Radiation emission in laser-wakefields driven by structured laser pulses with orbital angular momentum

Abstract

High-intensity X-ray sources are invaluable tools, enabling experiments at the forefront of our understanding of materials science, chemistry, biology, and physics. Laser-plasma electron accelerators are sources of high-intensity X-rays, as electrons accelerated in wakefields emit short-wavelength radiation due to betatron oscillations. While applications such as phasecontrast imaging with these betatron sources have already been demonstrated, others would require higher photon number and would benefit from increased tunability. In this paper we demonstrate, through detailed 3D simulations, a novel configuration for a laser-wakefield betatron source that increases the energy of the X-ray emission and also provides increased flexibility in the tuning of the X-ray photon energy. This is made by combining two Laguerre-Gaussian pulses with non-zero net orbital angular momentum, leading to a rotation of the intensity pattern, and hence, of the driven wakefields. The helical motion driven by the laser rotation is found to dominate the radiation emission, rather than the betatron oscillations. Moreover, the radius of this helical motion can be controlled through the laser spot size and orbital angular momentum indexes, meaning that the radiation can be tuned fully independently of the plasma parameters.

A Method to Generate Vector Beams with Adjustable Amplitude in the Focal Plane

“…It involves Fourier transforming the input signal and adding a phase, in accordance with the dispersion relation of the waves and distance between input and output planes, to determine the output signal. Mathematically, it is described in Equation (8).…”

“…For implementation of the Debye-Wolf integral, we used Equations (11)- (14) from Reference [30], with wavefront aberration determined according to Equation (9) and the complex amplitude of the beam, denoted by V lens , determined using Equations (8) and (10). The complex amplitude of the beam in the output plane, in the point R, is proportional to the complex amplitude of the plane-wave with direction given by the ray vector of the ray that passes through R. Since there is a one-to-one correspondence between the position in the transversal plane, after the focusing element, and the transversal projection of ray vector t, we can index the complex amplitude depending on t as shown in Equation 11.…”

“…Structured light and especially helical beams and cylindrical vector beams are deemed suitable for applications like optical trapping and particle manipulation [2][3][4], stimulated emission depletion (STED) microscopy [5,6], laser micromachining [7], or charged particle acceleration in plasma [8].…”

Attosecond betatron radiation pulse train

“…Several recent studies suggest methods for enhancing betatron radiation emission, mostly based on the increase of the betatron oscillation amplitude. This can be achieved by an axial magnetic field, either self-generated or external 17 , 18 ; by a delayed modulation laser pulse 19 ; by the interaction of the electron beam with a high intensity optical lattice formed by the superposition of two transverse laser pulses 20 ; by using structured laser pulses 21 ; or by the interaction of electrons with the tail of the plasma wave drive pulse 22 – 25 .…”

Conservation of angular momentum in second harmonic generation from under-dense plasmas

“…It has also been theoretically demonstrated that the angular momentum of light can be transferred to a plasma wakefield using a spatiotemporally shaped beam [17]. There are proposals to use intense OAM laser pulses for creating a suitable wake for electron/positron acceleration [18,19,20], ion acceleration [21], radiation emission [22], and magnetic field generation [23,24]. Most of these applications require vortex beams with relativistic intensity.…”