We discuss our experiments that apply ultrafast electron diffraction (UED) to study structural dynamics of the phase transition in single crystal tantalum ditelluride, TaTe2, a quasi-2D quantum material which exhibits a trimer superstructure at cryogenic temperatures. Intense near-infrared (NIR) pulses at 1030 nm are employed to quench the low temperature, atomically ordered state and the process is captured by ultrashort bunches of electrons as a function of pump-probe time delay. The diffraction signatures of the trimer superstructure recover on picosecond time scales. These measurements of TaTe2 underscore moreover the applicability of the HiRES UED beamline at Lawrence Berkeley National Laboratory (LBNL) to probe ultrafast structural dynamics of complex materials.
Nanoscale electron pulses are increasingly in demand, including as probes of nanoscale ultrafast dynamics and for emerging light source and lithography applications. Using electromagnetic simulations, we show that gold plasmonic lenses as multiphoton photoemitters provide unique advantages, including emission from an atomically at surface, nanoscale pulse diameter regardless of laser spot size, and femtosecond-scale response time. We then present fabrication of prototypes with sub-nm roughness via e-beam lithography, as well as electro-optical characterization using cathodoluminescence spectromicroscopy. Finally, we introduce a DC photogun at LBNL built for testing ultrafast photoemitters. We discuss measurement considerations for ultrafast nanoemitters and predict that we can extract tens of pA photocurrent from a single plasmonic lens using a Ti:Sa oscillator. Altogether, this lays the groundwork to develop and test a broad class of plasmon-enhanced ultrafast nanoemitters.
Ultrafast electron diffraction (UED) has become a leading technique for investigation of structural dynamics in solids providing high spatial and temporal resolutions. Radio frequency (RF) based photoinjectors providing Mega-electron-volt (MeV) scale electron beams are improving the source brightness and instrument versatility and are largely responsible for advancement of the field of structural dynamics. At Lawrence Berkeley National Laboratory (LBNL), an RF photoinjector gun for ultrafast structural studies using UED has been in development and is now producing high-quality scientific results. Here we describe some factors that enable UED of materials at LBNL and present some exemplary results.
We report on experimental activities on HiRES, a novel ultrafast electron diffraction beamline under development at LBNL. The instrument provides high-flux of relativistic electron pulses with sub-picosecond duration, which are then shaped in transverse and longitudinal phase space producing small spot sizes with femtosecond resolution. Alternatively beam shaping can be used to achieve large lateral coherence lengths for chemical and biological applications.
After the formidable results of X-ray 4th generation light sources based on free electron lasers around the world, a new revolutionary step is undergoing to extend the FEL performance from the present few hundred Hz to MHz-class repetition rates. In such facilities, temporally equi-spaced pulses will allow for a wide range of previously non-accessible experiments. The Advanced Photo-injector EXperiment (APEX) at the Lawrence Berkeley National Laboratory (LBNL), is devoted to test the capability of a novel scheme electron source, the VHF-Gun, to generate the required electron beam brightness at MHz repetition rates. In linac-based FELs, the ultimate performance in terms of brightness is defined at the injector, and in particular, cathodes play a major role in the game. Part of the APEX program consists in testing high quantum efficiency photocathodes capable to operate at the conditions required by such challenging machines. Results and status of these tests at LBNL are presented.
The APEX electron source at LBNL combines the high-repetition-rate with the high beam brightness typical of photoguns, delivering low emittance electron pulses at MHz frequency. Proving the high beam quality of the beam is an essential step for the success of the experiment, opening the doors of the high average power to brightness-hungry applications as X-Ray FELs, MHz ultrafast electron diffraction etc.. As first step, a complete characterization of the beam parameters is foreseen at the Gun beam energy of 750 keV. Diagnostics for low and high current measurements have been installed and tested, and measurements of cathode lifetime and thermal emittance in a RF environment with mA current performed. The recent installation of a double slit system, a deflecting cavity and a high precision spectrometer, allow the exploration of the full 6D phase space. Here we discuss the present layout of the machine and future upgrades, showing the latest results at low and high repetition rate, together with the tools and techniques used.
Polarized X-ray pulses at 0.6 Å have been generated via head-on collision of a laser pulse from the high-field laser
facility at Daresbury with a 30 MeV electron bunch in the ALICE energy recovery linear accelerator. The angular
distribution of the backscattered X rays was obtained in single-shot using a scintillation screen. The temporal profile of
the X ray yield as a function of the time delay between the laser pulse and electron bunch was measured and agreed well
with that expected from the collision point dependence of the laser-electron beam longitudinal overlap.
The SPARX project consists in an X-ray-FEL facility jointly supported by MIUR (Research Department of Italian
Government), Regione Lazio, CNR, ENEA, INFN and Rome University Tor Vergata. It is the natural extension of the
ongoing activities of the SPARC collaboration. The aim is the generation of electron beams characterized by ultra-high
peak brightness at the energy of 1 and 2 GeV, for the first and the second phase respectively. The beam is expected to
drive a single pass FEL experiment in the range of 13.5-6 nm and 6-1.5 nm, at 1 GeV and 2 GeV respectively, both in
SASE and SEEDED FEL configurations. A hybrid scheme of RF and magnetic compression will be adopted, based on
the expertise achieved at the SPARC high brightness photoinjector presently under commissioning at Frascati INFNLNF
Laboratories. The use of superconducting and exotic undulator sections will be also exploited. In this paper we
report the progress of the collaboration together with start to end simulation results based on a combined scheme of RF