nlight15 - abstracts of the talks

Development of X-ray synchrotron radiation sources, based of state-of-the-art laser technologies, attracts growing 
interest thanks to their numerous applications in science and medicine. New concepts of X-ray free electron lasers driven by the 
laser-plasma electron accelerators and based on optical and laser-plasma undulators are now explored by the leading research groups 
worldwide. Recently, an original scheme of the short-wavelength free electron laser based on the optical undulator created by two 
overlapped transverse laser beams was proposed and investigated [1,2]. We present a kinetic theory and an ad hoc numerical model 
of such amplifier, which allows to account for the finite energy spread, angular divergence and the spectral properties of the 
electron beam in the optical lattice. The theoretical findings are compared to the results of the one- and three-dimensional numerical 
modeling with the spectral free electron laser code [3]. The obtained results indicate that, traveling through the optical lattice, 
the laser-accelerated electron beam can efficiently amplify XUV light on centimeter scale.

[1] Ph. Balcou, Eur. Phys. J. D 59, 525 (2010).
[2] I.A. Andriyash, E. d’Humieres, V. T. Tikhonchuk, Ph. Balcou, Phys.Rev.Lett. 109, 244802 (2012).
[3] I.A. Andriyash, R. Lehe, and V. Malka, J. Comput. Phys. 282, 397 (2015).

The possibility of using high-intensity laser beams to create plasma waveguides for the long-distance 
transportation of the microwave radiation was proposed by Askaryan (1969). Theoretical consideration and experimental 
realization of such plasma waveguides can be found in [1,2].
In the present talk we discuss specific features of the plasma channel created by the intense UV femtosecond 
laser pulse arising from its strong nonequilibrium. In particular, the photoelectron energy spectrum formed 
in the process of gas multiphoton ionization consists of a number of peaks corresponding to the absorption of 
a certain number of photons. Such an electron energy distribution function (EEDF) is characterized by the energy 
intervals with inverse population that can be used for amplification of radio-frequency electromagnetic radiation 
in a plasma media with certain energy dependence of the transport cross section[3-6]. 
It is also worth to note that a strong nonequilibrium of photoelectron spectrum can also cause unusual refractive 
properties of the plasma channel, appearing in the process of multiphoton ionization of gas atoms by femtosecond 
laser pulse. In particular, the real part of the permittivity of such a plasma may be a greater than unity, i.e. 
plasma turns out to be optically denser medium in comparison with the surrounding unionized gas. Such a channel seems 
to be similar to the dielectric waveguide which can be used for both transportation and amplification of the microwave 
radiation. The process of guiding and amplification of the microwave radiation in a plasma channel formed by powerful 
femtosecond KrF laser pulse in xenon is studied numerically on the basis of self-consistent solution of the Boltzmann 
equation for the EEDF evolution in different spatial points of the gas media and the wave equation for the transported 
through the channel RF pulse[7]. The efficiency of amplification process in a nonequilibrium plasma channel depending 
on different parameters, i.e. plasma channel radius, RF pulse intensity etc, is analyzed. 

1) V. D. Zvorykin, A. O. Levchenko, I. V. Smetanin and N. N. Ustinovsky, JETP Lett., 91, 226 (2010)
2) V. D. Zvorykin, A. O. Levchenko, A. V. Shutov, E. V. Solomina, N. N. Ustinovsky and I. V. Smetanin, Phys. Plasmas, 19, 033509 (2012)
3) G. Bekefi, Y. L. Hirshfield and S. C. Brown, Phys. Fluids, 4, 173 (1961)
4) F. V. Bunkin, A. E. Kazakov, M. V. Fedorov, Sov. Phys. Usp., 15, 416 (1972)
5) A. V. Bogatskaya and A .M. Popov  JETP Lett., 97, 388 (2013)
6) A. V. Bogatskaya, E .A. Volkova and A .M. Popov  J.Phys.D, 47, 185202 (2014)
7) A.V. Bogatskaya, I.V. Smetanin, E.A. Volkova and A.M. Popov, Laser and Particle Beams (2014), doi:10.1017/S0263034614000755

In the frame of the EuroGammas proposal for the ELI-NP Gamma beam source, a new optical scheme has been proposed 
to reach required high spectral brightness performance. The Gamma Beam Source (GBS) is based on a warm linac delivering which 
accelerated 32 electron bunches at energy up to 700 MeV at 100 Hz repetition rate. Each electron bunches interacts with a 
single high peak power laser pulse delivered by 20 W Yb:Yag based laser system circulating inside a non-resonant laser cavity.

The GBS machine and particularly the interaction point module will be presented. The last development of the laser beam circulator 
will be reported.  

The pulses from X-ray free-electron lasers are a billion times brighter than the brightest synchrotron beams 
available today.  When focused to micron dimensions, such a pulse destroys any material, but the pulse terminates before 
significant atomic motion can take place.  This mode of "diffraction before destruction" yields structural information 
at resolutions better than 2 Angstrom, from proteins that cannot be grown into large enough crystals or are too radiation 
sensitive for high-resolution crystallography.  This has opened up a new methodology of serial femtosecond crystallography 
that yields radiation damage-free structures without the need for cryogenic cooling of the sample. The method has begun 
to yield new structures and has the potential to increase the rate at which structures can be solved. Ultrafast pump-probe 
studies of photoinduced dynamics in proteins or other materials can also be studied.  Irreversible reactions can be studied, 
synchronised with the short pulses, with new sample being constantly replenished.  We have yet to reach the limit of the 
smallest samples that can be studied this way, and many innovations indicate the feasibility of single molecule diffractive 
imaging.

