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ABSTRACT: Collisionless shocks are ubiquitous in astrophysics and in the lab. Recent
numerical simulations and experiments have shown how they can arise from the
encounter of two collisionless plasma shells. When the shells interpenetrate,
the overlapping region turns unstable, triggering the shock formation. As a
first step towards a microscopic understanding of the process, we analyze here
in detail the initial instability phase. On the one hand, 2D relativistic PIC
simulations are performed where two symmetric initially cold pair plasmas
collide. On the other hand, the instabilities at work are analyzed, as well as
the field at saturation and the seed field which gets amplified. For mildly
relativistic motions and onward, Weibel modes govern the linear phase. We
derive an expression for the duration of the linear phase in good agreement
with the simulations. This saturation time constitutes indeed a lower-bound for
the shock formation time.
03/2013;
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S. Jacquemot,
F. Amiranoff,
S. D. Baton,
J. C. Chanteloup,
C. Labaune,
M. Koenig,
D. T. Michel,
F. Perez,
H. P. Schlenvoigt,
B. Canaud, [......],
D. Batani,
J. R. Davies,
F. Fiuza,
R. A. Fonseca, L. O. Silva,
L. A. Gizzi,
P. Koester,
L. Labate,
J. Badziak,
O. Klimo
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ABSTRACT: Multi-dimensional particle-in-cell simulations are used to study the
generation of electrostatic shocks in plasma and the reflection of background
ions to produce high-quality and high-energy ion beams. Electrostatic shocks
are driven by the interaction of two plasmas with different density and/or
relative drift velocity. The energy and number of ions reflected by the shock
increase with increasing density ratio and relative drift velocity between the
two interacting plasmas. It is shown that the interaction of intense lasers
with tailored near-critical density plasmas allows for the efficient heating of
the plasma electrons and steepening of the plasma profile at the critical
density interface, leading to the generation of high-velocity shock structures
and high-energy ion beams. Our results indicate that high-quality 200 MeV
shock-accelerated ion beams required for medical applications may be obtained
with current laser systems.
01/2013;
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ABSTRACT: The theoretical model by Sorasio et al. (2006) for the steady state Mach
number of electrostatic shocks formed in the interaction of two plasma slabs of
arbitrary density and temperature is generalized for relativistic electron and
non-relativistic ion temperatures. We find that the relativistic correction
leads to lower Mach numbers, and as a consequence, ions are reflected with
lower energies. The steady state bulk velocity of the downstream population is
introduced as an additional parameter to describe the transition between the
minimum and maximum Mach numbers in dependence of the initial density and
temperature ratios. In order to transform the soliton-like solution in the
upstream region into a shock, a population of reflected ions is considered and
differences to a zero-ion temperature model are discussed.
01/2013;
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ABSTRACT: We show that monoenergetic ion beams can be accelerated by moderate Mach number collisionless, electrostatic shocks propagating in a long scale-length exponentially decaying plasma profile. Strong plasma heating and density steepening produced by an intense laser pulse near the critical density can launch such shocks that propagate in the extended plasma at high velocities. The generation of a monoenergetic ion beam is possible due to the small and constant sheath electric field associated with the slowly decreasing density profile. The conditions for the acceleration of high-quality, energetic ion beams are identified through theory and multidimensional particle-in-cell simulations. The scaling of the ion energy with laser intensity shows that it is possible to generate ∼200 MeV proton beams with state-of-the-art 100 TW class laser systems.
Physical Review Letters 11/2012; 109(21):215001. · 7.37 Impact Factor
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ABSTRACT: We assess the impact of non-thermally shock-accelerated particles on the
magnetohydrodynamic (MHD) jump conditions of relativistic shocks. The adiabatic
constant is calculated directly from first principle particle-in-cell
simulation data, enabling a semi-kinetic approach to improve the standard fluid
model and allowing for an identification of the key parameters that define the
shock structure. We find that the evolving upstream parameters have a stronger
impact than the corrections due to non-thermal particles. We find that the
decrease of the upstream bulk speed yields deviations from the standard MHD
model up to 10%. Furthermore, we obtain a quantitative definition of the shock
transition region from our analysis. For Weibel-mediated shocks the inclusion
of a magnetic field in the MHD conservation equations is addressed for the
first time.
