Poster presentations

A pump scheme with one long and two short pump pulses is proposed, 
in a grazing incidence pumping x-ray laser (XRL) configuration. A 360 ps long pulse, 
prepares a plasma in a low charge state. Then, an initial short pulse ionizes the plasma 
to an optimal charge state, while the second short pulse induces strong collisional excitation 
in the gain region. With only 200 mJ of pump energy on target, a compact Ag XRL at 13.9 nm 
with a gain coefficient of 55 cm(-1) was achieved.

R. Capdessus1, C. Ridgers2, P. McKenna1

1. Department of Physics SUPA, University of Strathclyde, Glasgow G4 0NG, United Kingdom
2. Department of Physics, University of York, Heslington, York YO10 5DD, United Kingdom

The emergence of petawatt-class lasers enables the exploration of new research frontiers, 
where many promising domains have not yet been experimentally investigated [1]. These new 
laser facilities will provide peak intensities beyond 1023 W/cm2. Amongst the many research 
topics to be explored, is the generation of strong electromagnetic fields and their key role 
in the synchrotron radiation produced by ultrarelativistic electrons. The radiation reaction 
effects are considered in the classical radiation dominated regime. The resonant interaction 
between longitudinal and transverse plasma waves is investigated on the basis of a relativistic 
theory accounting for the radiation reaction (RR) force, using Sokolov's model [2]. The 
associated non-linear permitivity is derived assuming that the initial plasma can be described 
by a relativistic Maxwell distribution. The link between the RR corrections and the synchrotron 
radiation emission is discussed. The role of the ions in the synchrotron radiation emsisson is 
also discussed. 
[1] A. Macchi et al. Rev. Mod. Phys., 85, 751 (2013).
[2] I. V. Sokolov, J. Exp. Theor. Phys. 109, 207 (2009)

Laser-gas interaction is an elegant way of energy conversion to higher frequencies through up-conversion 
mechanisms, as well as to lower frequencies through down-conversion processes. The reasonable size and 
high-repetition rates of the latest laser-based frequency converters make them highly promising for 
next-generation THz pulse devices. Spanning from the infrared to microwaves, the THz radiation spectrum 
is attractive for many applications in various fields of physics, biology and medicine, industry, 
security, communication, remote sensing and basic science with, for instance, molecular dynamic 
spectroscopy.
We present a one-dimensional model of THz emissions induced by laser-driven, time-asymmetric ionization 
and current oscillations in hydrogen, Helium and Argon gases. Our model highlights complex scalings 
of the THz fields with respect to the laser and gas parameters, in particular, a non-monotonic behavior 
against the laser parameters and plasma charged state Z. Analytical expressions of the transmitted and 
reflected fields are presented, explaining the THz spectra observed in particle-in-cell and 
forward-pulse propagation codes. 

X-ray production by inverse Compton scattering is under investigation for the 
ThomX X-ray source. In this project, a high average power pulsed laser periodically interacts 
with a picosecond relativistic electron beam to produce the X-ray beam. Very high photon fluxes 
can be achievable if sufficient power is stored in the Fabry-Perot cavity. Today main limitation 
is due to heat loads on the mirrors that lead to surface deformations. To this aim, extensive 
studies on thermal effects on the Fabry-Perot cavity mirror substrates have been performed, both 
with simulations and prototype tests. Various diagnostics are also investigated to measure mirrors 
deformation and cavity finesse in real time with a CW laser.

The interaction of high-contrast, intense laser pulses with grating targets was 
proven experimentally to strongly increase the energy of both electrons and protons emitted from 
the target surfaces. In particular, the maximum enhancement of the cutoff energy of TNSA-accelerated 
protons [1] was observed when the condition for matching to surface waves $lambda_{grating} 
simeq lambda_{laser}/(1 - sin(theta_{inc}))$ was satisfied.
This suggests a role of plasmonic resonance and local field enhancement.
In this contribution we discuss the effects of such field enhancement on High Harmonic Generation 
from gratings [2-4] via particle-in-cell simulations performed with the PICCANTE code [5]. 
Via simulations we also investigate particular target types (e.g. tapered waveguides) designed to 
exploit the coupling to gratings for achieving ultra-high laser intensities.