Laser-driven attosecond extreme ultraviolet (XUV) pulses are of great interest as a powerful tool for many 
potential applications. High-order harmonics generation from relativistic laser-irradiated plasma surface is considered 
as a promising approach to generate such source with high brilliance. Several radiation mechanisms have been identified, 
such as coherent wake emission, relativistic oscillating mirror, and coherent synchrotron emission. Here, we present a 
new mechanism for attosecond XUV pulse generation from relativistic laser-driven overdense plasma. With the help of 
particle-in-cell code, we find that an intense attosecond XUV pulse can be observed from the rear target side when a 
relativistic laser pulse normally incidents onto a thin solid foil. It is shown the emission is generated at the laser-illuminated 
side and transmit through the foil. Thus the source can be applied without spectral filtering. The generation mechanism is examined, 
in which wavebreaking induced large plasma oscillation plays a key role.

In this work we show through PIC simulations some studies on the Brillouin backscattering amplification of a 
short laser beam interacting in a plasma with a longer contra-propagating laser beam. The aim is to better understand the 
role on the amplification mechanism of each plasma parameter, such as the interaction length, the shape of the density profile, 
the duration of the long pump signal and the relative delay between the seed and pump signals. All the simulations reported have 
been carried out using the new PIC code SMILEI.

When an intense femtosecond laser pulse propagates in air, it will self-focus resulting in a continuous series 
of foci, or a filament. With a Ti-sapphire laser at a wavelength of 800nm (invisible), the color of the pulse turns white. 
This so-called chirped white light laser or supercontinuum could be used as a remote special purpose "illuminator". The intensity 
inside the filament core being high, nonlinear optical phenomena occur inside the filament. Some examples are ultrafast birefringence, 
THz generation, molecular excitation, population trapping, remote detection of chem-bio pollutants, lasing in air and snow and 
rain making in a cloud chamber. 

A discussion on an experimental campaign in which the generation of a narrow divergence (θγ < 2.5 mrad), 
multi-MeV (Emax approx 18 MeV) and ultrahigh peak brilliance (>1.8x1020 photons s-1 mm-2 mrad-2 0.1% BW) 
γ-ray beam from the scattering of an ultrarelativistic laser-wakefield accelerated electron beam in the field of a 
relativistically intense laser (dimensionless amplitude a0 approx 2) was carried out. The spectrum of the 
generated γ-ray beam is presented, with MeV resolution, seamlessly from 6 to 18 MeV, giving clear evidence of the onset 
of nonlinear relativistic Thomson scattering. To the best of our knowledge, this photon source has the highest peak brilliance 
in the multi-MeV regime ever reported in the literature.

The interaction between an ultra-intense laser pulse and an overdense plasma is a well-known source of copious 
coherent XUV emission. The latter follows either directly from the laser reflection off the so-called "relativistic plasma
mirror", or, indirectly, from the energetic electron bunches injected into the target, via coherent wake emission or optical 
transition radiation. For specific laser and target parameters, the electron boundary can be made to oscillate at relativistic 
velocities due to the strong laser pressure. As a result, the reflected light is Doppler-shifted to high-order harmonics of 
the laser frequency and compressed into a train of ultra-intense, 10-100 as pulses of great potential for time-resolved 
atomic spectroscopy. Yet their efficient production implies finely-controlled interaction conditions as well as a quantitative 
understanding of the underlying physics. Over the past 20 years, various models of high-order harmonic generation (HOHG) have 
been proposed, usually restricted to very strong laser fields (∼ 1020 W/cm2) and near-normal incidence angles, 
besides neglecting the fine structure of the plasma mirror oscillations. Only recently have self-consistent models been 
worked out that describe the textit{detailed} electron dynamics and the resulting HOHG, albeit still restricted to 
strongly-relativistic or normal-incidence interaction regimes. However, it is well-known from both numerical simulations and 
experiments that efficient HOHG can be obtained using moderate-intensity (∼ 1018 W/cm2) pulses impinging at
large incidence angles onto solid-density targets - a feature still awaiting quantitative interpretation. Besides, a unified 
picture of HOHG over a broad parameter range has, as yet, been missing. In this talk, we present a self-consistent model 
describing the fine structure of the plasma mirror dynamics in the general case of an obliquely incident laser pulse 
interacting with a steep-gradient plasma of arbitrary density. While this model does not adress electron bunching or finite 
density gradient effects, it gives the first full picture of HOHG in the case of laser-solid interaction for any laser intensity, 
incidence angle and plasma density. In particular, it reveals that, due to the complex plasma mirror dynamics, the high-order 
harmonic spectrum at intermediate frequencies can significantly deviate from the power-law shapes discussed in the literature. 
In addition, a comprehensive parametric study allows us to identify two regimes of plasma dynamics, namely the J × B regime 
and the cyclotron Brunel regime, which translate into distinct behaviors of HOHG. Finally, our theoretical predictions are shown 
to agree with 1-D particle-in-cell (PIC) simulations.