10/2012;
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ABSTRACT: It is shown through analytical modeling and numerical simulations that
external magnetic fields can relax the self-trapping thresholds in plasma based
accelerators. In addition, the transverse location where self-trapping occurs
can be selected by adequate choice of the spatial profile of the external
magnetic field. We also find that magnetic-field assisted self-injection can
lead to the emission of betatron radiation at well defined frequencies. This
controlled injection technique could be explored using state-of-the-art
magnetic fields in current/next generation plasma/laser wakefield accelerator
experiments.
10/2012;
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ABSTRACT: The effects of plasma ion motion in self-modulated plasma-based accelerators are examined. An analytical model describing ion motion in the narrow beam limit is developed and confirmed through multidimensional particle-in-cell simulations. It is shown that the ion motion can lead to the early saturation of the self-modulation instability and to the suppression of the accelerating gradients. This can reduce the total energy that can be transformed into kinetic energy of accelerated particles. For the parameters of future proton-driven plasma accelerator experiments, the ion dynamics can have a strong impact. Possible methods to mitigate the effects of the ion motion in future experiments are demonstrated.
Physical Review Letters 10/2012; 109(14):145005. · 7.37 Impact Factor
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ABSTRACT: We investigate the acceleration of light particles in perpendicular shocks
for plasmas consisting of a mixture of leptonic and hadronic particles.
Starting from the full set of conservation equations for the mixed plasma
constituents, we generalize the magneto-hydrodynamical jump conditions for a
multi-component plasma, including information about the specific adiabatic
constants for the different species. The impact of deviations from the standard
model of an ideal gas is compared in theory and particle-in-cell simulations,
showing that the standard-MHD model is a good approximation. The simulations of
shocks in electron-positron-ion plasmas are for the first time
multi-dimensional, transverse effects are small in this configuration and 1D
simulations are a good representation if the initial magnetization is chosen
high. 1D runs with a mass ratio of 1836 are performed, which identify the
Larmor frequency \omega_{ci} as the dominant frequency that determines the
shock physics in mixed component plasmas. The maximum energy in the non-thermal
tail of the particle spectra evolves in time according to a power-law
proportional to t^\alpha with \alpha in the range 1/3 < \alpha < 1, depending
on the initial parameters. A connection is made with transport theoretical
models by Drury (1983) and Gargate & Spitkovsky (2011), which predict an
acceleration time proportional to \gamma and the theory for small wavelength
scattering by Kirk & Reville (2010), which predicts a behavior rather as
proportional to \gamma^2. Furthermore, we compare different magnetic field
orientations with B_0 inside and out of the plane, observing qualitatively
different particle spectra than in pure electron-ion shocks.
06/2012;
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ABSTRACT: The transverse self-modulation of ultra-relativistic, long lepton bunches in
high-density plasmas is explored through full-scale particle-in-cell
simulations. We demonstrate that long SLAC-type electron and positron bunches
can become strongly self-modulated over centimeter distances, leading to wake
excitation in the blowout regime with accelerating fields in excess of 20 GV/m.
We show that particles energy variations exceeding 10 GeV can occur in
meter-long plasmas. We find that the self-modulation of positively and
negatively charged bunches differ when the blowout is reached. Seeding the
self-modulation instability suppresses the competing hosing instability. This
work reveals that a proof-of-principle experiment to test the physics of bunch
self-modulation can be performed with available lepton bunches and with
existing experimental apparatus and diagnostics.