[1]Ceccotti et al., Evidence of Resonant Surface-Wave Excitation in the Relativistic Regime through Measurements of Proton Acceleration from Grating Targets. , PRL 2013
[2]Lavocat-Dubuis et al., Numerical simulation of harmonic generation by relativistic laser interaction with a grating, PRE 2009
[3]Cerchez et al., Generation of Laser-Driven Higher Harmonics from Grating Targets, PRL 2013
[4]Yeung et al., Angularly separated harmonic generation from intense laser interaction with blazed diffraction gratings, Optics Letters 2011
[5]Sgattoni, Fedeli, Sinigardi, http://aladyn.github.io/piccante/ 

We study a new concept of compact X-rays Free Electron Laser (XFEL) based on the physics of 
free electron lasers, laser-plasma interaction, and nonlinear optics. This new scheme, the so 
called "Raman XFEL" [2], implies overlapping two identical laser pulses of the same frequency and 
polarization state to create an optical lattice at laser intensities high enough to induce the 
so-called strong field Kapitza-Dirac effect in relativistic regime. The ponderomotive force can 
trap a relativistic electron bunch in the wells and results in a transverse oscillations. 
This triggers a parametric process resulting emission of coherent radiation in the range of VUV or 
X-rays and the amplification of the Stokes component of the Raman-scattered. Analytical and numerical 
studies with the code EWOK have demonstrated that very high gain values, with gain lengths in the 
sub-mm range, and high photon numbers can be expected [2].
A particular code named RELIC [3], has been developed to study the injection of relativistic electrons 
and dynamics in the high intensity optical lattice. Several interesting phenomena related to the 
injection and the dynamics of electrons in ponderomotive potential wells are observed. In this poster, 
I will show some results obtained with RELIC code, especially the modeling of the interaction of a 
relativistic electron bunch produced by Laser Wakefield Acceleration (LWFA) and an intense optical 
lattice. The adiabatic and brutal injection cases will be addressed. 

References

[1] Ph. Balcou,Eur. Phys. J. D 59, 525 (2010).
[2] I.A. Andriyash, E. d'Humières, V.T. Tikhonchuk, and Ph. Balcou, Phys. Rev. Lett.109, 244802 (2012).
[3] M. Hadj-Bachir et al, Numerical proposal for the Kapitza-Dirac relativistic effect, (submitted)

Using a strong fundamental to drive Strong Field Ionisation and an
orthogonally polarised second harmonic to act as a gate for returning
trajectories, tunnel exit times could be extracted from the high
harmonic spectrum depending on the relative phase between the two
colours. We further investigate the influence of the ionic Coulomb
field during the propagation on the reconstruction process by
numerically calculating the trajectories of electrons ionised at
different times in the combined field of the two colours and the ion.
The results show that the two colour gate on the exit time for
returning trajectories is essentially independent of whether the ionic
Coulomb field is considered or neglected. However, the mapping between
harmonic energy and exit time is shifted to slightly higher energies
when including the Coulomb field. 

We investigate the dynamics of bulk electron heating and ionization in 
solid density plasmas driven by ultra-short relativistic laser pulses. During laser-plasma 
interactions, the solid plasma absorbs a fraction of laser energy and converts it 
into kinetic energy of electrons. A part of the electrons with high kinetic energy 
travels through the solid plasma and transfers energy into bulk electrons, which results 
in bulk electron heating by return current. The bulk electron temperatures in the interest 
of bulk region agrees very well with the theory based on Ohmic heating mechanism by treating 
the return current correctly. The bulk electron heating is finally translated into bulk 
ionization dynamics, which is modeled by Thomas-Fermi ionization mechanism in our studies. 

Laser-plasma acceleration with fs ultra-intense laser pulses is a
central research field for next generation particle accelerators. Due to
the intrinsic non-linearity of the processes and the extreme small-scale
compared to conventional accelerators, numerical studies are necessary
to enhance both the acceleration processes but also the diagnostics and
control during experiments.