Radiation spectra from Laser-Plasma interactions are straightforward to obtain in experiment, but due to the 
large number of simulated particles these are challenging to model ab-initio. Since the emitted spectra include the complete 
phase-space dynamics, they are a key to both designing and optimizing brilliant plasma-driven x-ray sources for applications 
such as ultra-short pump-probe experiments. We present angularly-resolved em-radiation spectra from all billions of particles 
in a laser-plasma simulation, including the full coherence properties from plasma structure and dynamics. These spectra range 
from far IR to X-rays. We show recent results from the multi-GPU, open-source code PIConGPU.

We use very similar methods, when designing optical free-electron lasers (OFELs) driven by Laser-wakefield accelerated (LWFA) 
electrons using both existing LWFA beams and OFEL driver lasers. Such optical FELs (OFELs) based on Traveling-wave Thomson
scattering (TWTS) optimally exploit the high spectral photon density in high-power laser pulses by spatially stretching the 
laser pulse and overlapping it with the electrons in a side scattering setup. The introduction of a laser pulse-front tilt 
provides for interaction lengths appropriate for FEL operation. With careful dispersion control, electrons witness an undulator 
field of almost constant strength and wavelength over hundreds to thousands of undulator periods, thus giving enough time for 
self-amplified spontaneous emission (SASE) to seed the FEL instability and the realization of large laser gains.

A simple setup for the generation of ultra-intense quasistatic magnetic fields, based on the generation of electron currents 
with a predefined geometry in a curved "escargot" target has recently been proposed [1]. Particle-In-Cell simulations and qualitative 
estimates show that giga-Gauss scale magnetic fields may be obtained with existent laser facilities. The described mechanism of the 
strong magnetic field generation may be useful in a wide range of applications, from laboratory astrophysics to magnetized ICF schemes.
The possibility to use this setup to optimize the generation of high energy radiation in high intensity laser plasma interaction has been 
studied using Particle-In-Cell simulations taking into account radiation losses [2]. The dependence of the characteristics of the radiation 
source on the choice of laser and target parameters will be discussed and the scaling to future ultra-high intensity laser facilities now 
being built in Europe will be presented.

[1] Ph. Korneev, E. d'Humières and V. Tikhonchuk, Giga-Gauss scale quasistatic magnetic field generation in an "escargot" target, submitted to Phys. Rev. E
[2] R. Capdessus, E. d'Humières and V. Tikhonchuk, Modeling of radiation losses in ultrahigh power laser-matter interaction, Physical Review E 86, 036401 (2012).

Raman amplification in plasma has been suggested as a possible mechanism for the creation of the next generation of ultrashort, 
ultraintense laser pulses.  However, experiments to date show a much lower efficiency than simulations (∼4% compared to ∼40%).  
One possible mechanism limiting the efficiency is plasma wave breaking, which limits energy transfer between the pump and probe.  
We investigate Raman amplification in the strong wavebreaking regime, which has applications in both understanding existing 
experimental results and in planning future experimental works.

Light waves in plasma can be compressed in time through nonlinear interactions with other 
light waves.  The most interesting application lies in achieving the next generation of laser intensities.
A variety of techniques have been proposed to achieve the amplification of the desired signal, while 
tuning out unwanted amplifications. Waves can also be compressed in plasma through adiabatic changes in 
time of the density of the plasma medium. Here the immediate applications are less apparent, but for 
unrelated reasons there are very large facilities being built specifically to compress plasma to incredible 
pressures. The physics of embedded waves might then become important.

Seeded operation of a Ne-like Titanium (32,6nm) soft x-ray laser has been demonstrated at LASERIX (University Paris sud). 
High brightness, low divergence, have been observed.  HHG delay scan enables a precise measurement of the plasma amplification 
temporal window. Moreover, the far-field detector dynamics was sufficiently large to record the harmonic beam profile, amplified 
or not. We will show that this set of data is of great interest to study the special features of this pumping configuration 
where ionisation and density gradient are indeed expected to change on a few picoseconds timescale. Presence of an electron 
density "bump" will be discussed.   
Reaching subfemtosecond pulse duration is the next frontier of plasma based soft x-ray laser. Temporal coherence of the present 
seeded source has been measured and will be presented. This result gives a precious input to discuss path toward shorter pulse 
duration. In this framework, preliminary results of a two-color pumping experiment shows dramatic change in the soft x-ray laser 
behaviour which might be the sign of lasing at higher electron density with a potential four time increase in bandwidth.    

