Bunch Compression Research Articles

GeV-Class Two-Fold CW Linac Driven by an Arc-Compressor

We present a study of an innovative scheme to generate high repetition rate (MHz-class) GeV electron beams by adopting a two-pass two-way acceleration in a super-conducting Linac operated in Continuous Wave (CW) mode. The beam is accelerated twice in the Linac by being re-injected, after the first pass, in opposite direction of propagation. The task of recirculating the electron beam is performed by an arc compressor composed by 14 Double Bend Achromat (DBA). In this paper, we study the main issues of the two-fold acceleration scheme, the electron beam quality parameters preservation (emittance, energy spread), together with the bunch compression performance of the arc compressor, aiming to operate an X-ray Free Electron Laser. The requested power to supply the cryogenic plant and the RF sources is also significantly reduced w.r.t a conventional one-pass SC Linac for the same final energy.

Using an alpha magnet to compress the electron bunches from a photocathode RF gun

We report here the design, simulation, and characterization of a compact electron injector for the formation of high-current, femtosecond electron bunches from a photocathode RF gun without mandatory use of ultra-short laser pulses and a high power RF supply. The proposed injector consists of a 1.6-cell S-band photocathode RF gun to produce the electron beam and a small bunch compressor to increase the peak current. The RF gun is designed to operate at a 65 MV/m accelerating field gradient, and the bunch compressor is a small α-magnet with a maximum magnetic field gradient of 340 G/cm. The output beam from the gun has 100 pC charge per-bunch that is equivalent to a beam current of ∼15 A, and output beam from the injector has an RMS bunch length of ∼ 439 fs, a peak current of 0∼ 12 A and an RMS normalized horizontal emittance of 1.21 mm-mrad.

Compression of Ultra-High Brightness Beams for a Compact X-ray Free-Electron Laser

The creation of the first X-ray free-electron laser at SLAC in 2009 introduced the scientific community to coherent photons of unprecedented high brightness. These photons were produced, however, at the cost of billion-dollar-class price tags and kilometer-scale machine footprints. This has meant that getting access to these photons is very difficult, and those who do get access do so with a strict time budget. Now, the development of critical enabling technologies, in particular high-field cryogenically cooled accelerating cavities and short-period, high-field undulator magnets, opens the door to an X-ray free-electron laser less than 30 m in length. We present here critical potential design elements for such a soft X-ray free-electron laser. To this end, simulation results are presented focusing on the problems associated with the process of bunch compression and novel ways in which those problems can be resolved.

Beam matching with space charge in energy recovery linacs

Abstract Matching with space charge of an Energy-Recovery Linac (ERL) arc into the subsequent RF structure is essential to preserve beam quality. We show how to match beam envelopes and dispersion along the bends and recirculation arcs of an ERL, including space charge forces, in order to adjust the beam to the parameters of the subsequent RF structure. For a qualitative analysis, we show that one can use a beam matrix approach together with the smooth focusing approximation but with longitudinal–transverse coupling. It is also shown that the space-charge-modified dispersion plays a key role for the adjustment of the momentum compaction R 56 required for both the isochronous and the non-isochronous recirculation mode of an ERL. In this work, a simple coupled transverse–longitudinal beam matrix approach for matching with space charge is employed and compared with particle tracking simulations using ELEGANT. As an example case, we use the 5 MeV low-energy, 18 0 ° injection arc, which also works as a bunch compressor, and matched to the subsequent first RF structure of the projected multi-turn Mainz Energy-recovering Superconducting Accelerator (MESA).

Parallel-plate waveguides for terahertz-driven MeV electron bunch compression.

We demonstrate the electromagnetic performance of waveguides for femtosecond electron beam bunch manipulation and compression with strong-field terahertz (THz) pulses. The compressor structure is a dispersion-free exponentially-tapered parallel-plate waveguide (PPWG) that can focus single-cycle THz pulses along one dimension. We show test results of the tapered PPWG structure using electro-optic sampling (EOS) at the interaction region with peak fields of at least 300 kV/cm, given 0.9 µJ of incoming THz energy. We also present a modified shorted design of the tapered PPWG for better beam manipulation and reduced magnetic field as an alternative to a dual-feed approach. As an example, we demonstrate that with 5 µJ of THz energy, the PPWG compresses a 2.5 MeV electron bunch by a compression factor of more than 4, achieving a bunch length of about 18 fs.

An alternative view of coherent synchrotron radiation induced microbunching development in multibend recirculation arcs

Abstract Coherent synchrotron radiation (CSR) induced microbunching instability has been one of the most challenging issues in the design of modern accelerators. We recently developed a semi-analytical Vlasov solver and applied it to the investigation of the physical processes associated with microbunching amplification for several types of recirculation arc lattices. In this study, we build on the concept of stage gain, proposed by Huang and Kim (2002), and develop an alternative view to characterize CSR microbunching development in terms of stage orders. The stage orders enable a fair and quantitative comparison of the impact of lattice optics on the microbunching development for different lattices under similar initial beam parameters. The multibend arcs considered in this study have a distinguishing feature of multistage amplification. The CSR microbunching gain grows as a six-stage amplification for the presented recirculation arcs with a total of 24 dipoles, while two-stage amplification was previously concluded for a three-dipole bunch compressor chicane. We also attempt to relate the lattice optics pattern with the obtained stage gain functions through a physical interpretation. The investigation in this paper may serve as a complementary discussion to our previous work (Tsai et al., 2017).

