Bunch Length Research Articles

Accelerated Deep Reinforcement Learning for Fast Feedback of Beam Dynamics at KARA

Coherent synchrotron radiation (CSR) is generated when the electron bunch length is in the order of the magnitude of the wavelength of the emitted radiation. The self-interaction of short electron bunches with their own electromagnetic fields changes the longitudinal beam dynamics significantly. Above a certain current threshold, the micro-bunching instability develops, characterized by the appearance of distinguishable substructures in the longitudinal phase space of the bunch. To stabilize the CSR emission, a real-time feedback control loop based on reinforcement learning (RL) is proposed. Informed by the available THz diagnostics, the feedback is designed to act on the radio frequency (RF) system of the storage ring to mitigate the micro-bunching dynamics. To satisfy low-latency requirements given by the longitudinal beam dynamics, the RL controller has been implemented on hardware (FPGA). In this article, a real-time feedback loop architecture and its performance is presented and compared with a software implementation using Keras-RL on CPU/GPU. The results obtained with the CSR simulation Inovesa demonstrate that the functionality of both platforms is equivalent. The training performance of the hardware implementation is similar to software solution, while it outperforms the Keras-RL implementation by an order of magnitude. The presented RL hardware controller is considered as an essential platform for the development of intelligent CSR control systems.

Production and Performance Evaluation of a Compact Deflecting Cavity to Measure the Bunch Length in the cERL

Production and Performance Evaluation of a Compact Deflecting Cavity to Measure the Bunch Length in the cERL

Broadband Imaging of Coherent Radiation as a Single-Shot Bunch Length Monitor with Femtosecond Resolution

Broadband Imaging of Coherent Radiation as a Single-Shot Bunch Length Monitor with Femtosecond Resolution

Bunch Length Characterizations for the Solaris Injector LINAC

Bunch Length Characterizations for the Solaris Injector LINAC

High-resolution longitudinal profile diagnostics for ultralow charges stored in a ring

Observations of optical synchrotron radiation emitted over millions of passes from a few electrons circulating in the particle accumulator ring at the Advanced Photon Source were performed with a digital CMOS camera and a synchroscan streak camera operating at 117.3 MHz. Discrete changes of integrated counts in the CMOS image region of interest are ascribed to single electron steps in a 10-s sensor integration time. Circulations of a single electron at 375 and at 425 MeV were demonstrated in the 12-bit digital CMOS camera images. The streak camera operating at the 12th harmonic of the fundamental revolution frequency at 9.77 MHz was used to measure the bunch length from 0.5-nC circulating charge down to tens of electrons or $<10\text{ }\text{ }\mathrm{aC}$. The measurements were performed with $\ensuremath{\sim}8\text{\ensuremath{-}}\mathrm{ps}$ temporal resolution uniquely at such ultralow charges. These results also serve as a proof-of-principle longitudinal profile diagnostic for the predicted effects of optical stochastic cooling at low charge in commissioning experiments at the Fermilab Integrable Optics Test Accelerator ring.

RF design of a compact, X-band travelling-wave RF photogun made from halves

This paper presents the RF design, thermal calculations and beam dynamics simulations of a compact, high repetition rate, travelling-wave RF photogun operating at the European X-band frequency of 11.994 GHz. The design is based on a CLIC prototype structure milled from copper halves and proposes to use the unique gap, introduced in the original X-band accelerating structure’s geometry, to couple in the photocathode laser without obstructing the outgoing electron bunches. Operating at an input RF power of 29 MW and combined with a single X-band structure downstream, the non-cylindrically symmetric travelling-wave RF photogun operates as a compact photo-injector able to provide 40 pC bunches with an energy of 34.4 MeV ± 0.07 %, a bunch length of 533 fs and a horizontal and vertical emittance of 0.368 and 0.338 mm mrad, respectively, at a repetition rate up to 1 kHz.