Development of advanced x-ray sources based on the laser-Thomson scattering mechanism is becoming important pushed by 
a strong demand for ultrashort hard x-ray pulses with a finite spectral bandwidth. The spectral characteristics of this x-ray beam 
depend on the interplay between interacting electron and laser beam parameters, i.e., electron beam energy spread and emittance, 
laser beam focusing geometry, bandwidth and intensity. We present high resolution angle- and energy-resolved measurements on the 
x-ray photon distribution generated by colliding picosecond electron bunches from the ELBE linear accelerator with counter-propagating 
ultrashort laser pulses from the 150 TW DRACO Ti:Sapphire laser system. The measured data and an ab-initio comparison with the 3D 
radiation code CLARA enable us to reveal parameter influences and correlation of both interacting beams. We conclude that in the low 
laser intensity interaction regime the electron angular distribution and the laser bandwidth, as in the case of ultrashort laser 
pulses, give a strong influence to the x-ray spectral shape and bandwidth. We also show the x-ray spectral broadening as the laser 
intensity increases indicating nonlinear interaction on the scattering process. Controlling these parameters is necessary for designing 
future Thomson x-ray sources with a specific bandwidth suited to an application

Ultra-intense laser-matter interactions are a major research area in in modern plasma physics, with many exciting challenges 
in terms of the fundamental physics, complex dynamics, and broad applications. The latter include, among others, isochoric heating of 
warm dense matter, particle acceleration, compact radiation sources, magnetized plasma phenomena for laboratory astrophysics, and fast 
igniter inertial fusion. One of the essential elements for all of these is the relativistic electron generation and transport dynamics. 
At present, a predictive understanding of high-intensity laser-matter interactions is severely hampered by the lack of self-consistent 
models for the ionization dynamics, coupled with the complex electron transport, and our inability to unravel this complexity with 
available experimental techniques in laser-only experiments. For example, ionization rates, scattering cross sections and shift energies 
of Kα and other self-radiation are not precisely known in a non-thermal non-static dense plasma. 
We establish the feasibility of using XFEL femtosecond X-ray sources to probe the spatial correlations inside of the solid-density 
plasma using small angle X-ray scattering (SAXS) and  resonant coherent diffraction, to obtain for the first time information on 
the spatial and temporal evolution of the electron density and ionization dynamics, with few fs and few nm resolution. The local 
and instantaneous ionization state can be measured when the X-ray beam is tuned to a bound-bound resonance of a particular charge 
state. The atomic scattering factor at the threshold of core electron excitation increases for example at Kalpha excitations in 
highly ionized Cu to a magnitude of more than 100 times the Thomson cross section per ion. 
Probing inside the solid-density plasma with X-rays will open entirely new ways to directly observe these extreme conditions, and 
will provide fundamental new data for developing improved models, as well as validate or falsify present and future treatments. 

An extended femtosecond filament is a nonlinear optical structure, which can emit a continuum of frequencies as well 
as quasi-isolated pulses in certain spectral ranges [1]. Generation of mid-infrared (MIR) ultrashort pulses can be enhanced by 
seeding the filament with the pulse at the central frequency close to its second harmonic [2]. With an 800 nm filament the coherent 
THz radiation can be delivered to the desired position far through the atmosphere, avoiding thereby strong water vapor absorption [3-5]. 
We have fully studied the long-wavelength part of 800 nm filament spectral continuum (0.8 < λ < 3000 μm) and identified new physical 
mechanisms supporting experimentally observed phenomena such as 3D Raman light bullet in the near-infrared range [1], single-cycle 
MIR pulse generation and enhancement [2], ring-type shape of the spatial distribution of terahertz radiation from the air-based 
plasma [6-8]. The search for the physical mechanisms has been performed numerically, based on the comprehensive unidirectional 
pulse propagation equation [9]. The lack of limitation on the range of frequencies and angles relative to the laser beam propagation 
axis allowed us to consider the frequencies down to 0.1 THz and angular divergence of terahertz radiation up to 15 degrees. We find 
that a 3D Raman bullet is created on the axis of the extended filament in a gas due to the Kerr nonlinearity. Its shift in central 
wavelength varies from ~840 to ~1000 nm with the propagation distance. The retarded Kerr effect leads to an increase in the energy 
converted into the bullet. In a single-color filament the radiation in the MIR range spreads out in a ring due to the medium dispersion. 
Here, we report an order of magnitude increase in the on-axis few-cycle MIR pulse energy if a visible seed pulse is added into a 800 nm 
filament. We show that the emission in the range ~0.1-15 THz in the (ω-2ω) filament follows mainly from the nonlinear nonstationary 
change of the photocurrent. With increasing frequency above 15 THz the Kerr nonlinearity due to neutrals contributes to THz generation 
through the 0=2ω-ω-ω process. THz radiation originates from the filament axis and diffracts on the plasma obstacle forming a ring in 
the far zone. By varying the relative contribution of the Kerr and the plasma nonlinearities, we have explicitly proved that this 
plasma-based THz generation scenario is of universal nature. The role of the Kerr nonlinearity in THz generation is limited to the 
intensity, hence favoring the action of the plasma. Our numerical simulation results explain and generalize recent experiments in 
this field [1,2,6-8]. 
We thank RFBR (15-02-99630, 14-02-31379), the Council of RF President for Support of Young Scientists (MK-4895.2014.2), RF President 
grant for Leading Scientific Schools (NSh-3796.2014.2), Dynasty Foundation, and CEA-France.
References:
[1] Y. Chen, F. Théberge, C. Marceau, H. Xu, N. Aközbek , О.G. Кosareva, S.L. Chin "Observation of filamentation-induced continuous self-frequency down shift in air", Appl. Phys. B 91, 219 (2008).
[2] T. Fuji and T. Suzuki, "Generation of sub-two-cycle mid-infrared pulses by four-wave mixing through filamentation in air," Opt. Lett. 32, 3330-3332 (2007).
[3] X. Xie, J. Dai, X.C. Zhang, "Coherent control of THz wave generation in ambient air", Phys. Rev. Lett., 96, 075005 (2006). 
[4] C. D'Amico, A. Houard, M. Franco, B. Prade, and A. Mysyrowicz, "Conical Forward THz Emission from Femtosecond-Laser-Beam Filamentation in Air", Phys. Rev. Lett. 98, 235002 (2007).
[5] L. Bergé, S. Skupin, C. Kohler, I. Babushkin, and J. Herrmann, "3D Numerical Simulations of THz Generation by Two-Color Laser Filaments", Phys. Rev. Lett. 110, 073901 (2013)
[6] Y. S. You, T. I. Oh, and K.Y. Kim, "Off-Axis Phase-Matched Terahertz Emission from Two-Color Laser-Induced Plasma Filaments", Phys. Rev. Lett. 109, 183902 (2012)
[7] P. Klarskov, A.C Strikwerda, K. Iwaszczuk, and P. Uhd Jepsen, "Experimental three-dimensional beam profiling and modeling of a terahertz beam generated from a two-color air plasma", New Journal of Physics 15, 075012 (2013).
[8] A. Gorodetsky, A.D. Koulouklidis, M. Massaouti, and S. Tzortzakis, "Physics of the conical broadband terahertz emission from two-color laser-induced plasma filaments", Phys.Rev. A 89, 033838 (2014)
[9] M. Kolesik, J.V. Moloney, "Nonlinear optical pulse propagation simulation: From Maxwell's to unidirectional equations", Phys. Rev. E, 70, 036604(2004).

