Next-generation Electron Sources

Tip-based photoemission electron sources offer unique properties for ultrafast imaging, diffraction, and spectroscopy experiments with highly coherent few-electron pulses. Extending this approach to increased bunch-charges requires a comprehensive experimental study on Coulomb interactions in nanoscale electron pulses and their impact on beam quality. For a laser-driven Schottky field emitter, we assess the transverse and longitudinal electron pulse properties in an ultrafast transmission electron microscope at a high photoemission current density. A quantitative characterization of electron beam emittance, pulse duration, spectral bandwidth, and chirp is performed. Due to the cathode geometry, Coulomb interactions in the pulse predominantly occur in the direct vicinity to the tip apex, resulting in a well-defined pulse chirp and limited emittance growth. Strategies for optimizing electron source parameters are identified, enabling advanced ultrafast transmission electron microscopy approaches, such as phase-resolved imaging and holography.

Ultrafast electron-based coherent radiation sources, such as free-electron lasers (FELs), ultrafast electron diffraction (UED) and Thomson-scattering sources, are becoming more important sources in today’s ultrafast science. Photocathode laser is an indispensable common subsystem in these sources that generates ultrafast electron pulses. To fully exploit the potentials of these sources, especially for pump-probe experiments, it is important to achieve high-precision synchronization between the photocathode laser and radio-frequency (RF) sources that manipulate electron pulses. So far, most of precision laser-RF synchronization has been achieved by using specially designed low-noise Er-fibre lasers at telecommunication wavelength. Here we show a modular method that achieves long-term (>1 day) stable 10-fs-level synchronization between a commercial 79.33-MHz Ti:sapphire laser oscillator and an S-band (2.856-GHz) RF oscillator. This is an important first step toward a photocathode laser-based femtosecond RF timing and synchronization system that is suitable for various small- to mid-scale ultrafast X-ray and electron sources.

Non-radioactive electron sources are of great interest in any application requiring the emission of electrons at atmospheric pressure, as they offer better control over emission parameters than radioactive electron sources and are not subject to legal restrictions. Recently, we published a simple electron source consisting only of a vacuum housing, a filament, and a single control grid. In this paper, we present improved control electronics that utilize this control grid in order to focus and defocus the electron beam, thus pulsing the electron emission at atmospheric pressure. This allows short emission pulses and excellent stability of the emitted electron current due to continuous control, both during pulsed and continuous operations. As an application example, this electron source is coupled to an ion mobility spectrometer. Here, the pulsed electron source allows experiments on gas phase ion chemistry (e.g., ion generation and recombination kinetics) and can even remove the need for a traditional ion shutter.

The Dielectric Laser Acceleration group at SLAC uses micro-fabricated dielectric grating structures and conventional infrared lasers to accelerator electrons. These structures have been estimated to produce an accelerating gradient up to 2 orders of magnitude greater than that produced by conventional RF accelerators. The success of the experiment depends on both the laser damage threshold of the structure and the timing overlap of femtosecond duration laser pulses with the electron bunch. In recent dielectric laser acceleration experiments, the laser pulse was shorter both temporally and spatially than the electron bunch. As a result, the laser is theorized to have interacted with only a small portion of the electron bunch. The detection of this phenomenon, referred to as partial population modulation, required a new approach to the data analysis of the electron energy spectra. A fitting function was designed to separate the accelerated electron population from the un-accelerated electron population. The approach was unsuccessful in detecting acceleration in the partial population modulation data. However, the fitting functions provide an excellent figure of merit for previous data known to contain signatures of acceleration.

Dielectric laser accelerator (DLA) plays a very important role in the development of on-chip particle accelerators, which has aroused the interest of many researchers. In all previous DLA schemes, each laser pulse can only be used once, which greatly limits the efficiency of the driving laser, as well as the acceleration length and energy gain of particles. Here we propose a DLA scheme based on the inverse cherenkov effect, which uses a pair of laser pulses to accelerate particles in multiple stages. The laser pulses are reflected and refracted multiple times, synchronizing with and accelerating the electron beam in each acceleration stage, just like a traveling wave acceleration in traditional rf accelerators. By this means, the acceleration efficiency can be greatly enhanced, which paves a way of developing high efficiency and high energy-gain particle accelerator on chip.

