Electron Energy Spread Research Articles

Characterisation of a Cs–Implanted Cu Photocathode

The generation of high-brightness electron beams is a crucial area of particle accelerator research and development. Photocathodes which offer high levels of quantum efficiency when illuminated at visible wavelengths are attractive as the drive laser technology is greatly simplified. The higher laser power levels available at longer wavelengths create headroom allowing use of manipulation techniques to optimise the longitudinal and transverse beam profiles, and so minimise electron beam emittance. Bi–alkali photocathodes which offer quantum efficiency ∼ 10 % under illumination at 532 nm are an example of this. Another solution is the use of modified photoemissive surfaces. Caesium has a low work function and readily photoemits when illuminated at green wavelengths (∼532nm). Caesium oxide has an even lower work function and emits at red wavelengths (∼635nm). We present data on our work to create a hybrid copper photocathode surface modified by implantation of caesium ions, measuring the surface roughness and probing its structure using MEIS. We measure the energy spread of photoemitted electrons, the QE as a function of illumination wavelength, and the practicality of this surface as a photocathode by assessing its lifetime on exposure to oxygen.

Suppressing of even harmonics in free electron lasers by modifying the energy spread of the beam

Using an analytical description of the evolution of the power of harmonics in free electron laser (FEL) undulators, we study the effect of electron beam parameters on the main characteristics of FEL radiation: the gain length, spectrum, and harmonic power. The possibility of suppressing even harmonics of the FEL as a light source in the study of materials and surfaces by analyzing the second harmonic nonlinear response (SHG) is being studied. The main factors influencing the radiation of even FEL harmonics are revealed; the influence of the beam cross section and the spread of electron energies on their radiation is analyzed. An example of a LEUTL FEL with visible light emission, which is used in studies of the optical properties of materials and the chemical state of molecules on surfaces and interfaces between media, is considered. In this context, the possibility of suppressing the second FEL harmonic is analyzed.

Three-pole wigglers at NSLS-II

The National Synchrotron Light Source-II (NSLS-II) at Brookhaven National Lab (BNL) is a third-generation medium-energy storage ring with 3 GeV and sub-nm-rad horizontal emittance, equipped with a top-off capability of 500 mA. It is designed to deliver an extremely intense photon beam across a wide spectral range, from the far-infrared to the very hard X-ray region. This outstanding performance is achieved through a combination of bending magnets (BM), three-pole wigglers (3 PW), and advanced insertion device (ID) sources. Six three-pole wigglers have been constructed at the NSLS-II IDs Lab. These devices are used to produce broadband radiation with lower angular power density and to monitor the electron beam emittance and energy spread. This paper describes the R&D activities focused on developing the required magnetic and mechanical designs, along with magnetic field optimization and the final magnetic measurement results. It also covers the spectral brightness, flux, power density, spatial and angular output properties of the 3 PWs, including their commissioning results and their effects on the performance of the NSLS-II storage ring.

All-Optical GeV Electron Bunch Generation in a Laser-Plasma Accelerator via Truncated-Channel Injection.

We describe a simple scheme, truncated-channel injection, to inject electrons directly into the wakefield driven by a high-intensity laser pulse guided in an all-optical plasma channel. We use this approach to generate dark-current-free 1.2GeV, 4.5% relative energy spread electron bunches with 120TW laser pulses guided in a 110mm-long hydrodynamic optical-field-ionized plasma channel. Our experiments and particle-in-cell simulations show that high-quality electron bunches were only obtained when the drive pulse was closely aligned with the channel axis, and was focused close to the density down ramp formed at the channel entrance. Start-to-end simulations of the channel formation, and electron injection and acceleration show that increasing the channel length to 410mm would yield 3.65GeV bunches, with a slice energy spread ∼5×10^{-4}.

The Influence of Increased Electron Energy Spread on the Radiation of the Second Harmonic in Free Electron Lasers

Free electron lasers (FELs) are becoming more and more popular as sources of radiation for research purposes in many fields of science. They are frequently employed in second harmonic generation (SHG) studies. SHG is an important effect of nonlinear responses of matter to irradiation. It should be separated from the second harmonic of the radiation source, FELs, for correct analysis of the response. Using an analytical model for harmonic powers evolution in a single-pass FEL, we demonstrate the dependence of FEL harmonics on the key parameters of beams and undulators and show that it is possible to reduce the second FEL harmonic content if the energy spread of an electron beam is increased. For LEUTL FEL radiation in the visible range, we analytically demonstrate a reduction in the second harmonic power by an order of magnitude if electron energy spread is increased twice, which still allows efficient bunching at the fundamental wavelength. The method is valid regardless of the photon energy and, therefore, the absorption edge of the target material, and as such, it is valid also in the X-ray band.

