Collimation of electrons via three-dimensional spatial intensity shaping of laser focal volume

Controlling the physics of laser-matter interactions depends on the properties of matter and the characteristic parameters of the laser beam. The dynamics of electrons in the focus of a laser pulse is governed by the spatial intensity profile. The time-averaged ponderomotive force arising from the inhomogeneities in the focal intensity distribution leads to the ejection of a diverging bunch of electrons. This research deals with reducing the divergence angle of the ejected electrons by spatially shaping the three-dimensional focal distribution of a high-intensity laser. A novel laser focus with a centrally peaked transverse focal intensity, transforming into an annular distribution along the laser-propagation direction, has been experimentally demonstrated. The longitudinal profile of such a shaped laser focal volume is approximately in the form of a "horseshoe". The horseshoe focus was realized experimentally by an incoherent, coaxial combination of Laguerre-Gaussian and Gaussian modes generated from segmented optical elements. A beam-shaping optical system consisting of reflective segmented optical elements was designed, custom fabricated and fielded on a 10 J, 0.5 ps multi-terawatt laser system. The near-field of the laser pulse was modulated by the reflective beam-shaping system, generating the horseshoe focus with an estimated peak intensity of 8 x 1018 W/cm2. Horseshoe and Gaussian focal volumes generated electrons by field-ionization of a low density, noble gas-jet target. The energy and angular distribution of electrons ejected from the focal volume was measured with a multi-angle magnetic electron spectrometer. The experimental data indicates that electrons generated from the horseshoe focus are ejected into lower angles (by ≈ 10°) with respect to the laser-propagation direction and higher energies (by ≈ 0.1 MeV) compared to those from the Gaussian focus of the same peak intensity. This corresponds to a relative decrease in minimum ejection angle and relative increase in energies of about 18% and 25% respectively. This experimental result validates the possibility of collimating electrons by three-dimensional spatial intensity shaping of the focal volume. The observed collimation is attributed to the focusing ponderomotive forces from the intensity cavity region of the horseshoe focus that are directed towards the laser axis. Monte-Carlo simulations of the ejected electron distribution reproduce the measured data and indicate that for ultra-high peak intensities (> 1020 W/ cm2), the collimation and focusing effect due to spatial shaping will be significantly enhanced, leading to forward-ejection of electrons to less than 20° with respect to the laser axis. Such a forward-directed, narrow divergence electron bunch may be useful for gamma-ray radiography, fast-ignition and other fundamental studies requiring high-brightness electron-beams.