Laser-plasma accelerator group

Abstract:

Betatron radiation is produced in laser plasma accelerators when the electrons are accelerated and simultaneously wiggle across the propagation axis [Rousse et al., Phys. Rev. Lett. 93, 135005 (2004)]. The mechanisms of electron acceleration and x-ray radiation production follow different scaling laws [Corde et al., Rev. Mod. Phys. 85, 1–48 (2013)], and the brightest x-ray radiation is often produced for an electron beam with a lower quality in terms of energy and divergence. Here, we report a laser-driven betatron x-ray source where the plasma density profile is tailored in order to separate the acceleration and wiggler stages, which allows for the independent optimizations of acceleration and x-ray production. We demonstrate this concept experimentally and show that the betatron photon energy can be controlled by adjusting the length of the plasma wiggler. This scheme offers a path to overcome the limitations of conventional betatron sources, enabling the production of bright, stable, energetic, and collimated x-ray beams.

Abstract:

Simultaneous space-time focusing (SSTF) is sometimes claimed to reduce the longitudinal extent of the high-intensity region near the focus, in contradiction to the original work on this topic. Here we seek to address this confusion by using numerical and analytical methods to investigate the degree of localization of the spatio-temporal intensity of an SSTF pulse. The analytical method is found to be in excellent agreement with numerical calculations and yields, for bi-Gaussian input pulses, expressions for the three-dimensional spatio-temporal intensity profile of the SSTF pulse, and for the on-axis bandwidth, pulse duration, and pulse-front tilt (PFT) of the SSTF pulse. To provide further insight, we propose a method for determining the transverse input profile of a non-SSTF pulse with equivalent spatial focusing. We find that the longitudinal variations of the peak axial intensities of the SSTF and spatially equivalent (SE) pulses are the same, apart from a constant factor, and hence that SSTF does not constrain the region of high intensity more than a non-SSTF pulse with equivalent focusing. We demonstrate that a simplistic method for calculating the pulse intensity exaggerates the degree of intensity localization, unless the spatio-temporal couplings inherent to SSTF pulses are accounted for.

Abstract:

<jats:p>Simultaneous space-time focusing (SSTF) is sometimes claimed to reduce the longitudinal extent of the high-intensity region near the focus, in contradiction to the original work on this topic. Here we seek to address this confusion by using numerical and analytical methods to investigate the degree of localization of the spatio-temporal intensity of an SSTF pulse. The analytical method is found to be in excellent agreement with numerical calculations and yields, for bi-Gaussian input pulses, expressions for the three-dimensional spatio-temporal intensity profile of the SSTF pulse, and for the on-axis bandwidth, pulse duration, and pulse-front tilt (PFT) of the SSTF pulse. To provide further insight, we propose a method for determining the transverse input profile of a non-SSTF pulse with equivalent spatial focusing. We find that the longitudinal variations of the peak axial intensities of the SSTF and spatially equivalent (SE) pulses are the same, apart from a constant factor, and hence that SSTF does not constrain the region of high intensity more than a non-SSTF pulse with equivalent focusing. We demonstrate that a simplistic method for calculating the pulse intensity exaggerates the degree of intensity localization, unless the spatio-temporal couplings inherent to SSTF pulses are accounted for.</jats:p>