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Register a new userRadiation from laser-microplasma-waveguide interactions in the ultra-intense regime
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When a high-contrast ultra-relativistic laser beam enters a micro-sized plasma waveguide, the pulse energy is coupled into waveguide modes, which remarkably modifies the interaction of electrons and electromagnetic wave. The electrons that pulled out of walls form a dense helical bunch inside the channel are efficiently accelerated by the transverse magnetic modes to hundreds of MeVs. In the mean time, the asymmetry in the transverse electric and magnetic fields provides significant wiggling that leads to a bright, well-collimated emission of hard X-rays. In this paper, we present our study on the underlying physics in the aforementioned process using 3D particle-in-cell simulations. The mechanism of electron acceleration and the dependence of radiation properties on different laser plasma parameters are addressed. A theoretical analysis model and basic scalings for X-ray emission are also presented by considering the lowest optical modes in the waveguide, which is adequate to describe the basic observed phenomenon. In addition, the effects of high order modes as well as laser polarization are also qualitatively discussed. The considered X-ray source have promising features that might serve as a competitive candidate for future tabletop synchrotron source.
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We discuss radiation reaction effects on charges propagating in ultra-intense laser fields. Our analysis is based on an analytic solution of the Landau-Lifshitz equation. We suggest to measure radiation reaction in terms of a symmetry breaking parameter associated with the violation of null translation invariance in the direction opposite to the laser beam. As the Landau-Lifshitz equation is nonlinear the energy transfer within the pulse is rather sensitive to initial conditions. This is elucidated by comparing colliding and fixed target modes in electron laser collisions.
Plasma high harmonics generation from an extremely intense short-pulse laser is explored by including the effects of ion motion, electron-ion collisions and radiation reaction force in the plasma dynamics. The laser radiation pressure induces plasma ion motion through the hole-boring effect resulting into the frequency shifting and widening of the harmonic spectra. Classical radiation reaction force slightly mitigates the frequency broadening caused by the ion motion. Based on the results and physical considerations, parameter maps highlighting optimum regions for generating a single intense attosecond pulse and coherent XUV radiations are presented.
The description of the dynamics of an electron in an external electromagnetic field of arbitrary intensity is one of the most fundamental outstanding problems in electrodynamics. Remarkably, to date there is no unanimously accepted theoretical solution for ultra-high intensities and little or no experimental data. The basic challenge is the inclusion of the self-interaction of the electron with the field emitted by the electron itself - the so-called radiation reaction force. We report here on the experimental evidence of strong radiation reaction, in an all-optical experiment, during the propagation of highly relativistic electrons (maximum energy exceeding 2 GeV) through the field of an ultra-intense laser (peak intensity of $4times10^{20}$ W/cm$^2$). In their own rest frame, the highest energy electrons experience an electric field as high as one quarter of the critical field of quantum electrodynamics and are seen to lose up to 30% of their kinetic energy during the propagation through the laser field. The experimental data show signatures of quantum effects in the electron dynamics in the external laser field, potentially showing departures from the constant cross field approximation.
Laser-accelerated electron beams have been created at a kHz repetition rate from the {it reflection} of intense ($sim10^{18}$ W/cm$^2$), $sim$40 fs laser pulses focused on a continuous water-jet in an experiment at the Air Force Research Laboratory. This paper investigates Particle-in-Cell (PIC) simulations of the laser-target interaction to identify the physical mechanisms of electron acceleration in this experiment. We find that the standing-wave pattern created by the overlap of the incident and reflected laser is particularly important because this standing wave can inject electrons into the reflected laser pulse where the electrons are further accelerated. We identify two regimes of standing wave acceleration: a highly relativistic case ($a_0~geq~1$), and a moderately relativistic case ($a_0~sim~0.5$) which operates over a larger fraction of the laser period. In previous studies, other groups have investigated the highly relativistic case for its usefulness in launching electrons in the forward direction. We extend this by investigating electron acceleration in the {it specular (back reflection) direction} and over a wide range of intensities ($10^{17}-10^{19}$ W cm$^{-2}$).
Backward terahertz radiation can be produced by a high-intensity laser normally incident upon an underdense plasma. It is found that terahertz radiation is generated by electrons refluxing along the bubble shell. These shell electrons have similar dynamic trajectories and emit backward radiations to vacuum. This scheme has been proved through electron dynamic calculations as well as by using an ionic sphere model. In addition, the bubble shape is found to influence the radiation frequency, and this scheme can be implemented in both uniform and up-ramp density gradient plasma targets. The terahertz radiation may be used for diagnosing the electron bubble shape in the interaction between an intense laser and plasma. All results are presented via 2.5 dimensional particle-in-cell simulations.
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