High current well-directed beams of super-ponderomotive electrons for laser driven nuclear physics applications
O. N. Rosmej, M. Gyrdymov, M. M. Günther, N. E. Andreev, P. Tavana, P. Neumayer, S. Zähter, N. Zahn, V. S. Popov, N. G. Borisenko, A. Kantsyrev, A. Skobliakov, … 展开作者
O. N. Rosmej, M. Gyrdymov, M. M. Günther, N. E. Andreev, P. Tavana, P. Neumayer, S. Zähter, N. Zahn, V. S. Popov, N. G. Borisenko, A. Kantsyrev, A. Skobliakov, V. Panyushkin, A. Bogdanov, F. Consoli, X. F. Shen, A. Pukhov
We report on new findings in a laser driven enhanced electron beam generation in the multi MeV energy range at moderate relativistic laser intensities and their applications. In our experiment, an intense sub-picosecond laser pulse propagates through a plasma of a near critical electron density (NCD) and direct laser acceleration (DLA) of electrons takes place. The breakthrough toward high current relativistic electron beams became possible due to application of low density polymer foams of sub-mm thickness. In foams, the NCD-plasma was produced by a mechanism of super-sonic ionization. Compared to NCD-plasmas generated by laser irradiation of conventional foils, the DLA acceleration path in foams was strongly enhanced. Measurements resulted into 11÷13 MeV of the effective electron temperature and up to 100 MeV maximum of the electron energy measured in the laser pulse propagation direction. The growth of the electron energy was accompanied by a strong increase of the number of super-ponderomotive electrons and a well-defined directionality of the electron beam that propagates in a divergence cone with a half angle of 12°. For the energy range above 7.5 MeV that is relevant for gamma-driven nuclear reactions, we estimate a charge carried by these well-directed electron beams as high as 50 nC and a corresponding efficiency of the laser energy conversion into electrons of 6%. The electron spectra generated by the DLA-mechanism in NCD-plasma at 1019 Wcm-2 laser intensity were compared with those measured in shots onto conventional metallic foils at ultra-relativistic laser intensities of 1021 Wcm-2 . In the last case, the twice lower effective electron temperature and the twice lower maximum of the electron energy were registered. The substantial difference in the electron spectra for these two cases presented itself in the isotope production yield.
It is shown that static longitudinal and transverse electric fields can significantly alter electron acceleration by a long laser beam in a sub-critical plasma, enabling generation of super-ponderomotive electrons. The role of the plasma fields in this regime is not to directly transfer substantial energy to the electron, but rather to reduce the axial dephasing rate between the electron and the laser beam. The reduced dephasing in both cases leads to a subsequent enhancement of the axial momentum and total electron energy. These mechanisms can be relevant to experiments with solid-density tar…
It is shown that electrons with momenta exceeding the `free electron' limit of $m_eca_0^2/2$ can be produced when a laser pulse and a longitudinal electric field interact with an electron via a non-wakefield mechanism. The mechanism consists of two stages: the reduction of the electron dephasing rate $γ-p_x/m_ec$ by an accelerating region of electric field and electron acceleration by the laser via the Lorentz force. This mechanism can, in principle, produce electrons that have longtudinal momenta that is a significant multiple of $m_eca_0^2/2$. 2D PIC simulations of a relatively simple laser-…
In laser-solid interactions, electrons may be generated and subsequently accelerated to energies of the order-of-magnitude of the ponderomotive limit, with the underlying process dominated by direct laser acceleration. Breaking this limit, realized here by a radially-polarized laser pulse incident upon a wire target, can be associated with several novel effects. Three-dimensional Particle-In-Cell simulations show a relativistic intense laser pulse can extract electrons from the wire and inject them into the accelerating field. Anti-dephasing, resulting from collective plasma effects, are shown…
We examine a regime in which a linearly-polarized laser pulse with relativistic intensity irradiates a sub-critical plasma for much longer than the characteristic electron response time. A steady-state channel is formed in the plasma in this case with quasi-static transverse and longitudinal electric fields. These relatively weak fields significantly alter the electron dynamics. The longitudinal electric field reduces the longitudinal dephasing between the electron and the wave, leading to an enhancement of the electron energy gain from the pulse. The energy gain in this regime is ultimately l…
Intense lasers can accelerate electrons to very high energy over a short distance. Such compact accelerators have several potential applications including fast ignition, high energy physics, and radiography. Among the various schemes of laser-based electron acceleration, vacuum laser acceleration has the merits of super-high acceleration gradient and great simplicity. Yet its realization has been difficult because injecting free electrons into the fast-oscillating laser field is not trivial. Here we demonstrate free-electron injection and subsequent vacuum laser acceleration of electrons up to…
We report observation of kHz-pulsed-laser-accelerated electron energies up to 3 MeV in the -$k_\text{laser}$ (backward) direction from a 3 mJ laser interacting at normal incidence with a solid density, flowing-liquid target. The electrons/MeV/s.r. >1 MeV recorded here using a mJ-class laser exceeds or equals that of prior super-ponderomotive electron studies employing lasers at lower repetition-rates and oblique incidence. Focal intensity of the 40-fs-duration laser is 1.5 $\cdot$ 10$^{18}$ W cm$^{-2}$, corresponding to only ~80 keV electron ponderomotive energy. Varying laser intensity confir…
