Tilted Snowplow Ponderomotive Electron Acceleration With Spatio-Temporally Shaped Ultrafast Laser Pulses

Abstract

We propose a novel scheme for using the ponderomotive force of a tilted ultrafast laser pulse to accelerate electrons in free space. The tilt of the intensity envelope results from the angular dispersion of the pulse's spectrum and slows down the interaction of the pulse with free electrons. The slower effective pulse velocity allows time for the electrons to accelerate from rest while remaining on the wave. We present both non-relativistic and relativistic analytic single-particle models in the adiabatic ponderomotive approximation, describing the process for an ideal infinite tilted pulse as well as a finite width beam. The analysis predicts the threshold intensity as a function of the pulse front tilt angle and shows that in the ideal case the output energy of the electrons is four times that of the ponderomotive potential at the capture threshold. Full-field simulations using the 2D OSIRIS 4.0 particle-in-cell code confirm the basic scheme. This tilted pulse acceleration scheme shows promise as a lab-scale method of accelerating electrons to the MeV level with good energy and angular resolution, to be used for ultrafast electron diffraction or injection into a second stage accelerator.

A 100-PW compressor based on single-pass single-grating pair

“…Figure 7(2c) shows that the introduced spatial chirp along the horizontal direction induces a slight pulse front tilt, which is the special property of spatiotemporal focusing. Usually, it has a detrimental impact for many laser-plasma interactions, but also it has an interesting impact on the energy and direction of accelerated electrons and the surface micromachining [27][28][29] . Figures 7(3a)-7(3d) show the intensity curves in the center along the horizontal axis of the figures above each one.…”

“…Figure 7(2c) shows that the introduced spatial chirp along the horizontal direction induces a slight pulse front tilt, which is the special property of spatiotemporal focusing. Usually, it has a detrimental impact for many laser–plasma interactions, but also it has an interesting impact on the energy and direction of accelerated electrons and the surface micromachining [ 27 – 29 ] . Figures 7(3a)–7(3d) show the intensity curves in the center along the horizontal axis of the figures above each one.

Figure 7 (1a) Near-field beam intensity without spatial chirp and spectral clipping.

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Space‐Time Wave Packets from Smith‐Purcell Radiation

“…We show that the space‐time wave profile can be further manipulated by shaping the electron bunch using existing nanotip emitters, [ 42–50 ] for instance, to control the position of the intensity hotspot. Our findings highlight the fundamental relationship between free‐electron radiation and space‐time wave packets, with potential applications in areas like ultrafast electron diffraction, [ 51–53 ] laser‐driven electron acceleration, [ 54–58 ] and Thomson scattering‐based X‐ray sources. [ 59–61 ]…”

Ponderomotive snowplow electron acceleration with high energy tilted ultrafast laser pulses

“…In recent years, there has been much interest in the optics community in spatio-temporal group velocity control [30][31][32]. Our group has worked with tilted pulses [33][34][35] and proposed using tilted pulses to control the interaction group velocity for ponderomotive acceleration 2 [36]. A 'flying focus' approach to ponderomotive acceleration, where a radial grating provides group velocity control through angular spectral dispersion in a Bessel focus, has also been proposed by Ramsey et al [37].…”

“…Sazegari [27] and Mendonca [28] both proposed using the dispersion of the plasma to reduce the group velocity; Robinson [29] recently showed that this requires unrealistically high density. Spatio-temporal shaping [36,37] provides for vacuum control over the group velocity. In our earlier paper [36], we developed a singleparticle theory which was confirmed by a 2-dimensional particle-in-cell code.…”