arXiv subjects

Francois Lemery

Publications and source records attributed to Francois Lemery.

15 recordsLinked to original sources

arXiv2026

Optical superradiance from single-digit-femtosecond electron beam structure

We report measurements of superradiant optical transition radiation in the 550-800 nm range produced by ultrashort relativistic electron bunches at a dielectric boundary. In the measured optical spectra, we observe photon production with quadratic charge dependence in the visible range, consistent with optical frequency coherence determined by the longitudinal electron bunch form factor. The measured spectral envelope is reproduced by a theoretical model of coherent transition radiation (CTR), which is consistent with a sub-femtosecond longitudinal feature within the electron bunch with characteristic scale $\tau_{\mathrm{FWHM}} = 1.2~\mathrm{fs}$. These results extend CTR from the terahertz into the visible spectrum without the use of undulators or externally seeded microbunching. This superradiant boundary emission in the optical range opens a route to tunable coherent radiation from charged particle beams and provides a platform for broadband coherent light generation, enabling new opportunities for phase-sensitive optical experiments.

physics.optics↗

arXiv2025

3D atomic structure determination with ultrashort-pulse MeV electron diffraction

Understanding structure at the atomic scale is fundamental for the development of materials with improved properties. Compared to other probes providing atomic resolution, electrons offer the strongest interaction in combination with minimal radiation damage. Here, we report the successful implementation of MeV electron diffraction for ab initio 3D structure determination at atomic resolution. Using ultrashort electron pulses from the REGAE accelerator, we obtained high-quality diffraction data from muscovite and $1T-TaS_2$, enabling structure refinements according to the dynamical scattering theory and the accurate determination of hydrogen atom positions. The increased penetration depth of MeV electrons allows for structure determination from samples significantly thicker than those typically applicable in electron diffraction. These findings establish MeV electron diffraction as a viable approach for investigating a broad range of materials, including nanostructures and radiation-sensitive compounds, and open up new opportunities for in-situ and time-resolved experiments.

cond-mat.mtrl-sci↗

arXiv2025

Radiation of a particle performing helical motion in a multilayer cylindrical waveguide

- An algorithm for calculating the radiation field of a charged point particle performing a spiral motion in an infinite cylindrical waveguide with a multilayer side wall is found. The number of layers and their filling is arbitrary. The axis of the spiral is aligned with the axis of the waveguide, so that the geometry of the problem has cylindrical symmetry. Explicit expressions for modal frequency distributions and equations for resonant frequencies for single-layer and double-layer waveguides are given. Examples of graphical constructions of modal frequency distributions of modes for single-layer (resistive), double-layer (metal-dielectric) and triple-layer (metal-dielectric with internal NEG coating) waveguides are presented.

physics.acc-ph↗

arXiv2024

Spectrally shaped THz pulses from tapered dielectric waveguides

In order to exploit the complete scientific potential of user-oriented accelerator facilities, it is necessary to provide adequate pump sources to enable pump-probe science. The users of the European XFEL have requested a THz pump source matching the X-ray repetition rate (10 Hz burst mode with up to 2700 bunches per burst) with a wide range of properties. The quest for suitable THz sources is thus a major development goal. The EuXFEL R\&D project STERN is exploring beam-based radiation generation methods using Cherenkov waveguides to satisfy these user requirements. In this work, custom waveguide geometries are designed to generate a THz pulse with arbitrary spectral content. The layer-thickness or inner radius of a wakefield structure is tapered to generate flattop or Gaussian pulses up to $1 \ \mathrm{THz}$, which can be scaled to higher frequencies.

physics.acc-ph↗

arXiv2023

Wakefield assisted bunch compression in storage rings

Equilibrium bunch lengths typical to modern day light sources of tens of picoseconds limit the spectral reach of superradiant approaches for exploring materials and biological samples. In particular, generation of terahertz radiation seems appealing for pump-probe experiments. Taking advantage of self-wakes generated by carefully chosen structure one can control the bunch length and shape the bunch profile to increase significantly the high-frequency spectral components. Numerical analysis indicates the presence of self-consistent steady states that can be obtained with passive corrugated or dielectric structures. This approach has potential applications for generating superradiant terahertz radiation and pulse shaping in circular accelerators.

physics.acc-ph↗

arXiv2022

Drive beam sources and longitudinal shaping techniques

Linear colliders are an attractive platform to explore high-precision physics of newly discovered particles. The recent significant progress in advanced accelerator technologies has motivated their applications to colliders which has been discussed in the {\sc alegro} workshop. In this paper we discuss one promising scheme, collinear wakefield acceleration. We especially discuss available drive and witness beam sources based on L and S-band radiofrequency technology, and also summarize available and forthcoming longitudinal shaping techniques to improve the overall acceleration efficiency via the transformer ratio.