06/2012;
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ABSTRACT: Three-dimensional (3D) particle-in-cell (PIC) simulations are used to
investigate the interaction of ultrahigh intensity lasers ($> 10^{20}$
W/cm$^{-2}$) with matter at overcritical densities. Intense laser pulses are
shown to penetrate up to relativistic critical density levels and to be
strongly self-focused during this process. The heat flux of the accelerated
electrons is observed to have an annular structure when the laser is tightly
focused, showing that a large fraction of fast electrons is accelerated at an
angle. These results shed light into the multi-dimensional effects present in
laser-plasma interactions of relevance to fast ignition of fusion targets and
laser-driven ion acceleration in plasmas.
05/2012;
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ABSTRACT: We describe how a new framework for coupling a full-PIC algorithm with a
reduced PIC algorithm has been implemented into the code OSIRIS. We show that
OSIRIS with this new hybrid-PIC algorithm can efficiently and accurately model
high energy density scenarios such as ion acceleration in laser-solid
interactions and fast ignition of fusion targets. We model for the first time
the full density range of a fast ignition target in a fully self-consistent
hybrid-PIC simulation, illustrating the possibility of stopping the laser
generated electron flux at the core region with relatively high efficiencies.
Computational speedups greater than 1000 times are demonstrated, opening the
way for full-scale multi-dimensional modeling of high energy density scenarios
and for the guiding of future experiments.
05/2012;
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ABSTRACT: The generation of DC magnetic fields in unmagnetized plasmas with velocity
shear is predicted for non relativistic and relativistic scenarios either due
to thermal effects or due to the onset of the Kelvin-Helmholtz instability
(KHI). A kinetic model describes the growth and the saturation of the DC field.
The predictions of the theory are confirmed by multidimensional
particle-in-cell simulations, demonstrating the formation of long lived
magnetic fields ($t \sim 100s \omega_{pi}^{-1}$) along the full longitudinal
extent of the shear layer, with transverse width on the electron length scale
($\sqrt{\gamma_0}c/\omega_{pe}$), reaching magnitudes
$eB_{\mathrm{DC}}/m_ec\omega_{pe}\sim \beta_0\sqrt{\gamma_0}$.
05/2012;
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ABSTRACT: The formation of non-relativistic collisionless shocks in laboratory with
ultrahigh intensity lasers is studied via \emph{ab initio} multi-dimensional
particle-in-cell simulations. The microphysics behind shock formation and
dissipation, and the detailed shock structure are analyzed, illustrating that
the Weibel instability plays a crucial role in the generation of strong
subequipartition magnetic fields that isotropize the incoming flow and lead to
the formation of a collisionless shock, similarly to what occurs in
astrophysical scenarios. The possibility of generating such collisionless
shocks in laboratory opens the way to the direct study of the physics
associated with astrophysical shocks.
04/2012;
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ABSTRACT: We present a computational study of dt fusion driven by Coulomb explosion
within a single, large, heteronuclear two-component D2/T2
nanodroplet, originating from kinematic overrun effects between deuterons
and tritons. Scaled electron and ion dynamics simulations have been used to
explore the size dependence and the isotopic composition dependence of the
intra-nanodroplet (INTRA) dt fusion yield in a composite D2n-2kT2k
nanodroplet, initially consisting of an inner sphere of D2 molecules
surrounded by an outer sphere of T2 molecules (n = 1.4×
108–2.0×109, k/n = 0.10–0.60, and initial radii R0
= 1100–2700Å) driven by a single, ultraintense, near-infrared,
Gaussian laser pulse (peak intensity 1020W cm-2, pulse length 25fs). INTRA dt fusion in D2n-2kT2k nanodroplets with neutron yields
of 30–90 (per nanodroplet, per laser pulse) were attained in the size
domain R0 = 2000–2700Å with the optimal composition in the
range of k/n = 0.2–0.4. INTRA yields in D2n-2kT2k nanodroplets
are similar (within 20–40%) to those in initially homogeneous
(DT)n nanodroplets of the same size. These INTRA yields are
sufficiently large to warrant experimental observation in a single
nanodroplet. The INTRA dt fusion can be distinguished from the
inter-nanodroplet dt fusion reaction, which occurs inside and outside the
macroscopic plasma filament, by the nanodroplet size dependence of the yield
and by the different energies of the neutrons produced in these two
channels.
