Electron Microbunches Research Articles

Simulation Study on Attosecond Inverse Compton Scattering Source from Laser Wakefield Acceleration with Near-Threshold Ionization Injection

We present the generation of attosecond gamma rays via inverse Compton scattering within the framework of laser wakefield acceleration through 2D Particle-In-Cell simulations. Utilizing the near-threshold ionization injection mechanism, an attosecond micro-bunched electron beam characterized by a comb-like current density profile can be achieved with a linearly polarized laser at an intensity of a0 = 1.5. The micro-bunched beam provides a beam energy of approximately 300 MeV and achieves a minimum relative energy spread of about 1.64% after undergoing 2 mm of acceleration. In the inverse Compton scattering scheme, these attosecond electron micro-bunches interact with the reflected driving laser pulse, resulting in the attosecond gamma-ray radiation exhibiting similar structures. Individual spatial-separated gamma-ray pulses exhibit a length of approximately 260–300 as, with a critical energy of 2.0 ± 0.2 MeV. The separated attosecond gamma-ray source owns a peak brilliance of ~1022 photons s−1 mm−2 mrad−2 0.1% BW. This brilliance is competitive in a laboratory for multi-MeV γ-ray sources with a laser intensity of I = 5 × 1018 W/cm2. Such attosecond gamma-ray radiation offers promising applications requiring ultrashort X-ray/gamma ray sources.

Shaping Micro-Bunched Electron Beams for Compact X-ray Free-Electron Lasers with Transverse Gradient Undulators

Laser-modulator-based micro-bunching of electron beams has been applied to many novel operating modes of X-ray free-electron lasers from harmonic generation to attosecond pulse production. Recently, it was also identified as a key enabling technology for the production of a compact XFEL driven by a relatively low-energy beam. In traditional laser modulator schemes with low-energy and high-current bunches, collective effects limit the possible working points that can be employed, and thus it is difficult to achieve optimal XFEL performance. We propose to utilize transverse longitudinal coupling in a transverse gradient undulator (TGU) to shape micro-bunched electron beams so as to optimize their performance in a compact X-ray free-electron laser. We show that a TGU added to a conventional laser modulator stage enables much more flexibility in the design, allowing one to generate longer micro-bunches less subject to slippage effects and even lower the slice emittance of the micro-bunches. We present a theoretical analysis of laser-based micro-bunching with an added TGU, simulation of compression with collective effects in such systems, and finally XFEL simulations demonstrating the gains in peak power enabled by the TGU. Although we focus on the application to compact XFELs, what we propose is a general phase space manipulation that may find utility in other applications as well.

Towards precise diagnosis time profile of ultrafast electron bunch trains using orthogonal terahertz streak camera.

Ultrafast electron microbunch trains have broad applications in which the individual bunch length and the bunch-to-bunch interval are critical parameters that need to be precisely diagnosed. However, directly measuring these parameters remains challenging. This paper presents an all-optical method that simultaneously measures the individual bunch length and the bunch-to-bunch spacing through an orthogonal THz-driven streak camera. For a 3 MeV electron bunch train, the simulation indicates that the temporal resolution of individual bunch length and the bunch-to-bunch spacing is 2.5 fs and 1 fs, respectively. Through this method, we expect to open a new chapter in the temporal diagnostic of electron bunch trains.

Beam-beam interaction in a dielectric laser accelerator electron-positron collider

We examine through numerical calculation the collision of counter-propagating trains of optically spaced electron or positron microbunches in a 1 TeV collider scenario for a dielectric laser accelerator. A time-dependent envelope equation is derived for arbitrary number of bunches in the classical limit, with inclusion of the radiation reaction force. Example parameters are examined based on a constrained luminosity relation that takes into account the bunch charge for optimal efficiency, material damage limits, and power constraints. We find that for initially identical counter-propagating Gaussian bunch trains, the periodic temporal structure leads to a peak in luminosity with number of bunches. For longer bunch trains, the enhancement then decreases inversely with number of bunches. The corresponding fractional energy loss of the beam is found to be of order 1.75%, which is reduced to 0.35% when the nonlinear radial dependence of the transverse force is included, with an average beamstrahlung parameter of 0.075, an important result considering that beamstrahlung losses are a critical concern for future TeV colliders.