Design, fabrication and relative bunch length measurement of S-band pre-buncher

A 10 MeV, 3 kW RF electron linear accelerator (linac) is in operation at EBC, Kharghar. A new horizontal linac rated for 10 MeV, 5 kW has been planned to be developed at EBC, Kharghar. The components of 10 MeV linac beam line consists of electron gun, a standing wave (SW) single cell pre-buncher cavity and a SW main linac cavity of length 0.9 m. In order to enhance the beam capture efficiency and to reduce the energy spread a pre-buncher cavity along with focusing elements will be introduced between the electron gun and the main linac cavity section. In this work the design simulation, fabrication, RF measurements and beam experimental trials of the 2856 MHz low Q, wide band pre-buncher cavity is presented. The measured RF parameters of pre-buncher cavity are f0=2854.4 MHz in air, QL=600, tuning range ±2.5 MHz. Electron beam transmission efficiency of 99% is achieved with minimum relative bunch length obtained at 400W of input RF power.

Quasi-isochronous triple-bend achromat with periodic stable optics and negligible coherent-synchrotron-radiation effects

In modern high-brightness electron accelerators, the goal of preserving the beam quality from the source has long been pursued to ensure the fulfillment of their scientific missions. However, when delivering the electron beam through a beam transfer line, which may consist of bending magnets, the performance of a high-brightness electron beam may be degraded by the coherent synchrotron radiation (CSR) and the residual longitudinal dispersion. It is of critical importance to preserve the transverse emittance and control the bunch length variation as well as maintain the longitudinal phase space distribution in such a beam transfer line. For a multipass beam transfer line, the achromatic cell designs with stable optics would bring great convenience. In this study, for the periodic stable symmetric triple-bend achromat designs with identical dipoles, it is discovered that the CSR-induced emittance growth and the longitudinal dispersion up to high orders can be minimized simultaneously when a certain transfer matrix relation is satisfied; to be specific, the matrix entries between the first two dipoles should follow $[{m}_{11},{m}_{21}({\mathrm{m}}^{\ensuremath{-}1})]\ensuremath{\approx}(\ensuremath{-}2,0)$. Numerical optimizations demonstrate that the normalized transverse emittance, the bunch length, and the longitudinal phase space distribution are well preserved. In addition, such a design strategy shows the potential merit of effectively mitigating the microbunching instability.

Compensation and improvement for bunch length diagnostics based on RF-phasing techniques

Reliable bunch length measurement for electron beams is critical for the successful development of high-brightness beam injectors. Compared with commonly used schemes involving complicated techniques and dedicated devices, RF-phasing techniques have advantages in low-energy injectors owing to their convenience and low cost. Although previous direct calculation formulas originating from RF-phasing techniques are feasible and efficient in low-energy injectors, non-negligible errors are inevitable in practical operations, which limits application. By recalling the analytical derivation procedure and reconsidering the approximate conditions, compensation methods were explored through theoretical analysis and numerical simulations. Virtual measurements using beam dynamics calculations were used to evaluate reliability, and experimental validations were conducted at the HUST (Huazhong University of Science and Technology) injector to verify the compensation. Both of these efforts demonstrate that the improvements in the RF-phasing-based diagnostics for bunch length are feasible and reliable in typical low-energy injectors in common operating conditions. With compensation, these diagnostic methods are a promising alternative to conventional methods. For beam injectors, there may be a tradeoff among a compact layout, low cost, and complete characterization functions.

First simultaneous detection of electron and positron bunches at the positron capture section of the SuperKEKB factory

The direct simultaneous detection of electron and positron bunch signals was successfully performed for the first time with wideband pickups and a detection system at the positron capture section of the SuperKEKB factory. The time interval between the electron and positron bunches, their bunch lengths, and bunch intensities depending on the phase of accelerating structures were measured to investigate their capture process and to maximally optimize the positron intensity. The results show that the time intervals were measured in the range of 135–265 ps, and the line-order switch of the electron and positron bunches in the axial direction was clearly observed as a function of the phase. The positron (electron) intensity was maximized at the optimal phase (180^{circ } shifted from the optimum). These series of measurements have never been experimentally conducted so far. It is demonstrated that the positron intensity can be systematically optimized with this system as functions of beam parameters in multidimensional spaces for any positron capture section.