  This poster presents numerical methods that will lead to better
quantitative predictions resulting from the complex dynamics of the
multi-physics of fundamental laser-plasma interactions and to diagnose
occurring plasma instabilities. The current extension of the
fully-relativistic particle-in-cell code PIConGPU with an adequate
treatment of the ionization and excitation rates computed by SCFLY will
be shown.

  A numerical method for synthetic in-situ probing of the
simulated experiment with x-ray photons, as in the upcoming HIBEF
experiments at the European XFEL, based on the on-the-fly calculated
scattering cross sections will be shown.

  Consequently, the resulting corrected evolution of ionization fronts,
opacity and target heating is discussed via an example of a large-scale
full 3D simulation of a mass-limited droplet target.

With multi-petawatt lasers it will be possible to achieve such 
a high intensity of laser pulses that quantum electrodynamics effects come 
into a play during the laser-matter interaction. Therefore it will be possible 
to observe emission of high-energy gamma photons leading to the generation of electron-positron pairs.

Using 2D simulations we compare interaction of two colliding, ultra-intense laser pulses 
with electron target for both types of polarization. As a consequence of different structure 
of the standing wave created by colliding laser pulses for each type of polarization, number 
of generated electron-positron pairs also differs. Since the standing wave produced by two 
linearly polarized laser pulses oscillates, electrons and photons achieve higher values of 
$ chi_{e} $ and $ chi_{gamma} $ parameters giving the probability of gamma photon emission and 
pair generation respectively. It is shown, that number of generated electron-positron pairs is 
much higher in the case of linear polarization.

Using 1 kHz, 9 mJ femtosecond laser pulses, we demonstrate laser-filamentation-induced 
spectacular snow formation in a supersaturated zone a cloud chamber. An intense convection and 
cyclone like actiona induced by the high repeated filaments was proved playing a significant role.
Using 1 kHz, 2 mJ femtosecond laser pulses,we observed laser-filamentation-induced snow formation 
also in a sub-saturated environment inside the cloud chamber. Calculation of the saturation ratio 
inside vortices formed below the filament shown a steady super-saturation state would be assisted 
inside the vortices,which led to the precipitation. Thermal effect of laser filamentation was 
supposed to be the main contributor to achieving its application in the rainmaking field.

We will present the status of the LASERIX facility, a 100 TW Ti:Sa laser installation of 
the Paris South University dedicated to the development of pump/probe experiments using soft x-ray 
lasers and high harmonic generation. 7 weeks per year are opened within the laserlab network to the 
user community.
Former source developments and applications campaigns will be presented together with 
future scientific program.

Intensities of the modern lasers will in the near future give us the possibility of 
observing essentially quantum electrodynamical effects in the laser field. One of the most fascinating 
effects (and probably the best studied one) is the particle production by colliding photons[1]. 
To provide such an event, the energy of the photons must be larger than two times the rest mass of 
the electron or a large amount of photons need to be used. Another interesting effect that has been 
described analytically is the nonlinear Compton scattering[2]. Electron in the electro-magnetic field 
can absorb multiple photons and emit one quantum of very high energy.

These two effects combined together can provide the following picture: once electron is put into strong 
laser field, it can scatter multiple photons to a single high-energy quantum. This photon, in turn, can 
interact with the incident field and provide electron-positron pair. Same processes can occur again, for 
all electrons and positrons including newly born ones. Globally, this becomes a cascade process (which 
is also called an avalanche). Electron-positron avalanches and their effect on the incident field have 
been studied by the number of authors mostly in particular field configurations where some approximations 
can be made[3,4].

In present work we provide the computational results on the pair production in the laser field of extreme 
intensity paying special attention to the case of extremely short pulses. We describe motion of particles 
between the acts of Compton scattering and pair production classically, while the events of nonlinear 
Compton scattering and pair production are described using Monte Carlo method. In order to obtain corresponding 
probability distributions constant crossed field approximation[5] is applied. We analyze electron and positron 
density distributions and the dynamics of the electron-positron avalanche in various field configurations.