physics.acc-ph↗

arXiv2022

Advanced RF Structures for Wakefield Acceleration and High-Gradient Research

Structure wakefield acceleration (SWFA) is one of the most promising AAC schemes in several recent strategic reports, including DOE's 2016 AAC Roadmap, report on the Advanced and Novel Accelerators for High Energy Physics Roadmap (ANAR), and report on Accelerator and Beam Physics Research Goals and Opportunities. SWFA aims to raise the gradient beyond the limits of conventional radiofrequency (RF) accelerator technology, and thus the RF to beam energy efficiency, by reducing RF breakdowns from confining the microwave energy in a short (on the order of about 10 ns) and intense pulse excited by a drive beam. We envision that the following research topics, within the scope of AF7, are of great interest in the next decade: advanced wakefield structures, terahertz and sub-terahertz (THz) structures, and RF breakdown physics. Research on SWFA in the above directions would directly contribute to long-term large-scale applications, including AAC-based linear colliders and compact light sources. There is also potentially a strong synergy between SWFA and other AAC concepts, when structures are combined with plasmas into hybrid AAC schemes. Research on novel structures is at the core of advancing SWFA, and is critical to future AAC-based linear colliders; at the same, it has a strong synergy with other directions, such as cavity designs, high-power microwave systems and sources, and compact light sources.

physics.acc-ph↗

arXiv2021

Self-calibration technique for characterization of integrated THz waveguides

Emerging high-frequency accelerator technology in the terahertz regime is promising for the development of compact high-brightness accelerators and high resolution-power beam diagnostics. One resounding challenge when scaling to higher frequencies and to smaller structures is the proportional scaling of tolerances which can hinder the overall performance of the structure. Consequently, characterizing these structures is essential for nominal operation. Here, we present a novel and simple self-calibration technique to characterize the dispersion relation of integrated hollow THz-waveguides. The developed model is verified in simulation by extracting dispersion characteristics of a standard waveguide a priori known by theory. The extracted phase velocity does not deviate from the true value by more than $ 9 \times 10^{-5} ~\%$. In experiments the method demonstrates its ability to measure dispersion characteristics of non-standard waveguides embedded with their couplers with an accuracy below $ \approx 0.5~\% $ and precision of $ \approx 0.05~\% $. Equipped with dielectric lining the metallic waveguides act as slow wave structures, and the dispersion curves are compared without and with dielectric. A phase synchronous mode, suitable for transverse deflection, is found at $ 275~\text{GHz} $.

physics.acc-ph↗

arXiv2021

A final focus system for injection into a laser plasma accelerator at the ARES linac

ARES (Accelerator Research Experiment at SINBAD) is a linear accelerator at the SINBAD (Short INnovative Bunches and Accelerators at DESY) facility at DESY. ARES was designed to combine reproducible beams from conventional RF-based accelerator technology with novel but still experimental acceleration techniques. It aims to produce high brightness ultra-short electron bunches in the range of sub fs to few fs, at a beam energy of 100-150 MeV, suitable for injection into novel acceleration experiments like Dielectric Laser Acceleration (DLA) and Laser driven Plasma Acceleration (LPA). This paper reports the conceptual design and simulations of a final focus system for injecting into a LPA experiment at ARES, including permanent magnetic quadrupoles (PMQ), sufficient longitudinal space for collinear laser and electron transport, space for required diagnostics and a LPA setup. Space-charge effects play a significant role and are included. Simulation results on the focusing of the ARES electron bunches into and their transport through the laser-driven plasma are presented. The effects of several errors have been simulated and are reported.

physics.acc-ph↗

arXiv2020

Longitudinal phase space synthesis with tailored 3D-printable dielectric-lined waveguides

Longitudinal phase space manipulation is a critical and necessary component for advanced acceleration concepts, radiation sources and improving performances of X-ray free electron lasers. Here we present a simple and versatile method to semi-arbitrarily shape the longitudinal phase space of a charged bunch by using wakefields generated in tailored dielectric-lined waveguides. We apply the concept in simulation and provide examples for radiation generation and bunch compression. We finally discuss the manufacturing capabilities of a modern 3D printer and investigate how printing limitations, as well as the shape of the input LPS affect the performance of the device.

physics.acc-ph↗

arXiv2020

Superradiant Cherenkov-Wakefield radiation as THz source for FEL facilities

An electron beam passing through a tube which is lined on the inside with a dielectric layer will radiate energy in the THz range due to the interaction with the boundary. The resonant enhancement of certain frequencies is conditioned by structure parameters as tube radius and permittivity of the dielectric layer. In low loss structures narrow-band radiation is generated which can be coupled out by suitable antennas. For higher frequencies the coupling to the resistive outer metal layer becomes increasingly important. The losses in the outer layer prohibit to reach high frequencies with narrow-band conditions. Instead short broad-band pulses can be generated with still attractive power levels. In the first section of the paper a general theory of the impedance of a two-layer structure is presented and the coupling to the outer resistive layer is discussed. Approximate relations for the radiated energy, power and pulse length for a set of structure parameters are derived and compared to numerical results in the following section. Finally first numerical result of the out-coupling of the radiation by means of a Vlasov antenna and estimates of the achieved beam quality are presented.