Electron acceleration of electrons using intense laser pulses that excite tens of gigavolt per meter fields in 
plasmas will be discussed and the path forward to practical machines. The potential impact of compact laser plasma accelerators 
(LPA) ranges from providing the capability of producing high energy, ultra-short electron bunches and associated radiation pulses 
for forefront science in a small laboratory setting, to medical and homeland security applications, to the development of high 
energy particle colliders for fundamental science into the origin of matter and energy.

An overview will be presented of radiation source development at Berkeley Lab that includes an LPA powered FEL and a compact 
gamma-ray source based on Thomson scattering. Since most applications require operation at higher repetition rate and hence 
higher average power lasers than currently available, a new concept for incoherent combining of laser pulses to drive plasma 
wakes will also be discussed.

With current solid-state technology it is possible to generate laser pulses of few fs duration with up to a Petawatt 
peak power, intensities of 1022 W/cm2 can be reached in the focal spot.  Most of the high-power systems today are based on 
the chirped pulse amplification (CPA) technique.  The limitation of these systems in terms of maximum achievable intensity is due 
to optical damage thresholds of the components. However reaching intensities of 1025 W/cm2 and beyond would open up the 
possibility to access for example nonlinear QED effects (e.g. pair-creation) directly with optical fields.
The use of plasma as an amplification medium is currently discussed because of the absence of a damage threshold in the classical sense. 
Via parametric scattering off a plasma oscillation the energy from a long pump pulse can be transferred into a short seed pulse. The 
plasma oscillation can either be an electron Langmuir wave (Raman scattering) or a low frequency ion wave (Brillouin scattering). 
We will discuss reduced mathematical models that allow insight into the amplification dynamics and describe similarities and differences 
between Raman and Brillouin amplification.  We will present that especially Brillouin scattering has the potential to become a robust 
amplification process, as it is less sensitive to plasma parameters compared to Raman scattering and still acts on a reasonably short 
time scale when used in the strongly-coupled regime.

Next-generation 10-PW-class lasers, such as the Apollon facility at CEA/Saclay and the ELI project, will deliver a few 
hundred Joules of energy in a few tens of femtoseconds, so as to reach on-target intensities close to 1023 W/cm2. They will open 
up a novel field of physics characterized by coupled ultra-relativistic plasma and electrodynamics (QED) effects. These unprecedented 
physical conditions will be exploited to design new concepts of laser-driven sources of high-energy photons and antimatter.

We will present a novel wire-target scheme optimized for efficient and tunable generation of intense collimated gamma-ray flashes 
using ultra-intense, femtosecond-duration, circularly-polarized laser pulses. In the case of a focal spot larger than the 
submicron-scale wire diameter, the laser wrapping around the target can efficiently drag out and accelerate several radiating 
electron bunches in the forward direction. Our study will be based on 3d particle-in-cell (PIC) simulations enriched with 
nonlinear Compton scattering radiative processes, using both the Sokolov radiation friction model and a quantum Monte-Carlo 
approach. For a laser intensity between 1022 W/cm2 and 1023 W/cm2, our simulations reveal important photon emission in an 
energy range of a few keV to several MeV. The emission is focused in a cone of angle less than 0.2 radians, with a conversion 
efficiency reaching 10% of the laser energy. Our parametric study will show how the emission properties can be tuned by adjusting 
the laser duration, intensity and spot size as well as the wire diameter.