Generation of ultrabright and low energy spread electron beams in laser wakefield acceleration in a uniform plasma

Generation of ultrabright and low energy spread electron beams in laser wakefield acceleration in a uniform plasma

Effects of a 4.3T superconducting wiggler on the electron beam optics and dynamics at NSLS-II

A superconducting wiggler (SCW) with a peak field of 4.3 T has recently been installed at NSLS-II. The wiggler generates a high-flux X-ray beam with a photon energy range from 20 keV to 200 keV for the High Energy Engineering X-ray Scattering beamline. This device induces significant distortions in the electron beam orbit and optics. We have corrected these adverse effects using feed-forward tables. The effects of the SCW on the electron beam emittance, energy spread, and bunch length were calculated and measured. The beam-induced heating of the cryogenic vacuum chamber was also studied. Finally, the superconducting wiggler has been commissioned for user operations.

Toward direct spatial and intensity characterization of ultra-high-intensity laser pulses using ponderomotive scattering of free electrons

Spatial distributions of electrons ionized and scattered from ultra-low-pressure gases are proposed and experimentally demonstrated as a method to directly measure the intensity of an ultra-high-intensity laser pulse. Analytic models relating the peak scattered electron energy to the peak laser intensity are derived and compared to paraxial Runge–Kutta simulations highlighting two models suitable for describing electrons scattered from weakly paraxial beams (f#>5) for intensities in the range of 1018−1021 W cm−2. Scattering energies are shown to be dependent on gas species, emphasizing the need for specific gases for given intensity ranges. Direct measurements of the laser intensity at full power of two laser systems are demonstrated, both showing a good agreement between indirect methods of intensity measurement and the proposed method. One experiment exhibited the role of spatial aberrations in the scattered electron distribution, motivating a qualitative study on the effect. We propose the use of convolutional neural networks as a method for extracting quantitative information on the spatial structure of the laser at full power. We believe the presented technique to be a powerful tool that can be immediately implemented in many high-power laser facilities worldwide.

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.

A novel design of a retarding field electron energy analyzer with a cavity electrode providing extremely high energy resolution

An electron energy analyzer based on the retarding electrostatic field generated by a cavity electrode was prototyped, where the designed energy resolution is smaller than 10 meV at acceleration voltages Vo less than 5 kV and a half angle spread θo less than 20 mrad for the incident electron beams with the scanning voltage range of 3 V. Precise energy resolution, however, has not been recognized experimentally because of the difficulties of equipping such a narrow energy spread electron source. This paper describes the principle of the newly developed energy analyzer and shows the obtained energy spectra from a Schottky emitter ZrO/W(100) with a tip of the curvature radius of 1.5 μm, which could highly reproduce the theoretically expected spectra at various emitter temperatures because of its predominantly thermal emission.

Transverse gradient undulator in a storage ring x-ray free electron laser oscillator

Modern electron storage rings produce bright x rays via spontaneous synchrotron emission, which is useful for a variety of scientific applications. The x-ray free-electron laser oscillator (XFELO) has the potential to amplify this output, both in terms of peak power and photon coherence. However, even current fourth generation storage rings (4GSRs) lack the requisite electron beam brightness to drive the XFELO due to its electron energy spread. The transverse gradient undulator (TGU) can overcome this issue, thus providing a practical means to couple the 4GSR to the XFELO. In this study, we first examine the theoretical basis of the TGU interaction by deriving the 3D TGU gain formula in the low gain approximation. Then, we perform an optimization study of the gain formula in order to determine optimal beam and machine parameters. Finally, we construct a hypothetical storage ring TGU-XFELO based on near-optimal parameters and report on its projected performance using multistage numerical simulation. We also discuss potential implementation challenges associated with the ring-FEL coupling.

