Bunch Train Research Articles

CETASim: A numerical tool for beam collective effect study in storage rings

We developed a 6D multi-particle tracking program CETASim in C++ to simulate intensity-dependent effects in electron storage rings. The program can simulate the beam collective effects due to short-range/long-range wakefields for single/coupled-bunch instability studies. It also features the simulation of interactions among charged ions and the trains of electron bunches, including both fast ion and ion trapping effects. The bunch-by-bunch feedback is also included so that the user can simulate the damping of the unstable motion when its growth rate is faster than the radiation damping rate. The particle dynamics is based on the transfer maps from sector to sector, including the nonlinear effects of amplitude-dependent tune shift, high-order chromaticity, and second-order momentum compaction factor. Users can also introduce a skew quadrupole useful for emittance sharing and exchange studies. This paper describes the code structure, the physics models, and the algorithms used in CETASim. We also present the results of its application to the PETRA-IV storage ring.

Design and simulation of an S-band RF photogun for a linac-injector for the SYLA FEL

Currently,the 4th generation synchrotron radiation source SYLA (Synchrotron + Laser)is being developed at NRC «Kurchatov Institute». TheSYLAwillincludeof a free electron laser (FEL) and a 6 GeV storage ring with a 7BA lattice, which will allow obtaining a transverse emittance value of about 50 pm·rad. A linear accelerator for full energy is considered as both the main top-up injector into the storage ring and a driver for FEL. To generate high-brightness bunch trains for FEL, an S-band radiofrequency (RF) gun with a photocathode based on 3.5-cells π-mode standing wave accelerating structure at operating frequency of 2.8 GHz will be used. In this paper, the general concept of the RF photogun is proposed and briefly discussed. An optimization of beam dynamics is performed, electrodynamics parameters are found, fields distribution in the cavity and coaxial RF coupler are provided. Thermal analysis of the structure presented is performed.

Quasi-coherent radiation from a train of electron bunches inside a waveguide partially filled with dielectric

We investigate the quasi-coherent radiation from a train of electron bunches moving along the axis of a cylindrical waveguide, assuming that a part of the waveguide is filled with a dielectric medium. For the permittivity of the latter, the general case of dispersion is considered. It is shown that under certain conditions on the permittivity of the medium and on the values of the problem parameters, the waveguide modes become equidistant. As a consequence, quasi-coherent Cherenkov radiation from the train of bunches may be generated on the first several waveguide modes simultaneously. An example of dispersion law is provided for which the corresponding Cherenkov radiation is suppressed.

Breakdown insensitive acceleration regime in a metamaterial accelerating structure

A new regime in radiofrequency (rf) breakdown, named the breakdown insensitive acceleration regime (BIAR), was observed in an 11.7 GHz metamaterial structure for wakefield acceleration driven by rf pulses with a duration of a few nanoseconds. In the BIAR, rf breakdown occurs without interrupting potential beam acceleration, resulting in greater resilience to breakdown. We have investigated the possibility that BIAR can support higher gradients by characterizing the breakdown in a high-power test. The peak gradient reached 190 MV/m when the structure was powered by 6 ns long rf pulses with 115 MW peak power. The short rf pulses were extracted from 65 MeV electron bunch trains with a total charge of up to 210 nC. This work has revealed the benefits of short-pulse acceleration by characterizing rf breakdown in the previously unexplored parameter space. Published by the American Physical Society 2024

Longitudinal compression of a macrorelativistic electron beam

We present a novel concept of longitudinal bunch train compression that can manipulate a relativistic electron beam across hundreds of meters. This concept holds the potential to compress the electron beam produced by a conditional linear accelerator at a high ratio, elevating its power to a level comparable with large induction accelerators. The method employs the spiral motion of electrons in a uniform magnetic field to fold hundreds-of-meters-long trajectories into a compact setup. The interval between bunches can be fine-tuned by modulating their spiral movement. We explore this method with the particle dynamic simulation. Compared to setups of similar size, such as a chicane, our method can compress bunches at considerably larger scales. Consequently, it opens up new possibilities for generating high-power beams using compact devices at lower costs. Published by the American Physical Society 2024

