As
the field of laser wakefield acceleration (LWFA) 1 matures, emphasis is
shifting toward utilizing LWFA as a source of electron beams and x-rays for
applications.  There is an increasing
emphasis on producing electron beams from LWFAs that can meet the stringent
beam requirements (narrow divergence, small emittance, narrow energy spread)
necessary for use in staged plasma accelerators 2 and free electron lasers. Simultaneously,
betatron x-rays from LWFA are being utilized for applications 3-7, which
places an emphasis on optimizing LWFA to produce these x-rays.  Even though these applications require
optimization of different electron beam properties, all applications benefit
from a more-complete understanding of the dynamics of electron energy gain in
LWFA and how those dynamics affect properties such as electron beam energy, divergence,
source size, shape, and energy spread.

For the
range of plasma densities (mid-1018 to a few 1019 cm-3)
and laser pulse durations (35-45 femtoseconds full width at half maximum) that
are typically used in many current LWFA experiments in the forced or
quasi-blowout regimes, the laser pulse length is on the order of the wake wavelength;
therefore it may occupy the entire first bucket of the wake.  In such experiments, the wakefield structure
has a desirable transverse and longitudinal field structure for generating a self-injected
electron bunch, but it also has the conditions needed for direct laser
acceleration (DLA) 8, 9 if there is an overlap between the accelerating
electrons and the transverse electric field of the laser pulse 10-16.  It is therefore important to understand the
role that not only the longitudinal electric field of the wake, but also the
other fields—namely, the transverse fields of the ion column and of the laser
itself—play in determining the ultimate energy gained by the electrons.  In this paper, we show through experiments
direct, observable signatures in the produced electron beams that indicate that
DLA makes a significant contribution to the electrons’ energy in LWFAs operated
in such a configuration. Three-dimensional (3D) particle-in-cell (PIC) simulations are used
to elucidate the energy dynamics that lead to this contribution.

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