Relativistic electrons are prodigious sources of photons. Beyond classical accelerators, ultra-intense laser interactions 
are of particular interest as they allow the coherent motion of relativistic electrons to be controlled and exploited as sources of 
radiation. Here we report that bright extreme ultraviolet (XUV) radiation was observed when double foil targets separated by a low 
density plasma were irradiated by a PW-class laser. Simulations show that a dense sheet of relativistic electrons is formed during 
the interaction of the laser with the tenuous plasma between the two foils. The coherent motion of the electron sheet as it transits 
the second foil results in a subcycle XUV pulse, consistent with our experimental observations. 

At intensities of  I > 1018 W/cm2, present-day laser pulses drive target electrons close 
to the velocity of light, generating relativistic laser plasma. These electrons radiate themselves, forming novel sources of 
intense and ultrashort (attosecond) secondary light pulses. Here we review different options such as incoherent emission 
(up to MeV photon energies) by means of Thomson backscattering and electrons making betatron oscillations in plasma wakes 
and self-focusing plasma channels. Special emphasis is given to coherent emission (up to 10 keV photon energies) from solid 
surfaces and ultrathin (nanometer) electron sheets acting as relativistic mirrors. Recent experiments are discussed that 
have verified some of the concepts. These novel sources of x-ray and gamma-ray pulses compete with synchrotron and X-FEL 
sources. They are attractive because of their compact size and have many applications in material sciences, biology, 
and medicine.

We would like to discuss our experimental work on the production of plasma focus by femtosecond laser pulses with 
analysis of spectra and scattered images of filamentations. We are developing theory which combines nonlinear propagation and 
photoionization to describe propagation and scattering of femtosecond laser by a plasma channel created by photoionized gas to 
simulate the spectra and images. We make use of our recent theoretical works on photoionization with arbitrary laser pulses, 
using refined Keldysh theory with inclusion of relativistic effect and Stark shift. Current effort is also on understanding 
the momentum of photoelectron and ion. Finally, we briefly share our proposal of a scheme to produce superintense laser fields 
up to Schwinger limit. 

I will report on a new LPI research program being developed around the ongoing project on a 100-TW CO2 laser. 
New opportunities emerge in several science fields ranging from laser wakefield electron acceleration and shock-wave ion 
acceleration to THz radiation and compact all optical Compton x-ray sources. These new endeavors discussed at a recent ATF 
Upgrade Workshop held at BNL Oct.16-17. 2014 will be highlighted in my talk.  

Recent progress in creation of ionized channels in gases by high intensity short laser pulses makes it possible to 
study a lot of new physical processes and their practical applications. Among them is the generation of extreme ultraviolet and 
soft x-ray radiation and, in particular, pulses of attosecond duration produced in the process of the recombination of the 
photoelectron with the parent ion, when it comes back in the electric field of the laser pulse. 
In this talk new approach is proposed to generate VUV or XUV radiation from a plasma channel formed in a gas media by high-intensity 
laser pulse. It is based on the interference stabilization (IS) phenomenon and population trapping in high-lying excited (Rydberg) 
states near the continuum boundary resulting in the inverse population between the set of Rydberg states and the ground state. 
Estimations for the possible gain factor are performed and it is demonstrated that for typical conditions of IS the gain factor 
for the VUV and XUV frequency range can reach ~ 0.1 - 1 cm-1 that makes it possible to obtain VUV and XUV lasing emission in 
plasma channels produced by high intensity IR laser pulse. We would also note that the VUV or XUV lasing effect based on the 
IS stabilization phenomenon probably can be observed in filaments at rather far away distances from the source of intense IR 
laser radiation.

An undulator based on control of the focusing forces inside a laser-plasma accelerator is proposed. Controlling 
the focusing force is achieved by inducing laser pulse centroid oscillations in a plasma channel. The period of such a plasma 
undulator is proportional to the Rayleigh length of the laser pulse and can be sub-millimeter range. The electron trajectories 
inside the plasma undulator are examined, expressions for the undulator strength are presented, and the spontaneous radiation 
is calculated. Multimode and multicolor laser pulses are considered for greater tunability.

The next generation of particle accelerators will utilize plasmas as source for the seed or in the acceleration stage. 
This requires sophisticated tools to monitor injection into plasma acceleration structures, e.g. wakefields driven by a particle 
beam or laser pulses, and the subsequent acceleration. 
To be able to resolve the acceleration process diagnostics well-suited for this plasma environment need to be designed and realized.  
Sub 10 fs probe pulses enable the time-resolved monitoring of the transient accelerating structure in the plasma, i.e. the plasma wave. 
Increasing the temporal resolution even further by using few cycle pulses the transition from the non-relativistic oscillation of the 
electrons forming a linear plasma wave to a highly relativistic regime including a curving of the wave front and ultimately the 
damping of the trailing wave oscillations except for the leading one, the so-called plasma bubble was observed experimentally. 
Essential characteristics for the laser plasma interaction were derived by a detailed characterization of the transformation of 
the plasma wave during wave breaking and electron injection, which form the basis for a future optimization of the acceleration 
process.
Further, plasma instabilities like Raman forward and backward scattering or Hosing, which can limit the acceleration process are 
discussed and their influence shown.  

