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.
Highlights
One of the issues facing the use of laser pulses to accelerate electrons is the relative difference in the velocity of the electrons and of the light wave
When a pulse is focused with angular chirp, the intensity is localized along the optical axis due to simultaneous spatial and temporal focusing (SSTF) [7, 8]
Angular spatial chirp leads to a complementary effect, known as simultaneous spatial and temporal focusing (SSTF)
Summary
One of the issues facing the use of laser pulses to accelerate electrons is the relative difference in the velocity of the electrons and of the light wave. When a pulse is focused with angular chirp, the intensity is localized along the optical axis due to simultaneous spatial and temporal focusing (SSTF) [7, 8] This intensity localization is extremely useful for micromachining [9, 10] and laser surgery [11]. We consider the acceleration at densities sufficiently low that space charge is not a factor; in later work we will consider higher densities, including the wakefield regime While it is beyond the scope of this initial paper to fully evaluate this scheme for applications, an ultrafast source of moderate energy electron pulses can have application to ultrafast electron diffraction [13], generation of coherent light, and injection into wakefield accelerators.
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