online storage ring optimization using dimension-reduction and genetic algorithms

Particle storage rings are a rich application domain for online optimization algorithms. The Cornell Electron Storage Ring (CESR) has hundreds of independently powered magnets, making it a high-dimensional test-problem for algorithmic tuning. We investigate algorithms that restrict the search space to a small number of linear combinations of parameters ("knobs") which contain most of the effect on our chosen objective (the vertical emittance), thus enabling efficient tuning. We report experimental tests at CESR that use dimension-reduction techniques to transform an 81-dimensional space to an 8-dimensional one which may be efficiently minimized using one-dimensional parameter scans. We also report an experimental test of a multi-objective genetic algorithm using these knobs that results in emittance improvements comparable to state-of-the-art algorithms, but with increased control over orbit errors.

Accelerator design for the Cornell High Energy Synchrotron Source upgrade

During the summer and fall of 2018 the Cornell High Energy Synchrotron Source (CHESS) is undergoing an upgrade to increase high-energy flux for x-ray users. The upgrade requires replacing one-sixth of the Cornell Electron Storage Ring (CESR), inverting the polarity of half of the CHESS beam lines, and switching to single-beam on-axis operation. The new sextant is comprised of six double-bend achromats (DBAs) with combined-function dipole-quadrupoles. Although the DBA design is widely utilized and well understood, the constraints for the CESR modifications make the CHESS-U lattice unique. This paper describes the design objectives, constraints, and implementation for the CESR accelerator upgrade for CHESS-U.

Optical diffraction radiation for position monitoring of charged particle beams

In the framework of the future linear collider collaboration (CLIC, ILC), non-intercepting beam monitoring instruments are under development for very low emittance and high charge density beams. Optical diffraction radiation (ODR) was studied and developed during the last years focussing on beam size measurements. We propose in the paper to consider the use of diffraction radiation for ultra relativistic beams as position monitors with applications for the centering of scrapers, collimators and targets with high resolution. We present the experimental results obtained using small aperture slits on the ATF2 extraction beam line at KEK and on the Cornell Electron Storage Ring with 1.2GeV and 2.1GeV electrons respectively.

Minimization of Emittance at the Cornell Electron Storage Ring With Sloppy Models

Minimization of Emittance at the Cornell Electron Storage Ring With Sloppy Models

Beam induced electron cloud resonances in dipole magnetic fields

The buildup of low energy electrons in an accelerator, known as electron cloud, can be severely detrimental to machine performance. Under certain beam conditions, the beam can become resonant with the cloud dynamics, accelerating the buildup of electrons. This paper will examine two such effects: multipacting resonances, in which the cloud development time is resonant with the bunch spacing, and cyclotron resonances, in which the cyclotron period of electrons in a magnetic field is a multiple of bunch spacing. Both resonances have been studied directly in dipole fields using retarding field analyzers installed in the Cornell Electron Storage Ring (CESR). These measurements are supported by both analytical models and computer simulations.

ExclusiveDssemileptonic branching fraction measurements

We measure absolute branching fractions for six exclusive ${D}_{s}$ semileptonic decays. We use data collected in the CLEO-c detector from ${e}^{+}{e}^{\ensuremath{-}}$ annihilations delivered by the Cornell Electron Storage Ring with a center-of-mass energy near 4170 MeV. We find $\mathcal{B}({D}_{s}\ensuremath{\rightarrow}\ensuremath{\phi}e\ensuremath{\nu})=(2.14\ifmmode\pm\else\textpm\fi{}0.17\ifmmode\pm\else\textpm\fi{}0.08)%$, $\mathcal{B}({D}_{s}\ensuremath{\rightarrow}\ensuremath{\eta}e\ensuremath{\nu})=(2.28\ifmmode\pm\else\textpm\fi{}0.14\ifmmode\pm\else\textpm\fi{}0.19)%$, and $\mathcal{B}({D}_{s}\ensuremath{\rightarrow}{\ensuremath{\eta}}^{\ensuremath{'}}e\ensuremath{\nu})=(0.68\ifmmode\pm\else\textpm\fi{}0.15\ifmmode\pm\else\textpm\fi{}0.06)%$ for the largest modes, where the first uncertainties are statistical and the second are systematic. We also obtain $\mathcal{B}({D}_{s}\ensuremath{\rightarrow}{K}^{0}e\ensuremath{\nu})=(0.39\ifmmode\pm\else\textpm\fi{}0.08\ifmmode\pm\else\textpm\fi{}0.03)%$, $\mathcal{B}({D}_{s}\ensuremath{\rightarrow}{K}^{*}e\ensuremath{\nu})=(0.18\ifmmode\pm\else\textpm\fi{}0.04\ifmmode\pm\else\textpm\fi{}0.01)%$, and $\mathcal{B}({D}_{s}\ensuremath{\rightarrow}{f}_{0}e\ensuremath{\nu},{f}_{0}\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\pi})=(0.13\ifmmode\pm\else\textpm\fi{}0.03\ifmmode\pm\else\textpm\fi{}0.01)%$ for ${f}_{0}$ masses within 60 MeV of 980 MeV. We use our results to determine the $\ensuremath{\eta}\ensuremath{-}{\ensuremath{\eta}}^{\ensuremath{'}}$ and ${f}_{0}$ mixing angles with $s\overline{s}$, and we combine our results with lattice calculations to estimate $|{V}_{cs}|$. This measurement improves upon the ${D}_{s}$ semileptonic branching ratio precision and provides a new approach for future work that eliminates the ${D}_{s}^{*}$ daughter photon reconstruction.