Excitation of strongly nonlinear plasma wakefield by electron bunches

We propose a new method for analytical self-consistent description of the excitation of a strongly nonlinear wakefield (a bubble) excited by an electron bunch. This method makes it possible to calculate the shape of the bubble and the distribution of the electric field in it based only on the properties of the driver, without relying on any additional parameters. The analytical results are verified by particle-in-cell simulations and show good correspondence. A complete analytical solution for cylindrical drivers and scaling laws for the properties of the bubble and other plasma accelerator parameters depending on the bunch charge and length are derived.

Numerical solution of the coupled Haïssinski equations for the equilibrium state of an arbitrary bunch train in an electron storage ring

We study the effects of the beam loadings in the main and harmonic cavities and short-range wakefields on the bunch lengthening performance, in an electron storage ring. The compensation of the main cavity beam loading is modeled by adjusting the RF generator voltage so that the total voltage from the main cavity for each bunch can get as close as possible to the desired effective accelerating voltage under the ideal main cavity assumption. A system of coupled Haïssinski equations, which determine the bunch positions and profiles in the equilibrium state for arbitrary beam-fill patterns, are derived and solved numerically by a relaxation iteration. The proposed method has advantages of being a simple algorithm, displaying effectiveness as well as good convergence. The parameters of the planned Advanced Light Source upgrade (ALS-U) storage ring are used to test the method and good agreement is obtained comparing with particle tracking simulations.

Beam blow up due to beamstrahlung in circular e^+e^- colliders

After the discovery of the Higgs boson in 2012, several future circular colliders — Higgs factories — are proposed, such as FCC-ee and CEPC. At these e^+e^- colliders, beamstrahlung can greatly affect the equilibrium bunch length and energy spread. If the dispersion function at the collision point is not zero, beamstrahlung will also increase the transverse emittances. In this paper, we derive analytical formulae for the equilibrium beam parameters, considering the varying electromagnetic field during the collision, and taking into account hourglass effect, crossing angle, and crab waist. We illustrate the importance of beamstrahlung, including the increase of bunch length and the implied tolerance on the spurious dispersion function at the collision point, by considering a few examples.

Épocas de desponte de cachos na variedade de uva rainha Itália (Vitis vinifera L.), produzida em ambiente protegido na Serra Gaúcha

The objective of the work was to evaluate the production and quality of the cultivar Italia produced in a protected environment, submitted to three seasons of topping of bunch. The experiment was carried out in 2015, in Caxias do Sul - RS. The experimental design used blocks at random with four treatments, being: without topping of bunch, topping of bunch in pre-flowering, topping of bunch in full bloom, topping of bunch in post-blooming. The variables analyzed were: production per plant, bunch weight, bunch length, berry diameter and number of bunches soluble solids and titratable acidity. Regardless of the pruning time of the bunches, the factors of production and the evaluated chemical physics were not affected, obtaining bunches with weight and shape according to market requirements.

Coherent Smith-Purcell radiation for minimally invasive bunch length measurement at the subpicosecond time scale

We have designed a tool to measure the bunch length of an electron beam in a minimally invasive way by means of coherent Smith-Purcell radiation (SPR). The technique has been employed successfully at a test apparatus for the Mainz Energy-recovery Superconducting Accelerator MESA, demonstrating it is possible to determine the bunch length while losing less than 0.6% of the electron beam. The impact of the space charge on the bunch length can be reduced while tuning the longitudinal bunch preparation system during a live measurement at beam currents up to 1 mA. Doing so, RMS bunch lengths of $70\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ can be achieved in a typical operating mode of the low-energy beam transport system of MESA. In addition to the bunch length measurements, typical properties of the generated SPR are demonstrated.