Measurement of tunable terahertz drive laser pulse train generation using streak camera

A pulse-train with tunable terahertz (THz) repetition rate can be used for driving a photocathode radio-frequency (RF) electron gun to generate electron microbunch train, which is indispensable for exciting intense narrowband THz radiation. In this paper, a train of 8 laser micro-pulses is experimentally obtained and measured by a 3-stage polarization-beam-splitter-based pulse stacker and a streak camera respectively. In addition, we performed measurements of the generation of tunable THz pulse-train with the interpulse spacings of 12.0, 9.8, and 6.3 ps, corresponding to modulated repetition frequencies of 0.089, 0.103, and 0.148 THz, respectively. As a result, preliminary experiments prove the feasibility and applicability of tunable THz laser pulse train generation based on the polarization-beam-splitter-based pulse stacking system and the streak camera measurement system.

Three-Dimensional Radiative Effects in the Compression of Ultra-Short Electron Micro-Bunches

Three-Dimensional Radiative Effects in the Compression of Ultra-Short Electron Micro-Bunches

Quasi-phase-matched laser wakefield acceleration of electrons in an axially density-modulated plasma channel

Quasi-phase matching in corrugated plasma channels has been proposed as a way to overcome the dephasing limitation in laser wakefield accelerators. In this study, the phase-lock dynamics of a relatively long electron bunch injected in an axially-modulated plasma waveguide is investigated by performing particle simulations. The main objective here is to obtain a better understanding of how the transverse and longitudinal components of the wakefield as well as the initial properties of the beam affect its evolution and qualities. The results indicate that the modulation of the electron beam generates trains of electron microbunches. It is shown that increasing the initial energy of the electron beam leads to a reduction in its final energy spread and produces a more collimated electron bunch. For larger bunch diameters, the final emittance of the electron beam increases due to the stronger experienced transverse forces and the larger diameter itself. Increasing the laser power improves the maximum energy gain of the electron beam. However, the stronger generated focusing and defocusing fields degrade the collimation of the bunch.

Efficiency enhancement of THz radiation from an electron bunch in a waveguide due to low-frequency stabilization

Simultaneous generation of pulses of high-frequency Super-Radiance and low-frequency Coherent Spontaneous Radiation from an ultra-relativistic electron bunch moving in a waveguide placed in a spatially periodic and / or uniform longitudinal magnetic field is studied. The low-frequency radiation can weaken expansion of the bunch and decrease a velocity spread. This significantly improves electron micro-bunching and increases energy of high-frequency pulses. Such effect makes possible fairly efficient high-frequency cyclotron radiation of the bunch near an autoresonance regime, when the wave phase velocity is very close to speed of light.

Simultaneous high-frequency Super-Radiance and low-frequency Coherent Spontaneous Radiation from ultrarelativistic electrons in a waveguide

Terahertz generation of high- and low-frequency pulses from an ultrarelativistic electron bunch, which move in a waveguide placed in an undulator, has been studied. If the length of the bunch is less than the low-frequency wavelength, its radiation under certain conditions stabilizes the length of the bunch and largely compensates for the velocity spread introduced by the particle repulsion. This leads to a significant improvement in the micro-bunching of electrons and increases the energy of the radiated high-frequency pulses.

Method of spatial size measurement of relativistic electrons beams with small bunch length

We investigate the practical implementation of a previously proposed method for determining the transverse dimensions of an electron beam on a target by measuring the two-dimensional angular distributions of diffracted transition radiation of relativistic electrons for two distances between the crystal where the radiation is generated and the coordinate detector for femtosecond bunches. We show that determining electron microbunches with small longitudinal sizes requires an increased photon energy, achieved by decreasing the observation angle. The sensitivity limits of the method and the influence of the emission of secondary electrons and photons from the detector pixels are discussed.