Effects of cavity pre-detuning on RF power transients at injection into the LHC

At injection into the Large Hadron Collider (LHC), the radio frequency (RF) system is perturbed by beam-induced voltage resulting in strong RF power transients and the instant detuning of the cavities. The automatic tuning system, however, needs time for the mechanical compensation of the resonance frequency to take place. Acting back on the beam, the transients in RF power are expected to limit the maximum injected intensity by generating unacceptable beam loss. Reducing them is therefore essential to reach the target intensity during the High Luminosity (HL) LHC era. At LHC flat bottom, the cavities are operated using the half-detuning beam-loading compensation scheme. As implemented today, the tuner control algorithm starts acting only after the injection of the first longer bunch train which causes the bunches for this injection to experience the largest power spikes. This contribution presents an adapted detuning scheme for the RF cavities before injection. It was proposed as a path to decrease the transients, hence increasing the available intensity margin for the available RF power. The expected gain is evaluated in particle tracking simulations and measurements acquired during operation.

Monochromatic backward transition radiation from a tilted grating with adaptive strips produced by an electron bunch train

Nowadays, Smith–Purcell radiation is used for bunch length diagnostics. This paper presents experimental results of investigating radiation from a specific tilted grating. We propose using a grating consisting of adaptive strips, allowing to change a grating profile. Such a grating offers additional advantages compared to the traditional Smith–Purcell radiation geometry and radiation from a tilted grating. The results of measurements on the electron bunch train show that this advantage consists in the ability to increase the radiation intensity for a given tilt angle of the grating.

Single radio frequency bucket injection in the 88-Inch Cyclotron using a pulsed high voltage chopper.

A single radio frequency bucket of the 88-Inch Cyclotron is filled using a fast chopper located in the axial line. The bucket then accelerates until it reaches the deflector, at which point, it is extracted as a train of bunches. This phenomenon can be attributed to the characteristic multi-turn extraction of the cyclotron and, by simplifying the complex dynamics of a cyclotron, corresponds to the conceptual transfer function of the cyclotron. The confirmation of the single radio frequency bucket injection was achieved by operating the cyclotron in the third harmonic mode and observing the absence of intermediate bunches during the multiple-bunch extraction.

Future ESS upgrade to Medium Pulse Length – What are the technical challenges for the accelerator and target?

A compression of the ESS proton pulse from the present 2.86 milliseconds to a medium pulse length of a few tens of microseconds which is better matched to the moderator time-constant of thermal neutrons would considerably boost the performance for many instruments at ESS. Generating such a proton pulse with preserved instantaneous beam power requires a storage ring to be added to the ESS accelerator. Such a ring has been studied within the ESSnuSB neutrino super-beam study. The proton pulse length extracted in single turn extraction from this ring would be approximately 1 microsecond long which could be destructive for the present ESS target and is very short compared to the moderator time constant. The more desirable medium length pulse could possibly be generated by multi-turn extraction and a new concept using a cyclotron like extraction scheme in a synchrotron or a FFA. Another way to generate the longer pulses is to extract a bunch train using fast strip line kickers but this would require a larger storage ring. Using a “bunch train” has been successfully applied at the CERN ISOLDE facility to avoid destruction of sensitive liquid metal targets used for Nuclear Physics experiments. Other challenges are linked to the injection into the storage rings and the understanding of the target, moderator and neutron extraction systems with short and medium pulse length.

Bunch lifetime analysis based on the injection interval at SuperKEKB

We study the bunch lifetime dependence on the bunch position at the SuperKEKB main ring. The bunch lifetime ratio is evaluated from the injection interval. The lifetime imbalance is observed between the forward and backward of the bunch train at the positron ring. It becomes larger when the beam current becomes larger. The luminosity dependence on the RF-bucket spacing can be explained with the lifetime dependency which we determined in this analysis. The result indicates the existence of the unknown phenomenon which varies the Touschek lifetime with the RF-bucket spacing in the electron ring.