Fast ion instability at the Cornell Electron Storage Ring Test Accelerator

Fast ion instability can lead to deterioration of an electron beam (increasing emittance and instability of a train of bunches) in storage rings and linacs. We study this at the Cornell electron storage ring test accelerator using a 2.1 GeV low emittance beam. As the source of ions is residual gas, our measurements are conducted at various pressures, including nominal vacuum as well as injected gas (Ar, Kr). We experiment with mitigation techniques, including changing the bunch pattern to have mini-trains instead of one long train, as well as increasing the initial vertical emittance of the beam. We also check to ensure that ion-trapping is not a substantial effect in our measurements. We measure turn-by-turn vertical bunch size and position, as well as the multibunch power spectrum. Our measurements confirm fast ion instability under all vacuum conditions. A detailed simulation is then used to compare theory with observations.

In-situ measurements of the secondary electron yield in an accelerator environment: Instrumentation and methods

The performance of a particle accelerator can be limited by the build-up of an electron cloud (EC) in the vacuum chamber. Secondary electron emission from the chamber walls can contribute to EC growth. An apparatus for in-situ measurements of the secondary electron yield (SEY) in the Cornell Electron Storage Ring (CESR) was developed in connection with EC studies for the CESR Test Accelerator program. The CESR in-situ system, in operation since 2010, allows for SEY measurements as a function of incident electron energy and angle on samples that are exposed to the accelerator environment, typically 5.3GeV counter-rotating beams of electrons and positrons. The system was designed for periodic measurements to observe beam conditioning of the SEY with discrimination between exposure to direct photons from synchrotron radiation versus scattered photons and cloud electrons. The samples can be exchanged without venting the CESR vacuum chamber. Measurements have been done on metal surfaces and EC-mitigation coatings. The in-situ SEY apparatus and improvements to the measurement tools and techniques are described.

Measurements of electron cloud growth and mitigation in dipole, quadrupole, and wiggler magnets

Retarding field analyzers (RFAs), which provide a localized measurement of the electron cloud, have been installed throughout the Cornell Electron Storage Ring (CESR), in different magnetic field environments. This paper describes the RFA designs developed for dipole, quadrupole, and wiggler field regions, and provides an overview of measurements made in each environment. The effectiveness of electron cloud mitigations, including coatings, grooves, and clearing electrodes, are assessed with the RFA measurements.

Characterization of the International Linear Colliderdamping ring optics

A method is presented for characterizing the emittance dilution and dynamic aperture for an arbitrary closed lattice that includes guide field magnet errors, multipole errors and misalignments. This method, developed and tested at the Cornell Electron Storage Ring Test Accelerator (CesrTA), has been applied to the damping ring lattice for the International Linear Collider (ILC). The effectiveness of beam based emittance tuning is limited by beam position monitor (BPM) measurement errors, number of corrector magnets and their placement, and correction algorithm. The specifications for damping ring magnet alignment, multipole errors, number of BPMs, and precision in BPM measurements are shown to be consistent with the required emittances and dynamic aperture. The methodology is then used to determine the minimum number of position monitors that is required to achieve the emittance targets, and how that minimum depends on the location of the BPMs. Similarly, the maximum tolerable multipole errors are evaluated. Finally, the robustness of each BPM configuration with respect to random failures is explored.

Comparison of electron cloud mitigating coatings using retarding field analyzers

In 2008, the Cornell Electron Storage Ring (CESR) was reconfigured to serve as a test accelerator (CesrTA) for next generation lepton colliders, in particular for the ILC damping ring. A significant part of this program has been the installation of diagnostic devices to measure and quantify the electron cloud effect, a potential limiting factor in these machines. One such device is the Retarding Field Analyzer (RFA), which provides information on the local electron cloud density and energy distribution. Several different styles of RFAs have been designed, tested, and deployed throughout the CESR ring. They have been used to study the growth of the cloud in different beam conditions, and to evaluate the efficacy of different mitigation techniques. This paper will provide an overview of RFA results obtained in a magnetic field free environment.

Measurement and compensation of horizontal crabbing at the Cornell Electron Storage Ring Test Accelerator

In storage rings, horizontal dispersion in the rf cavities introduces horizontal-longitudinal ($xz$) coupling, contributing to beam tilt in the $xz$ plane. This coupling can be characterized by a ``crabbing'' dispersion term ${\ensuremath{\zeta}}_{a}$ that appears in the normal mode decomposition of the 1-turn transfer matrix. ${\ensuremath{\zeta}}_{a}$ is proportional to the rf cavity voltage and the horizontal dispersion in the cavity. We report experiments at the Cornell Electron Storage Ring Test Accelerator where $xz$ coupling was explored using three lattices with distinct crabbing properties. We characterize the $xz$ coupling for each case by measuring the horizontal projection of the beam with a beam size monitor. The three lattice configurations correspond to (i) 16 mrad $xz$ tilt at the beam size monitor source point, (ii) compensation of the ${\ensuremath{\zeta}}_{a}$ introduced by one of two pairs of rf cavities with the second, and (iii) zero dispersion in rf cavities, eliminating ${\ensuremath{\zeta}}_{a}$ entirely. Additionally, intrabeam scattering is evident in our measurements of beam size vs rf voltage.