Magnetic Bunch Compressor Research Articles

Suppression of Microbunching Instability based on optimized velocity bunching in linac-driven FELs

The microbunching instability (MBI) driven by beam collective effects can cause significant electron beam quality degradation in advanced x-ray free electron lasers. Typically, multiple stage magnetic bunch compressors used to generate high peak current electron beam will dramatically amplify the microbunching instability. In this paper, by redesigning the solenoid elaborately and adopting a dual-mode buncher cavity with the third harmonic mode used to correct the RF curvature, it is potential for the electron beam to be further compressed in velocity bunching (VB) process. Therefore, a VB plus one bunch compressor could be a promising alternative scheme to achieve moderate peak current beam for high-repetition-rate X-ray FELs to suppress the additional MBI gain due to multi-stage magnetic bunch compressors.

Numerical Simulation of an Electron Beam for Magnetic Bunch Compressor Commissioning at PITZ

A THz free electron laser (FEL) prototype has been developed at the Photo Injector Test Facility at DESY in Zeuthen (PITZ) for obtaining high intensity radiation for THz-pump and X-ray-probe experiments at the European XFEL. In this development, a magnetic chicane was recently installed to optimize the THz FEL performance. The aim of this study is to investigate the beam dynamics in the chicane for a trajectory commissioning by tracking the electron beam via ASTRA using a 3-dimensional (3-D) magnetic field of the chicane simulated with CST-EM Studio. The simulated results indicate the possibility to obtain a minimum-momentum dispersion of an electron beam after chicane with the high bunches charge of electron beam transportation.

Start-to-end simulations for bunch compressor and THz SASE FEL at PITZ

The magnetic bunch compressor was designed as part of a THz accelerator source being developed at the Photo Injector Test facility at DESY in Zeuthen (PITZ) as a prototype for pump-probe experiments at the European XFEL. As an electron bunch is compressed to achieve higher bunch currents for the THz source, the beam dynamics in the bunch compressor was studied by numerical simulations. A start-to-end simulation optimizer including coherent synchrotron radiation (CSR) effects has been developed by combining the use of ASTRA, OCELOT, and GENESIS to support the design of the THz source prototype. In this paper, we present simulation results to explore the possibility of improving the performance of the THz FEL at PITZ by using the developed bunch compressor.

Novel bunch compressor chicane: The five-bend chicane

Four dipole magnetic chicanes are typically used in high-performance accelerators to compress electron bunches to their final peak currents but subject the beam to emittance dilution due to the coherent synchrotron radiation (CSR) effect. In this paper, we present a new magnetic bunch compressor geometry designed to locally suppress the CSR-induced emittance degradation called the five-bend chicane. We will include a simulation study comparing a five-bend chicane to the nominal four-bend chicane of the LCLS-II-HE's second bunch compressor (BC2) beamline. The study will consist of emittance and energy spread growth due to CSR using a one-dimensional, elegant, and two-dimensional, csrtrack, CSR algorithm in free space, and the free-electron laser performance using genesis.

CoSyR: A novel beam dynamics code for the modeling of synchrotron radiation effects

The self-consistent nonlinear dynamics of a relativistic charged particle beam interacting with its complete self-fields is a fundamental problem underpinning many of the accelerator design issues in high brightness beam applications, as well as the development of advanced accelerators. Particularly, synchrotron radiation induced effects in a magnetic dispersive beamline element can lead to collective beam instabilities and emittance growth. A novel beam dynamics code is developed based on a Lagrangian method for the calculation of the particles’ radiation near-fields using wavefront/wavelet meshes via the Green’s function of the Maxwell equations. These fields are then interpolated onto a moving mesh for dynamic update of the beam. This method allows radiation co-propagation and self-consistent interaction with the beam in 2D/3D simulations at greatly reduced numerical errors. Multiple levels of parallelisms are inherent in this method and implemented in our code CoSyR to enable at-scale simulations of nonlinear beam dynamics on modern computing platforms using MPI, multi-threading, and GPUs. The current 2D implementation of CoSyR has been used to evaluate the transverse and longitudinal coherent radiation effects on the beam and to investigate beam optics designs proposed for mitigation of beam brightness degradation in a magnetic bunch compressor. In this paper, the design of CoSyR, as well as the benchmark with other coherent synchrotron radiation models, are described and discussed. Extension of the core algorithms to 3D is possible and planned.