Ultrafast Plasma Electron Dynamics: A Route to Terahertz Pulse Shaping

Intense ultrafast laser interaction with solid-density plasma can lead to relativistic transient electron dynamics that are favorable for the generation of high-field terahertz (THz) pulses. We investigate the temporal characteristics of such THz pulses. Single-shot electro-optic measurements enable us to record the complete temporal profiles of the THz pulses emanating from planar $\mathrm{Cu}$ and aligned $\mathrm{Cu}$-nanorod-array plasma. The temporal properties of the THz pulses exhibit several transients. Fully relativistic two-dimensional particle-in-cell simulations corroborate experimental observations and reveal that these features originate from electron microbunch emission from the target. Our results demonstrate that target nanostructuring provides a route to control such ultrafast electron dynamics and hence THz-pulse properties in the time domain.

Generation and Characterization of Attosecond Microbunched Electron Pulse Trains via Dielectric Laser Acceleration

Dielectric laser acceleration is a versatile scheme to accelerate and control electrons with the help of femtosecond laser pulses in nanophotonic structures. We demonstrate here the generation of a train of electron pulses with individual pulse durations as short as 270±80 attoseconds (FWHM), measured in an indirect fashion, based on two subsequent dielectric laser interaction regions connected by a free-space electron drift section, all on a single photonic chip. In the first interaction region (the modulator), an energy modulation is imprinted on the electron pulse. During free propagation, this energy modulation evolves into a charge density modulation, which we probe in the second interaction region (the analyzer). These results will lead to new ways of probing ultrafast dynamics in matter and are essential for future laser-based particle accelerators on a photonic chip.

Microbunched electron cooling with amplification cascades

The Microbunched Electron Cooling (MBEC) is a promising cooling technique that can find applications in future hadron and electron-ion colliders to counteract intrabeam scattering that limits the maximum achievable luminosity of the collider. To minimize the cooling time, one would use amplification cascades consisting of a drift section followed by a magnetic chicane. In this paper, we first derive and optimize the gain factor in an amplification section for a simplified one-dimensional model of the beam. We then deduce the cooling rate of a system with one and two amplification cascades. We also analyze the noise effects that counteract the cooling process through the energy diffusion in the hadron beam. Our analytical formulas are confirmed by numerical simulations for a set of model parameters.

Micro-bunching for generating tunable narrow-band THz radiation at the FAST photoinjector

This paper presents expected THz radiation spectra emitted by micro-bunched electron beams produced using a slit-mask placed within a magnetic chicane in the FAST (Fermilab Accelerator Science and Technology) electron injector at Fermilab. Our purpose is to generate tunable narrow-band THz radiation with a simple scheme in a conventional photo-injector. Using the slit-mask in the chicane, we create a longitudinally micro-bunched beam after the chicane by transversely slicing an energy chirped electron bunch at a location with horizontal dispersion. In this paper, we discuss the theory related to the micro-bunched beam structure, the beam optics, the simulation results of the micro-bunched beam and the bunching factors. Energy radiated at THz frequencies from two sources: coherent transition radiation and from a wiggler is calculated and compared. We also discuss the results of a simple method to observe the micro-bunching on a transverse screen monitor using a skew quadrupole placed in the chicane.

Cooling rate for microbunched electron cooling without amplification

The Microbunched Electron Cooling (MBEC) proposed by D. Ratner is a promising cooling technique that can find applications in future hadron and electron-ion colliders. In this paper, we develop a new framework for the study of MBEC which is based on the analysis of the dynamics of microscopic 1D fluctuations in the electron and hadron beams during their interaction and propagation through the system. Within this framework, we derive an analytical formula for the cooling rate and benchmark it against 1D computer simulations with a agreement between the analytical and numerical results. We then calculate the expecting cooling time for a set of parameters of the proposed electron-ion collider eRHIC in a simple cooling system with one chicane in the electron channel. While the cooling rate in this system turns out to be insufficient to counteract the intra-beam scattering in the proton beam, we discuss how the electron signal can be amplified by two orders of magnitude through the use of plasma effects in the beam.

