Bunch Compression System Research Articles

Electron Energy Spectrometer for MIR-THz FEL Light Source at Chiang Mai University

The linear accelerator system of the PBP-CMU Electron Linac Laboratory has been designed with the aim of generating free-electron lasers (FELs) in the mid-infrared (MIR) and terahertz (THz) regions. The quality of the radiation is strongly dependent on the properties of the electron beam. Among the important beam parameters, the electron beam energy and energy spread are particularly important. To accurately measure the electron beam energy, the first dipole magnet in the bunch compressor system and the downstream screen station are employed as an energy spectrometer. The A Space Charge Tracking Algorithm (ASTRA) software is used for the design and optimization of this system. Simulation results demonstrate that the developed spectrometer is capable of accurately measuring the energy within the 5–25 MeV range. The screen station system is designed and constructed to have the ability to capture a beam size with a resolution of 0.1 mm per pixel. This resolution is achieved with a screen-to-camera distance of 1.2 m, which proves sufficient for precise energy measurement. The systematic error in energy measurement is found to be less than 10%, with a minimum energy spread of 0.4% achievable when the horizontal beam size remains below 3 mm.

Phase drift compensation between injector linac master oscillator and ring master oscillator for stable beam injection at SuperKEKB

The SuperKEKB injector linac that includes the positron damping ring (DR) delivers low emittance electron and positron beams to the main rings. The beam injection phase from linac to the rings should be stabilized for long-term stable beam injection. The frequencies of the linac master oscillator (MO) and ring MO are 571.2 and 508.9 MHz, respectively. These two different frequency signals are monitored using the direct sampling technique with a common sampling rate. A phase drift of several degrees is observed between the linac and ring MOs. For stable beam injection, a phase drift compensation system is implemented at linac. The system performance was evaluated by the beam energy stability after the bunch compression system from DR to linac.

Low-charge, hard x-ray free electron laser driven with anX-band injector and accelerator

After the successful operation of the Free Electron Laser in Hamburg (FLASH) and the Linac Coherent Light Source (LCLS), soft and hard x-ray free electron lasers (FELs) are being built, designed, or proposed at many accelerator laboratories. Acceleration employing lower frequency rf cavities, ranging from $L$-band to $C$-band, is usually adopted in these designs. In the first stage bunch compression, higher-frequency harmonic rf system is employed to linearize the beam's longitudinal phase space, which is nonlinearly chirped during the lower frequency rf acceleration process. In this paper, a hard x-ray FEL design using an all $X$-band accelerator at 11.424 GHz (from photocathode rf gun to linac end) is presented, without the assistance of any harmonic rf linearization. It achieves LCLS-like performance at low charge using $X$-band linac drivers, which is more versatile, efficient, and compact than ones using $S$-band or $C$-band rf technology. It employs initially 42 microns long (rms), low-charge (10 pC) electron bunches from an $X$-band photoinjector. An overall bunch compression ratio of roughly 100 times is proposed in a two stage bunch compressor system. The start-to-end macroparticle 3D simulation employing several computer codes is presented in this paper, where space charge, wakefields, and incoherent and coherent synchrotron radiation effects are included. Employing an undulator with a short period of 1.5 cm, a Genesis FEL simulation shows successful lasing at a wavelength of 0.15 nm with a pulse length of 2 fs and a power saturation length as short as 20 meters, which is equivalent to LCLS low-charge mode. Its overall length of both accelerators and undulators is 180 meters (much shorter than the effective LCLS overall length of 1230 meters, including an accelerator length of 1100 meters and an undulator length of 130 meters), which makes it possible to be built in places where only limited space is available.

Bunch compression efficiency of the femtosecond electron source at Chiang Mai University

A femtosecond electron source has been developed at the Plasma and Beam Physics Research Facility (PBP), Chiang Mai University (CMU), Thailand. Ultra-short electron bunches can be produced with a bunch compression system consisting of a thermionic cathode RF-gun, an alpha-magnet as a magnetic bunch compressor, and a linear accelerator as a post acceleration section. To obtain effective bunch compression, it is crucial to provide a proper longitudinal phase-space distribution at the gun exit matched to the subsequent beam transport system. Via beam dynamics calculations and experiments, we investigate the bunch compression efficiency for various RF-gun fields. The particle distribution at the RF-gun exit will be tracked numerically through the alpha-magnet and beam transport. Details of the study and results leading to an optimum condition for our system will be presented.