Summary form only given.The results of experimental and theoretical research on interaction of femtosecond laser pulses at a wavelength 800 nm and 400 nm with a copper foil are presented. Ti:Sapphire laser intensity on foil surface was in the rang of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> -3*10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> in these experiments that were carried out at University of Alberta (Canada). The calculations of femtosecond laser pulses interaction with a matter and study of the foil blow-off are performed with using of ID ERA code developed in RFNC-VNIITF. The hydrodynamic and electron conductivity with degeneracy effects were taken into account. Wide-range equations of state for solid, gas and plasma were used in ERA-code simulations. Two- temperature case was considered in these calculations by taking into account of energy exchange between electrons and phonons (ions). Laser energy absorption was calculated from Helmholtz equation with complex dielectric permittivity. The scattering of electrons on phonons and free electrons was taken into account in calculation of dielectric permittivity. The results of ERA code calculations are in a good agreement with data on laser light reflection that have been obtained earlier in experiments on interaction of 150-fs laser pulses with copper foils at University of Alberta. ERA code simulations of foil dynamic under action of 100-fs laser pulse with wavelength of 800 nm and intensity of 3*10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> were performed as with low-intensity nanosecond prepulse as without it. It is shown that nanosecond prepulse at intensity of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> - 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> have no significant influence on copper foils.

ABSTRACTWe report on the development of an ultrafast Transmission Electron Microscope based on a laser-driven cold-field emission source. We first describe the instrument before reporting on numerical simulations of the laser-driven electron emission. These simulations predict the temporal and spectral properties of the femtosecond electron pulses generated in our ultrafast electron source. We then discuss the effects that contribute to the spatial, temporal and spectral broadening of these electron pulses during their propagation from the electron source to the sample and finally to the detectors of the electron microscope. The spectro-temporal properties are then characterized in an electron/photon cross-correlation experiment based on the detection of electron energy gains. We finally illustrate the potential of this instrument for ultrafast electron holography and ultrafast electron diffraction.

Ultrafast electron dynamics determines how physical and chemical changes occur at a fundamental level. The investigation of such processes requires (isolated) attosecond pulses [1]. On all time scales down to the attosecond regime, there are a number of physical processes that depend on the electric field, rather than just the intensity envelope of a driving pulse. There have been numerous proposals to produce ultrashort pulses in free-electron lasers (FELs) [2]. However, the temporal coherence of the output radiation is limited. In order to solve this problem, we proposed a robust method for producing waveform-controlled CEP-stable attosecond pulses in the EUV spectral range in a FEL [3]. Here, we investigate the feasibility of our setup when a laser-plasma accelerator (LPA) is used as the electron source, rather than a LINAC.

In dielectric laser acceleration, nanostructures etched into silicon are used to convert free-space ultrashort laser pulses, incident from the side and parallel to the wafer substrate, to accelerate particles. This current approach is experimentally challenging and, as it turns out, not quite necessary for most experiments and practical applications. Here, we experimentally demonstrate and numerically verify the efficacy of top-illuminated structures, and measure a maximum acceleration gradient of 49.2 ± 3.1 MeV/m. We discuss how, in practice, this approach proves superior to the current standard in the field, and expect it to become the definitive choice for nanophotonic particle laser acceleration.

Temporal gating allows high accuracy time-resolved measurements of a broad range of ultrafast processes. By manipulating the interaction between an atom and an intense laser field, we extend gating into the nonlinear medium in which attosecond optical and electron pulses are generated. Our gate is an amplitude gate induced by ellipticity of the fundamental pulse. The gate modulates the spectrum of the high harmonic emission and we use the measured modulation to characterize the sub-laser-cycle dynamics of the recollision electron wave packet.

The interaction of high-intensity laser pulses and solid targets provides a promising way to create compact, tunable, and bright XUV attosecond sources that can become a unique tool for a variety of applications. However, it is important to control the polarization state of this XUV radiation and to do so in the most efficient regime of generation. Using the relativistic electronic spring (RES) model and particle-in-cell simulations, we show that the polarization state of the generated attosecond pulses can be tuned in a wide range of parameters by adjusting the polarization and/or the angle of incidence of the laser radiation. In particular, we demonstrate the possibility of producing circularly polarized attosecond pulses in a wide variety of setups.