Alongside those large-scale and expensive high-brilliance sources and compact plasma-based x-ray lasers arouse great 
interest, notably due to their ability to deliver the highest demonstrated energy per pulse [1] within a narrow linewidth.  But 
also because they display excellent optical properties when seeded with a high-harmonic source [2].
However, those sources have been limited to the picosecond range [3] for more than a decade, which restrained the scope of 
applications.
We proposed and demonstrated an original technique, based on "Collisional Ionization Gating", which allowed us not only to achieve 
femtosecond pulse duration for the first time, but also to report a boost in output energy. The combination of those made possible 
a remarkable enhancement of total peak intensity by about two orders of magnitude compared to the present state of the art. 
Our scheme relies on increasing the plasma density to quench the gain duration of the plasma amplifier, which also leads to an 
increase in saturation intensity and laser gain [4]. At the reported high densities, guiding techniques prove to be pivotal to 
counterbalance refraction [5]. 
The demonstrated plasma-based soft X-ray laser was implemented focusing an ultra-intense IR pule into a krypton gas and pumping 
the atomic transition of Ni-like species at 32.8nm [6].
Tailoring the plasma waveguide for higher densities and longer amplifiers holds great promises to further outdistance previous p
erformances of plasma-based soft X-ray lasers

References 
[1] Rus, B. et al., Phys. Rev. A. 66, 063806 (2002).  
[2] Goddet, J. P. et al. Opt. Lett. 34, 16 (2009).
[3] Wang Y. et al. Nat. Phot. 2, 94-98 (2008).
[4] Mocek, T. et al., Phys. Rev. A. 95, 173902 (2005).
[5] Chou, M. C. et al., Phys. Rev. Lett. 99, 063904 (2007).
[6] Zeitoun, P. et al, Nature 431, 426-429 (2004).

Narrowband x- and gamma-ray sources based on inverse Compton scattering of laser light suffer from a limitation 
of the allowed laser intensity due to the onset of nonlinear effects, which limits the photon yield. At high laser intensity 
the ponderomotive force changes the electrons' longitudinal velocity and leads to a variable red-shift during the scattering 
such that the scattered radiation's spectral bandwidth increases. In this talk I will discuss the possibilities to use chirped 
laser pulses to compensate this ponderomotive broadening and to reduce the bandwidth of the spectral lines, which would allow 
to operate narrowband Compton sources in the high-intensity regime [1]. The optimal frequency modulation of the initial laser 
pulse is derived from the strong-field QED scattering matrix element for nonlinear Compton scattering in the Furry picture, 
where the electron recoil and spin are taken into account [2].

[1] B. Terzić et. al., Phys. Rev. Lett. 112, 074801 (2014).
[2] D. Seipt et. al., arXiv:1412.2659.

The interaction of the extreme intensities associated with state-of-the-art laser and particle beams brings to 
the laboratory conditions also present in extreme astrophysical scenarios. Under these conditions, the physics is very rich 
and highly nonlinear and can only be fully grasped resorting to large scale numerical simulations.  New multi-scale models, 
combined with massively parallel high performance computing, are driving new discoveries and developments. I will review the 
models and some of the recent results from the in silico exploration of extreme plasma physics scenarios, ranging from collisionless 
shocks driven by intense beams, to fundamental magnetic field generation processes, or to the physics and the dynamics of 
intense fields when QED and the relativistic dynamics of electron-positron plasmas start to be relevant. I will also discuss 
how we addressing the challenges posed by the next generation of exaflop supercomputers and their potential impact on 
computational plasma sciences.

A method based on the reflection of a "source" laser pulse by a counterpropagating relativistic "mirror" boosted 
by a "driver" pulse is proposed for generating a single sub-three-cycle and multi-petawatt reflected laser pulse [1]. In our 
scheme, the short duration of the reflected pulse is achieved by employing an ultraintense source pulse, which abruptly disperses 
the plasma mirror after only the first few cycles. Our multidimensional PIC simulations show that even few-cycle source pulses 
can be further shortened (both temporally and in the number of laser cycles) with pulse amplification. A single sub-three-cycle, 
two-petawatt laser pulse with six joules of energy and with a peak intensity exceeding 1023 W/cm2 can be generated already 
by employing next-generation high-power laser systems. In addition, the carrier-envelope phase of the generated few-cycle pulse 
can be tuned provided that the carrier-envelope phase of the initial source pulse is controlled. Such ultrastong and few-cycle 
laser pulse is suitable for probing and potentially controlling strong-field QED processes in the yet unexplored regime of 
ultrashort duration, where qualitatively new features are expected [2].

