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Laser-driven electron acceleration in infinite vacuum

I first review basic models for laser-plasma
interaction that explain electron acceleration and beam confinement
in plasma. Next, I discuss ponderomotive electron acceleration in
infinite vacuum, showing that the transverse scattering angle of
the accelerated electron may be kept small with a proper choice of
parameters. I then analyze the direct (a.k.a. linear) acceleration
of an electron in infinite vacuum by a pulsed radially-polarized
laser beam, consequently demonstrating the possibility of
accelerating an initially-relativistic electron in vacuum without
the use of ponderomotive forces or any optical devices to terminate
the laser field. As the Lawson-Woodward theorem has sometimes been
cited to discount the possibility of net energy transfer from a
laser pulse to a relativistic particle via linear acceleration in
unbounded vacuum, I derive an analytical expression (which I verify
with numerical simulation results) defining the regime where the
Lawson-Woodward theorem in fact allows for this. Finally, I propose
a two-color laser-driven direct acceleration scheme in vacuum that
can achieve electron acceleration exceeding 90% of the one-color
theoretical energy gain limit, over twice of what is possible with
a one-color pulsed beam of equal total energy and pulse
duration.