Electron-bunch compression using a dynamical nonlinearity correction for a compact x-ray free-electron laser

We propose a novel method to achieve a highly brilliant electron bunch with a peak current of several kA in an x-ray free-electron laser (XFEL) by efficiently correcting nonlinearity of the bunch-compression system. In this method, a second-order positive energy chirp is imprinted to the electron bunch by a correction cavity installed at the beginning of the system, namely the nonlinearity is initially overcorrected. The imprinted chirp is enhanced in a compression process at a bunch compressor due to electron bunch rotation in longitudinal phase space, and then continuously corrects the second-order nonlinearity induced in the following sections. Since the frequency and field amplitude of the correction cavity can be significantly lowered to the available technology level, this method enables the use of a high-frequency accelerator, i.e., a high-gradient accelerator, which is indispensable to realize a compact XFEL facility.

Microbunching instability in a chicane: Two-dimensional mean field treatment

We study the microbunching instability in a bunch compressor by a parallel code with some improved numerical algorithms. The two-dimensional charge/current distribution is represented by a Fourier series, with coefficients determined through Monte Carlo sampling over an ensemble of tracked points. This gives a globally smooth distribution with low noise. The field equations are solved accurately in the lab frame using retarded potentials and a novel choice of integration variables that eliminates singularities. We apply the scheme with parameters for the first bunch compressor system of FERMI{at}Elettra, with emphasis on the amplification of a perturbation at a particular wavelength. Gain curves agree with those of the linearized Vlasov model at long wavelengths, but show some deviation at the smallest wavelengths treated.

Control of Emittance Growth in a Bunch Compressor for the International Linear Collider

We present a short two-stage bunch compressor system that has been selected as an alternative design in the ILC (International Linear Collider). The designed bunch compressor system has two rf sections and two chicanes in which each chicane consists of four bending magnets. Each bunch compressor stage includes a 34 m lattice of bending magnets that is relatively short compared to the baseline design with a 150 m lattice in the ILC. Beams with bunch length of 6 mm rms and 9 mm rms can be compressed to 0.15 mm rms and 0.3 mm rms in the bunch compressor system, respectively. We show the optimized parameters of the bunch compressor system and results of transverse and longitudinal beam trackings in the compressor system. We examined the influences of coherent and incoherent synchrotron radiations in the chicanes on the transverse emittance. Control of the emittance growth due to machine errors is an important issue in the performance of the bunch compressor. Thus, we performed the lattice corrections such as corrections of orbit distortion, dispersion function and skew component to control the emittance growths due to several machine errors in the bunch compressor system. It is shown that the system is error tolerant and the lattice corrections are very effective to suppress growth of the emittance in the bunch compressor system.

A compact free-electron laser for generating coherent radiation in the extreme ultraviolet region

Single-pass free-electron lasers based on self-amplified spontaneous emission1,2,3,4 are enabling the generation of laser light at ever shorter wavelengths, including extreme ultraviolet5, soft X-rays and even hard X-rays6,7,8. A typical X-ray free-electron laser is a few kilometres in length and requires an electron-beam energy higher than 10 GeV (refs 6, 8). If such light sources are to become accessible to more researchers, a significant reduction in scale is desirable Here, we report observations of brilliant extreme-ultraviolet radiation from a 55-m-long compact self-amplified spontaneous-emission source, which combines short-period undulators with a high-quality electron source operating at a low acceleration energy of 250 MeV. The radiation power reaches saturation at wavelengths ranging from 51 to 61 nm with a maximum pulse energy of 30 µJ. The ultralow emittance (0.6π mm mrad) of the electron beam from a CeB6 thermionic cathode9 is barely degraded by a multiple-stage bunch compression system that dramatically enhances the beam current from 1 to 300 A. This achievement expands the potential for generating X-ray free-electron laser radiation with a compact 2-GeV machine. Free-electron lasers can produce powerful pulses of radiation at very short wavelengths, even in the hard-X-ray region. In general, however, they comprise facilities several kilometres in length. A 55-m-long laser could open up the technology to a broader range of researchers.

