Ultrafast Relativistic Electron Nanoprobes

Abstract

One of the frontiers in electron scattering is to couple ultrafast temporal resolution with highly localized probes to investigate the role of microstructure on material properties. Here, taking advantage of the unprecedented average brightness of the APEX electron gun providing relativistic electron pulses at high repetition rates, we demonstrate for the first time the generation of ultrafast relativistic electron beams with picometer-scale emittance and their ability to probe nanoscale features on materials with complex microstructures. At the sample plane, the APEX beam is tightly focused by a custom in-vacuum lens system based on permanent magnet quadrupoles, and its evolution around the waist is tracked by a knife-edge technique, allowing accurate reconstruction of the beam shape and local density. We then use the focused beam to characterize a Ti-6 wt% Al polycrystalline sample by correlating the diffraction and imaging modality, showcasing the capability to locate grain boundaries and map adjacent crystallographic domains with sub-micron precision. This work provides a new paradigm for ultrafast electron instrumentation, demonstrating the ability to generate relativistic beams with ultrasmall transverse phase space volumes enabling novel characterization techniques such as relativistic ultrafast electron nano-diffraction and ultrafast scanning transmission electron microscopy.Since the discovery of the particle-wave duality 1 , electrons have been extensively used to probe matter at atomic scales. Owing to their very short (sub-Å) wavelength and large scattering cross section compared to X-rays, electron diffraction and imaging are today well established techniques for structure determination. More recently, the advent of ultrafast lasers sparked the development of intense ultrashort electron sources which, in turn, paved the way to a new generation of time-resolved electron scattering techniques such as ultrafast electron diffraction and microscopy (UED/M) 2-4 . These are now capable of probing atomic-scale structural dynamics with femtosecond-scale temporal accuracy.Recent developments in this field include the introduction of methods and technology common in particle accelerator science. Radio frequency (RF) based electron sources have been successfully used for generating few-femtosecond electron probe beams 5, 6 and for gathering information about ultrafast structural changes in solids and gases 7, 8 . Here, electrons are generated and rapidly accelerated to relativistic energies by using high accelerating gradients, increasing the maximum achievable electron current density 9, 10 and minimizing the temporal broadening caused by Coulomb repulsions and initial energy bandwidth, which are the main challenges for low-energy electron sources.Notwithstanding this significant progress, ultrafast electron-based instrumentation is still far from reaching spatial resolution similar to what can be achieved in static electron microscopes. At low energies, setups using tip-based photoemission guns in standard tra...

Adaptive Deep Learning for Time-Varying Systems With Hidden Parameters: Predicting Changing Input Beam Distributions of Compact Particle Accelerators

“…In the present work we utilized an adaptive CNN-based inverse model for mapping output beam measurements measurements directly to the learned principal components that represent beam inputs at the High Repetition-rate Electron Scattering apparatus (HiRES, shown in Fig. 3) beamline at Lawrence Berkeley National Laboratory (LBNL), which accelerates pC-class, subpicosecond long electron bunches up to one million times a second (MHz), providing some of the most dense 6D phase space among accelerators at unique repetition rates, making it an ideal test bed for advanced algorithm development 36,48 . The first step was to collect data to learn a basis with which to represent a distribution of interest.…”

“…The High Repetition-rate Electron Scattering apparatus (HiRES, shown in Fig. 3) at Lawrence Berkeley National Laboratory (LBNL), accelerates pC-class, sub-picosecond long electron bunches up to one million times a second (MHz), providing some of the most dense 6D phase space among accelerators at unique repetition rates, making it an ideal test bed for advanced algorithm development 36,48 .…”

Knife-edge based measurement of the 4D transverse phase space of electron beams with picometer-scale emittance

“…First, the laser spot on the cathode was tightly focused down to 50 um RMS, which minimizes the initial transverse emittance. Second, the combination of the gun solenoid and a following 500 um aperture cuts the high divergence part of the beam and decreases the transverse normalized emittance to 3 nm [8]. Then, the beam was transported through the dogleg energy collimator: two sets of quadrupole triplets lenses were utilized to compensate the energy dispersion and minimize the transverse emittance growth due to the energy spread.…”

Plasmonic Lenses for Ultrafast Electron Nanoemission

“…As ultrafast science moves towards more nanoscale and heterogeneous systems, bright sources of femtosecond and nanoscale electron pulses are in demand [1]. While capable tip emitters have been demonstrated [2], they are limited in some experimental parameters; for instance, they provide few electrons per pulse and are susceptible to breakdown under high extraction fields like those used in radiofrequency MeV electron guns.…”