Single-bunch Beam Research Articles

Tailoring transverse beam characteristics with the new CERN PS Booster charge-exchange injection system

A key aspect of the LHC Injectors Upgrade project is the connection of the PSB to the newly built Linac4 and the related installation of a new 160 MeV charge-exchange injection system. The new injection system was commissioned in winter 2020/21 and is now used operationally to tailor the transverse characteristics for the various beam types at CERN, such as high-intensity fixed target beams, LHC single bunch beams, and high-brightness beams for LHC. This contribution outlines the different injection strategies for producing the various beam types and discusses the application of numerical optimization algorithms to adjust injection settings in operation efficiently.

Method for solving bunch head-tail overlapping in HLS-II using a new trigger scanning module of streak-camera measurement system

This study proposed a trigger scanning module for a streak camera to overcome head–tail overlapping of bunch to achieve absolute bunch length monitoring based on the unique machine properties of the Hefei Light Source-II (HLS-II). Furthermore, precise measurement of longitudinal bunch characteristic parameters and study of longitudinal beam dynamics is crucial, and this significantly contributes for observing longitudinal bunch behavior and evolution associated with current potential-well distortion and microwave instability. First, the scanning module is calibrated thoroughly to test its performance and applicability. Afterward, the module is applied to measure the dependence of bunch length on current under single-bunch operation. Moreover, potential-well distortion and bunch lengthening are observed. Results showed that the designed trigger scanning module is appropriate for the exploration of the physical mechanism of single-bunch beam behavior, which is crucial for the investigation of bunch characteristics and beam instability in storage ring light sources.

Commissioning results of single bunch selection system for the RAON heavy-ion accelerator facility

To enable neutron time-of-flight experiments at the RAON heavy-ion accelerator facility, we tested a single bunch beam selection method by combining an RF chopper and a double gap buncher in the low-energy beam transport section. The RF chopper converts a CW beam into a hundreds-nanosecond pulsed beam. Then, the double gap buncher performs bunching to shorten the pulse length to less than one radio frequency quadrupole (RFQ) cycle. Ideally, a single isolated bunch can be achieved after the RFQ. In this study, we discuss the design concept of the single bunch selection system and present initial beam commissioning results.

Online optimization of NSLS-II dynamic aperture and injection transient

The goal of the NSLS-II online optimization project is to improve the beam quality for the user experiments. To increase the beam lifetime and injection efficiency, we have developed a model-independent online optimization of nonlinear beam dynamics using advanced algorithms, such as Robust Conjugate-Gradient Algorithm (RCDS). The optimization objective is the injection efficiency and optimization variables are the sextupole magnet strengths. Using the online optimization technique, we increased the NSLS-II dynamic aperture and reduced the amplitude-dependent tune shift. Recently, the sextupole optimization was successfully applied to double the injection efficiency up to above 90% for the high-chromaticity lattice being developed to improve the beam stability and to increase the single-bunch beam intensity. Minimizing the beam perturbation during injection is the second objective in this project, realized by online optimization of the injection kickers. To optimize the full set of kicker parameters, including the trigger timing, amplitude, and pulse width, we upgraded all kicker power supplies with the capability of tunable waveform width. As a result, we have reduced the injection transient by a factor of 29, down to the limit of 60 μm.

Design study of a single bunch selection method for neutron TOF experiment at RAON heavy ion accelerator

To enable neutron time-of-flight (TOF) experiments at the RAON heavy-ion accelerator facility, we propose a single bunch beam selection method by combining a fast chopper and double gap buncher in the low-energy beam transport (LEBT) section. The fast chopper operates with switching times of tens of ns to convert a CW beam into a pulsed beam.Then, the double gap buncher performs bunching to shorten the beam pulse duration to less than one radio frequency quadrupole (RFQ) cycle.Ideally, a single isolated bunch can be achieved after the RFQ.In this study, we discuss the system design of the proposed single bunch selection scheme and present detailed beam dynamics simulations.

Combined effect of chromaticity and feedback on transverse head-tail instability

The resonant interaction of a bunched particle beam with short-range wakefields is the cause of head-tail instability, which is a major limitation for single-bunch beam intensity in circular accelerators. There are several processes that suppress this type of instability, such as fast chromatic damping and Landau damping due to machine nonlinearity. Beam-based feedback systems (transverse dampers) provide active suppression of the beam oscillations. The combined effect of the transverse dampers and chromaticity is discussed in this article. A brief overview of theoretical and experimental studies of the head-tail instability and its mitigation is presented. Results of experimental studies of the transverse mode coupling carried out at National Synchrotron Light Source-II are compared with the theoretical model.