Beam Dynamics in a Linear Accelerator for Generations of Short Electron Beams and Femtosecond Hard X-ray Pulses

We investigate a linear accelerator system capable of generating short electron beams and femtosecond hard X-ray pulses. We show a detailed design for a two-stage bunch compressor to generate the short electron beams in the linear accelerator. The bunch compressor system consists of two chicanes with a short system length that can compress an electron bunch of 0.6 nC and beam energy of 162 MeV, from 3 to 0.5 ps rms. One important design issue in the bunch compressor is to make as small growths of the emittance and energy spread as possible. The normalized horizontal emittance of 3 mm mrad is increased by approximately 10% due to coherent synchrotron radiation in the designed bunch compressor. Lattice distortions due to machine errors associate with quadrupole magnets, bending magnets and beam position monitors in the linear accelerator were investigated. It is shown that the lattice distortions due to the machine errors can be easily compensated by performing both orbit correction and dispersion correction in the linear accelerator. We have performed tolerance studies due to the various jitter sources in the linear accelerator to examine their sensitivities on the beam quality. From these results, it is shown that the linear accelerator system provides sufficient tolerances to maintain stable electron beams. We also investigated the generation of femtosecond hard X-ray pulses that may be provided by the interactions at 90° of the short electron beams in the linear accelerator with a laser system. It is shown that 3.4×106 photons within 10% bandwidth at 0.04 Å wavelength in about 350 fs rms pulse may be provided using the linear accelerator system. We presented studies on beam dynamics in the linear accelerator system that may provide the short beams and intense X-ray pulses.

Calculation of coherent synchrotron radiation in the TTF-FEL bunch compressor magnet chicanes

The bunch compression system in the TTF-FEL decreases the bunch length in order to achieve the high peak currents needed for the SASE FEL process. Along the curved trajectories in the bunch compressor magnet chicanes, these short bunches will start to radiate coherently. In this paper, a numerical calculation of the characteristics of this radiation is presented. One-dimensional bunch models are not sufficient for near-field effects, in particular, cannot completely explain the power flow to the EM field, but they can be used to estimate the radiation in the far region. For the calculation of the power flow, in presence of shielding by horizontal conducting planes, and of the far field, a two-dimensional bunch with time-dependent shape was simulated.

Emittance growth due to wake fields on curved bunch trajectories

Very short bunches with high peak currents are required in linac based X-ray Free Electron Lasers (SASE-FELs) and in the next generation of linear colliders. In most designs, short bunches are produced by lingitudinal compression in magnet chicanes, where particles with different energies have different path lengths so that a bunch with an energy distribution correlated with longitudinal particle position can shrink in length. However, care has to be taken that the low emittance beams are not blown up by wake field effects. Especially the wake field calculation for very short bunches on curved trajectories with relatively small bending radii (of the order of meters) requires consideration of “overtaking effects”. “Overtaking” means, that on a curved trajectory the wake of the bunch tail may reach the head because the field travels on a shorter straight path. We present a treatment to calculate these fields considering transient regimes and shielding effects as well as calculations on the emittance growth for the case of the TTF-FEL bunch compression system.

Nonlinear subpicosecond electron-bunch compressor

The minimum bunch length achievable using a magnetic bunch compression system is related to the nonlinear dispersion of the compressor and the nonlinear energy-phase correlation of the bunch's longitudinal phase space. For electron bunches with medium or high charge (on the order of 1 nC, or higher), the longitudinal wakefields in the accelerating structure cause a large nonlinearity in the initial energy-phase correlation. Specific nonlinearities in the dispersion of the bunch compressor must then be introduced in order to maximally compress the bunch length, which is done by a chicane-style compressor by increasing the bend angle. This nonlinear dispersion actually decreases the rms longitudinal-beam emittance, by compensating for the nonlinearities in the beam's energy-phase correlation. In this paper, we report on the compression theory, design, and operation of such a compressor. For this compressor, introducing nonlinear dispersion reduced the minimum compressed bunch length by about a factor of 5, resulting in measured FWHM bunch lengths of 2 3 ps for bunch charges as large as 1 nC.

The bunch compression system at the TESLA test facility FEL

Abstract A SASE-FEL [A.M. Kontradenko and E.L. Saldin, Particle Accelerators 10 (1980) 207; R. Bonifacio, C. Pellegrini and I.M. Narducci, Opt. Commun. 50 (1884) 373] requires extremely high peak currents which cannot be achieved by electron guns. The bunch length therefore has to be reduced along the accelerating linac, the bunch has to be compressed. In the TTF-FEL this is done with the help of bending magnet chicanes in three stages. We present the lay-out of the scheme as well as first beam dynamics calculations.