1. A. R. Bell and J. G. Kirk, Phys. Rev. Let. 101, 200403 (2008)
2. F. Makenroth, A. Di~Piazza, Phys. Rev. A 83, 032106 (2011)
3. N. Elkina et. al, Phys. Rev. ST Accel. Beams 14, 054401 (2011) 
4. E. N. Nerush et al, Phys. Rev. Lett. 106, 035001 (2011)
5. A. I. Nikishov and V. I. Ritus, Sov. Phys. JETP 19, 529 (1964)

Experimental results of high-order harmonic generation using focusing 0.5mJ 60fs pulses produced by a kHz 
Ti:Sapphire laser system at PALS Center are presented. Two types of targets were employed: a gas cell of 
variable length filled with Ar or Ne gas, and a pulsed valve with multiple nozzles. The influence of target 
geometry and gas pressure of both types of targets was studied. We believe that using the multi-jet target 
the density modulation of the medium enabling periodic correction of phase mismatch (quasi-phase matching) 
allowed significant signal increase of some harmonics.

The excitation of nonlinear electrostatic shock waves by ultraintense laser interaction with 
overdense plasmas and related ion acceleration are investigated using PIC simulations. Linearly 
polarized pulses with dimensionless amplitude $a_0 sim n_e/n_c$ drives the generation of shocks, 
solitary waves or multi-peak structures depending on the laser pulse duration. Such nonlinear waves 
drive secondary ion acceleration in the plasma bulk, the acceleration dynamics being more complex 
than "specula" reflection of ions from the wave front. Ions located deep in the plasma can be accelerated 
both by these electrostatic solitary as well as by shock waves. Stability of solitons and formation of 
shock waves is strongly dependent on the velocity distribution of ions. An increase of initial ion 
temperature to some finite value, effects the reflection of ions which in turns the electroststic 
solitary waves into electrostatic shock waves. Stable monoenergetic ion reflection is occurred only 
with some "optimal" value of initial ion distribution. A further increase of the initial ion temperature, 
reflection becomes "linear" in the sense that there always have some ions in the tail of the distribution 
function which can be reflected and there occurs "steady" ion reflection. 

The amount of energy converted from the laser pulse into gamma rays in 
the interaction of ultraintense lasers with solids and plasmas is widely studied. 
However, the situation gets complicated if a relativistic electron beam interacts with 
a laser pulse. The energies from both the relativistic electron beam and laser pulse may 
be converted into the gamma rays. We present the numerical results of particle-in-cell (PIC) 
simulations of energy conversion from the laser pulse and electron beam into gamma rays. 
The electron beam is generated by a first laser pulse targeting a solid target via an 
am-bipolar electric created by electrons pushed into the target.  The energetic electron 
beam then interacts with a second laser pulse. The electron dynamics in the second laser 
pulse is simulated by including the radiation damping effect proposed by I. V. Sokolov [1]. 
In his radiation damping model, the electron momentum arising from the external field and 
recoil of the emitted photon is incorporated. The electron trajectories, time evolutions of 
electron energy, radiation spectrum will be presented in detail. The optimum condition for 
laser energy conversion into gamma ray will be discussed. 

[1] I. V. Sokolov et al., Dynamics of Emitting Electrons in Strong Laser Fields, Phys. Plasmas 16, 093115 (2009).

We present simulated far field radiation spectra from plasma based light sources such 
as high harmonics generation (HHG) during laser foil irradiation and betatron oscillation during 
laser-wakefield-acceleration (LWFA). The synthetic spectra allow quantifying both total radiation 
flux of these light sources and the occurring radiation background.
We also present applications of far field radiation as spectroscopic diagnostic of the plasma 
dynamics occurring during LWFA and a variety of plasma instabilities, such as Kelvin-Helmholtz 
and the Rayleight-Taylor instability. This so-called synthetic diagnostic allows probing the 
electron dynamics by predicting the emitted radiation spectra thus emulating real experiments. 

