High-brightness Electron Beam Source Research Articles

Experimental characterization of the optical klystron effect to measure the intrinsic energy spread of high-brightness electron beams

The intrinsic energy spread of electron beams needs to be measured to characterize and optimize high-brightness electron beam sources such as those driving x-ray free-electron lasers (FELs). We demonstrate the use of the optical klystron effect as a precise and high-resolution method to measure the electron beam energy spread. The optical klystron setup consists of undulator modules and magnetic chicanes placed between them. The energy spread is found by measuring the radiation power produced in the undulators as a function of the chicanes’ strengths. High resolution and simplicity are the advantages of this approach, in contrast to the standard method, which measures the longitudinal phase space of the electron beam with a transverse deflector. The demonstration was performed at Athos, the soft x-ray FEL beamline of SwissFEL, for which we measured energy spreads below 1 MeV at a central beam energy of 3.4 GeV. We have verified the consistency of the method for different parameters (radiation wavelengths, undulator polarization configurations, and electron bunch durations) and we have benchmarked it against the standard measurement approach using a transverse deflector. Our results confirm the optical klystron as a valid approach to measure the electron beam energy spread. The method can be especially useful to reconstruct low energy spread values, where the conventional approach may be resolution limited, such as in ultra high-brightness radiofrequency photoinjectors or plasma sources, or when transverse deflectors are not available. Published by the American Physical Society 2024

Energy spread blowup by intrabeam scattering and microbunching at the SwissFEL injector

We present high-resolution measurements of the electron beam energy spread at the SwissFEL injector as a function of the electron bunch charge, the optics, and the longitudinal dispersion of the lattice. The measured values are in general an order of magnitude higher than what is predicted by standard simulation codes. The measured dependences indicate that the energy spread blowup is caused primarily by intrabeam scattering and microbunching instabilities, effects not covered in the conventional modeling of radio-frequency injectors. We present further a numerical model that qualitatively reproduces the experimental data and an approach to mitigate the energy spread deterioration. The work underlines the importance of considering the energy spread in the optimization and design of high-brightness electron beam sources and the need to develop new models to adequately understand and simulate the observed physics effects.

Cs2Te photocathode fabrication system at Peking University

A cesium telluride (Cs2Te) photocathode with a quantum efficiency of 13% at 253.7 nm (radiant incidence 200 μW/cm2) is fabricated by tellurium and cesium vapor deposition onto a stainless-steel substrate. The cesium telluride cathode will be used to provide a high-brightness electron beam source for the 3+1/2 photo-injector at Peking University. The design of the system, the fabrication procedures and the preliminary experimental results are presented in this paper.

Advanced Laser Technologies for High-brightness Photocathode Electron Gun

A laser-excited photocathode RF gun is one of the most reliable high-brightness electron beam sources for XFELs. Several 3D laser shaping methods have been developed as ideal photocathode illumination sources at SPring-8 since 2001. To suppress the emittance growth caused by nonlinear space-charge forces, the 3D cylindrical UV-pulse was optimized spatially as a flattop and temporally as squarely stacked chirped pulses. This shaping system is a serial combination of a deformable mirror that adaptively shapes the spatial profile with a genetic algorithm and a UV-pulse stacker that consists of four birefringent α-BBO crystal rods for temporal shaping. Using this 3D-shaped pulse, a normalized emittance of 1.4 π mm mrad was obtained in 2006. Utilizing laser's Z-polarization, Schottky-effect-gated photocathode gun was proposed in 2006. The cathode work functions are reduced by a laser-induced Schottky effect. As a result of focusing a radially polarized laser pulse with a hollow lens in vacuum, the Z-field (Z-polarization) is generated at the cathode.

Plasma density transition trapping as a possible high-brightness electron beam source

Plasma density transition trapping as a possible high-brightness electron beam source

The SPARC project: a high-brightness electron beam source at LNF to drive a SASE-FEL experiment

The Project Sorgente Pulsata e Amplificata di Radiazione Coerente (SPARC), proposed by a collaboration among ENEA–INFN–CNR–Universita’ di Tor Vergata–INFM–ST, was recently approved by the Italian Government and will be built at LNF. The aim of the project is to promote an R&D activity oriented to the development of a coherent ultra-brilliant X-ray source in Italy. This collaboration has identified a program founded on two main issues: the generation of ultra-high peak brightness electron beams and of resonant higher harmonics in the SASE-FEL process, as presented in this paper.