Transient electric fields, which are an important but hardly explored parameter of laser plasmas, can now be diagnosed experimentally with combined ultrafast temporal resolution and field sensitivity, using femtosecond to picosecond electron or proton pulses as probes. However, poor spatial resolution poses great challenges to simultaneously recording both the global and local field features. Here, we present a direct 3D measurement of a transient electric field by time-resolved electron schlieren radiography with simultaneous 80-μm spatial and 3.7-ps temporal resolutions, analyzed using an Abel inversion algorithm. The electric field here is built up at the front of an aluminum foil irradiated with a femtosecond laser pulse at 1.9 × 10(12) W/cm(2), where electrons are emitted at a speed of 4 × 10(6) m/s, resulting in a unique "peak-valley" transient electric field map with the field strength up to 10(5) V/m. Furthermore, time-resolved schlieren radiography with charged particle pulses should enable the mapping of various fast-evolving field structures including those found in plasma-based particle accelerators.

THz radiation by beating two femtosecond laser pulses in magnetized and collisional clustered plasma for different laser profiles is investigated. Laser pulses interaction with clustered plasma gives rise to transverse nonlinear current at beating frequencies and is responsible for THz generation. In this mechanism, cluster plasmon resonance significantly enhances the efficiency and the phase matching amplifies that further. Variation of electric field amplitude on directivity of generated waves is examined analytically, as well as numerically for different laser profiles. The numerical findings indicated that laser pulse profile is an important tool to control plasma dynamics and a technique to devise for THz radiation harness for especial applications. The angular distribution of the radiation pattern of THz radiation in forward direction is obtained and the effect of external magnetic field, frequency of plasma, cluster radius and electron collision frequency on generated THz wave patterns have been considered. The study shows that external magnetic field induced more electron current density leading to an increase in radiation power with no change in directivity diagram. Substantial enhancement in magnitude of the angular distribution of THz radiation in clustered plasma compared with un-clustered plasma was noted.

Electron pulse radiolysis is a modern, versatile technique for investigating the dynamic properties of physical, chemical and biological change. The technique requires an electron-accelerating machine to produce an intense (0.1-5A) beam of high energy (1-20MeV) electrons in a single pulse (10-9 - 10-5>s). Such a beam, when incident upon a target can create physical and chemical perturbations in sufficient amounts to be directly observed in real time by one of several physical methods. In this work the target liquid occupies the cavity of an optical kinetic spectrometer capable of detecting small changes in light intensity with a resolution of a few nanoseconds. By coupling the output current of the photo-detector to a mini-computer via a fast ADC, and incorporation of sundry digital interfaces, computer control of experimental settings, data acquisition and data-analysis is effected. Using such a device, changes in the nature and concentration of transient species generated during passage of the electron beam through the target medium can be conveniently followed. Molecular excited states are efficiently generated on electron beam bombardment of aromatic liquids such as benzene through direct excitation or fast electron-hole recombination. Both Sl(lB2u) and Tl(3Blu) states are formed which can undergo energy transfer reactions to minor solute components thus populating the respective solute states.

In this paper, the electrons energy spectrum produced by stochastic acceleration in the interaction of an intense laser pulse with the underdense plasma is described by employing the fully kinetic 1D-3 V particle-in-cell simulation. In this way, two finite laser pulses with the same length 200 fs and with two different rise times 30 and 60 fs are typically selected. It is shown that the maximum energy of electrons in the laser pulse with the short rise time (30 fs) is about eight times greater than the maximum energy of the electrons with the long rise time (60 fs). Furthermore, unlike the pulse with the short rise time, the shape of energy spectrum and the electrons temperature in the long rise time laser pulse are approximately unchanged over the time. These results originated from the fact that in the case of long rise time laser pulse, all electrons are accelerated by the one chaotic mechanism because of the scattered fields generated in the plasma, but in the case of short rise time laser pulse, three different mechanisms accelerate the electrons: first, the stochastic acceleration because of the nonlinear wave breaking via plasma-vacuum boundary effect; second, the stochastic acceleration initiated by the wave breaking; and third, the direct laser acceleration of the released electrons.