In order to obtain these results, we developed an in-situ method to compute spectrally and 
angularly resolved far field radiation based on Liénard-Wiechert potentials. By computing radiation 
concurrently to simulating the plasma dynamics, using the particle-in-cell code PIConGPU, we are 
able to take into account all $sim10^{10}$ electrons simulated, thus allowing to quantify both 
coherent and incoherent radiation. Furthermore, spectra can be computed for thousand of observation 
directions and frequencies simultaneously. We show that the code’s capability of resolving radiation 
processes temporally is an indispensable tool for linking the evolution of the plasma dynamics to 
the emitted radiation. 

We present experimental results on laser-driven electron acceleration achieved with the POLARIS 
laser delivering pulses of 2.4 J in 160-170 fs. Here, we observed the generation of quasi-monoenergetic electron 
pulses by self-modulated laser wakefield acceleration.
We found a clear correlation between the accelerating length and the peak electron energies.

Surface high harmonic generation in the relativistic and nonrelativistic regime 
will be discussed for the imaging of dense laser-generated plasmas. In particular, we emphasize 
the use of ultrashort XUV pulses by surface high harmonic generation for the time-resolved imaging 
of solid-density hydrogen plasmas. A typical application scenario is presented.

Linearly polarized solitary waves are naturally excited during the interaction of 
an intense pulse with a plasma. New localized structures in the form of exact, nonlinear solutions 
of the one-dimensional Maxwell-fluid model for a cold plasma with fixed ions are presented. These 
solutions exhibit a breather-like behavior and a periodic exchange of electromagnetic and electron 
kinetic energy at twice the frequency of the wave. The spatio-temporal structure of the waves and 
the frequency-amplitude relationship are presented. Direct fluid simulations suggest that these 
solitary waves propagate without change a long time.

Amplification of ultra-short laser pulses via strongly-coupled Brillouin scattering is 
a promising technique for the generation of multi-Petawatt to Exawatt laser pulses. 

Energy is transferred from a long pump pulse (ps to ns duration) to a short seed pulse (about 100fs duration) 
via a resonant interaction with an ion quasi-mode. We present an envelope model to investigate this process 
in multi-dimensional geometry. 

In particular we focus on the influence of a frequency chirp of the pump pulse. The chirp affects the 
resonance condition for frequency matching of pump, seed and plasma wave and thus the energy transfer 
from pump to seed. A residual chirp will always be present in experiments and we find that chirping 
the pump pulse has a major impact on the amplification efficiency in the nonlinear phase of the 
amplification process.

Ultrafast shadowgraphy utilizes few cycle probe pulses in order to image density gradients in a plasma. 
This allows to probe structures, such as laser-driven wakes, which move close to the speed of light. 
Here we study the process of shadowgraphic image formation through three-dimensional particle-in-cell 
(PIC) simulations of the interaction of a few cycle probe pulse with a laser-driven wake. The output 
of the PIC code is post-processed in order to take into account the effect of a typical imaging setup. 
This allows to construct simulated shadowgrams and to study their dependence on various parameters such 
as probe pulse duration, wavelength and delay. The simulations thus allow to establish a connection 
between shadowgraphic images and density gradients in the plasma and facilitates the interpretation 
of recent experiments which study electron injection in an evolving wakefield.

Optical free-electron lasers (OFELs) from the ultra violet to x-ray range 
can be realized with Traveling-Wave Thomson-Scattering (TWTS). This becomes possible in 
TWTS by increasing the photon scattering efficiency of standard Thomson scattering geometries 
more than one order of magnitude by changing the interaction geometry. In TWTS a side-scattering 
geometry is used where the laser and electron propagation directions enclose the interaction 
angle $phi$. Together with a tilt of the laser pulse front the interaction distance is 
increased in TWTS beyond the limits of head-on Thomson scattering. TWTS implements dispersion 
control of the laser pulse to compensate for variations of the optical undulator period originating 
from the pulse-front tilt.  Altogether, the combination of side-scattering, pulse-front tilt and 
dispersion control in TWTS allows for meter-scale interaction distances in which the electron 
beam becomes microbunched and OFEL operation is achieved. These TWTS OFELs provide transverse 
coherence as well as brilliances an order of magnitude enhanced over standard head-on Thomson 
scattering geometries. 
We present the scaling laws of TWTS OFELs derived from a fully analytic theory of the electron 
laser interaction in TWTS scattering geometries. TWTS OFELs can be realized in an all-optical 
setup with a meter-scale footprint using laser wakefield accelerated electrons featuring both 
ultralow transverse emittances and large energy spreads on the one percent level.