The high-brightness ultra-short pulsed electron beam source at Peking University

In this paper the characteristics of an ongoing new high-brightness ultra-short pulsed electron beam source in Peking University is reported. The results of the experiment on the DC laser-driven electron gun are shown, together with the preparation of the photo-cathode, which has a much improved performance. Based on these achievements the new electron source will adopt a superconducting accelerating section, thus it is expected to obtain electron beams of 1–2 MeV as a first step, and it is expected to reach a higher energy range in the future.

Design of high‐voltage and high‐brightness pseudospark‐produced electron beam source for a Raman free‐electron laser

The design of a pseudospark chamber, a high-voltage (.100 kV), high-current density, low-emittance and high-brightness electron beam source which can provide high-quality beams for free-electron la- sers (FELs), driven by a modified pulse line accelerator, is presented. Based on the hollow cathode (HC) effect and the pseudospark (PS) empirical formula, the HC parameters, the cathode-anode separation, and the working gas pressure region are determined. The general ex- perimental setup of the high-brightness PS-produced electron beam source is shown. © 1996 Society of Photo-Optical Instrumentation En- gineers. Subject terms: high-brightness electron beam; pseudospark; pulse line accelera- tor.

High brightness electron beam sources for FEL applications

Abstract A new generation of field emitter array (FEA) cathode materials is under development at SAIC, in collaboration with NRL and GTE Laboratories. The emitter structures under consideration consist of large area ( ∼ 1 cm 2 ) arrays of large numbers ( ∼ 10 6 ) of microscopic field emitting tips. The structures can be fabricated so as to choose an emitter tip microstructure that is a solid cone, a hollow cylinder, or a variety of other shapes. These microstructures evidence very high local field enhancement factors, controllable from a factor of ∼ 200 to > 2000 . This large local field enhancement allows quantum field emission of significant current from the large area array while the applied macroscopic electric field is still quite low ( ∼ 20 kV/cm ). Single-tip, noninteracting particle, multigrid simulations indicate that the beam brightnesses B n = I/π 2 ϵ n 2 >10 10 A/cm 2 rad 2 may be possible. Beams with such high brightness allow for a greatly expanded field of FEL applications, including high gain and harmonic operation in the FIR wavelength regime. Experiments have so far demonstrated DC average current densities > 1 A/cm 2 , uniform emission, and improved characteristics when run for long periods of time ( > 100 h, DC ). Our present efforts are concentrated on optimizing the available cathode current density, measuring the actual beam brightness, and including self-field and 3-D effects in the numerical simulations.

Physics design of the high brightness linac for the advanced free-electron laser initiative at Los Alamos

Abstract Free-electron lasers and high energy physics accelerators have increased the demand for very high brightness electron-beam sources. This paper describes the design of an accelerator that can produce beams of greater than 7×10 11 A/m 2 (brightness equal to 21/ϵ 2 , with ϵ = 90% normalized emittance, equivalent to four times the rms emittance). The beam emittance growth in the accelerator is minimized by: producing a short electron bunch in a high gradient rf cavity, using a focusing solenoid to correct the emittance growth caused by space charge, and designing the coupling slots between accelerator cavities to minimize quadrupole effects. The simulation code PARMELA was modified for this design effort. This modified version uses SUPERFISH output files for the accelerator cavity fields, MAFIA output files for the 3-D perturbation fields caused by the coupling slots in the accelerator cells, and POISSON output files for the solenoid field in the gun region. The results from simulations are, at 2.3 nC, a peak current of 180 A and a 90% emittance of 6.4π mm mrad, and, at 4 nC a peak current of 300 A and a 90% emittance of 9.4π mrad. The exit energy from the linac is 20 MeV for both cases. A magnetic pulse compressor can be used to further increase the peak current.

High-brightness electron-beam generation and transport

Experimental results for a high-brightness electron-beam source are compared to results of numerical and analytic calculations. The 4-MV, 5-kA beam is generated from a velvet cathode and focused by a solenoidal magnet. Data for the diode impedance and the beam focusing length and spot size agree well with numerical simulations. The minimum spot size is consistent with a normalized Lapostolle emittance of 0.12 cm rad, yielding a brightness of 3.5×108 A/m2 rad2 . Transverse beam oscillations are observed in streak photographs and are thought to be caused by electromagnetic dipole modes in the diode cavity. The oscillation amplitude is significantly reduced by making the current rise more slowly.