physics.acc-ph↗

arXiv2019

35 megawatt multicycle THz pulses from a homemade periodically poled macrocrystal

High-power multicycle THz radiation is highly sought after with applications in medicine, imaging, spectroscopy, characterization and manipulation of condensed matter, and could support the development of next-generation compact laser-based accelerators with applications in electron microscopy, ultrafast X-ray sources and sub-femtosecond longitudinal diagnostics. Multicycle THz-radiation can be generated by shooting an appropriate laser through a periodically poled nonlinear crystal, e.g. lithium niobate (PPLN). Unfortunately, the manufacturing processes of PPLNs require substantially strong electric fields $\mathcal{O}(10~kV/mm)$ across the crystal width to locally reverse the polarization domains; this limits the crystal apertures to below 1 cm. Damage threshold limitations of lithium niobate thereby limits the laser power which can be shone onto the crystal, which inherently limits the production of high-power THz pulses. Here we show that in the THz regime, a PPLN crystal can be mechanically constructed in-air by stacking lithium niobate wafers together with 180$^{\circ}$ rotations to each other. The relatively long (mm) wavelengths of the generated THz radiation compared to the small gaps ($\sim$10 $μ$m) between wafers supports a near-ideal THz transmission between wafers. We demonstrate the concept using a Joule-class laser system with $\sim$50 mm diameter wafers and measure up to 1.3 mJ of THz radiation corresponding to a peak power of $\sim$35 MW, a 50 times increase in THz power compared to previous demonstrations. Our results indicate that high-power THz radiation can be produced with existing and future high-power lasers in a scalable way, setting a course toward multi-gigawatt THz pulses. Moreover the simplicity of the scheme provides a simple way to synthesize waveforms for a variety of applications.

physics.acc-ph↗

arXiv2018

Passive Ballistic Microbunching of Non-Ultrarelativistic Electron Bunches using Electromagnetic Wakefields in Dielectric-Lined Waveguides

Temporally-modulated electron beams have a wide array of applications ranging from the generation of coherently-enhanced electromagnetic radiation to the resonant excitation of electromagnetic wakefields in advanced-accelerator concepts. Likewise producing low-energy ultrashort microbunches could be useful for ultra-fast electron diffraction and new accelerator-based light-source concepts. In this Letter we propose and experimentally demonstrate a passive microbunching technique capable of forming a picosecond bunch train at $\sim 6$~MeV. The method relies on the excitation of electromagnetic wakefields as the beam propagates through a dielectric-lined waveguide. Owing to the non-ultrarelativistic nature of the beam, the induced energy modulation eventually converts into a density modulation as the beam travels in a following free-space drift. The modulated beam is further accelerated to $\sim20$~MeV while preserving the imparted density modulation.

physics.acc-ph↗

arXiv2017

An Adiabatic Phase-Matching Accelerator

We present a general concept to accelerate non-relativistic charged particles. Our concept employs an adiabatically-tapered dielectric-lined waveguide which supports accelerating phase velocities for synchronous acceleration. We propose an ansatz for the transient field equations, show it satisfies Maxwell's equations under an adiabatic approximation and find excellent agreement with a finite-difference time-domain computer simulation. The fields were implemented into the particle-tracking program {\sc astra} and we present beam dynamics results for an accelerating field with a 1-mm-wavelength and peak electric field of 100~MV/m. The numerical simulations indicate that a $\sim 200$-keV electron beam can be accelerated to an energy of $\sim10$~MeV over $\sim 10$~cm. The novel scheme is also found to form electron beams with parameters of interest to a wide range of applications including, e.g., future advanced accelerators, and ultra-fast electron diffraction.

physics.acc-ph↗

arXiv2015

Tailored electron bunches with smooth current profiles for enhanced transformer ratios in beam-driven acceleration

Collinear high-gradient ${\cal O} (GV/m)$ beam-driven wakefield methods for charged-particle acceleration could be critical to the realization of compact, cost-efficient, accelerators, e.g., in support of TeV-scale lepton colliders or multiple-user free-electron laser facilities. To make these options viable, the high accelerating fields need to be complemented with large transformer ratios $>2$, a parameter characterizing the efficiency of the energy transfer between a wakefield-exciting "drive" bunch to an accelerated "witness" bunch. While several potential current distributions have been discussed, their practical realization appears challenging due to their often discontinuous nature. In this paper we propose several alternative current profiles which are smooth which also lead to enhanced transformer ratios. We especially explore a laser-shaping method capable of generating one the suggested distributions directly out of a photoinjector and discuss a linac concept that could possible drive a dielectric accelerator.

physics.acc-ph↗