Selective excitation and control of coherent terahertz Smith-Purcell radiation by high-intensity period-tunable train of electron micro-bunches

We report the observation and studies of selective excitation and control of terahertz (THz) coherent Smith-Purcell radiation (cSPr) from a train of sub-picosecond-period electron micro-bunches. The coherence of the radiation from such a train has been demonstrated. The spectrum of cSPr was measured and the selective excitation of the first and the second harmonics was observed, respectively. We also demonstrate experimentally that radiation pulses generated by each electron micro-bunch interfere coherently with the maximum intensity of cSPr observed at the frequency equal to the frequency of the micro-bunch spacing. The experimental results greatly contribute to the understanding of coherent Smith-Purcell radiation from the train of electron micro-bunches, as well as the development of THz diagnostics for accelerators.

Theoretical and simulation study of ‘Comb’ electron beam and THz generation

A compact accelerator based super-radiant THz source is under development at Inter University Accelerator Centre (IUAC), New Delhi. The facility is based on the principle of pre-bunched Free Electron Laser (FEL) which will produce THz radiation in the range of ∼0.18 to 3 THz from a modulated electron beam. A photocathode electron gun will generate a short train of micro-bunches (a “comb” beam) driven by a fibre laser system capable of producing multi micro-pulse laser beam with variable separation (“comb” laser pulse). Upon acceleration, the electron beam will be injected in to a compact undulator magnet tuned to the same frequency as the separation of the electron micro-bunches. The paper discusses the process of enhancement of super-radiant emission of radiation due to modulation in the comb beam and the conditions required to achieve maximum enhancement of the radiation power. The feasibility study of generating a comb beam at the photocathode and its transport through the beamline while preserving its temporal structure has been reported. To evaluate the characteristics of the radiation emitted from the comb beam, a C++ based particle tracker and Liẽnard–Wiechert field solver has been developed. The conceptual understanding of the emission of radiation from comb beam is shown to conform with the numerical results. The code has been used to calculate the radiation pulse energy emitted into the central cone of undulator for various comb beam configurations.

Trains of electron micro-bunches in plasma wake-field acceleration

Plasma-based charged particle accelerators have been intensively investigated in the past three decades due to their capability to open up new horizons in accelerator science and particle physics yielding electric field accelerating gradient more than three orders of magnitudes higher than in conventional devices. At the current stage the most advanced and reliable mechanism for accelerating electrons is based on the propagation of an intense laser pulse or a relativistic electron beam in a low density gaseous target. In this paper we concentrate on the electron beam-driven plasma wake-field acceleration and demonstrate using 3D PiC simulations that a train of electron micro-bunches with ∼10 fs period can be generated behind the driving beam propagating in a density down-ramp. We will discuss the conditions and properties of the micro-bunches generated aiming at understanding and study of multi-bunch mechanism of injection. It is show that the periodicity and duration of micro-bunches can be controlled by adjusting the plasma density gradient and driving beam charge.

Observation of coherent Smith-Purcell and transition radiation driven by single bunch and micro-bunched electron beams

Generation of coherent Smith-Purcell (cSPr) and transition/diffraction radiation using a single bunch or a pre-modulated relativistic electron beam is one of the growing research areas aiming at the development of radiation sources and beam diagnostics for accelerators. We report the results of comparative experimental studies of terahertz radiation generation by an electron bunch and micro-bunched electron beams and the spectral properties of the coherent transition and SP radiation. The properties of cSPr spectra are investigated and discussed, and excitations of the fundamental and second harmonics of cSPr and their dependence on the beam-grating separation are shown. The experimental and theoretical results are compared, and good agreement is demonstrated.

Generation of a femtosecond electron microbunch train from a photocathode using twofold Michelson interferometer

The interest in producing ultrashort electron bunches has risen sharply among scientists working on the design of high-gradient wakefield accelerators. One attractive approach generating electron bunches is to illuminate a photocathode with a train of femtosecond laser pulses. In this paper we describe the design and testing of a laser system for an rf gun based on a commercial titanium-sapphire laser technology. The technology allows the production of four femtosecond laser pulses with a continuously variable pulse delay. We also use the designed system to demonstrate the experimental generation of an electron microbunch train obtained by illuminating a cesium-telluride semiconductor photocathode. We use conventional diagnostics to characterize the electron microbunches produced and confirm that it may be possible to control the main parameter of an electron microbunch train.