There has been much interest in recent times on the topic of the nonlinear 
interaction of an ultra-intense laser with a counter streaming electron beam. While laser 
intensities have been increasing with the construction of modern high powered lasers, we 
propose a twin pulse system which may potentially eliminate the need for such high powered 
systems. A first laser pulse impinges on a target plasma, creating an imbalance in the 
ion-electron density. This in turn produces a strong ambipolar field within the target which 
could greatly accelerate the electrons over MeV range speeds. In this time, a second laser 
pulse is fired and interacts with the counter streaming electrons to illicit gamma-ray 
production. We present the numerical simulation of such a scheme, and show the energy evolution 
of the system, along with the frequency spectrum of the released radiation. We will examine 
the feasibility of such a scheme and compare it to traditional single-pulse high intensity 
systems. At the same time, we would like to examine the applicability of several mathematical 
models of the radiation damping as a function of laser intensity.

We investigate terahertz radiation in gases by strongly focused pulsed laser 
beams. The interaction between the pulse and the laser-induced micro-plasma is studied using 
the in-house PIC code OCEAN. The code incorporates ionization via the quasi-static ADK theory. 
Charge separation effects and the full vectorial character of the electromagnetic field is 
taken into account beyond the paraxial regime. In addition, a simplified analytical modeling 
is provided in order to understand the separate effects on the THz spectral characteristics. 
In particular, the impact of ionization and dimensional effects are investigated.

Plasma surface dynamics play an important role in the process of interaction of 
intense short laser pulses with overdense targets. For example, reflection of the laser pulse 
from self-generated oscillating plasma surface leads to efficient generation of high-order 
harmonics with frequencies up to the water window and beyond. Using a single particle analytical 
model and 1D particle-in-cell simulations with Graphic Processing Units (GPUs) we have studied the 
effects of carrier-envelope phase (CEP) of the two-cycle laser pulse on the surface motion and on 
temporal structure of the generated harmonics. We present an analytical expression for the optimal 
CEP value that leads to the isolation of a single attosecond pulse.

Continuum generation pumped by femtosecond pulses generates light fields with more 
than an octave bandwidth. When performed in gases, either guided in hollow fibers or a filament, 
the resulting pulses can be compressed to sub-5 fs and are the basis of single attosecond pulse 
generation. Continua from bulk materials are ideal seed sources for optical parametric amplifiers 
and also compressible to sub-5 fs. At the same time it has not yet been possible to compress the 
latter continuum itself. We now show that this is due to the nonlinear pulse propagation of the 
newly generated wavelengths in the later part of the crystal. This leads to strongly wavelength 
dependent beam evolution in free space and the inability to image all colors simultaneously. A 
fully achromatic and astigmatism free Schiefspiegler telescope is essential for this characterization. 
When the continuum generation is moved from the beginning of the sapphire crystal to the very end, 
all colors can be imaged simultaneously to the same spot size. As a consequence the full bandwidth 
of the continuum can be utilized. In particular, we find and fully characterize that the continuum 
emerging from the sapphire is chirp-free and a 7 fs pulse results without external compressor.

We investigate the equations of propagation of ultrashort intense laser pulses in the atmosphere. 
The aim is to develop general propagation equations for the attosecond laser pulses through plasma channel 
generated by photoionization rate from multiphoton ionization to tunnel ionization regimes. The spatial and 
temporal evolution of the laser pulse, plasma and current density are analyzed under the influence of 
Raman process, multiphoton absorption and nonlinear Kerr effect.