[1] M. Tamburini, A. Di Piazza, T.V. Liseykina, and C.H. Keitel, Phys. Rev. Lett. 113, 025005 (2014).
[2] A. Di Piazza et al., Rev. Mod. Phys. 84, 1177 (2012);
 S. Meuren et al., Phys. Rev. Lett 107, 260401 (2011);
 A. I. Titov et al., Phys. Rev. Lett 108, 240406 (2012);
 F. Mackenroth et al., Phys. Rev. A 83, 032106 (2011);
 M. Boca et al., Phys. Rev. A 86, 013414 (2012).

Our time resolved x-ray spectroscopy setup is based on a sub-picosecond hard x-ray continuum spectrum from 
a water plasma and a highly efficient energy-dispersive detector. The water-jet plasma target combines a flexible sample 
environment, easy debris handling, and a smooth photon source usable up to 10 keV with sufficient flux to saturate our 
detector. The novel approach is the use of a single photon measuring cryogenic micro-calorimeter array as a high-resolution 
energy-dispersive detector allowing an optics free approach to laboratory pump-probe spectroscopy. With count rates of up 
to 16 kHz, resolution of up to 1.6 eV at 6 keV and efficiencies 1-2 orders of magnitude above most wavelength dispersive 
approaches ultra-fast table-top x-ray spectroscopy becomes available for weaker/faster sources. We present spectra from 
the hard- and soft x-ray range taken with this detector. Iron Kα and Kβ emission peaks as well as the K-edge 
EXAFS is of interest to the design and optimization of novel transition metal based light sensitizers under development 
in our laboratories. The fine structure of nitrogen K$alpha$ emission is the basis to study many highly active catalysts 
and security application. The presented design is a paradigm change in table-top spectroscopy allowing studies from the 
soft x-ray range over tender x-rays into the hard x-ray range with relatively weak sources and without any mayor changes 
to the setup. 

J. Vieira1, R. Trines2, E.P. Alves1, J.T. Mendonça1, R. Bingham2, L.O. Silva1
1 GoLP/Instituto Superior Tecnico, Universidade de Lisboa, Portugal
2 CLF/STFC Rutherford Appleton Laboratory

Since the seminal work by L. Allen et al [1], an abundance of new applications has been identified for laser beams 
with Orbital Angular Momentum (OAM), including super-resolution microscopy, quantum computation or ultra-fast optical 
communications. The potential of OAM light for laser-plasma interactions, however, is yet to be fully realized. One of 
the topics where OAM lasers could have a major impact is laser-wakefield acceleration [2]. Laser-wakefield acceleration 
requires a high intensity (above 1018 W/cm2) laser beam to excite relativistic plasma waves with electric fields in 
excess of 1 GV/cm. The use of intense OAM light has thus been recently proposed as a driver for large amplitude 
plasma waves capable to accelerate electrons and positrons to high energies [3].
Typical configurations for OAM light generation, however, have not yet been able to produce laser beams with 
OAM at the high intensities required for laser plasma interactions. To this end, we explore stimulated Raman 
backscattering of OAM lasers in plasmas [4]. We show analytically and through three-dimensional particle-in-cell 
simulations using the code Osiris [5], that Raman amplification can be used to amplify OAM modes to the required 
intensities for relativistic laser plasma interactions. In addition, we demonstrate that Raman backscattering can 
be used to generate lasers with new, well-defined OAM modes, which are not present initially in the laser beams 
that drive the instability [6]. We also demonstrate generation and amplification of light with arbitrarily large 
OAM states through cascading processes. This work opens new perspectives for the use of OAM light in relativistic 
laser plasma interactions.

[1] L. Allen et al, Phys. Rev. A 8185 (1992).
[2] T. Tajima and J. Dawson, Phys. Rev. Lett. 43, 267 (1979).
[3] J. Vieira and J.T. Mendonça, Phys. Rev. Lett. 112 215001 (2014).
[4] J. T. Mendonça et al, Phys. Rev. Lett. 102 185005 (2009).
[5] R.A. Fonseca et al, Lecture Notes in Computer Science, Vol. 2331, p. 342 (2002).
[6] J. Vieira et al, in preparation (2014).

Terahertz (THz) wave is an electromagnetic radiation with a general frequency interval from 0.1 to 10 THz. 
Due to its unique properties, THz science and technology based on table-top laser system is finding use in an increasingly 
wide variety of important applications: information and communications technology, global environmental monitoring, 
biology and medical sciences, homeland security, etc.. However, the delivery of energetic THz at a remote position in 
the atmosphere is, as of now, limited by linear diffraction and the strong attenuation due to water vapor. Broadband 
and rather powerful THz pulses generated from femtosecond laser filamentation in air would provide a new prospective 
tool for remote THz nonlinear optics and spectroscopy because the technique allows the generation of intense near 
single-cycle THz pulses at long distance by controlling the remote onset of the filament via controlling the initial 
laser parameters. 
In this talk, we briefly review our recent progress on intense, broadband THz generation from femtosecond laser filaments 
in air. There results include pulse chirp control, molecular alignment control, external DC filed control, external 
focusing control on the optimization of THz radiation. Towards remote intense THz generation, we have demonstrated 
few μJ THz radiation at a distance of few tens meters away. As for an application, a spatially resolved measurement 
of plasma density along femtosecond laser filament in air via THz spectroscopy has been demonstrated recently and 
will be mainly discussed.