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Register a new userRelativistic high-harmonic generation and correlated electron acceleration from plasma mirrors at 1~kHz repetition rate
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We report evidence for the first generation of XUV spectra from relativistic surface high-harmonic generation (SHHG) on plasma mirrors at a kilohertz repetition rate, emitted simultaneously and correlated to the emission of energetic electrons. We present measurements of SHHG spectra and electron angular distributions as a function of the experimentally controlled plasma density gradient scale length $L_mathrm{g}$ for three increasingly short and intense driving pulses: 24~fs (9 optical cycles) and $a_0=1.1$, 9~fs (3.5 optical cycles) and $a_0=1.8$, and finally 4~fs (1.7 optical cycles) and $a_0approx2.0$. For all driver pulses, we observe relativistic SHHG in the range $L_mathrm{g}in[lambda/25,lambda/10]$, with an optimum gradient scale length of $L_mathrm{g}approxlambda/15$.
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High intensity laser-plasma interactions produce a wide array of energetic particles and beams with promising applications. Unfortunately, high repetition rate and high average power requirements for many applications are not satisfied by the lasers, optics, targets, and diagnostics currently employed. Here, we address the need for high repetition rate targets and optics through the use of liquids. A novel nozzle assembly is used to generate high-velocity, laminar-flowing liquid microjets which are compatible with a low-vacuum environment, generate little to no debris, and exhibit precise positional and dimensional tolerances. Jets, droplets, submicron thick sheets, and other configurations are characterized with pump-probe shadowgraphy to evaluate their use as targets. To demonstrate a high repetition rate, consumable liquid optical element, we present a plasma mirror created by a submicron thick liquid sheet. This plasma mirror provides etalon-like anti-reflection properties in the low-field of 0.1% and high reflectivity as a plasma, 69%, at a repetition rate of 1 kHz. Practical considerations of fluid compatibility, in-vacuum operation, and estimates of maximum repetition rate, in excess of 10 kHz, are addressed. The targets and optics presented here enable the use of relativistically intense lasers at high average power and make possible many long proposed applications.
A new parameter regime of laser wakefield acceleration driven by sub-petawatt femotsecond lasers is proposed, which enables the generation of relativistic electron mirrors further accelerated by the plasma wave. Integrated particle-in-cell simulation including the mirror formation and Thomson scattering demonstrates that efficient coherent backscattering up to keV photon energy can be obtained with moderate driver laser intensities and high density gas targets.
Experiments on the excitation of propagating surface plasmons (SPs) by ultrashort, high intensity laser interaction with grating targets are reviewed. At intensities exceeding $10^{19}~mbox{W cm}^{-2}$ on target, i.e. in the strongly relativistic regime of electron dynamics, multi-MeV electrons are accelerated by the SP field as dense bunches collimated in a near-tangent direction. By the use of a suitable blazed grating, the bunch charge can be increased up to $simeq $660 picoCoulomb. Intense XUV high harmonics (HH) diffracted by the grating are observed when a plasma with sub-micrometer scale is produced at the target surface by a controlled prepulse. When the SP is excited, the HH are strongly enhanced in a direction quasi-parallel to the electrons. Simulations show that the HH are boosted by nanobunching in the SP field of the electrons which scatter the laser field. Besides the static and dynamic tailoring of the target density profile, further control of electron and HH emission might be achieved by changing the SP duration using a laser pulse with rotating wavefront. This latter technique may be capable to produce nearly single-cycle SPs.
We report the enhancement of individual harmonics generated at a relativistic ultra-steep plasma vacuum interface. Simulations show the harmonic emission to be due to the coupled action of two high velocity oscillations -- at the fundamental $omega_L$ and at the plasma frequency $omega_P$ of the bulk plasma. The synthesis of the enhanced harmonics can be described by the reflection of the incident laser pulse at a relativistic mirror oscillating at $omega_L$ and $omega_P$.
A method for the controlled generation of intense high frequency electromagnetic fields by a breaking Langmuir wave (relativistic flying mirrors) in a gradually inhomogeneous plasma is proposed. The wave breaking threshold depends on the local plasma density gradient. Compression, chirping and frequency multiplication of an electromagnetic wave reflected from relativistic mirrors is demonstrated using Particle-In-Cell simulations. Adjusting the shape of the density profile enables control of the reflected light properties.
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