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The application high intensity ultrafast lasers to compact plasma-based electron accelerators has recently been an extremely active area of research. Here, for the first time, we show experimentally and theoretically that carefully sculpting an intense ultrafast pulse in the spatio-temporal domain allows ponderomotive pressure to be used for direct acceleration of electron bunches from rest to relativistic energies. With subluminal group velocity and above-threshold intensity, a laser pulse can capture and accelerate electrons, pushing on them like a snowplow. Acceleration of electrons from rest requires a substantial reduction of group velocity. In this demonstration experiment, we achieve a group velocity of ∼0.6c in a tilted pulse by focusing the output of a novel asymmetric pulse compressor we developed for the petawatt-class ALEPH system at Colorado State University. This direct laser-electron approach opens a route towards exploiting optical spatio-temporal control techniques to sculpt electron beams with desired properties such as narrow energy and angular distributions. The tilted-pulse snowplow technique can be scaled from small-scale to facility-scale amplifiers to produce short electron bunches in the 10 keV−10 MeV range for applications including ultrafast electron diffraction and efficient injection into laser wakefield accelerators for acceleration beyond the GeV level.

 

While there has been success in Wakefield acceleration of electrons, there are a number of applications that could benefit from acceleration to modest energy (~MeV) by the laser field, for example, ultrafast electron diffraction and injection into higher-energy laser-driven accelerators. Here we outline our scheme for ponderomotive acceleration of electrons (and in principle, positrons) in which we control the group velocity of ultrafast pulses through pulse front tilt. Provided the intensity is above the threshold for capture of electrons, the leading part of the pulse front effectively acts like a moving mirror whose shape is controlled by the spatio-temporal topology of the intensity profile. Our analytic models of the propagation of spatially-chirped beams, simple relativistic single-particle models of the laser-electron interaction and our implementation of these beams in particle-in-cell simulations help to predict the output electron energy and direction. We are preparing experiments on the ALEPH laser system at Colorado State University in which we will use the diagnostic techniques that we have developed to align our scaled-up design of a high-energy pulse compressor that will deliver spatially chirped pulses. 

The study of the physics of naturally occurring electrostatic discharges (ESDs) at early times is challenged by the difficulty in overcoming pre-trigger requirements of laser probes. In this work, ultraviolet (UV) pulses from a diode-pumped solid-state, Q-switched laser system are used to trigger ESDs. We use an open-air spark gap with a gap voltage held near threshold. The laser intensity is in the microjoule range so that seed electrons are produced through the photoelectric effect on the cathode. In contrast to laser-triggered spark gaps, the resulting discharges are anticipated to be very similar to those produced by random seed electrons. The triggering produces ESDs with a yield of >65%. While there is ~10ns jitter, co-recording of the current pulse will allow for time-resolved experimental diagnostics with ns timing resolution. Early results show a relatively short delay between triggering and the arc discharge (~100ns), indicating that collisional UV generation is a more likely source of secondary electrons than ion return current. Our experiment will be compared to our numerical models for plasma temperature and species evolution measurements in ESDs. Future experiments will be completed in a discharge chamber which allows for control of the gas composition and pressure. 

Using a novel pulse compressor for the CSU ALEPH facility, we demonstrate direct ponderomotive acceleration of electrons with 1.5J, tilted ultrafast pulses. The < 500keV electrons are directed normal to the tilted pulse front as predicted. 

We introduce a self-referenced system that retrieves the full spatio-temporal profile of an ultrashort pulse using a Shack-Hartmann and second harmonic generation FROG. The key feature is the precise co-location of a spectral phase measurement at one spatial position with the spectrally resolved spatial measurements.