Magnetic Bunch Compressor Research Articles

Femtosecond Electron Pulses Production System

The SURIYA project is designed to generate femtosecond (fs) electron pulses at the Fast Neutron Research Facility (FNRF), Thailand. The fs electron pulses production system consists mainly of a thermionic cathode RF-gun, a magnetic bunch compressor in form of an alpha magnet (α-magnet), a linear accelerator (linac), a beam transport line, and various electron beam diagnostic instruments. This system aims to produce a 20-25 MeV electron beam with micropulses of less than 100 fs in length. Theses pulses can be used either for direct experimentation or to produce fs pulses of intense coherent far infrared radiation (FIR) and/or x-ray. In this paper, an overview of the system and characteristics of its major components will be presented.

Theory of space-charge waves on gradient-profile relativistic electron beam: An analysis in propagating eigenmodes

We developed an exact analytical treatment for space-charge waves within a relativistic electron beam in terms of (self-reproducing) propagating eigenmodes. This result is of obvious theoretical relevance as it constitutes one of the few exact solution for the evolution of charged particles under the action of self-interactions. It also has clear numerical applications in particle accelerator physics where it can be used as the first self-consistent benchmark for space-charge simulation programs. Today our work is of practical relevance in FEL technology in relation with all those schemes where an optically modulated electron beam is needed and with the study of longitudinal space-charge instabilities in magnetic bunch compressors.

Start-to-end simulations of SASE FEL at the TESLA Test Facility, phase 1

Phase 1 of the vacuum ultra-violet free-electron laser (FEL) at the TESLA Test Facility recently concluded operation. It successfully demonstrated the saturation of a SASE FEL in the wavelength range of 80–120nm. We present a posteriori start-to-end numerical simulations of this FEL. These simulations are based on the programs Astra and elegant for the generation and transport of the electron distribution. An independent simulation of the intricate beam dynamics in the magnetic bunch compressor is performed with the program CSRtrack. The SASE FEL process is simulated with the code FAST. From our detailed simulations and the resulting phase space distribution at the undulator entrance, we found that the FEL was driven only by a small fraction (slice) of the electron bunch. This “lasing slice” is located in the head of the bunch, and has a peak current of approximately 3kA. A strong energy chirp (due to the space charge field after compression) within this slice had a significant influence on the FEL operation. Our study shows that the radiation pulse duration is about 40fs (FWHM) with a corresponding peak power of 1.5GW. The simulated FEL properties are compared with various experimental data and found to be in excellent agreement.

Performance simulation and design of the magnetic bunch compressor for the infrared free-electron laser at the Pohang light source test linac

A chicane bunch compressor was designed for the infrared free-electron laser project at the Pohang Light Source test linac. The bunch compressor has triplet quadrupoles before and after the chicane to obtain a transversely uncorrelated beam at the center of the bunch compressor. In this paper, we describe the design considerations of the chicane that provides a compression from 3 ps rms to 250 fs rms a beam energy of 100 MeV . Simulations provide predictions of emittance variation with the chicane parameters and the linac RF phase in the precompressor. The simulations also indicate that the normalized emittance in the designed bunch compressor increases from 5 mm mrad to about 5.5 mm mrad.

Klystron instability of a relativistic electron beam in a bunch compressor

In this paper, we consider a klystron-like mechanism of amplification of parasitic density modulations in an electron bunch passing a magnetic bunch compressor. Analytical expressions are derived for the small-signal gain. The effect of wakefields in front of the bunch compressor is analyzed by using a model of linear compression which assumes linear correlated energy chirp and linear dependence of a path length on energy deviation. Analysis of the density modulation growth due to coherent synchrotron radiation inside bends of the magnetic bunch compressor is done for the simplified case of no correlated energy chirp (no compression). Analytical results of this paper can be used for benchmarking numerical simulation codes.

