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Relativistic plasma optics enabled by near-critical density nanostructured material
Abstract and Figures
The nonlinear optical properties of a plasma due to the relativistic electron
motion in an intense laser field are of fundamental importance for current
research and the generation of brilliant laser-driven sources of particles and
photons1-15. Yet, one of the most interesting regimes, where the frequency of
the laser becomes resonant with the plasma, has remained experimentally hard to
access. We overcome this limitation by utilizing ultrathin carbon nanotube
foam16 (CNF) targets allowing the strong relativistic nonlinearities at near-
critical density (NCD) to be exploited for the first time. We report on the
experimental realization of relativistic plasma optics to spatio-temporally
compress the laser pulse within a few micrometers of propagation, while
maintaining about half its energy. We also apply the enhanced laser pulses to
substantially improve the properties of an ion bunch accelerated from a
secondary target. Our results provide first insights into the rich physics of
NCD plasmas and the opportunities waiting to be harvested for applications.
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We have performed a detailed study on the interaction of ultra-intense, short
laser pulse with under-dens plasma. The underlying interaction physics is
outlined and key topics like laser absorption and electron acceleration are
addressed. This study is assisted by the extensive 1D3V particle-in-cell (PIC)
simulations over a wide range of initial plasma densities, , ( is the critical
density) and laser intensities, . It is noticed that the steady propagation of
a short-pulse through a low density plasma is violated in proportion to the
expression ( and are electron density laser gamma factor). Accordingly, when
the plasma density rises toward the critical value, a new physical regime
appears which has not been adequately explored, previously. Using general
conservation laws it is demonstrated that due to the radiation pressure, strong
wave-breaking (phase mixing) occurs in this regime. The electron acceleration
is described in terms of the wave-breaking followed by the direct laser
acceleration (DLA). A new physical model, provides estimates for the total
absorption, saturation time and electron temperature, is proposed to describe
the light evolution in the plasma. This model predictions are in close
agreement with the global trends observed in simulations. The overall
absorption and electron temperature are found to be mainly affected by the
saturation time which inversely relates to the amount of anomalous plasma
light-scattering. In this way, both the absorption and temperature decrease
against .
Plasmas can serve as damage-less optics for amplifying and focusing light pulses to very high intensity. This provides a way to overcome the limitations of solid-state optical materials as a damage threshold in the classical sense is absent. The amplification process relies on parametric processes in plasmas exploiting the coupling of transverse electromagnetic waves to a longitudinal plasma wave. The plasma response can either be an electron plasma wave ( stimulated Raman scattering), an ion-acoustic wave (stimulated Brillouin scattering) or a more complicated non-resonant feature in the case of very short pulses.
We present a new regime to generate high-energy quasimonoenergetic proton beams in a ``slow-pulse'' regime, where the laser group velocity ${v}_{g}<c$ is reduced by an extended near-critical density plasma. In this regime, for properly matched laser intensity and group velocity, ions initially accelerated by the light sail (LS) mode can be further trapped and reflected by the snowplough potential generated by the laser in the near-critical density plasma. These two acceleration stages are connected by the onset of Rayleigh-Taylor-like (RT) instability. The usual ion energy spectrum broadening by RT instability is controlled and high quality proton beams can be generated. It is shown by multidimensional particle-in-cell simulation that quasimonoenergetic proton beams with energy up to hundreds of MeV can be generated at laser intensities of $1{0}^{21}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$.
Overdense plasmas are usually opaque to laser light. However, when the
light is of sufficient intensity to drive electrons in the plasma to
near light speeds, the plasma becomes transparent. This process--known
as relativistic transparency--takes just a tenth of a picosecond. Yet
all studies of relativistic transparency so far have been restricted to
measurements collected over timescales much longer than this, limiting
our understanding of the dynamics of this process. Here we present
time-resolved electric field measurements (with a temporal resolution of
~ 50fs) of the light, initially reflected from, and subsequently
transmitted through, an expanding overdense plasma. Our result provides
insight into the dynamics of the transparent-overdense regime of
relativistic plasmas, which should be useful in the development of
laser-driven particle accelerators, X-ray sources and techniques for
controlling the shape and contrast of intense laser pulses.
The development of ultra-intense lasers has facilitated new studies in laboratory astrophysics and high-density nuclear science, including laser fusion. Such research relies on the efficient generation of enormous numbers of high-energy charged particles. For example, laser-matter interactions at petawatt (1015 W) power levels can create pulses of MeV electrons with current densities as large as 1012 A cm-2. However, the divergence of these particle beams usually reduces the current density to a few times 106 A cm-2 at distances of the order of centimetres from the source. The invention of devices that can direct such intense, pulsed energetic beams will revolutionize their applications. Here we report high-conductivity devices consisting of transient plasmas that increase the energy density of MeV electrons generated in laser-matter interactions by more than one order of magnitude. A plasma fibre created on a hollow-cone target guides and collimates electrons in a manner akin to the control of light by an optical fibre and collimator. Such plasma devices hold promise for applications using high energy-density particles and should trigger growth in charged particle optics.
The following is a comment on the published paper shown on the preceding page.
Overdense plasmas are usually opaque to incident laser light. But when
the light is of sufficient intensity to drive electrons in the plasma to
near light speeds, the plasma becomes transparent. In the physical
picture, as the electrons reach near light speeds their mass increases
due to relativistic effect. The increase in electron mass in turn slows
their motion such that they can no-longer shield the plasma from the
incident laser, making the plasma subsequently transparent to the
incident laser. This process -- known as relativistic transparency (RT)
-- takes just a tenth of a picosecond. Yet all studies of RT to date
have been restricted to measurements collected over time-scales much
longer than this, limiting our understanding of the dynamics of this
process. Here we present optical signatures of relativistic transparency
by measuring the time-resolved electric fields and temporal phases (with
temporal resolution ˜50 fs) of the light, initially reflected
from, and subsequently transmitted through, an expanding overdense
plasma due to temporal evolution of RT. These measurements are done
using a single-shot Frequency-Resolved-Optical-Gating (FROG) technique.