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.

Bunch length and energy measurements in the bunch compressor of a free-electron laser

At SwissFEL, a fully non-invasive characterization of the beam energy distribution can be performed by means of a Synchrotron-Radiation-Monitor (SRM) imaging in the visible the transverse profile of the electron beam in a magnetic chicane. Under conditions of off-crest acceleration in the SwissFEL injector, according to a first order Taylor series expansion, the distribution of the relative energy of the electron beam can be linearly expressed as a function of the distribution of the electron longitudinal coordinates via the coefficient of the rf induced beam energy chirp. It is hence possible to express the distribution of the electron longitudinal coordinates at the entrance of the magnetic chicane of SwissFEL as a function of the distribution of the dispersed electron trajectories in the horizontal plane of the magnetic chicane. Machine parameters and instrument data at SwissFEL can be beam-synchronously acquired. A shot-by-shot correlation between the analysis results of the SRM camera images and the linear coefficient of the beam energy chirp resulting from the rf parameters of the injector can be hence established. Finally, taking into account the compression factor of the magnetic chicane, the electron bunch length at the exit of the magnetic chicane can be expressed as a function of the horizontal size of the electron beam imaged by the SRM. Bunch-length measurements performed by means of the SRM in the first magnetic chicane of SwissFEL will be presented.

FEL performance achieved at PAL-XFEL using a three-chicane bunch compression scheme.

PAL-XFEL utilizes a three-chicane bunch compression (3-BC) scheme (the very first of its kind in operation) for free-electron laser (FEL) operation. The addition of a third bunch compressor allows for more effective mitigation of coherent synchrotron radiation during bunch compression and an increased flexibility of system configuration. Start-to-end simulations of the effects of radiofrequency jitter on the electron beam performance show that using the 3-BC scheme leads to better performance compared with the two-chicane bunch compression scheme. Together with the high performance of the linac radiofrequency system, it enables reliable operation of PAL-XFEL with unprecedented stability in terms of arrival timing, pointing and intensity; an arrival timing jitter of better than 15 fs, a transverse position jitter of smaller than 10% of the photon beam size, and an FEL intensity jitter of smaller than 5% are consistently achieved.

Design and beam dynamic simulation of the thermionic RF electron gun with external resonant cavity for transverse emittance reduction

Abstract The accelerator components and radiation production systems at the PBP-CMU Electron Linac Laboratory of the Plasma and Beam Physics Research Facility are currently being developed and upgraded to make it possible for generation of the mid-infrared free-electron laser (MIR FEL) and the coherent intense terahertz (THz) radiation via transition radiation and superradiant undulator radiation. To generate a high brightness radiation, a high quality electron beam is crucially required. One of the important keys for this issue is the good performance and reliable electron source . Development of a new thermionic radio-frequency (RF) electron gun is foreseen at our facility. The design of the new RF gun was conducted with the aim to reduce transverse emittance while keeping the longitudinal phase space distribution that is suitable for the bunch compression with the alpha magnet. The present design of the PBP-CMU RF electron gun, which has 1-1/2 cell TM 010 reentrant cylindrical standing-wave resonant cavities and a side-coupling cavity, was used as the starting model for this work. The shape and dimensions of the new RF gun were optimized and the electromagnetic field distribution was simulated by using the program CST Microwave Studio 2016. Electron beam dynamic simulations were done with program ASTRA. The initial particle distribution from thermionic emission process was created by using the self-developed MATLAB script that provides the thermal emittance values corresponding well to the theoretical values. The results of this work suggest that by adding an external resonant cavity with a rounded pill-box shape to the present RF gun model can reduce the transverse emittance. This additional cavity has an optimal length of 25 mm and is connected to the present RF gun at the position 13 mm downstream the second cavity exit. The RF wave is coupled from the second cavity to the external cavity via the second side-coupling cavity. The entire structure finally has a resonant frequency of 2856.03 MHz, an unloaded quality factor of 15623 and a shunt impedance of 1.18 M Ω . The maximum electric field values in the first, the second and the external cavity are 32.8, 65.0 and 29.8 MV/m, respectively. The results from beam dynamic simulations show that this new design RF gun should be able to produce electron bunches with a charge of 170 pC per bunch, a maximum kinetic energy of 2.72 MeV and an energy spread of 540 keV. The simulated horizontal and vertical normalized emittances of 90% beam were respectively reduced by 11.6% and 4.5% compared to the values obtained from the original PBP-CMU RF gun. Furthermore, it was found that the transverse emittance is conserved at the distance longer than 60 cm downstream the external cavity exit.