An analytical description of longitudinal phase space distortions in magnetic bunch compressors

In this paper we consider a possible mechanism of strong distortions of longitudinal phase space due to collective effects in an electron bunch passing a bunch compressor. Analytical expressions are derived for the case of a linear compression. Estimates for the TESLA Test Facility at DESY are presented.

Design and performance simulations of the bunch compressor for the Advanced Photon Source Low-Energy Undulator Test Line free-electron laser

A magnetic bunch compressor was designed and commissioned to provide higher peak current for the Advanced Photon Source's Low-Energy Undulator Test Line free-electron laser [S. V. Milton et al., Phys. Rev. Lett. 85, 988 (2000)]. Of great concern is limiting emittance growth due to coherent synchrotron radiation. Tolerances must also be carefully evaluated to find stable operating conditions and ensure that the system can meet operational goals. Automated matching and tolerance simulations allowed consideration of numerous configurations, pinpointing those with reduced error sensitivity. Simulations indicate significant emittance growth up to 600 A peak current, for which the normalized emittance will increase from 5 to about $8.5\ensuremath{\mu}\mathrm{m}$. The simulations also provide predictions of emittance variation with chicane parameters and precompressor linac phase, which we hope to verify experimentally.

Simple method for particle tracking with coherent synchrotron radiation

Coherent synchrotron radiation (CSR) is of great interest to those designing accelerators as drivers for free-electron lasers (FELs). Although experimental evidence is incomplete, CSR is predicted to have potentially severe effects on the emittance of high-brightness electron beams. The performance of an FEL depends critically on the emittance, current, and energy spread of the beam. Attempts to increase the current through magnetic bunch compression can lead to increased emittance and energy spread due to CSR in the dipoles of such a compressor. The code elegant was used for design and simulation of the bunch compressor [M. Borland et al., in Proceedings of the 2000 Linear Accelerator Conference, Monterey, CA (SLAC, Menlo Park, CA, 2001), p. 863] for the low-energy undulator test line (LEUTL) FEL [S. V. Milton et al., Phys. Rev. Let. 85, 988 (1999)] at the Advanced Photon Source (APS). In order to facilitate this design, a fast algorithm was developed based on the 1D formalism of Saldin and co-workers [E. L. Saldin, E. A. Schneidmiller, and M. V. Yurkov, Nucl. Instrum. Methods Phys. Res., Sect. A 398, 373 (1997)]. In addition, a method of including CSR effects in drift spaces following the chicane magnets was developed and implemented. The algorithm is fast enough to permit running hundreds of tolerance simulations including CSR for 50 000 particles. This article describes the details of the implementation and shows results for the APS bunch compressor.

Measurement of far-infrared subpicosecond coherent radiation for pulse radiolysis

Using a magnetic bunch compression method, a 26.5 MeV subpicosecond electron single bunch was generated with the L-band linac of Osaka University. The coherent transition radiation emitted from the subpicosecond single bunch was observed at wavelengths from 100 to 700 μm. The intensity was 7.9×10 9 times higher than that of the incoherent transition radiation obtained by calculation. The length of the compressed electron bunch was evaluated to be roughly 50 fs (rms) from the analysis of the spectra of the transition radiation. The coherent transition radiation has high enough intensity to be applied to pulse radiolysis as a pulsed light source.

Femto-second electron pulses from a linear accelerator

An overview of the research program and recent experimental results from the SUNSHINE (Stanford University short intense electron source) facility will be presented and discussed. This facility consists of an rf-gun and a magnetic bunch compressor followed by a 30 MeV linear accelerator and associated diagnostic instrumentation. The goal of this research activity is to produce sub picosecond electron pulses, study limitations and use those bunches to generate coherent far infrared radiation or for single pass free electron laser development. High intensity electron bunches with a peak current of 298 A at a bunch length of 70 μm rms or 235 fs rms have been produced at a microbunch charge of 0.17 nC while the shortest bunches reached so far last about 100 fs rms at about 10% of maximum intensity. The experimental setup, numerical simulations for bunch compression, sub picosecond bunch length measurements and limitations on achievable short bunches will be discussed.

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.