Superponderomotive regime of tunneling ionization

Abstract

Ultra-relativistic photoelectron spectra exhibit unexpected characteristics in a paraxial laser focus. The photoelectron energy scales super-ponderomotively, and the usual parabolic momentum distribution is distorted into a variety of intricate patterns, depending on the location of the ion. These patterns include discrete contours, which in some cases can be easily identified with a particular sub-cycle burst of ionization current. An analytical formula for the maximum photoelectron energy in a paraxial radiation field is given, and high resolution momentum distributions with narrowly peaked features are presented. These narrowly peaked features suggest application to electron injection into plasma based accelerators.

Special unitary particle pusher for extreme fields

“…Vay [3], Higuera-Cary (HC) [4], quadratic-unitary, exact-unitary, and SO (3,1) pushers. The advantages of the SO(3,1) pusher, which is mathematically equivalent to the exact-unitary pusher, are discussed in [5] and in the supplementary material of [9] (the term "covariant pusher" is used in these references). These articles show that the SO(3,1) pusher gives the correct solution in orders of magnitude fewer steps than the standard Boris pusher.…”

“…Electromagnetic particle-in-cell codes utilize a variety of algorithms to update the particle momentum [1,2,3,4,5,9,6]. The accuracy of the standard Boris pusher [1,8] is reduced when eA/mc 2 1, where A is the vector potential [7].…”

“…References [5,9] give a closed form expression for the time translation operator that is exact in a uniform field, and therefore eliminates the condition on the time step (1). The primary disadvantage of the expression in [5,9] is that it is unwieldy.…”

“…References [5,9] give a closed form expression for the time translation operator that is exact in a uniform field, and therefore eliminates the condition on the time step (1). The primary disadvantage of the expression in [5,9] is that it is unwieldy. Reference [6] gives another closed form expression for the time translation operator that is exact in a uniform field, but simplifies the expression by transforming to a special Lorentz frame.…”

“…The expressions from [5,9,6] operate on four-vectors, which are representations of the group of transformations known as SO (3,1). As is well known, the group SU(2)×SU( 2) is isomorphic to SO (3,1).…”