Longitudinal phase-space diagnostics using a stripline-type beam-timing monitor at the KEKB injector linac

Abstract A new beam-timing monitor (BTM) has been developed to analyze the longitudinal beam dynamics and bunching characteristics at the pre-injector of the KEKB injector linear accelerator (linac). The BTM is a conventional stripline-type beam monitor with two pickup electrodes. It measures the relative time delay between the arrival time of the beam signal and 1st-zero-cross time of the fundamental accelerating radio frequency (rf) signal (2856 MHz) immediately after the detection of the beam signal. The beam tests were performed to evaluate the performance and characteristics of the BTM for primary single-bunched electron beams with high bunch charges of ∼ 8 nC for positron production. It was found that the results show that the resolution in the delay-time measurement is σ ∼ 1.2 ps in one standard deviation. This report describes the basic design of the BTM system, the beam tests, and results in detail, to investigate the sensitivity and stability of the beam timing based on the longitudinal phase-space parameters at the pre-injector and location A1 of the injector linac, along with the calibration method in the pulse-height variation of the BTM signal.

Observation of coherent Smith-Purcell and transition radiation driven by single bunch and micro-bunched electron beams

Generation of coherent Smith-Purcell (cSPr) and transition/diffraction radiation using a single bunch or a pre-modulated relativistic electron beam is one of the growing research areas aiming at the development of radiation sources and beam diagnostics for accelerators. We report the results of comparative experimental studies of terahertz radiation generation by an electron bunch and micro-bunched electron beams and the spectral properties of the coherent transition and SP radiation. The properties of cSPr spectra are investigated and discussed, and excitations of the fundamental and second harmonics of cSPr and their dependence on the beam-grating separation are shown. The experimental and theoretical results are compared, and good agreement is demonstrated.

PiC code KARAT simulations of Coherent THz Smith-Purcell Radiation from diffraction gratings of various profiles

Generation of coherent THz Smith-Purcell radiation by single electron bunch or multi-bunched electron beam was simulated for lamellar, sinusoidal and echelette gratings. The dependences of the CSPR intensity of the corrugation gratings depth were investigated. The angular and spectral characteristics of the CSPR for different profiles of diffraction gratings were obtained. It is shown that in the case of femtosecond multi-bunched electron beam with 10 MeV energy sinusoidal grating with period 292 μm and groove depth 60 μm has the uniform angular distribution with high radiation intensity.

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.

Beam-based model of broad-band impedance of the Diamond Light Source

In an electron storage ring, the interaction between a single-bunch beam and a vacuum chamber impedance affects the beam parameters, which can be measured rather precisely. So we can develop beam-based numerical models of longitudinal and transverse impedances. At the Diamond Light Source (DLS) to get the model parameters, a set of measured data has been used including current-dependent shift of betatron tunes and synchronous phase, chromatic damping rates, and bunch lengthening. A matlab code for multiparticle tracking has been developed. The tracking results and analytical estimations are quite consistent with the measured data. Since Diamond has the shortest natural bunch length among all light sources in standard operation, the studies of collective effects with short bunches are relevant to many facilities including next generation of light sources.

Demonstration of cathode emittance dominated high bunch charge beams in a DC gun-based photoinjector

We present the results of transverse emittance and longitudinal current profile measurements of high bunch charge (≥100 pC) beams produced in the DC gun-based Cornell energy recovery linac photoinjector. In particular, we show that the cathode thermal and core beam emittances dominate the final 95% and core emittances measured at 9–9.5 MeV. Additionally, we demonstrate excellent agreement between optimized 3D space charge simulations and measurement, and show that the quality of the transverse laser distribution limits the optimal simulated and measured emittances. These results, previously thought achievable only with RF guns, demonstrate that DC gun based photoinjectors are capable of delivering beams with sufficient single bunch charge and beam quality suitable for many current and next generation accelerator projects such as Energy Recovery Linacs and Free Electron Lasers.

High gradient limits due to single bunch beam breakup in a collinear dielectric wakefield accelerator

A study into the maximum acceleration gradient, ${E}_{z}^{\mathrm{max}}$, in a collinear dielectric wakefield accelerator due to single bunch beam breakup (SBBU) is presented. The longitudinal wakefield is proportional to charge over radius squared (${E}_{z}\ensuremath{\sim}Q/{a}^{2}$) which implies small $a$ is favorable for fixed $Q$. However, when the transverse wakefield (${E}_{\ensuremath{\perp}}\ensuremath{\sim}Q/{a}^{3}$) is also considered then the SBBU instability severely limits the charge that can pass through a long structure as required for a large transfer of energy from the drive bunch to the witness bunch and hence limits the sustained acceleration that can be obtained.