The measured electric fields show the temporal chopping nature of RT in
expanding overdense plasma from nanofoils. In addition the temporal
phases of the corresponding electric fields record the plasma critical
surface movement via Doppler-shift in reflection and plasma refractive
index in transmission. Our result provides insight into the dynamics of
the transparent-overdense-regime (TOR) of relativistic plasmas, which
should be useful in the development of laser-driven particle
accelerators, x-ray sources, and techniques for controlling the shape
and contrast of intense laser pulses.
GeV electron accelerators are essential to synchrotron radiation facilities and free electron lasers, and as modules for high-energy particle physics. Radio frequency based accelerators are limited to relatively low accelerating fields (10-50 MV/m) and hence require tens to hundreds of meters to reach the multi-GeV beam energies needed to drive radiation sources, and many kilometers to generate particle energies of interest to the frontiers of high-energy physics.Laser wakefield accelerators (LWFA) in which particles are accelerated by the field of a plasma wave driven by an intense laser pulse produce electric fields several orders of magnitude stronger (10-100 GV/m) and so offer the potential of very compact devices. However, until now it has not been possible to maintain the required laser intensity, and hence acceleration, over the several centimeters needed to reach GeV energies.For this reason laser-driven accelerators have to date been limited to the 100 MeV scale. Contrary to predictions that PW-class lasers would be needed to reach GeV energies, here we demonstrate production of a high-quality electron beam with 1 GeV energy by channeling a 40 TW peak power laser pulse in a 3.3 cm long gas-filled capillary discharge waveguide. We anticipate that laser-plasma accelerators based on capillary discharge waveguides will have a major impact on the development of future femtosecond radiation sources such as x-ray free electron lasers and become a standard building block for next generation high-energy accelerators.
A multifluid implicit plasma simulation code has been used to study the transport of hot electrons generated by an intense \(>=3×1018 W/cm 2\) short-pulse 1.06 mum laser into plasma targets over a broad range of densities [\(0.35-200\)ncrit], as arising in the Fast Ignitor approach to inertial confinement fusion. The most intense (16-250 MG) magnetic fields generated in this interaction are traced to the ponderomotive push on background electrons, and tardy electron shielding. These fields can focus the heated electrons toward the axis of the beam, while impeding the direct return flow of background electrons.
Transmission of a subpicosecond relativistic laser pulse is observed through solid foils and preformed overcritical plasmas. Transmission rates near 10% for densities above 10×nc are measured. A moderately relativistic strong threshold in intensity is found in order to observe this effect. The experimental results as well as preliminary particle-in-cell simulations suggest that for thin solid foils the observed transmission is explicable by rapid heating and expansion to transmissive conditions during the pulse. Self-induced transparency and hole boring processes apply to thicker preformed plasmas. These results have important implications in fast ignition for inertial confinement fusion.
The nonlinear frequency shift of a strong electromagnetic wave in a plasma, due to weak relativistic effects and the v[over →]×B[over →] force, can cause modulation and self-focusing instabilities. These processes are explored, and their relation to self-focusing driven by the ponderomotive force is described.
Extreme ultraviolet (XUV) and X-ray harmonic spectra produced by intense
laser-solid interactions have, so far, been consistent with Doppler
upshifted reflection from collective relativistic plasma
oscillations--the relativistically oscillating mirror mechanism. Recent
theoretical work, however, has identified a new interaction regime in
which dense electron nanobunches are formed at the plasma-vacuum
boundary resulting in coherent XUV radiation by coherent synchrotron
emission (CSE). Our experiments enable the isolation of CSE from
competing processes, demonstrating that electron nanobunch formation
does indeed occur. We observe spectra with the characteristic spectral
signature of CSE--a slow decay of intensity, I, with high-harmonic
order, n, as I(n)n-1.62 before a rapid efficiency rollover.
Particle-in-cell code simulations reveal how dense nanobunches of
electrons are periodically formed and accelerated during
normal-incidence interactions with ultrathin foils and result in CSE in
the transmitted direction. This observation of CSE presents a route to
high-energy XUV pulses and offers a new window on understanding
ultrafast energy coupling during intense laser-solid density
interactions.
Interaction of relativistically strong laser pulses with plasmas is investigated by a multi-dimensional particle-in-cell (PIC) code VLPL (Virtual Laser Plasma Laboratory) [Bull. Am. Phys. Soc. 41, 1502 (1996)]. Acceleration of background electrons to multi-MeV energies, generation of 100 MG magnetic fields, and dynamics of ion channel boring are studied. It is shown that direct v×B push by the laser pulse in the presence of an azimuthal dc magnetic field effectively accelerates background plasma electrons to energies significantly higher than the ponderomotive potential. The authors call this novel effect “B-loop” acceleration mechanism. It is dominant in near-critical plasma, or when plasma waves disappear due to wavebreaking. Laser channeling in under- and overdense plasmas is also studied. Energy spectra of the accelerated electrons and ions and the laser energy conversion efficiency at the critical surface are presented. It is shown that the accelerated electrons propagate in the form of magnetized jets. This physics is crucial for the fast ignitor concept in inertial confinement fusion. © 1998 American Institute of Physics.