Beam Size Monitor Research Articles

Experimental comparison of three methods to measure electron source properties for synchrotron radiation

Measuring the electron source size is essential for determining the emittance of synchrotron radiation. Pinhole imaging is the most common technique which measures the source profile in all transverse directions. The grating interferometry technique measures the coherence function of the x-ray beam, which indirectly determines the source size. The newly developed phase-space beam position and size monitor (ps-BPM) system provide information on electron source properties: position, angle, size, and divergence. These three techniques were used to determine the source size at a bend magnet beamline at the Canadian Light Source. In this work, we compare experimental results from these measurements at the same time, which is used as a cross-calibration procedure for each method.

Design of the X-Ray Beam Size Monitor for the Advanced Photon Source Upgrade

Design of the X-Ray Beam Size Monitor for the Advanced Photon Source Upgrade

Design of High Energy Photon Source Booster beam size monitor

High Energy Photon Source (HEPS) Booster is designed to ramp up the energy of the beam from linac and inject high quality electron beam to the storage ring. To measure the transverse beam size, emittance and energy spread of HEPS Booster, we designed two beam diagnostic beamlines at visible-UV spectrum region. The source points are at two bending magnets with different chromatic dispersion, one has no dispersion while the other has large dispersion. By measuring the transverse beam sizes with synchrotron imaging system, beam emittance and energy spread can also be calculated. This paper introduces the extraction of visible-UV light, and the optical imaging system, the spatial resolution. It also introduces the design of beam spot changes measurement during beam ramping process.

Transverse Beam Emittance Measurement by Undulator Radiation Power Noise.

Generally, turn-to-turn power fluctuations of incoherent spontaneous synchrotron radiation in a storage ring depend on the 6D phase-space distribution of the electron bunch. In some cases, if only one parameter of the distribution is unknown, this parameter can be determined from the measured magnitude of these power fluctuations. In this Letter, we report an absolute measurement (no free parameters or calibration) of a small vertical emittance (5-15nm rms) of a flat beam by this method, under conditions, when it is unresolvable by a conventional synchrotron light beam size monitor.

A sub-micron resolution, bunch-by-bunch beam trajectory feedback system and its application to reducing wakefield effects in single-pass beamlines

A high-precision intra-bunch-train beam orbit feedback correction system has been developed and tested in the ATF2 beamline of the Accelerator Test Facility at the High Energy Accelerator Research Organization in Japan. The system uses the vertical position of the bunch measured at two beam position monitors (BPMs) to calculate a pair of kicks which are applied to the next bunch using two upstream kickers, thereby correcting both the vertical position and trajectory angle. Using trains of two electron bunches separated in time by 187.6 ns, the system was optimised so as to stabilize the beam offset at the feedback BPMs to better than 350 nm, yielding a local trajectory angle correction to within 250 nrad. The quality of the correction was verified using three downstream witness BPMs and the results were found to be in agreement with the predictions of a linear lattice model used to propagate the beam trajectory from the feedback region. This same model predicts a corrected beam jitter of c. 1 nm at the focal point of the accelerator. Measurements with a beam size monitor at this location demonstrate that reducing the trajectory jitter of the beam by a factor of 4 also reduces the increase in the measured beam size as a function of beam charge by a factor of c. 1.6.

Source size measurement options for low-emittance light sources

Radiation-based techniques for measuring electron source sizes are widely used as emittance diagnostics at existing synchrotron sources. Three of these techniques, namely, pinhole imaging, double-slit interferometry, and a K-edge filter-based beam position and size monitor system (ps-BPM), are evaluated for measuring source sizes at low-emittance storage rings. Each technique is reviewed with a detailed system description, design optimization, and practical considerations targeted for small source sizes. Pinhole imaging has the simplest setup and gives the beam profile in both transverse dimensions but with limited resolution. Double-slit interferometry has the highest resolution but with a limited detectable size range. The ps-BPM system shows reasonable resolution for monitoring small source sizes and divergence and can give real-time information of the source position and angle. New facilities may consider an integrated system that combines some or all of these techniques.

Application of a phase space beam position and size monitor for synchrotron radiation source characterization

A phase space beam position and size monitor (ps-BPM) system was introduced previously to measure the electron source vertical position and angular motion along with the vertical source size and angular size at a single location in a synchrotron bend magnet beam line. The system was validated experimentally at the Canadian Light Source and demonstrated by a simulation that it is suitable for low-emittance light sources. In this work, the potential practicality of the ps-BPM system is explored with real-case studies. By combining the analysis in the time and frequency domain, information on the beam motion and size can be extracted and identified from both the source and the beam-line optics. Applications of the ps-BPM system at the Canadian Light Source are demonstrated by studying beam vibrations and machine insertion device field changes. Results of these studies provide guidelines for implementing a dedicated ps-BPM system at existing and planned synchrotron facilites.

