Search

We demonstrate experimentally that the trident process $e^-\rightarrow e^-e^+e^-$ in a strong external field, with a spatial extension comparable to the effective radiation length, is well understood theoretically. The experiment, conducted at CERN, probes values for the strong field parameter $\chi$ up to 2.4. Experimental data and theoretical expectations using the Local Constant Field Approximation show remarkable agreement over almost 3 orders of magnitude in yield., Comment: 4 pages, 2 figures

A semi-classical approach is used to calculate radiation emission in the collision of an electron with an intense focused laser pulse. The results are compared to predictions from the locally constant field and locally monochromatic approximations. It is found that simulations employing the semi-classical approach capture features in the energy spectra, such as subharmonics and bandwidth structure, which are beyond local approaches. The formation length is introduced as a diagnostic to select between approaches as the electron is propagated through the pulse.

The Cornell Electron-Positron Storage Ring Test Accelerator (CesrTA) has been utilized to probe the interaction of the electron cloud with a 2.1 GeV stored positron beam. Recent experiments have characterized any dependence of beam'electron cloud (EC) interactions on the bunch length (or synchrotron tune) and the vertical chromaticity. The measurements were performed on a 30-bunch positron train with 14 nsec spacing between bunches, at a fixed current of 0.75 mA/bunch. The dynamics of the stored beam, in the presence of the EC, was quantified using 20 turn-by-turn beam position monitors in CESR to measure the correlated bunch-by-bunch dipole motion. In this paper we report on the observations from these experiments and analyze the coupling of di-pole motion from bunches within the train to subsequent bunches, caused by the EC., Proceedings of the North American Particle Accelerator Conf., NAPAC2016, Chicago, IL, USA

Earlier experiments at the Cornell Electron-Positron Storage Ring Test Accelerator (CesrTA) have probed the interaction of the electron cloud (EC) with a 2.1 GeV stored positron beam. Since a very low EC density is expected with the electron bunches, these results characterize the dependence of beam-vacuum chamber impedance interactions, which are common to both positron and electron beams. The experiments were performed on a 30-bunch electron train with a 14 ns spacing, at a fixed current of 0.75mA/bunch, at two different vertical chromaticity settings and for four different bunch lengths (or synchrotron tunes.) The beam dynamics of the stored beam, in the presence of the electron cloud, was quantified using: 20 turn-by-turn beam position monitors in CESR to measure the correlated bunch-by-bunch dipole motion and 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 analysis of the observations from these experiments and compare them with effects of the EC on the positron beam's dipole motion and coupling of the motion from each bunch to its succeeding bunches., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

A visible-light beam size monitor has been built and commissioned to measure transverse beam profiles at CESR-TA*. In order to eliminate beam jitter and to study bunch-by-bunch beam dynamics, a fast-gating camera has been utilized to measure single bunch transverse beam profiles. The minimum camera gate width is ~ 3ns which allows us to resolve single bunch beam dynamics along a CesrTA bunch train. Using single bunch interferometry at different bunch currents, we found that the horizontal beam sizes measured by gated camera are consistently less than those measured by a conventional CCD camera, demonstrating the elimination of turn-by-turn beam jitter with single shot capability. By stepping the camera trigger delay, we collected transverse beam profile images from each bunch in a 14ns-spacing 30-bunch train. The horizontal motion of each bunch as well as the horizontal beam size increases dramatically along an electron train but not along positron bunch trains under the same machine condition. The difference in single bunch horizontal dynamics may be a signature for the difference between electron cloud build-up for positron bunch trains versus ions present for electron bunch trains., Proceedings of the 7th Int. Particle Accelerator Conf., IPAC2016, Busan, Korea

CesrTA is a test accelerator facility at Cornell University that has been configured to study physics associated with electron and positron damping rings. Electron cloud effects is a concern for positron beams for such damping rings. The presentation will give an overview of recent measurements and simulation results for CesrTA. The measurement conditions were set up in order to study single bunch phenomena by observing a "Witness bunch" behind a train of positron bunches. The beam size and the turn by turn spectra were obeserved for the witness bunch under different conditions. Simulations were performed under similar conditions using the program CMAD., Proceedings of the 6th Int. Particle Accelerator Conf., IPAC2015, Richmond, VA, USA

At the Cornell Electron-Positron Storage Ring Test Accelerator (CesrTA) experiments have been studying the interaction of the electron cloud (EC) with 2.1 GeV stored electron and positron beams. These experiments are intended to characterize the dependence of beam–EC interactions on various beam parameters, such as bunch spacing and vertical chromaticity. Most experiments were performed with 30 or 45-bunch trains, at a fixed current of 0.75 mA/bunch. Earlier experiments with positrons had varied the bunch spacing between 4 and 56 ns at three different vertical chromaticity settings. More recent measurements have included electron-bunch trains to contrast the build up of EC between electron and positron beams. The dynamics of the stored beam was quantified using: a gated Beam Position Monitor (BPM) and spectrum analyzer to measure the frequency spectrum of bunches in the trains; an x-ray beam size monitor to record the bunch-by-bunch, turn-by-turn vertical size of each bunch within the trains. We report on recent ob-servations from these experiments and additional studies, using witness bunches trailing 30 or 45-bunch positron trains, which were used for the generation of the ECs., Proceedings of the 5th Int. Particle Accelerator Conf., IPAC2014, Dresden, Germany

The beam position monitor (BPM) system at the Cornell Electron Storage Ring (CESR) has been upgraded for use in both CESR Test Accelerator (CesrTA) and Cornell High Energy Synchrotron Source (CHESS) operations. CesrTA operates with electron and positron bunch trains with as little as 4ns bunch spacing. CHESS operates with simultaneous counter-rotating electron and positron trains with 14ns bunch spacing. The upgraded BPM system provides high resolution measurement capability as is needed for the CesrTA ultra low emittance operations, turn-by-turn digitization of multiple bunches for beam dynamics studies, and the capability for real-time dual beam monitoring in CHESS conditions. In addition to standard position measurement capability, the system is also required to measure betatron phase by synchronous detection of a driven beam for optics diagnosis and correction. This paper describes the characteristics of the BPM hardware upgrade, performance figures of the electronics designed for this purpose and the overall status of the upgrade effort. Examples of key measurement types and the analysis of data acquired from the new instruments will also be presented., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

During the last several years CESR has been studying the effects of electron clouds on stored beams in order to understand their impact on future linear-collider damping ring designs. One of the important issues is the way that the electron cloud alters the dynamics of bunches within the train. Techniques for observing the dynamical effects of beams interacting with the electron clouds have been developed. These methods will be discussed and examples of measurements will be presented., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

The Cornell Electron Storage Ring Test Accelerator (CesrTA) program has included extensive measurements of coherent tune shifts for a variety of electron and positron beam energies, bunch current levels, and bunch train configurations. The tune shifts have been shown to result primarily from the interaction of the beam with the space-charge field of the beam-induced low-energy electron cloud in the vacuum chamber. Comparison to several advanced electron cloud simulation program packages has allowed determination of the sensitivity of these measurements to physical parameters such as the synchrotron radiation flux, its interaction with the vacuum chamber wall, the beam emittance and lattice optics, as well as to those of the various contributions to the electron secondary yield model. We report on progress in understanding the cloud buildup and decay mechanisms in magnetic fields and in field-free regions, addressing quantitatively the precise determination of the physical parameters of the modelling. Validation of these models will serve as essential input in the design of damping rings for future high-energy linear colliders., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan