An ultracold low emittance electron source

A preliminary design of a mega-electron-volt (MeV) monochromator with 10−5 energy spread for ultrafast electron diffraction (UED) and ultrafast electron microscopy (UEM) is presented. Such a narrow energy spread is advantageous in both the single shot mode, where the momentum resolution in diffraction is improved, and the accumulation mode, where shot-to-shot energy jitter is reduced. In the single-shot mode, we numerically optimized the monochromator efficiency up to 13% achieving 1.3 million electrons per pulse. In the accumulation mode, to mitigate the efficiency degradation caused by the shot-to-shot energy jitter, an optimized gun phase yields only a mild reduction of the single-shot efficiency, therefore the number of accumulated electrons nearly proportional to the repetition rate. Inspired by the recent work of Qi et al. (Phys Rev Lett 124:134803, 2020), a novel concept of applying reverse bending magnets to adjust the energy-dependent path length difference has been successfully realized in designing a MeV monochromator to achieve the minimum energy-dependent path length difference between cathode and sample. Thanks to the achromat design, the pulse length of the electron bunches and the energy-dependent timing jitter can be greatly reduced to the 10 fs level. The introduction of such a monochromator provides a major step forward, towards constructing a UEM with sub-nm resolution and a UED with ten-femtosecond temporal resolution. The one-to-one mapping between the electron beam parameter and the diffraction peak broadening enables a real-time nondestructive diagnosis of the beam energy spread and divergence. The tunable electric–magnetic monochromator allows the scanning of the electron beam energy with a 10−5 precision, enabling online energy matching for the UEM, on-momentum flux maximizing for the UED and real-time energy measuring for energy-loss spectroscopy. A combination of the monochromator and a downstream chicane enables “two-color” double pulses with femtosecond duration and the tunable delay in the range of 10 to 160 fs, which can potentially provide an unprecedented femtosecond time resolution for time resolved UED.

Heat treatment up to 1500 degrees C in an ultrahigh-vacuum furnace, along with processing of cavities and temperature mapping was used to suppress field emission and analyze emitter properties. In 27 tests of one-cell 1500-MHz fired accelerating cavities, on the average the accelerating field, E/sub acc/, increased to 24 MV/m (H/sub pk/=1250 Oe) from 13 MV/m with chemical treatment alone; the highest E/sub acc/ reached was 30.5 MV/m. Emitter properties, efficiency of He processing, and results of the exposure of well-processed RF surfaces to various media are also discussed.

The development of ultrafast gas electron diffraction with nonrelativistic electrons has enabled the determination of molecular structures with atomic spatial resolution. It has, however, been challenging to break the picosecond temporal resolution barrier and achieve the goal that has long been envisioned—making space- and-time resolved molecular movies of chemical reaction in the gas-phase. Recently, an ultrafast electron diffraction (UED) apparatus using mega-electron-volt (MeV) electrons was developed at the SLAC National Accelerator Laboratory for imaging ultrafast structural dynamics of molecules in the gas phase. The SLAC gas-phase MeV UED has achieved 65 fs root mean square temporal resolution, 0.63 Å spatial resolution, and 0.22 Å−1 reciprocal-space resolution. Such high spatial-temporal resolution has enabled the capturing of real-time molecular movies of fundamental photochemical mechanisms, such as chemical bond breaking, ring opening, and a nuclear wave packet crossing a conical intersection. In this paper, the design that enables the high spatial-temporal resolution of the SLAC gas phase MeV UED is presented. The compact design of the differential pump section of the SLAC gas phase MeV UED realized five orders-of-magnitude vacuum isolation between the electron source and gas sample chamber. The spatial resolution, temporal resolution, and long-term stability of the apparatus are systematically characterized.

To harness the full potential of the ultrafast electron diffraction (UED) and microscopy (UEM), we must know accurately the electron beam properties, such as emittance, energy spread, spatial-pointing jitter, and shot-to-shot energy fluctuation. Owing to the inherent fluctuations in UED/UEM instruments, obtaining such detailed knowledge requires real-time characterization of the beam properties for each electron bunch. While diagnostics of these properties exist, they are often invasive, and many of them cannot operate at a high repetition rate. Here, we present a technique to overcome such limitations. Employing a machine learning (ML) strategy, we can accurately predict electron beam properties for every shot using only parameters that are easily recorded at high repetition rate by the detector while the experiments are ongoing, by training a model on a small set of fully diagnosed bunches. Applying ML as real-time noninvasive diagnostics could enable some new capabilities, e.g., online optimization of the long-term stability and fine single-shot quality of the electron beam, filtering the events and making online corrections of the data for time-resolved UED, otherwise impossible. This opens the possibility of fully realizing the potential of high repetition rate UED and UEM for life science and condensed matter physics applications.