Optimization of a phase-space beam position and size monitor for low-emittance light sources.

The recently developed vertical phase-space beam position and size monitor (ps-BPM) system has proven to be able to measure the electron-source position, angle, size and divergence simultaneously in the vertical plane at a single location of a beamline. The optimization of the ps-BPM system is performed by ray-tracing simulation to maximize the instrument sensitivity and resolution. The contribution of each element is studied, including the monochromator, the K-edge filter, the detector and the source-to-detector distance. An optimized system is proposed for diffraction-limited storage rings, such as the Advanced Photon Source Upgrade project. The simulation results show that the ps-BPM system can precisely monitor the source position and angle at high speed. Precise measurements of the source size and divergence will require adequate resolution with relatively longer integration time.

Deep Learning Applied for Multi-Slit Imaging Based Beam Size Monitor

Deep Learning Applied for Multi-Slit Imaging Based Beam Size Monitor

First measurements of the vertical beam size with an X-ray beam size monitor in SuperKEKB rings

This paper reports first measurements of the vertical beam size using an X-ray Beam Size Monitor (XRM) for each of the SuperKEKB rings (Low Energy Ring, LER, and High Energy Ring, HER) during Phase 1 and 2 commissioning of SuperKEKB. The XRM device is dedicated to measuring the e−/e+ vertical beam size using X-ray synchrotron radiation, and will eventually have the capability for single-shot (single bunch, single turn) measurements. The preparation of a deep Si detector and high-speed readout electronics for single-shot measurements is underway. In the meantime for Phase 1 and 2, we used a scintillator screen and CCD camera for multi-bunch measurements. The XRM has been installed in each ring and images X-rays from a bending magnet that pass through optical elements (a single pinhole and coded apertures). Several studies (geometrical scale factors, emittance control knob, and beam lifetime study) were carried out during the commissioning. In Phase 1 (February–June 2016), the measured vertical emittances εy are ∼10 pm for LER (consistent with the optical estimation) and ∼35 pm for HER (3.5 × greater than the optical estimation). Analysis of the beam size and lifetime measurements suggests unexpectedly large point spread functions (PSF), particularly in the HER. The spatial resolution of the imaging system (defect of focus, diffraction, and spherical aberration) and scattering in the beamline (EGS5 simulation) contribute ∼6µm of PSF, which does not adequately account for the observed PSF. The Phase 2 commissioning commenced in May–July 2018, with thinner Be filters, new optical elements, scintillator, and CCD camera with the purpose to reduce the PSF in HER. The Phase 2 study results suggest the PSF σs∼6.6µm in HER (corresponding to the spatial resolution of the imaging system and scattering in the beamline), which is ∼5 × smaller than in Phase 1. The overall performances of XRMs in LER and HER are accurate. The XRM in LER will be able to measure the design beam size at the zero current (14µm). For the HER, the PSF that we observed during the Phase 2 indicates that the minimum measured beam size corresponds to the emittance at the design current (12.9 pm) and the XRM will be able to measure the design beam size at the zero current (7 µm).

Performance of beam size monitor based on Kirkpatrick–Baez mirror at SSRF

High-energy photon source (HEPS) is a 6 GeV ultralow emittance storage ring light source to be built in Beijing, China. Both the horizontal and vertical beam sizes of the HEPS storage ring are below 10 µm. It is a challenge to measure such a small beam size in both directions. To this end, measurement by a Kirkpatrick–Baez (KB) mirror imaging system was evaluated. A test KB system for the Shanghai Synchrotron Radiation Facility storage ring was designed and tested. Two crossed cylindrical mirrors were used to image the dipole source point. Both mirrors can be moved in and out so that the monitor is interchangeable with the original X-ray pinhole monitor. The aberration and point spread function, which would cause image blur, were evaluated. A beam-based calibration scheme was used by varying the beam size with different quadrupole settings and fitting them with the corresponding theoretical values. We updated the original X-ray camera with a new camera having a 5-μm-thick LuAG/Ce scintillator, and the imaging result shows greatly decreased image blur.

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.

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.