Investigation of Bunch Compressor and Compressed Electron Beam Characteristics by Coherent Transition Radiation

A magnetic chicane bunch compressor for a new compact accelerator-based terahertz (THz) radiation source at the Institute of Advanced Energy, Kyoto University, was completely installed in March 2016. The chicane is employed to compress an electron bunch with an energy of 4.6 MeV generated by a 1.6-cell photocathode radio frequency (RF)-gun. The compressed bunch is injected into a short planar undulator for THz generation by coherent undulator radiation (CUR). The characteristics of the bunch compressor and the compressed bunch were investigated by observing the coherent transition radiation (CTR). The CTR spectra, which were analyzed by using a Michelson interferometer, and the compressed bunch length were also estimated. The results were that the chicane could compress the electron bunch at a laser injection phase less than 45 degrees, and the maximum CTR intensity was observed at a laser injection phase around 24 degrees. The optimum value of the first momentum compaction factor was around −45 mm, which provided an estimated rms bunch length less than 1 ps.

Electron beam dynamics in the 3D magnetic field of alpha magnet at the PBP-CMU Electron Linac Laboratory

The electron injector system for a future mid-infrared free-electron laser (MIR FEL) and coherent terahertz (THz) radiation source at the Plasma and Beam Physics (PBP) Research Facility of Chiang Mai University (CMU) in Thailand will consists mainly of a thermionic cathode radio-frequency (RF) electron gun, an alpha magnet, a traveling-wave linear accelerator (linac) structure and two magnetic bunch compressor systems. The detailed study of all components in the injector system is very important for generation of high quality electron beam and radiation. This paper focuses on the alpha magnet, which is used as a magnetic bunch compressor and an energy filter before the beam being accelerated in a post-acceleration linac structure. We report its characteristics and the electron beam dynamics in its three-dimensional (3D) magnetic field distribution that was simulated with the program CST EM Studio 2016. The simulated alpha magnet’s fields are comparable with the measurement data. The beam dynamic simulations in 3D magnetic field were performed by using programs ASTRA and ELEGANT. The simulation results allow us to investigate the influence of the fringe field, the vertical focusing and the space charge effect in the alpha magnet. This is significantly important for electron beam manipulation in the injector system. The study outputs reveal that the proper injection angle in the practical beam operation to compensate the fringe field effect should be 42∘ that is 1.29∘ larger than the ideal injection angle. We also found that the ratio of the vertical focal length to the total path length in the alpha magnet is 0.544, which is independent on the magnetic field gradient. Moreover, the space charge force in the electron bunch leads to the rotation of transverse phase spaces while the beam traverses in the alpha magnet’s field. This effect has considerably small influence on the longitudinal phase space, especially for the beam with a bunch charge not exceeding 0.2 nC.

Study of the saturation of radiation energy caused by the space charge effect in a compact THz coherent radiation source

To generate an intense quasi-monochromatic Terahertz Coherent Undulator Radiation (THz-CUR), a compact linac system, which employs a magnetic electron bunch compressor with a beam energy of 4.6 MeV, has been constructed at Kyoto University. The THz-CUR has successfully been generated in a frequency range from 0.16 to 0.65 THz with a bunch charge of 60 pC. The maximum micro-pulse energy of THz radiation was observed higher than 1 μJ at 0.16 THz with 160 pC. However, when a bunch charge was higher than 80 pC, the micro-pulse energy of THz radiation gradually went to the saturation because of bunch lengthening and degradation of electron beam quality due to the space charge effect. The dependence of a bunch length on a bunch charge has been studied by GPT simulation and compared with CTR and CUR experiments. The trends of the measured results from CUR and CTR are in good agreement with the GPT simulation.

Coherent transition radiation from femtosecond electron bunches at the accelerator-based THz light source in Thailand

A THz radiation source based on electron linear accelerator (linac) has been built and commissioned at the Plasma and Beam Physics (PBP) Research Facility, Chiang Mai University (CMU) in Thailand since 2005. The accelerator system consists of a thermionic cathode RF electron gun, a magnetic bunch compressor in a form of an alpha-magnet, an S-band travelling-wave linac structure, beam steering and focusing magnets, beam and radiation diagnostic instruments, control system and other support components. Electron bunches with an average kinetic energy of about 8 MeV, a bunch charge of 100 pC, and a bunch length of 300 fs can be produced from the PBP-CMU electron linac system. Coherent THz radiation generated via transition radiation technique is used as a source of the THz spectroscopy and THz imaging applications. The measured power spectrum indicates that the radiation obtained from short electron bunches covers the frequency range of 0.15–1.80 THz or the wavenumber range of 5–60 cm−1. Experimental results reveal that a peak radiation power as high as 0.2 MW per micropulse and an average radiation power of 2.5 mW can be obtained. Generation and characterization of the coherent THz transition radiation as well as some examples of experiments to achieve electron bunch length and radiation power spectrum are reported and discussed in this paper.