The European Physical Journal D 04/2012; 54(1):71-75. · 1.48 Impact Factor
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ABSTRACT: We examine the influence of non-ideal plasma-density and non-Gaussian
transverse laser-intensity profiles in the laser wakefield accelerator
analytically and numerically. We find that the characteristic amplitude and
scale length of longitudinal density fluctuations impacts on the final energies
achieved by electron bunches. Conditions that minimize the role of the
longitudinal plasma density fluctuations are found. The influence of higher
order Laguerre-Gaussian laser pulses is also investigated. We find that higher
order laser modes typically lead to lower energy gains. Certain combinations of
higher order modes may, however, lead to higher electron energy gains.
04/2012;
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ABSTRACT: Compact and affordable ion accelerators based on laser-produced plasmas have potential applications in many fields of science and medicine. However, the requirement of producing focusable, narrow-energy-spread, energetic beams has proved to be challenging. Here we demonstrate that laser-driven collisionless shocks can accelerate proton beams to similar to 20MeV with extremely narrow energy spreads of about 1% and low emittances. This is achieved using a linearly polarized train of multiterawatt CO(2) laser pulses interacting with a gas-jet target. Computer simulations show that laser-heated electrons launch a collisionless shock that overtakes and reflects the protons in the slowly expanding hydrogen plasma, resulting in a narrow energy spectrum. Simulations predict the production of similar to 200MeV protons needed for radiotherapy by using current laser technology. These results open a way for developing a compact and versatile, high-repetition-rate ion source for medical and other applications.
Nature Physics 01/2012; 8(1):95-99. · 18.97 Impact Factor
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ABSTRACT: The proton bunch-driven plasma wakefield acceleration (PWFA) has been proposed as an approach to accelerate an electron beam to the TeV energy regime in a single plasma section. An experimental program has been recently proposed to demonstrate the capability of proton-driven PWFA by using existing proton beams from the European Organization for Nuclear Research (CERN) accelerator complex. At present, a spare Super Proton Synchrotron (SPS) tunnel, having a length of 600 m, could be used for this purpose. The layout of the experiment is introduced. Particle-in-cell simulation results based on realistic SPS beam parameters are presented. Simulations show that working in a self-modulation regime, the wakefield driven by an SPS beam can accelerate an externally injected similar to 10 MeV electrons to similar to 2 GeV in a 10-m plasma, with a plasma density of 7 x 10(14) cm(-3).
Journal of Plasma Physics. 01/2012; 78:347-353.
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ABSTRACT: The role of laser frequency chirps in the laser wakefield accelerator is
examined. We show that in the linear regime, the evolution of the laser pulse
length is affected by the frequency chirp, and that positive (negative) chirp
compresses (stretches) the laser pulse, thereby increasing (decreasing) the
peak vector potential and wakefield amplitude. In the blowout regime, the
frequency chirp can be used to fine tune the localized etching rates at the
front of the laser. In our simulations, chirped laser pulses can lead to 15%
higher self-trapped electrons, and 10% higher peak energies as compare to the
transform-limited pulse. Chirps may be used to control the phase velocity of
the wake, and to relax the self-guiding conditions at the front of the laser.
Our predictions are confirmed by multi-dimensional particle-in-cell simulations
with OSIRIS.
12/2011;
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ABSTRACT: Raman amplification in plasma has been promoted as a means of compressing picosecond optical laser pulses to femtosecond duration to explore the intensity frontier. Here we show for the first time that it can be used, with equal success, to compress laser pulses from nanosecond to picosecond duration. Simulations show up to 60% energy transfer from pump pulse to probe pulse, implying that multikilojoule ultraviolet petawatt laser pulses can be produced using this scheme. This has important consequences for the demonstration of fast-ignition inertial confinement fusion.
Physical Review Letters 09/2011; 107(10):105002. · 7.37 Impact Factor
