Revisiting Experimental Signatures of the Ponderomotive Force

Abstract

The classical theory of single-electron dynamics in focused laser pulses is the foundation of both the relativistic ponderomotive force (RPF), which underlies models of laser-collective-plasma dynamics, and the discovery of novel strong-field radiation dynamics. Despite this bedrock importance, consensus eludes the community as to whether acceleration of single electrons in vacuum has been observed in experimental conditions. We analyze an early experiment on the RPF with respect to several features that were neglected in modeling and that can restore consistency between theory predictions and experimental data. The right or wrong pulse profile function, laser parameters, or initial electron distribution can each make or break the agreement between predictions and data. The laser phase at which the electron’s interaction with the pulse begins has a large effect, explaining why much larger energies are achieved by electrons liberated in the focal region by photoionization from high-Z atoms and by electrons ejected from a plasma mirror. Finally, we compute the difference in a typical electron spectrum arising from fluctuating focal spot size in state-of-the-art ultra-relativistic laser facilities. Our results emphasize the importance of thoroughly characterizing laser parameters in order to achieve quantitatively accurate predictions and the precision required for discovery science.

Ponderomotive snowplow electron acceleration with high energy tilted ultrafast laser pulses

“…It is the dominant mechanism for setting up the space charge distribution in the laser wakefield accelerator (LWFA), first proposed by Tajima and Dawson [11] and demonstrated by a number of groups [12][13][14]. Accelerating electrons more directly by the laser beam without reliance on plasma effects has drawn much interest using several approaches (though the mechanisms listed here can be blended [15]). Many previous proposals and demonstrations are phase-sensitive: for example, excess kinetic energy resulting from tunneling ionization of high-charge ions can seed subsequent ponderomotive acceleration out of the focal spot [16,17]; a beam can be structured to have a longitudinal electric field [18][19][20][21]; electrons can be injected at an angle into a Gaussian beam focus [22], and the nonlinear (⃗ v × ⃗ B) Lorentz force can be exploited for acceleration [18,[23][24][25].…”

Data on Stark Broadening of N VI Spectral Lines

“…On the first BN target is focused a laser beam, which generates protons that collide with the other BN target triggering nuclear reactions. In order to optimize the fusion yield, a plasma diagnostic is needed [6] and Stark broadening data for N VI may be useful for such purposes. Also, the presence of the multiply charged boron ions B IV, B V and B VI is clearly identified [7], so that broadening of N VI by collisions with multiply charged boron ions is also of interest.…”