A THz Spectroscopy System Based on Coherent Radiation from Ultrashort Electron Bunches

A spectroscopy system will be discussed for coherent THz transition radiation emitted from short electron bunches, which are generated from a system consisting of an RF gun with a thermionic cathode, an alpha-magnet as a magnetic bunch compressor, and a linear accelerator for post-acceleration. The THz radiation is generated as backward transition radiation when electron bunches pass through an aluminum foil. The emitted THz transition radiation, which is coherent at wavelengths equal to and longer than the electron bunch length, is coupled to a Michelson interferometer. The performance of the spectroscopy system employing a Michelson interferometer is discussed. The radiation power spectra under different conditions are presented. As an example, the optical constant of a silicon wafer can be obtained using the dispersive Fourier transform spectroscopy (DFTS) technique.

Design, construction and measurements of an alpha magnet as a solution for compact bunch compressor for the electron beam from Thermionic RF Gun

The beam produced by a thermionic RF gun has wide energy spread that makes it unsuitable for direct usage in photon sources. Here in the present work, we optimize the extracted beam from a thermionic RF gun by a compact economical bunch compressor. A compact magnetic bunch compressor (Alpha magnet) is designed and constructed. A comparison between simulation results and experimental measurements shows acceptable conformity. The beam dynamics simulation results show a reduction of the energy spread as well as a compression of length less than 1 ps with 2.3 mm-mrad emittance.

Passive Linearization of the Magnetic Bunch Compression Using Self-Induced Fields.

In linac-driven free-electron lasers, colliders, and energy recovery linacs, a common way to compress the electron bunch to kiloampere level is based upon the implementation of a magnetic dispersive element that converts particle energy deviation into a path-length difference. Nonlinearities of such a process are usually compensated by enabling a high harmonic rf structure properly tuned in amplitude and phase. This approach is however not straightforward, e.g., in C-band and X-band linacs. In this Letter we demonstrate that the longitudinal self-induced field excited by the electron beam itself is able to linearize the compression process without any use of high harmonic rf structure. The method is implemented at the FERMI linac, with the resulting high quality beam used to drive the seeded free-electron laser during user experiments.

Commissioning experience and beam physics measurements at the SwissFEL Injector Test Facility

The SwissFEL Injector Test Facility operated at the Paul Scherrer Institute between 2010 and 2014, serving as a pilot plant and testbed for the development and realization of SwissFEL, the X-ray Free-Electron Laser facility under construction at the same institute. The test facility consisted of a laser-driven rf electron gun followed by an S-band booster linac, a magnetic bunch compression chicane and a diagnostic section including a transverse deflecting rf cavity. It delivered electron bunches of up to 200 pC charge and up to 250 MeV beam energy at a repetition rate of 10 Hz. The measurements performed at the test facility not only demonstrated the beam parameters required to drive the first stage of an FEL facility, but also led to significant advances in instrumentation technologies, beam characterization methods and the generation, transport and compression of ultra-low-emittance beams. We give a comprehensive overview of the commissioning experience of the principal subsystems and the beam physics measurements performed during the operation of the test facility, including the results of the test of an in-vacuum undulator prototype generating radiation in the vacuum ultraviolet and optical range.

Transverse to longitudinal phase space coupling in an electron beam for suppression of microbunching instability

The microbunching instability developed during the beam compression process in the linear accelerator (LINAC) of a free-electron laser (FEL) facility has always been a problem that degrades the lasing performance, and even no FEL is able to be produced if the beam quality is destroyed too much by the instability. A common way to suppress the microbunching instability is to introduce extra uncorrelated energy spread by the laser heater that heats the beam through the interaction between the electron and laser beam, as what has been successfully implemented in the Linac Coherent Light Source and Fermi@Elettra. In this paper, a simple and effective scheme is proposed to suppress the microbunching instability by adding two transverse gradient undulators (TGU) before and after the magnetic bunch compressor. The additional uncorrelated energy spread and the density mixing from the transverse spread brought up by the first TGU results in significant suppression of the instability. Meanwhile, the extra slice energy spread and the transverse emittance can also be effectively recovered by the second TGU. The magnitude of the suppression can be easily controlled by varying the strength of the magnetic fields of the TGUs. Theoretical analysis and numerical simulations demonstrate the capability of the proposed technique in the LINAC of an x-ray free-electron laser facility.

Beam Dynamics Investigation for the Compact Seeded THz-FEL Amplifier

A compact seeded terahertz (THz) free-electron laser (FEL) amplifier is under development at the Institute of Advanced Energy, Kyoto University. The system consists of a 1.6-cell S-band BNL-type photocathode radio frequency (RF) gun, a focusing solenoid magnet, a magnetic bunch compressor, focusing quadrupoles, a short planar undulator and a THz parametric generator for seeding. The accelerator system will generate a high brightness ultra-short electron bunch injected to the short undulator. Since characteristics of the emitted radiation strongly depend on the electron beam properties, the beam dynamics of the system has been investigated. As the result, the system should be operated at the high accelerating voltage of 80 MV/m to avoid the space-charge effect which causes serious bunch lengthening. The laser with the uniform temporal distribution and the injection phase of 12 degree is suitable for the bunch compression in the chicane. This condition provides the electron beam injected to the undulator with the peak current of more than 300 A and the FWHM bunch length of 0.36 ps at the bunch charge of 200 pC.