Beam dynamics optimization for high gradient beam driven plasma wakefield acceleration at SPARC-LAB

The SPARC_LAB test facility at the LNF (Laboratori Nazionali di Frascati, Rome) holds a high brightness photo-injector used to investigate advanced beam manipulation techniques. High brightness electron bunch trains (so-called comb beams) can be generated striking on the photo-cathode of a Radio Frequency (RF) photo-injector with a ultra-short UV laser pulse train in tandem with the velocity bunching technique. Beam dynamics studies have been performed with the aim of optimizing the dynamics of the double beam (driver and witness) used to perform particle driven plasma wake field acceleration (PWFA). In this scenario different scans on beam parameters were carried on adopting the ASTRA simulation code, in order to optimize the witness beam quality and improve the plasma booster stage performances. A benchmark of the simulations has been then performed, reproducing the experimental data obtained from the optimization of machine performances, and a good agreement was found.

Comparison of measurements and simulation results of dose for the flash radiation therapy beamline at PITZ

The high-brightness electron beam at the Photo Injector Test facility at DESY in Zeuthen (PITZ) is now also used for FLASHlab@PITZ: an R.&D platform for studying radiation biology and the FLASH effect in radiation therapy. The available parameter space of the electron beam with a momentum of 22 MeV/c allows bunch charges from 1 pC up to 5 nC, bunch durations of 0.160 ps and bunch train lengths up to 1 ms. The number of bunches in the single train can currently be varied between 1 and 1000 bunches, with an upgrade to 4500 foreseen in 2023. Radiation biology studies require accurate dose predic-tion, therefore Monte Carlo simulations based on the FLUKA code were performed. According to estimations, dose delivery of 0.002 Gy (low charge case 1 pC) and 10 Gy (high charge case 5 nC) is possible, if the beam is confined to a circular area with a radius of 5 mm with a lead collimator. For the Monte Carlo simulations, the experimental setup was accurately modelled, including the exit window, lead collimator, etc. Dose measurements were used to compare simulations with experiments. Dose profiles were experimentally measured with Gafchromic films and then compared with Monte Carlo simulations. The first experiments at FLASHlab@PITZ in 2023 have demonstrated flexible dose options for studying the FLASH effect and radiation biology studies.

First results of running the LHC with lead ions at a beam energy of 6.8 Z TeV

A two-day test of operation with Pb ion beams was carried out in the CERN Large Hadron Collider (LHC) in 2022, with the aim of gaining experience in view of the future high luminosity heavy-ion physics runs from 2023 onwards. The LHC experiments received the first Pb-Pb collisions at a record energy of 5.36 TeV centre-of-mass energy per colliding nucleon pair (beam energy 6.8 Z TeV). Bunch trains created with a new production scheme in the injectors, including slip-stacking, were injected into the LHC, with the collimation of nuclear beams with bent crystals tested along with a new collimation scheme for collision products. This paper describes the conditions and outcomes of these tests, which are critical steps in the upgrade to higher luminosity.

Overview of FLASHlab@PITZ: the new R&D platform for FLASH radiation therapy and radiation biology

An R&D platform for electron FLASH radiation therapy and radiation biology is being prepared at the Photo Injector Test facility at DESY in Zeuthen (FLASHlab@PITZ). This platform is based on the unique beam parameters available at PITZ: ps scale electron bunches of up to 22 MeV with up to 5 nC bunch charge at MHz bunch repetition rate in bunch trains of up to 1 ms in length repeating at 1 to 10 Hz. It works together with the Technical University of Applied Sciences Wildau (TH Wildau) as partner in close vicinity for the biological resources.A startup beamline has been installed to allow dosimetry studies and irradiation experiments on chemical, biochemical and biological samples after a 60-degree dispersive arm. The measured dose and dose rates under different beam conditions and first experimental results will be reported in this paper. In addition, a dedicated beamline for FLASHlab@PITZhas been designed for better control of the high brightness electron beams. This includes a dogleg to translate the beam and a 2D kicker system to scan the tiny beam focused by quadrupoles across the samples within less than 1 ms. Simulation studies will be presented to demonstrate the extremely flexible dose parameters with various irradiation options for electron FLASH radiation therapy and radiation biology studies.