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 diffraction enables the study of molecular structural dynamics with atomic resolution at subpicosecond timescales, with applications in solid-state physics and rational drug design. Progress with ultrafast electron diffraction has been constrained by the limited transverse coherence of high-current electron sources. Photoionization of laser-cooled atoms can produce electrons of intrinsically high coherence, but has been too slow for ultrafast electron diffraction. Ionization with femtosecond lasers should in principle reduce the electron pulse duration, but the high bandwidth inherent to short laser pulses is expected to destroy the transverse coherence. Here we demonstrate that a two-colour process with femtosecond excitation followed by nanosecond photoionization can produce picosecond electron bunches with high transverse coherence. Ultimately, the unique combination of ultrafast ionization, high coherence and three-dimensional bunch shaping capabilities of cold atom electron sources have the potential for realising the brightness and coherence requirements for single-shot electron diffraction from crystalline biological samples.

Ultrafast electron diffraction (UED) is a powerful technique for observing atomic-scale structural dynamics in materials. Electron beam parameters—beam size, divergence, energy spread, and bunch length—determine spatio-temporal resolution. Traditional diagnostic methods require complex instrumentation that cannot be integrated into routine workflows, particularly for high-repetition-rate facilities. We present a machine learning approach enabling comprehensive, non-invasive extraction of electron beam parameters directly from diffraction patterns. Deep neural networks trained on physics-based simulations decode signatures that beam parameters imprint on diffraction images. The method exploits distinct physical mechanisms: geometric effects from beam size, angular distortions from divergence, chromatic aberrations from energy spread, and temporal convolution from bunch length. This enables bunch length measurement without dedicated temporal diagnostics—traditionally one of the most challenging parameters to access non-invasively. The trained models can be deployed across UED facilities using standard imaging detectors, democratizing access to advanced diagnostics. This approach eliminates expensive specialized equipment and enables real-time beam monitoring and optimization, enhancing experimental throughput and data quality for ultrafast materials characterization.

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.

The laser processing method has proven to produce surfaces while ensuring a low secondary electron yield of oxygen-free high-conductivity copper (OFHC) samples, making it attractive for electron cloud mitigation in next-generation particle accelerators and neutron tubes. In this work, the laser processing method is proposed to OFHC targets for the first time, aiming to reduce the secondary electrons in the neutron tube. The secondary electron yields (SEYs) and the thermal conductivities of Ti film and quaternary Ti–Zr–V–Hf films with unprocessed and laser processed OFHC substrates are investigated. Our results highlight that the thermal conductivity of Ti film with laser processed OFHC substrates is in proximity to the cleaned bare OFHC sample, especially at high temperatures. Moreover, the SEY of coated OFHC substrates are higher than that of coated laser processed substrates, which indicates the better secondary electron suppression capability of coated laser processed substrates. Therefore, the thermal conductivity and SEY results illustrate that the application of Ti and Ti–Zr–V–Hf coated laser processed OFHC can be considered to improve the neutron yield in neutron tubes in the future.

A compact accelerator-base source of THz Coherent Undulator Radiation (THz-CUR) at the Kyoto University has been developed with the purpose of providing intense quasimonochromatic and tunable THz-CUR at frequencies below 1 THz. The source is driven by a compact accelerator using a photocathode RF gun, which emits an electron beam with a fixed energy of 4.6 MeV and high bunch charge. The THz-CUR from our source can be generated when a compressed electron bunch passes through a planar undulator. In this study, we investigate the properties of this radiation, including the total radiation energy, spatial distribution, and power spectrum. With an electron beam of 160 pC bunch charge, the total radiation energy of THz-CUR at 0.16 THz was 1.2 µJ in the micropulse. The THz-CUR covering the frequency range from 0.16 THz to 0.65 THz could be produced by changing the magnetic field of the undulator at a 60 pC bunch charge. Due to the influence of the space charge forces causing the degradation of electron beam qualities, saturation of the radiation energy occurred, and the radiation power spectrum at a frequency of 0.65 THz could not be observed in the high charge condition (160 pC). The effects of bunch lengthening, energy spread, and emittance growth and the influence of the phase error on the generation of THz radiation are also discussed in this paper. This opens up the possibility of understanding the generation of THz-CUR and sheds further light on the enhancement of the radiation power.

Despite the fact that DOE provided only a fraction of the requested funds, the goals we defined in the proposal on which award ER41783 was based were essentially all met. This was partially due to the fact that other funding agencies, which supported our collaborators (especially from Italy and Korea) contributed as well, and partially due to the effective solutions that were developed to compensate for the fact that the detector we had proposed to build had to be scaled down. The performance of the SuperDREAM calorimeter is better than anything that has been built or proposed so far. This has of course not gone unnoticed in the scientific community. Scientists who are preparing experiments for the proposed new generation of particle accelerators (FCCee, CPEC,..) are all very seriously considering the technology developed in this project. Several new collaborations have formed which aim to adapt the dual-readout calorimeter principles to the demands of a 4 environment. Preliminary measurements using silicon photomultipliers as light sensors have already been carried out. This type of readout would make it possible to operate this detector in a magnetic field, and it would also allow for a longitudinal segmentation into electromagnetic and hadronic sections, if so desired. In addition, SiPM readout would eliminate the need for “forests” of fibers sticking out of the rear end of the calorimeter (Figure 1), and obtain an arbitrary fine lateral segmentation, which might be very important for recognizing electrons inside jets. The improvements in our understanding of the fundamental structure of matter and the forces that govern its behavior have always hinged on the availability of detectors that make it possible to explore the possibilities of new, more powerful particle accelerators to the fullest extent. We believe that the SuperDREAM project has created a quantum leap in detector technology, which may turn out to be crucially important for future discoveries in particle physics.