Effects of mirror distortion by thermal deformation in an interferometry beam size monitor system at PLS-II

Extraction mirrors installed at the most upstream position of interferometry beam size monitor are frequently used for measuring the beam size in storage rings. These mirrors receive the high power synchrotron radiation and are distorted owing to the heat distribution that depends on the position on the mirror surface. The distortion of the mirror changes the effective separation of the slit in the interferometry beam size monitor. Estimation of the effects of the front-end mirror distortion is important for measuring the beam size accurately. In this paper, we present the result of the numerical simulation of the temperature distribution and thermal expansion of the front-end mirror using ANSYS code, the theoretical basis of the effects of mirror distortion and compare with experimental results from Pohang Light Source II (PLS-II) at the Pohang Accelerator Laboratory (PAL). The equipment in the beam diagnosis line in PLS-II and experimental set-up for measuring the distortion of the front-end mirror using a multi-hole square array Hartmann screen are described.

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.

Operation of the CESR-TA vertical beam size monitor at [formula omitted

We describe operation of the CESR-TA vertical beam size monitor (xBSM) with e± beams with Eb=4GeV. The xBSM measures vertical beam size by imaging synchrotron radiation x-rays through an optical element onto a detector array of 32 InGaAs photodiodes with 50μm pitch. The device has previously been successfully used to measure vertical beam sizes of 10–100μm on a bunch-by-bunch, turn-by-turn basis at e± beam energies of ~2GeV and source magnetic fields below 2.8kG, for which the detector required calibration for incident x-rays of 1–5keV. At Eb=4.0GeV and B=4.5kG, however, the incident synchrotron radiation spectrum extends to ~20keV, requiring calibration of detector response in that regime. Such a calibration is described and then used to analyze data taken with several different thicknesses of filters in front of the detector. We obtain a relative precision of better than 4% on beam size measurement from 15 to 100 μm over several different ranges of x-ray energy, including both 1–12keV and 6–17keV. The response of an identical detector, but tilted vertically by 60°in order to increase magnification without a longer beamline, is measured and shown to improve x-ray detection above 4keVwithout compromising sensitivity to beam size. We also investigate operation of a coded aperture using gold masking backed by synthetic diamond.

Design and performance of coded aperture optical elements for the CESR-TA x-ray beam size monitor

We describe the design and performance of optical elements for an x-ray beam size monitor (xBSM), a device measuring e+ and e− beam sizes in the CESR-TA storage ring. The device can measure vertical beam sizes of 10–100μm on a turn-by-turn, bunch-by-bunch basis at e± beam energies of ~2–5GeV. x-rays produced by a hard-bend magnet pass through a single- or multiple-slit (coded aperture) optical element onto a detector. The coded aperture slit pattern and thickness of masking material forming that pattern can both be tuned for optimal resolving power. We describe several such optical elements and show how well predictions of simple models track measured performances.

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.

Vertical beam size measurement in the CESR-TA [formula omitted] storage ring using x-rays from synchrotron radiation

We describe the construction and operation of an X-ray beam size monitor (xBSM), a device measuring e+ and e− beam sizes in the CESR-TA storage ring using synchrotron radiation. The device can measure vertical beam sizes of 10–100μm on a turn-by-turn, bunch-by-bunch basis at e± beam energies of ~2GeV. At such beam energies the xBSM images X-rays of ϵ≈1–10keV (λ≈0.1–1nm) that emerge from a hard-bend magnet through a single- or multiple-slit (coded aperture) optical element onto an array of 32 InGaAs photodiodes with 50μm pitch. Beamlines and detectors are entirely in-vacuum, enabling single-shot beam size measurement down to below 0.1mA (2.5×109 particles) per bunch and inter-bunch spacing of as little as 4ns. At Eb=2.1GeV, systematic precision of ~1μm is achieved for a beam size of ~12μm; this is expected to scale as ∝1/σb and ∝1/Eb. Achieving this precision requires comprehensive alignment and calibration of the detector, optical elements, and X-ray beam. Data from the xBSM have been used to extract characteristics of beam oscillations on long and short timescales, and to make detailed studies of low-emittance tuning, intra-beam scattering, electron cloud effects, and multi-bunch instabilities.

Laserwire: A high resolution non-invasive beam profiling diagnostic

A laserwire is a non-invasive, high resolution particle beam size monitor based on Compton scattering that is required for future planned colliders and can also be used to estimate the size of the source in wakefield acceleration experiments. We present recent results from the high resolution laserwire transverse electron beam diagnostic installed at the Accelerator Test Facility 2 (ATF2) electron accelerator at KEK in Japan. Full characterisation of the propagation of the 150mJ, 77ps laser beam is used to deconvolve the transverse laserwire profile demonstrating the successful measurement of 1µm scale vertical electron beam sizes, even with extreme aspect ratios. We also present progress in the development of high energy photonic crystal fibre based laser systems for laserwire measurements at megahertz repetition rates, suitable for intra-train scanning for planned accelerators such as the International Linear Collider, or beam size measurement in laser or particle driven plasma accelerators.