First Experimental Measurements of the Caustic Nature of Trajectories in Bunch Compressors

First Experimental Measurements of the Caustic Nature of Trajectories in Bunch Compressors

Modeling coherent synchrotron radiation with a discontinuous Galerkin time-domain method

Coherent synchrotron radiation (CSR) remains a computationally difficult aspect in the numerical modeling of modern electron accelerators. In this paper, we present the theory and numerical implementation of a high-order discontinuous Galerkin method for computing CSR wake fields in a bunch compressor while including geometrical effects. We incorporate this method in our code: CSRDG (Coherent Synchtrotron Radiation with Discontinuous Galerkin). Using this code, wake potentials and associated quantities are calculated for the simple case of an electron bunch propagating along a single bend structure as well as for a complete bunch compressor chamber of the European XFEL.

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.

Proposition of a femtosecond pulse radiolysis with terahertz probe pulses

A new concept of pulse radiolysis based on a photoinjector, which can synchronously offer femtosecond (fs) pump electron beams and intense Terahertz (THz) probe pulses, is proposed and studied in this paper. The two electron beams are generated in the radio-frequency (rf) electron gun by illuminating the photocathode with two laser pulses, then compressed and accelerated in two linear accelerators respectively. By applying the brake-applied velocity bunching bunch compression method, a few hundreds of fs electron beams with nano-Coulomb (nC) bunch charge can be obtained. The front beam can generate THz pulse with MW peak power for THz probe, and the rear beam is used to pumping the radiation induced phenomena. This new concept pulse radiolysis scheme provides a promising way for high time resolution pump-probe experiments with a novel experimental methodology.

Self-compression of dense photo-injector electron bunches

Several different mechanisms of compression of dense electron bunches in self fields are described. It is shown that the spontaneous radiation from a short (shorter than the radiation wavelength) electron bunch can result in a significant axial compression of the bunch under effect of the rf field of the radiated wave. This radiative self-compression can be provided in regimes of both cyclotron and undulator emission. Self-compression in the case of the “negative-mass” regime of the electron motion in an undulator of a special type is also described.

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.

Manipulation of Laser Distribution to Mitigate the Space-Charge Effect for Improving the Performance of a THz Coherent Undulator Radiation Source

A THz coherent undulator radiation (THz-CUR) source has been developed at the Institute of Advanced Energy, Kyoto University. A photocathode Radio-Frequency (RF) gun and a bunch compressor chicane are used for generating short-bunch electron beams. When the electron beam energy is low, the space-charge effect strongly degrades the beam quality, such as the bunch length and the energy spread at the high bunch charge condition at around 160 pC, and results in the reduction of the highest frequency and the maximum radiated power of the THz-CUR. To mitigate the space charge effect, we have investigated the dependence of the electron beam quality on the laser distribution in transverse and longitudinal directions by using a numerical simulation code, General Particle Tracer GPT. The manipulation of the laser distribution has potential for improving the performance of the THz-CUR source. The electron bunch was effectively compressed with the chicane magnet when the laser transverse distribution was the truncated Gaussian profile, illuminating a cathode. Moreover, the compressed electron bunch was shortened by enlarging the laser pulse width. Consequently, an enhancement of the radiated power of the THz-CUR has been indicated.

Bunch Compression and Turnaround Loops in the FCC-ee Injector Complex

The Future Circular e+ e- Collider (FCC-ee) requires two 180-degree turnaround loops to transport the positron beam from the damping ring to the lower energy section of the linac. In addition bunch compression is required to reduce the RMS bunch length from 5 mm to 0.5 mm, prior to injection into the linac. A dogleg bunch compressor comprised of two triple bend achromat (TBAs) can achieve this compression. Sextupole magnets are incorporated into the bunch compressor design for chromaticity correction as well as optimisation of the second-order longitudinal dispersion, T566, and to linearize the longitudinal phase space distribution. In this paper we present the design of the transport line and the bunch compressor. Measures to limit emittance growth due to coherent synchrotron radiation (CSR) are also discussed, because despite the relatively long bunch length, the large degree of bending required introduces cause for consideration of CSR.

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.

Few-femtosecond electron beam with terahertz-frequency wakefield-driven compression

We propose and demonstrate a novel method to produce few-femtosecond electron beam with relatively low timing jitter. In this method a relativistic electron beam is compressed from about 150 fs (rms) to about 7 fs (rms, upper limit) with the wakefield at THz frequency produced by a leading drive beam in a dielectric tube. By imprinting the energy chirp in a passive way, we demonstrate through laser-driven THz streaking technique that no additional timing jitter with respect to an external laser is introduced in this bunch compression process, a prominent advantage over the conventional method using radio-frequency bunchers. We expect that this passive bunching technique may enable new opportunities in many ultrashort-beam based advanced applications such as ultrafast electron diffraction and plasma wakefield acceleration.