Herein, we propose a technique for megaelectronvolt ultrafast electron diffraction that can achieve a few-femtosecond time resolution. The technique modulates a relativistic electron beam using two radially polarized laser pulses of varying frequencies, which enable the condensation of the electrons into an ultrashort pulse. Moreover, our study shows that this technique can suppress the arrival-time jitter of the electron beam at the sample location. Numerical simulations show that using realistic laser parameters corresponding to the current gigawatt-power-level laser systems, a 3.3-fs (rms over 45% charge) electron beam can be obtained and the arrival-time jitter can be 1.35 fs (rms). This technique may have a considerable impact on ultrafast electron diffraction, microscopy, and other ultrafast research facilities that require few-femtosecond electron beams with a small time jitter.

In this article, we report our proof-of-principle design and experimental commissioning of a broadly tunable and low-cost transverse focusing lens system for MeV-energy electron beams. The lens system based on electromagnetic (EM) quadrupoles has been built as a part of the existing instrument for ultra-fast electron diffraction (UED) experiments at the Accelerator Test Facility II (ATF-II) at Brookhaven National Laboratory (BNL). We experimentally demonstrated the independent control of the size and divergence of the beam with the charge ranging from 1 to 13 pC. The charge density and divergence of the beam at the sample are the most important factors determining the quality of the Bragg-diffraction image (BDI). By applying the Robust Conjugate Directional Search (RCDS) algorithm for online optimization of the quadrupoles, the transverse beam size can be kept constant down to 75 µm from 1 to 13 pC. The charge density is nearly two orders of magnitude higher than the previously achieved value using a conventional solenoid. Using the BDI method we were able to extract the divergence of the beam in real-time and apply it to the emittance measurement for the first time. Our results agree well with simulations and with the traditional quadrupole scan method. The real-time divergence measurement opens the possibility of online optimization of the beam divergence (<0.2 mrad) at the sample with the increased beam charge. This optimization is crucial for the future development of single-shot ultra-fast electron microscope (UEM). Finally, we demonstrated BDI with significant improvement, up to 3 times higher peak intensity and 2 times sharper Bragg-diffraction peaks at 13 pC. The charge is now limited by the laser power and increasing charge may improve the quality of BDI further. The capability we demonstrated here provides us with opportunities for new sciences using near-parallel, bright and ultrafast electron beams for single-shot imaging, to directly visualize the dynamics of defects and nanostructured materials, or even record molecular movie, which are impossible using present electron-beam technologies.

Conventional metal-tip-based laser-driven electron sources are normally constrained by a trade-off between energy spread and pulse width due to optical-field-induced free electron acceleration. This makes it challenging to surpass the current state-of-the-art, which exhibits energy spreads exceeding 1 eV and pulse durations of hundreds of femtoseconds. Here we report an unconventional delayed emission from a one-dimensional carbon-nanotube-based electron source. By utilizing a special pump-probe approach, we apply 7-fs laser pulses to the carbon-nanotube emitters and observe free electron emission tens of femtoseconds after the pulse. This delayed emission results in a substantially reduced energy spread of approximately 0.3 eV and an electron pulse width of about 13 fs. Through time-dependent density functional theory calculations, we find that the delayed emission is driven by the interplay of collective oscillations and electron-electron interactions. Our results may provide a promising technology for developing cutting-edge ultrafast electron sources.

The structural anisotropy of layered materials leads to disparate lattice responses along different crystallographic directions following femtosecond photoexcitation. Ultrafast scattering methods are well-suited to resolving such responses, though probe size and specimen structure and morphology must be considered when interpreting results. Here we use ultrafast electron microscopy (UEM) imaging and diffraction to study the influence of individual multilayer terraces and few-layer step-edges on acoustic-phonon dynamics in 1T-TaS2 and 2H-MoS2. In TaS2, we find that a multilayer terrace produces distinct, localized responses arising from thickness-dependent c-axis phonon dynamics. Convolution of the responses is demonstrated with ultrafast selected-area diffraction by limiting the probe size and training it on the region of interest. This results in a reciprocal-space frequency response that is a convolution of the spatially separated behaviors. Sensitivity of phonon dynamics to few-layer step-edges in MoS2 and the capability of UEM imaging to resolve the influence of such defects are also demonstrated. Spatial frequency maps from the UEM image series reveal regions separated by a four-layer step-edge having 60.0 GHz and 63.3 GHz oscillation frequencies, again linked to c-axis phonon propagation. As with ultrafast diffraction, signal convolution is demonstrated by continuous increase of the size of the selected region of interest used in the analysis.