Control of electron beam current, charge, and energy spread using density downramp injection in laser wakefield accelerators

Density downramp injection has been demonstrated to be an elegant and efficient approach for generating high-quality electron beams in laser wakefield accelerators. Recent studies have demonstrated the possibilities of generating electron beams with charges ranging from tens to hundreds of picocoulombs while maintaining good beam quality. However, the plasma and laser parameters in these studies have been limited to specific ranges or attention has been focused on separate physical processes such as beam loading, which affects the uniformity of the accelerating field and thus the energy spread of the trapped electrons, the repulsive force from the rear spike of the bubble, which reduces the transverse momentum p⊥ of the trapped electrons and results in small beam emittance, and the laser evolution when traveling in the plasma. In this work, we present a comprehensive numerical study of downramp injection in the laser wakefield, and we demonstrate that the current profile of the injected electron beam is directly correlated with the density transition parameters, which further affects the beam charge and energy evolution. By fine-tuning the plasma density parameters, electron beams with high charge (up to several hundreds of picocoulombs) and low energy spread (around 1% FWHM) can be obtained. All these results are supported by large-scale quasi-three-dimensional particle-in-cell simulations. We anticipate that the electron beams with tunable beam properties generated using this approach will be suitable for a wide range of applications.

An Investigation of the Resonant and Non-Resonant Angular Time Delay of e-C60 Elastic Scattering

Time delay in electron scattering depends on both the scattering angle θ and scattered electron energy E. A study on the angular time delay of e-C60 elastic scattering was carried out in the present work. We employed the annular square well (ASW) potential to simulate the C60 environment. The contribution from different partial waves to the total angular time delay profile was examined in detail. The investigation was performed for both resonant and non-resonant energies, and salient characteristics in the time delay profile for each case were studied.

Digital low-level RF system for the infrared free-electron laser facility

The infrared free-electron laser facility (IR-FEL) is a new free-electron oscillator user facility in the University of Science and Technology of China (USTC), which has come into user operation in September 2021. To get a low energy spread electron beam, we designed and fabricated a digital low-level radio frequency system (DLLRF) based on the MTCA.4 platform to monitor and control the microwave field. The simulation for RF system is implemented to estimate the feedback coefficients and analyze the influence of driving chain noise and measurement noise in this case. Beam tests show that the pulse-to-pulse PI feedback eliminates the slow drift effectively and improves the stability of the phase from 3.4° to 0.19° (RMS). The application of the PI-type feed-forward decreases intra-pulse ripples of phase from 5° to 1° (peak to peak).

A Laser Frequency Transverse Modulation Might Compensate for the Spectral Broadening Due to Large Electron Energy Spread in Thomson Sources

Compact laser plasma accelerators generate high-energy electron beams with increasing quality. When used in inverse Compton backscattering, however, the relatively large electron energy spread jeopardizes potential applications requiring small bandwidths. We present here a novel interaction scheme that allows us to compensate for the negative effects of the electron energy spread on the spectrum, by introducing a transverse spatial frequency modulation in the laser pulse. Such a laser chirp, together with a properly dispersed electron beam, can substantially reduce the broadening of the Compton bandwidth due to the electron energy spread. We show theoretical analysis and numerical simulations for hard X-ray Thomson sources based on laser plasma accelerators.

A multivariate approach to determine electron beam parameters for a Monte Carlo 6 MV Linac model: Statistical and machine learning methods.

This study aimed to determine the optimal initial electron beam parameters of a Linac for radiotherapy with a multivariate approach using statistical and machine-learning tools. For MC beam commissioning, a 6MV Varian Clinac was simulated using the Geant4 toolkit. The authors investigated the relations between simulated dose distribution and initial electron beam parameters, namely, mean energy (E), energy spread (ES), and radial beam size (RS). The goodness of simulation was evaluated by the slope of differences between the simulated and the golden beam data. The best-fit combination of the electron beam parameters that minimized the slope of dose difference was searched through multivariate methods using conventional statistical methods and machine-learning tools of the scikit-learn library. Simulation results with 87 combinations of the electron beam parameters were analyzed. Regardless of being univariate or multivariate, traditional statistical models did not recommend a single parameter set simultaneously minimizing slope of dose differences for percent depth dose (PDD) and lateral dose profile (LDP). Two machine learning classification modules, RandomForestClassifier and BaggingClassifier, agreed in recommending (E=6.3MeV, ES = ±5.0%, RS=1.0mm) for predicting simultaneous acceptance of PDD and LDP. The machine learning with random-forest and bagging classifier modules recommended a consistent result. It was possible to draw an optimal electron beam parameter set using multivariate methods for MC simulation of a radiotherapy 6 MV Linac.