Synchrobetatron resonance of crab crossing scheme with large crossing angle and finite bunch length

Crab crossing scheme is an essential collision scheme to achieve high luminosity for the future colliders with large crossing angles. However, when bunch length of one or both colliding beams is comparable with the wavelength of the crab cavity voltage, the nonlinear dependence of the crabbing kick may present a challenge to the beam dynamics of the colliding beams and impact the beam quality as well as the luminosity lifetime. In this paper, the results of nonlinear dynamics in the crab crossing scheme are presented, using both analytical and numerical studies. The result indicates that higher-order synchro-betatron resonances may be excited in the crab crossing scheme with large crossing angle, which causes the beam quality deterioration and luminosity degradation. The studies also reveal possible countermeasures to suppress the synchro-beta resonance, hence mitigate the degradation of beam quality and luminosity.

Design, fabrication and cold-test results of a 3.6 cell C-band photocathode RF gun for SXFEL

The increasing demands for electron beams with short bunch length and high brightness for Soft X-ray Free Electron Laser (SXFEL) have stimulated active efforts to develop suitable photoinjectors for these accelerators. One promising direction in this regard is increasing the gradient of rf gun. The rf design, beam dynamics and cold-test results of a novel 3.6 cell C-band 150 MV/m rf photocathode gun are presented in this paper. The rf gun and its beam dynamics modeling have been done using a combination of codes including POSSION, CST, and ASTRA. The impact of first half-cell length, cell number, and solenoid on beam dynamic are analyzed and described. The gun has state-of-the-art rf features including: elliptical irises profile; a non-brazed cathode plate; and a rectangular mode launcher. The fabrication and cold-test of the rf gun prototype are successfully implemented, and the measured solenoid field agrees well with simulations.

Longitudinal equilibrium density distribution of arbitrary filled bunches in presence of a passive harmonic cavity and the short range wakefield

We propose a novel scheme for the calculation of bunch profiles and synchronous phase shifts for arbitrary fill patterns in electron storage rings considering both the effects of the passive harmonic cavity and the short range wakefield. This scheme treats the bunch as a charge distribution rather than a point charge with a bunch form factor, and divides the voltage induced in the cavity into two groups: that induced by the previous passages and that by the present passage, where the latter is integrated with the short range wakefield from all the other vacuum components. An iteration loop for computing the bunch profiles, including a subloop using Newton iteration for calculating the synchronous phase deviations, is built and implemented into a matlab-based code. Using this code, we study the bunch lengthening and centroid transient effects for several possible fill patterns in the Hefei Advanced Light Facility storage ring under design. We find the method is very efficient, and the CPU time for the whole calculation on a regular personal computer is less than 30 seconds for each case even with more than 700 bunches. Using the case of a long gap fill as a benchmark example, both the results given by our code and the multiparticle tracking simulation with elegant are in very good agreement.

Development of a seven-cell S-band standing-wave RF-deflecting cavity for Tsinghua Thomson scattering X-ray source

A 2856-MHz, π-mode, seven-cell standing-wave deflecting cavity was designed and fabricated for bunch length measurement in Tsinghua Thomson scattering X-ray source (TTX) facility. This cavity was installed in the TTX and provided a deflecting voltage of 4.2 MV with an input power of 2.5 MW. Bunch length diagnoses of electron beams with energies up to 39 MeV have been performed. In this article, the RF design of the cavity using HFSS, fabrication, and RF test processes are reviewed. High-power operation with accelerated beams and calibration of the deflecting voltage are also presented.

Design and Optimization of the High Order Modes Damper for a 1.5 GHz Superconducting Harmonic Cavity

The installation of a superconducting radio-frequency (SRF) cavity can improve the beam lifetime via increasing the electron bunch length. A prototype 1.5-GHz SRF passive harmonic cavity working at 4.5 K was designed for The Taiwan Photon Source (TPS). As the beam might become unstable because of the coupled-bunch instability resulting from Higher Order Modes (HOMs) of the SRF harmonic cavity, HOMs damping of this cavity is an important issue. Ferrite absorbers placed in the beam tubes are designed to damp the HOMs. A theoretical calculation for the instability threshold for the HOMs is given. A tracking method is used to simulate the instabilities for operation with this HOMs-damped SRF harmonic cavity. The design and optimization of the HOMs damper, as well as the results of instability studies for this SRF harmonics cavity, are presented here.