Advances in ultra-intense laser technology are enabling, for the first time, relativistic intensities at mid-infrared (mid-IR) wavelengths. Anticipating further experimental research in this domain, we present high-resolution two dimensional Particle-in-Cell (PIC) simulation results using the Large- Scale Plasma (LSP) code that explore intense mid-IR laser interactions with dense targets. We present the results of thirty PIC simulations over a wide range of intensities (0.03 < $a_0$ < 39) and wavelengths (λ =780 nm, 3 μm, and 10 μm). Earlier studies, limited to λ =780 nm and $a_0 \sim$ 1 [1, 2…
The physics governing electron acceleration by a relativistically intense laser are not confined to the critical density surface, they also pervade the sub-critical plasma in front of the target. Here, particles can gain many times the ponderomotive energy from the overlying laser, and strong fields can grow. Experiments using a high contrast laser and a prescribed laser pre-pulse demonstrate that development of the pre-plasma has an unexpectedly strong effect on the most energetic, super-ponderomotive electrons. Presented 2D particle-in-cell simulations reveal how strong, voluminous magnetic …
We consider a situation in when the interaction of relativistically intense EM waves with an isotropic electron positron plasma takes place, i.e.,we consider short pulse lasers with intensity up to 1021 W/cm2, in which the photon density is of the order of 1030cm3 and the strength of electric field E = 10^9 statvolt/cm. Such a situation is possible in astrophysical and laboratory plasma which are subject to intense laser radiation, thus leading to non thermal equilibrium field radiations. Such interaction of the super-strong laser radiation with an isotropic pair plasma leads to the generation…
We report on the experimental studies of laser driven ion acceleration from double-layer target where a near-critical density target with a few-micron thickness is coated in front of a nanometer thin diamond-like carbon foil. A significant enhancement of proton maximum energies from 12 to ~30 MeV is observed when relativistic laser pulse impinge on the double-layer target under linear polarization. We attributed the enhanced acceleration to superponderomotive electrons that were simultaneously measured in the experiments with energies far beyond the free-electron ponderomotive limit. Our inter…
Electric field dynamics and ion acceleration in the self-channeling of a superintense laser pulse
本文用一维静电有质动力学模型研究了相对论强度的激光脉冲在低密度等离子体中传播时产生电场和离子径向加速的动力学过程，该模型用于解释电场的实验测量[S. Kar等人，New J. Phys. *9*，402（2007）]。离子在空间上聚焦在电荷位移通道的边缘，导致流体动力学破裂，这又导致电子加热和电场中的“回波”效应。在通道的中心区域中的完全电子耗尽的开始导致到“库仑爆炸”制度的平滑过渡和离子加速的饱和。
Relativistic temperature electrons higher than 0.5 MeV are generated typically with laser intensities of about 10$^{18}$ W/cm$^{2}$. Their generation with high repetition rate lasers that operate at non-relativistic intensities ($\simeq$10$^{16}$ W/cm$^{2}$) is cardinal for the realization of compact, ultra-short, bench-top electron sources. New strategies, capable of exploiting different aspects of laser-plasma interaction, are necessary for reducing the required intensity. We report here, a novel technique of dynamic target structuring of microdroplets, capable of generating 200 keV and 1 Me…
The electron dynamics in counter-propagating laser waves is investigated by employing a novel approach, where the new Hamiltonian is time-independent when the perturbative laser wave is absent. The physical picture of stochastic electron dynamics is clearly revealed and the threshold values of the amplitude of the perturbative laser field for triggering stochastic electron acceleration are derived for different laser polarization directions and initial electron momentum. It demonstrates that the dephasing rate (new Hamiltonian) between the electron and the dominant laser can be randomly reduce…
Theory of Relativistic Surface Plasmon Excitation on Smooth Surface by High-Intensity Laser
The University of Liverpool（利物浦大学）；Cockcroft Institute（科克克罗夫特研究所）；Center for Applied Physics and Technology, HEDPS, and SKLNPT, School of Physics, Peking University（北京大学物理学院应用物理与技术中心、HEDPS和SKLNPT）；Helmholtz Institute Jena（耶拿亥姆霍兹研究所）；Friedrich-Schiller-Universität Jena（耶拿弗里德里希·席勒大学）；The University of Manchester（曼彻斯特大学）
We briefly report on the recently proposed [G. Fiore, R. Fedele, U. de Angelis, Phys. Plasmas 21 (2014), 113105], [G. Fiore, S. De Nicola, arXiv:1509.04656] electron acceleration mechanism named "slingshot effect": under suitable conditions the impact of an ultra-short and ultra-intense laser pulse against the surface of a low-density plasma is expected to cause the expulsion of a bunch of superficial electrons with high energy in the direction opposite to that of the pulse propagation; this is due to the interplay of the huge ponderomotive force, huge longitudinal field arising from charge se…
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 …
The generation of super-high energetic electrons influenced by pre-plasma at relativistic intensity laser-matter interaction is studied in a one-dimensional slab approximation with particle-in-cell simulations. Different pre-plasma scale-lengths of $1\ μ\text{m}$, $5\ μ\text{m}$, $10\ μ\text{m}$ and $15\ μ\text{m}$ are considered, showing an increase in both particle number and cut-off kinetic energy of electrons with the increase of pre-plasma scale-length, and the cut-off kinetic energy greatly exceeding the corresponding laser ponderomotive energy. A two-stage electron acceleration model is…
Analytic model of stable shock-like structures in laser interaction with underdense plasma for identifying of phase and polarization dependent regime of laser wakefield accelerators
