The spatio-temporal and polarisation properties of intense light is important in wide-ranging topics at the forefront of intense light-matter interactions, including laser-driven particle acceleration. In the context of experiments to optimize transparency-enhanced ion acceleration in expanding ultrathin foils, we investigate the polarisation and temporal properties of intense light measured at the rear of the target. An effective change in the angle of linear polarisation of the light results from a superposition of coherent radiation, generated by a directly accelerated bipolar electron distribution, and the light transmitted due to the onset of relativistic self-induced transparency. Simulations show that the generated light has a high-order transverse electromagnetic mode structure in both the first and second laser harmonics that can evolve on intra-pulse time-scales. The mode structure and polarisation state vary with the interaction parameters, opening up the possibility of developing this approach to achieve dynamic control of structured light fields at ultrahigh intensities [1].
We also report on frequency-resolved optical gating measurements of the light which demonstrate a novel and simple approach to diagnose the time during the interaction at which the foil becomes transparent to the laser light. This is a key parameter for optimising ion acceleration in expanding ultrathin foils. Coherent transition radiation produced at the foil rear interferes with laser light transmitted through the foil producing spectral fringes. The fringe spacing enables the relative timing of the onset of transmission with respect to the transition radiation generation to be determined. This self-referencing approach to spectral interferometry provides a route to optically controlling and optimising ion acceleration from ultrathin foils undergoing transparency [2].
[1] M.J. Duff et al., Scientific Reports 10, 105 (2020)
[2] S.D.R. Williamson et al., Phys. Rev. Applied 14, 034018 (2020)
The radiation pressure of next generation high-intensity lasers could efficiently accelerate ions to GeV energies. However, nonlinear quantum-electrodynamic effects play an important role in the interaction of these lasers with matter. We show that these quantum-electrodynamic effects lead to the production of a critical density pair-plasma which completely absorbs the laser pulse and consequently reduces the accelerated ion energy and efficiency by 30-50%.
An investigation of the effects of the radiation reaction force on radiation pressure acceleration is presented. Through 1D(3V) PIC code simulations, it is found that radiation reaction causes a decrease in the target velocity during the interaction of an ultra-intense laser pulse with a solid density thin foil of varying thickness. This change in the target velocity can be related to the loss of backwards-directed electrons due to cooling and reflection in the laser field. The loss of this electron population changes the distribution of the emitted synchrotron radiation. We demonstrate that it is the emission of radiation which leads to the observed decrease in target velocity. Through a modification to the light sail equation of motion (which is used to describe radiation pressure acceleration in thin foils), which accounts for the conversion of laser energy to synchrotron radiation, we can describe this change in target velocity. This model can be tested in future experiments with ultra-high intensity lasers, and will lead to a better understanding of the process of relativistically induced transparency in the new intensity regime.
We consider interaction of two counter-propagating homogeneous sub-relativistic plasma beams with no external magnetic field applied. In numerical simulations performed with a particle-in-cell code three stages of evolution can be identified. The shock formation is initiated with development of the electron two stream and Weibel-like micro-instabilities, followed by fast electron heating and ion deceleration and heating. We present a theoretical analysis of the instabilities development and the nonlinear saturation to explore the origins of the heating and the magnetic field generation. From the dispersion relation, the instabilities are characterized and the dependence on the electron temperature and ion velocity is studied. The growth rate and the characteristic scales of instability are compared to simulation results.
Radiation energy losses of electrons in ultra-intense laser fields constitute a process of major importance when
considering laser-matter interaction at intensities of the order of and above 1022 W/cm2. Radiation losses can strongly modify the electron (and ion) dynamics, and are associated with intense and directional emission of high energy photons. Accounting for such effects is therefore necessary for modeling of, electron and ion acceleration and creation of secondary photon on the forthcoming ultra-high power laser facilities. To account for radiation losses in the particle-in-cell code PICLS, we have introduced the radiation friction force using a renormalized
Lorentz-Abraham-Dirac model.10 Here, we present a study of the effect of radiation friction on the electron and photon energy distribution in a semi-infinite and overdense plasma. A possibillity to create a collisonless shock
using an ultra intense laser field, in the context of laboratory astrophysics is discussed. The influence of the radiation reaction on the plasma dynamics is demonstrated.
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