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The predominant source of nonlinearity in most existing accelerators are sextupoles, which introduce nonlinear resonances. When the horizontal tune of an accelerator is near such a resonance line (nν_{x}), stable fixed points (SFPs) may appear in the horizontal phase space to form a second closed orbit different from the “zero” closed orbit. The stable islands in phase space surrounding the SFPs are also referred as transverse resonance island “buckets” (TRIBs). However, the TRIBs are not always present near a resonance line in a storage ring. Necessary conditions for TRIBs formation will be discussed in this paper. A 6-GeV lattice with the horizontal tune near a third-order resonance at 3ν_{x}=50 is then designed for the Cornell Electron Storage Ring (CESR). After loading this lattice into CESR, the TRIBs are observed while adjusting the horizontal tune near the third-order resonance, consistent with particle tracking simulations. TRIBs rotation in the horizontal phase space are also experimentally demonstrated with adjustment of a sextupole knob. These results are in good agreement with the theoretical calculation.

In preparation for a demonstration of optical stochastic cooling in Cornell Electron Storage Ring, 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.

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 multipass 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.

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.

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.

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 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 ζ_{a} that appears in the normal mode decomposition of the 1-turn transfer matrix. ζ_{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 ζ_{a} introduced by one of two pairs of rf cavities with the second, and (iii) zero dispersion in rf cavities, eliminating ζ_{a} entirely. Additionally, intrabeam scattering is evident in our measurements of beam size vs rf voltage.

Intrabeam scattering (IBS) limits the emittance and single-bunch current that can be achieved in electron or positron storage ring colliders, damping rings, and light sources. Much theoretical work on IBS exists, and while the theories have been validated in hadron and ion machines, the presence of strong damping makes IBS in lepton machines a different phenomenon. We present the results of measurements at CesrTA of IBS-dominated beams, and compare the data with theory. The beams we study have parameters typical of those specified for the next generation of wiggler-dominated storage rings: low emittance, small bunch length, and an energy of a few GeV. Our measurements are in good agreement with IBS theory, provided a tail-cut procedure is applied.