Design Study on Electron Energy Spread Effect on Performance of Linac Irradiation Facility with the Aid of Electron Energy Spectrum Online Analyzer

Design Study on Electron Energy Spread Effect on Performance of Linac Irradiation Facility with the Aid of Electron Energy Spectrum Online Analyzer

Beam-based characterization of higher-order-mode driven coupled-bunch instabilities in a fourth-generation storage ring

Longitudinal coupled-bunch instabilities are driven by monopole higher-order modes (HOMs) in the active and passive radio-frequency (rf) cavities at the MAX IV 3 GeV electron storage ring. By combining different beam and rf-based techniques, we have performed a systematic survey of the properties of these resonant modes as a function of cavity temperatures and fundamental mode frequency. This information was then used to setup a HOM model that allowed us to infer optimized temperature ranges for the cavities. An important feature of the optimization method is that it takes into account not only the need to minimize the growth rates (given by the resistive component of the sum of HOM impedances) from each individual cavity but also aims to minimize the total reactive component of the impedance of HOMs that are present in more than one cavity. The resulting small real coherent tune shifts allow the coupled-bunch modes driven by these HOMs to be effectively Landau damped. Further optimization of the cavity temperatures was performed by minimizing the measured electron beam energy spread within the temperature ranges defined by the HOM model. The optimum temperature search was performed at 300 mA stored beam current and in multi-bunch mode using the RCDS (Robust Conjugate Direction Search) algorithm and succeeded in bringing the electron energy spread to within 10% of the zero current value determined solely by quantum excitation and radiation damping.

Electron-molecule scattering in a bichromatic elliptically polarised laser field: Plateau structures and two-centre interference minima

Scattering of electrons off diatomic molecules in a bichromatic elliptically polarised laser field is considered by applying the S-matrix theory within the second Born approximation. Two characteristic plateaus appear in the energy spectrum of scattered electrons. The higher plateau is observed in the low-energy part of the spectrum and describes the single scattering, while the lower plateau extends to the high-energy part of the spectrum and describes the double scattering. Scattering and rescattering of electrons off molecular targets may occur at any molecular centre and in any order. Interference of contributions results in increasing/decreasing the differential cross section in particular regions of the energy spectrum of scattered electrons. Two contributions for single scattering and four contributions for double scattering exist in the case of diatomic molecules. For some molecular orientations, a sequence of declining maxima may be observed instead of the plateaus in the electron energy spectrum. We have also observed parabolic structures in the angle-resolved energy spectra. Analytical formulas that explain these structures have been provided. We have also explored the impact of the laser-field ellipticity on the scattered-electron energy spectra. The bichromatic ω– and ω– laser fields have been considered.

Scattering effect of very low frequency transmitter signals on energetic electrons in Earth’s inner belt and slot region

Whistler mode very low frequency (VLF) waves from man-made ground-based transmitters in a frequency range of 10–30 kHz are mainly used for submarine communication, and they propagate primarily in the Earth-lower ionosphere waveguide and part of their energy can leak into the inner magnetosphere, leading the energetic electrons in inner radiation belt and slot region to precipitate into atmosphere and then affect the energetic electron dynamics in the near-Earth space. The scattering effects of artificial VLF signals from NWC, NAA and DHO38 transmitters on energetic electrons in Earth’s inner belt and slot region are investigated in detail in this work. Based on the quasi-linear theory and the Full Diffusion Code, we calculate the bounce-average pitch angle diffusion coefficients induced by NWC, NAA and DHO38 VLF transmitter signals, for which the resonance harmonics |<i>N</i>| ≤ 10 are considered, respectively. We further implement the one-dimensional Fokker-Planck diffusion simulations by using the available pitch angle diffusion rates to model the dynamic evolutions of energetic electrons caused by the scattering of the VLF transmitter signals in the inner belt and slot region in 200 d. The simulation results indicate that the NWC VLF transmitter signals are dominant in scattering ~100 keV electrons with pitch angles less than 60° at <i>L</i> ≤ 1.8, and the mainly scattered electron energy values increase with <i>L</i>-shell decreasing , from <i>L</i> = 1.8 to <i>L </i>= 1.5, the mainly scattered electron energy increases from 90–120 keV to 550–650 keV. The NAA and DHO38 VLF transmitter signals are important in scattering < 20 keV electrons with pitch angles less than 70° at higher <i>L</i>-shells (2.2 ≤ <i>L</i> ≤ 2.7), from <i>L</i> = 2.2 to <i>L</i> = 2.7, the mainly scattered electron energy decreases from 10–20 keV to several keV. The VLF transmitter signals are found to have a slight influence on the loss of energetic electrons with pitch angles larger than 80°.