A beam line setup for flash radiation therapy with focused electron beams at the PITZ facility at desy in zeuthen: Basic concept and dosimetry simulations

The objective of this study is demonstration of the principal possibility to increase the electron beam dose deposition at the certain depth of the sample for radiation therapy purposes. Electron bunches of 22 MeV within train generated at PITZ are focused inside the sample using a dedicated fast deflector and a solenoid magnet. To explore the capabilities of the proposed setup, dose distributions are calculated for multiple electron bunches focused in a single point inside a water phantom. Electron beam focusing produces dose peaks with a tunable maximal dose depth which is interesting for healthy tissue sparing at the surface and enhancing treatment quality. The duration of the full bunch train is 1 ms. During this time interval, the FLASH effect could be efficiently triggered inside the irradiated target volume. Monte Carlo simulations based on the FLUKA code were performed to evaluate the depth dose curves distributions in a water phantom. Using the PITZ electron beam parameters, simulations have shown the possibility to produce a peak dose in water seven times higher than compared to the dose at the surface. Moreover, the RMS size homogeneous area around the maximal dose is approximately 25 mm.

(Invited) Photo-Assisted Electron Emission from Silicon-Based Electron Emission Devices

Ultrafast pulsed electron sources with spatial coherence have attracted significant interest in emerging applications such as scientific instruments of ultrafast electron microscopy [1-3] and electron beam lithography [4], and also as sources of X-ray [5] and micro- and THz-waves [6-9]. In particular, high-frequency electron bunch trains generated directly from the cathode surface can not only dramatically reduce dimensions and weight through the elimination of the pre-modulation circuit, but can also provide extremely high efficiency and improve the power in the electron beam devices due to coherence effects.One solution is employing photoemission from electron sources driven by ultrashort laser pulses for such emerging applications. Metallic flat photocathodes and recently metallic nanotips have been employed. However, a low quantum efficiency (QE), typically of approximately 0.01 %, and a large work function require a high-power UV laser to generate an intense electron beam. Negative electron affinity (NEA) semiconductor photocathodes such as NEA GaAs can provide QE as high as ~ 10 % [10], operating using visible laser light irradiation. However, the NEA surface is fragile and easily damaged by residual gas adsorption or beam extraction.In this presentation, we introduce the research and development of gated Si-FEAs with a volcano structure and planar-type electron emission devices based on a MOS diode structure and discuss the potential of the devices as a photocathode of the devices operating without NEA surface treatment. We investigate the optical properties of emitted electrons from the devices under visible laser light irradiation. The results indicate that Si-based electron emission devices with a fully optimized structure hold promise as a photocathode with a high QE and a high-speed response for generating a train of ultrashort electron bunches, operating without NEA surface treatment.

Spectral-angular distribution of radiation generated by a train of electron bunches passing through the centre of a ball

We present the results of theoretical investigations of the spectral and angular distributions of the radiation generated by a train of electron bunches crossing a dielectric ball. The numerical examples are given for a dielectric ball made of fused quartz. It is shown that strong peaks appear in the spectral-angular distribution of the radiation intensity for certain values of the problem parameters. The heights of the peaks in the spectral density of the radiated energy are essentially increased when the frequency of the succession of the bunches in the train becomes equal to the respective resonant frequency of the ball.

