Experiments were performed on the Omega EP laser facility to study laser pulse propagation, channeling phenomena and electron acceleration from high-intensity, high-power laser interactions with underdense plasma. A CH plasma plume was used as the underdense target and the interaction of the laser pulse channeling through the plasma was imaged using proton radiography. High-energy electron spectra were measured for different experimental laser parameters. Structures observed along the channel walls are interpreted as having developed from surface waves, which are likely to serve as an injection mechanism of electrons into the cavitated channel for acceleration via direct laser acceleration mechanisms. Two-dimensional particle-in-cell simulations give good agreement with these channeling and electron acceleration phenomena. IntroductionLaser-based plasma accelerators have become a highly promising alternative to conventional accelerators in recent years. Wakefield acceleration can be driven by a laser pulse or particle beam propagating through an underdense plasma, which produces a plasma wave with a phase velocity close to the speed of light, and can transfer energy to 'surfing' electrons trapped in the waves [1][2][3]. With the reduction of the laser pulse duration, a regime where the laser pulse duration matched the plasma frequency was achieved and this allowed significant advances in controlling and producing narrow energy spread electron beams [4][5][6]. Furthermore, transverse oscillations of the high-energy electron beams within the plasma wave structure leads to a very bright, directional x-ray source [7]. Using laser pulses of longer pulse duration produce a more complicated interaction, with the leading edge of the pulse producing plasma waves. However, if the laser pulse is intense enough, the ponderomotive force of the laser pulse expels the electrons from the regions of highest intensity to form a cavitated channel. Once the channel has formed, plasma waves are no longer present, but electrons are able to gain energy through direct laser acceleration (DLA) mechanisms [8,9].The study of this channel formation, the energy exchange from the laser pulse to electrons and the subsequent transport and dissipation of the energy is of specific relevance to the hole boring fast ignition inertial confinement fusion scheme [10]. A high-intensity laser pulse is used to form a channel though the low-density coronal plasma of the compressed fuel, so that a second laser pulse can be guided to the dense fuel and strongly heat the electrons in this region to ignite the system. The aim of this study is to gain a better understanding of the energy transfer and electron heating mechanisms in such systems.Several DLA mechanisms have been identified using particle-in-cell (PIC) simulations to accelerate electrons to energies exceeding the ponderomotive potential. The transfer of laser energy to the electrons can occur either through a stochastic acceleration mechanism [11,12], or via the coupling of quasi-static electric o...
Superponderomotive-energy electrons are observed experimentally from the interaction of an intense laser pulse with a relativistically transparent target. For a relativistically transparent target, kinetic modeling shows that the generation of energetic electrons is dominated by energy transfer within the main, classically overdense, plasma volume. The laser pulse produces a narrowing, funnel-like channel inside the plasma volume that generates a field structure responsible for the electron heating. The field structure combines a slowly evolving azimuthal magnetic field, generated by a strong laser-driven longitudinal electron current, and, unexpectedly, a strong propagating longitudinal electric field, generated by reflections off the walls of the funnel-like channel. The magnetic field assists electron heating by the transverse electric field of the laser pulse through deflections, whereas the longitudinal electric field directly accelerates the electrons in the forward direction. The longitudinal electric field produced by reflections is 30 times stronger than that in the incoming laser beam and the resulting direct laser acceleration contributes roughly one third of the energy transferred by the transverse electric field of the laser pulse to electrons of the super-ponderomotive tail.
The line emission of n=7-->3, 6-->3, 5-->3, and 4-->3 transitions in Ni- to Kr-like gold ions produced in the Livermore electron beam ion traps EBIT-I and EBIT-II has been recorded with an x-ray crystal spectrometer and a photometrically calibrated microcalorimeter. The plasmas had either monoenergetic electron beams with E(beam)=2.66, 3.53, or 4.54 keV or an experimentally simulated thermal electron distributions with T(e)=2.5 keV. The electron densities were approximately 10(12)cm(-3). The measured spectra have been compared to atomic structure calculations and synthetic spectra provided by the Hebrew University Lawrence Livermore Atomic Code atomic data package. Line identifications and accurate photon energy measurements have been made for many collisionally excited transitions. Approximately 140 lines have been identified in nine charge states. Agreement within 20-30 % exists between the measured and modeled line intensities for most lines excited by the monoenergetic electron beam plasmas, although some larger discrepancies can be found for some weaker features.
Experimental measurements using the OMEGA EP laser facility demonstrated direct laser acceleration (DLA) of electron beams to (505 ± 75) MeV with (140 ± 30) nC of charge from a low-density plasma target using a 400 J, picosecond duration pulse. Similar trends of electron energy with target density are also observed in self-consistent two-dimensional particle-in-cell simulations. The intensity of the laser pulse is sufficiently large that the electrons are rapidly expelled along the laser pulse propagation axis to form a channel. The dominant acceleration mechanism is confirmed to be DLA and the effect of quasi-static channel fields on energetic electron dynamics is examined. A strong channel magnetic field, self-generated by the accelerated electrons, is found to play a comparable role to the transverse electric channel field in defining the boundary of electron motion.
Energy distribution of high-power, short-pulse laser produced fast electrons was experimentally and numerically studied using high-energy bremsstrahlung x-rays. The hard x-ray photons and escaping electrons from various metal foils, irradiated by the 50 TW Leopard laser at Nevada Terawatt Facility, were recorded with a differential filter stack spectrometer that is sensitive to photons produced by mainly 0.5–2 MeV electrons and an electron spectrometer measuring >2 MeV electrons. The experimental bremsstrahlung and the slope of the measured escaped electrons were compared with an analytic calculation using an input electron spectrum estimated with the ponderomotive scaling. The result shows that the electron spectrum entering a Cu foil could be continuous single slope with the slope temperature of ∼1.5 MeV in the detector range. The experiment and analytic calculation were then compared with a 2D particle-in-cell code, PICLS, including a newly developed radiation transport module. The simulation shows that a two-temperature electron distribution is generated at the laser interaction region, but only the hot component of the fast electrons flow into the target during the interaction because the low energy electron component is trapped by self-generated magnetic field in the preformed plasma. A significant amount of the photons less than 100 keV observed in the experiment could be attributed to the low energy electrons entering the foil a few picoseconds later after the gating field disappears.
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