Phase modulation of the bucket stops bunch oscillations at the Fermilab Tevatron

Bunches in the Tevatron are known to exhibit longitudinal oscillations which persist indefinitely. These oscillations are colloquially called 'dancing bunches.' Although the dancing proton bunches do not cause single bunch emittance growth or beam loss at injection, they lead to bunch lengthening at collisions. In Tevatron operations, a longitudinal damper has been built which stops this dance and damps out coupled bunch modes. Recent theoretical work predicts that the dance can also be stopped by an appropriate change in the bunch distribution. This paper describes the Tevatron experiments which support this theory.

Simulation calculations of transmissions for neutron total cross-section measurements of MAs and LLFPs by complementary use of a compact accelerator-driven neutron source

We performed simulation calculations of neutron transmission spectra for minor actinide and long-lived fission product samples with pulsed neutron beams produced by the two neutron sources: the Accurate Neutron-Nucleus Reaction measurement Instrument (ANNRI) at the Japan Spallation Neutron Source (JSNS) that provides the world's most intense pulsed neutron beam, and a compact accelerator-driven neutron source. For the both cases, we assumed to put a neutron detector at downstream of the sample and took into account the detector efficiency of neutrons to estimate statistics. ANNRI gives high statistical neutron transmission spectra. Therefore, it is useful for obtaining the total cross-section data through the transmission spectra for the small amount sample or the small cross-section regions between the resonances not only for monitoring the beam condition during the neutron capture-reaction measurements at ANNRI. On the other hand, a compact neutron source shows transmission spectra at resonances with less deformation by its single bunch beam. This implies that the compact neutron source is useful for assigning the resonances and the resonance analysis.

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.

Cs–Te photocathode RF electron gun for applied research at the Waseda University

A compact electron accelerator based on photo-cathode RF electron gun is under development for applied research at the Waseda University. Higher charge and higher energy are key issues for applications such as pulse radiolysis experiment and laser-Compton X-ray generation. The new RF-gun cavity with cesium telluride (Cs–Te) photo-cathode which has higher quantum efficiency (Q.E.) compared with metal cathodes has been installed. The evaluation of Q.E. of the photo-cathode has been performed and the value obtained is more than 5% at the preparation stage. For understanding the Cs–Te cathode properties and the higher Q.E. operation, fundamental studies by single bunch beam operation have been performed resulting in more than 6 nC bunch charge with a high Q.E. of 2.9%. Additionally, the new RF-gun cavity was improved for obtaining a higher Q value. Higher beam energy, i.e. shunt impedance compared with old type cavity, was observed. The shunt impedance increased by about 20%. These successful results will considerably contribute to the applied research.

Short-Range Longitudinal Wake Field and Its Effect on the Energy Spectrum in a Linear Accelerator

We have studied the effects of the wake field on the energy spectrum of a high-intensity single-bunch electron beam of 28 nC charge accelerated in an L-band accelerating structure of 1.3 GHz frequency both by experiment and by calculation. Energy spectra measured as a function of the accelerating phase show significant changes, including appearance of multiple peaks owing to the short-range longitudinal wake field. The empirical wake function is derived for the specific accelerating structure using the simulation code, ABCI. Energy spectra calculated with the wake function agree quite well with the energy spectra measured over the wide range of the accelerating phase. The effects of the wake field on the energy spectrum of the high-intensity single-bunch beam accelerated with the L-band linac at Institute of Scientific and Industrial Research (ISIR), Osaka University, are quantitatively explained in terms of the wake field theory with the empirical wake function.

Observation of THz coherent transition radiation from single-bunch beam at KURRI-LINAC as an intense pulsed light source

A single-bunch beam has been generated using a developed high-speed avalanche-type pulser in KURRI-LINAC in order to lift restrictions of the spectral resolution in the spectroscopic study and the delay time in the time-resolved measurement. Both of the rise and fall times of the developed pulser are 110ps. The observation of CTR has confirmed the single-bunch beam. The degree of impurity of single bunch has been estimated to be 1.5% by the analysis of the interferogram.

LWFA OF ATTO-SECOND AND FEMTO-SECOND BUNCHES FOR PULSE RADIOLYSIS

Pulse radiolysis spectroscopically studies the initial stage of atomic and molecular reactions induced by electron beams. Single-bunched beams with bunch lengths on the order of femtosecond or shorter are fervently requested for this purpose. The laser wake field acceleration is readily applicable now, which is able to realize beams with bunch length of 1 fs, energy of 30 MeV, repetition rate of 1 Hz, bunch charge of 10 pC, energy width of 10%, etc.