Status and future plans for C3 R&D

C3 is an opportunity to realize an e + e - collider for the study of the Higgs boson at √s = 250 GeV, with a well defined upgrade path to 550 GeV while staying on the same short facility footprint [2,3]. C3 is based on a fundamentally new approach to normal conducting linear accelerators that achieves both high gradient and high efficiency at relatively low cost. Given the advanced state of linear collider designs, the key system that requires technical maturation for C3 is the main linac. This paper presents the staged approach towards a facility to demonstrate C3 technology with both Direct (source and main linac) and Parallel (beam delivery, damping ring, ancillary component) R&D.The primary goal of the C3 Demonstration R&D Plan is to reduce technical and cost risk by building and operating the key components of C3 at an adequate scale. This R&D plan starts with the engineering design, and demonstration of one cryomodule and will culminate in the construction of a 3 cryomodule linac with pre-production prototypes. This R&D program would also demonstrate the linac rf fundamentals including achievable gradient and gradient stability over a full electron bunch train and breakdown rates. It will also investigate beam dynamics including energy spread, wakefields, and emittance growth. This work will be critical to confirm the suitability of the C3 beam parameters for the physics reach and detector performance in preparation for a Conceptual Design Report (CDR), as well as for follow-on technology development and industrialization.The C3 Demonstration R&D Plan will open up significant new scientific and technical opportunities based on development of high-gradient and high-efficiency accelerator technology. It will push this technology to operate both at the GeV scale and mature the technology to be reliable and provide high-brightness electron beams.The timeline for progressing with C3 technology development will be governed by practical limitations on both the technical progress and resource availability. It consists of four stages: Stage 0) Ongoing fundamental R&D on structure prototypes, damping and vibrations. Stage 1) Advancing the engineering maturity of the design and developing start-to-end simulations including space-charge and wakefield effects. This stage will include testing of strucutres operating at cryogenic temperatures. Beam tests would be performed with high beam current to test full beam loading. Stage 2) Production and testing of the first cryomodule at cryogenic temperatures. This would provide sufficient experimental data to compile a CDR and it is anticipated for Stage 2 to last 3 years and to culminate with the transport of photo-electrons through the first cryomodule. Stage 3) Updates to the engineering design of the cryomodules, production of the second and third cryomodule and their installation. Lower charge and lower emittance beams will be used to investigate emittance growth. The successful full demonstration of the 3 cryomodules to deliver up to a 3 GeV beam and achieve the C3five gradient will allow a comprehensive and robust evaluation of the technical design of C3 as well as mitigate technical, schedule, and cost risks required to proceed with a Technical Design Report (TDR).

Generation and optimization of electron bunch trains from nonlinear longitudinal space charge oscillation

Generation and optimization of electron bunch trains from nonlinear longitudinal space charge oscillation

Beam dynamics simulation in the linear accelerator used as an injector for the 4-th generation Specialized Synchrotron Radiation Source SSRS-4

At present, a fourth-generation synchrotron radiation source USSR (Synchrotron & Laser), which will the one of the world’s largest scientific centers is being designed in Russia. Its creation will make it possible to carry out experiments to study the structures of a wide range of objects in various disciplines at a qualitatively new level compared to the previous generation of sources. The overall facility layout includes a 6 GeV main storage ring and a linear electron accelerator (linac) at full energy. It is proposed to use one linear accelerator with two RF guns. A RF gun with a photocathode can be used to generate a bunch train for a Free Electron Laser (FEL), a RF gun with a thermionic cathode can be used for injection into a storage ring. Both injectors will operate with the same regular part of the linear accelerator, consisting of 100-120 identical sections. The purposed transverse emittance in the main storage ring will be 50–100 pm.rad. The development of a general linear accelerator scheme in order to minimize the beam energy spread and the output transverse emittance, the optimization of the geometric and electrodynamics parameters of the accelerating structures, and the beam dynamics analysis in this linear accelerator will be discussed in the article. The beam dynamics simulation was performed using the BEAMDULAC code developed at the Department of Electrophysical Facilities of the National Research Nuclear University MEPhI.