Generation of Terahertz Waves Using Ultrashort Electron Beams from a Photocathode Radio‐Frequency Gun Linac

SUMMARYUltrashort (picosecond or femtosecond) electron beams are useful for electromagnetic radiation production in the terahertz (THz) range because the inverse of a 1 ps bunch length corresponds to a frequency of 1 THz. The electron beams were generated by a photocathode radio‐frequency (RF) gun linac. Electron beams with an energy of about 4 MeV at the RF gun exit were accelerated to about 30 MeV in a linac. The accelerated electron beams were compressed by a magnetic bunch compressor, resulting in femtosecond pulse width. THz radiation of the order of 0.1 THz based on coherent Cherenkov radiation (CCR) using hollow dielectric tubes and Smith−Purcell radiation (SPR) using a metallic grating or metasurface are described.

Analytical bunch compression studies for a linac-based electron accelerator

The current paper deals with analytical bunch compression studies for FLUTE whose results are compared to simulations. FLUTE is a linac-based electron accelerator with a design energy of approximately 40 MeV currently being constructed at the Karlsruhe Institute of Technology. One of the goals of FLUTE is to generate electron bunches with their length lying in the femtosecond regime. In the first phase this will be accomplished using a magnetic bunch compressor. This compressor forms the subject of the studies presented. The paper is divided into two parts. The first part deals with pure geometric investigations of the bunch compressor where space charge effects and the backreaction of bunches with coherent synchrotron radiation are neglected. The second part is dedicated to the treatment of space charge effects. The upshot is that the analytical results in the two parts agree quite well with what is obtained from simulations. This paper shall form the basis for future analytical studies of the FLUTE bunch compressor and of bunch compression, in general.

Measurements and analysis of a high-brightness electron beam collimated in a magnetic bunch compressor

A collimator located in a magnetic bunch compressor of a linear accelerator driven x-ray free electron laser has many potential applications, such as the removal of horns in the current distribution, the generation of ultrashort beams, and as a diagnostic of the beam slice emittance. Collective effects, however, are a major concern in applying the technique. Systematic measurements of emittance and analysis were performed using a collimator in the first bunch compressor of the Linac Coherent Light Source (LCLS). In the nominal, undercompressed configuration using the collimator we find that the $y$ emittance (nonbending plane) is not increased, and the $x$ emittance (in the bending plane) is increased by about 25%, in comparison to the injector emittance. From the analysis we conclude that the parasitic effects associated with this method are dominated by coherent synchrotron radiation (CSR), which causes a ``systematic error'' for measuring slice emittance at the bending plane using the collimation method. In general, we find good agreement between the measurements and simulations including CSR. However, for overcompressed beams at smaller collimator gaps, an extra emittance increase is found that does not agree with 1D simulations and is not understood.

Measurement of<20 fsbunch length using coherent transition radiation

By monitoring coherent transition radiation (CTR) with a Michelson interferometer, bunch length measurement was investigated. Less-than-20-fs electron bunches were generated by a photocathode-based linear accelerator (linac) and a magnetic bunch compressor. From the femtosecond electron bunch of 2.1 pC and 32 MeV, CTR was measured using a Michelson interferometer that had two detectors with different spectral ranges, which enhanced the detection spectral range of CTR (ranging from 3 to 50 THz) and enabled us to determine the bunch length of the electron bunch. A model based on the interferometer's frequency sensitivity was proposed and introduced to analyze the measured interferograms. The measured interferograms and the frequency spectra were explained using the model.

Generation of Terahertz-waves using Ultrashort Electron Beams from a Photocathode Radio-frequency Gun Linac

Ultrashort picosecond or femtosecond electron beams are useful for electromagnetic radiation production in the terahertz THz range because the inverse of a 1 ps bunch length corresponds to a frequency of 1 THz. The electron beams were generated by a photocathode radio-frequency RF gun linac. Electron beams with an energy of about 4 MeV at the RF gun exit were accelerated to about 30 MeV in a linac. The accelerated electron beams were compressed by a magnetic bunch compressor, resulting in femtosecond pulse width. THz radiation of the order of 0.1 THz based on coherent Cherenkov radiation CCR using hollow dielectric tubes and Smith-Purcell radiation SPR using a metallic grating or metasurface are described.