Betatron radiation from laser-plasma accelerators reproduces the principle of a synchrotron on a millimeter scale, but featuring femtosecond duration. Here we present the outcome of our latest developments, which now allow us to produce stable and polarized X-ray bursts. Moreover, the X-ray polarization can simply be adjusted by tuning the polarization of the laser driving the process. The excellent stability of the source is expressed in terms of pointing, flux, transverse distribution and critical energy of the spectrum. These combined features make our betatron source particularly suitable for applications in ultrafast X-ray science.
In this presentation we will describe the generation process, relying on the ionization injection scheme for laser-plasma acceleration. We will show experimental measurements, numerical results and first applications in time-resolved spectroscopy.
One of the key ingredients of laser-plasma accelerators is their injector, which defines how electrons are trapped into the laser-driven plasma wave. The stability and control of laser-plasma electron bunches strongly depends on this injection stage. Self-injection is a convenient way to achieve the electron trapping and is the most widely used injector. Here we demonstrate, by using a variable length gas cell, that injection can be achieved by either longitudinal or transverse self-injection, giving rise to very different electron beam features. The results are supported by 3 dimensional particle-in-cell simulations.
High intensity femtosecond laser pulses can be used to generate X-ray radiation. In the laser wakefield process, when a
high intensity laser pulse (<1018 W/cm2) is focused onto a gas jet target, it interacts with the instantaneously created under-dense plasma and excites a wakefield wave. In the wakefield electrons are trapped and accelerated to high energies in short distances. The electrons trapped in the wakefield can perform Betatron oscillations across the
propagation axis and emit X-ray photons. The Betatron X-ray beam is broadband as the radiation emission has a
synchrotron distribution. The X-ray beam is collimated and its pulse duration is femtosecond. For high resolution and
phase contrast X-ray imaging applications, the important feature of the X-ray Betatron beam is the μm source size.
Using ALLS 100 TW class laser system we demonstrate that the Betatron X-ray beam is both energetic and bright
enough to produce single laser shot phase contrast imaging of complex objects located in air.
We are exploring the use of the ultra-high contrast 200 TW ALLS facility (5 J, 28 fs, 10 Hz repetition rate) as a
basic tool to image in real time with X-rays (generated by the laser) tumors during their irradiation by protons
(accelerated by the same laser). The feasibility of phase contrast imaging in in-line geometry and proton acceleration
with 100 TW (3 J, 30 fs) on targets is studied and presented in the present paper. We demonstrate here that phase
contrast x-ray imaging, of tests and complex objects located in air at 1m from the X-ray source, can be achieved in one
shot using our betatron x-ray source generated in a supersonic gas jet. Using solid targets (thin and thick foils) our
experiments indicate that protons are accelerated at a maximum energy of 12 MeV.
Beams of X-rays of few keV energy have been produced from laser-supersonic gas jet interaction. Betatron X-ray
radiation is generated when energetic electrons are accelerated and experience betatron oscillations in the ion channel
produced in the wake of a high intensity femtosecond laser pulse. Experiments took place at the 200 TW laser system (5
J, 25 fs, 10 Hz) of the Advanced Laser Light source facility (ALLS). Thanks to the laser system performance these
preliminary results are the first steps to an expected improvement of the X-rays beams characteristics (collimation,
brightness and energy above the keV range).
In this paper, we present the first temporal characterization of betatron X-ray radiation. Results obtained
from time resolved x-ray diffraction experiments, for which the ultra fast phase transition of non thermal melting
of InSb was used, indicates that the x-ray pulse duration is less than 1 ps. We then propose a novel technique
to improve the spectral and flux properties of the x-ray source. The energy and the flux can be enhanced when
the electron beam propagates and oscillates in a tailored plasma density profile.
We propose an accelerator based 4th generation source to provide coherent femtosecond light pulses down to the soft X
ray range to the user community. The project is based on a CW 700 MeV to 1 GeV superconducting linear accelerator
delivering high charge, subpicosecond, low emittance electron bunches with high repetition rate. This facility allows for
providing High Gain Harmonic Generation seeded with high harmonics in gases, covering a spectral range down to
0.8 nm. In addition, two beam loops are foreseen to increase the beam current in using the energy recovery technique.
They will accommodate fs synchrotron radiation sources in the IR, VUV and X ray ranges together with a FEL oscillator
in the 10 nm range. A particular emphasis is put on the synergy between accelerator and laser communities. In particular,
electron plasma acceleration will be tested. Hard X ray femtosecond radiation will be produced by Thomson Scattering.
The first phase of the project, ARC-EN-CIEL phase 1, is now under study. A general overview is given.
We present the experimental and theoretical studies of the optical response from the single-crystal of bismuth to the excitation by the femtosecond laser pulse. The experimental results revealed a complex, first - positive and a few picoseconds later - negative, change in time-dependent reflectivity, which could not be explained in the light of the existing theories. It is shown that reflectivity oscillations are related to the excitation of coherent phonons by the pulse with duration shorter of all relaxation times. We demonstrate that swiftly heated electrons are responsible for the phonon excitation due to the fast modification of the attractive (electronic) part of inter-atomic potential. The electronic perturbation of potential is also responsible for the red shift of phonon frequency and for the increase in the amplitude of phonons. The coherent phonon excitation process as well as the change in the reflectivity is related mainly to the modification of the electron-phonon momentum exchange frequency. The comparison between the theory and experiments shows an excellent agreement. Moreover, the reflectivity measurements allow direct recovery of the electron-phonon coupling rate in bismuth crystal, which has not been measured before.
G. Lambert, B. Carre, M. Couprie, M. Desmons, O. Chubar, B. Gilquin, D. Garzella, M. Jablonka, M. Labat, A. Loulergue, J. Marques, J. Ortega, F. Meot, P. Monot, A. Mosnier, L. Nahon, A. Rousse
The French project of a fourth generation light source, ARC-EN-CIEL (Accelerator-Radiation for Enhanced Coherent Intense Extended Light), is a unique facility providing the user community with coherent femtosecond light pulses covering the UV, VUV and soft X ray spectral range. It is based on a CW 1 GeV superconducting linear accelerator delivering high charge, subpicosecond, low emittance electron bunches with high repetition rate (1 kHz), and adjustable polarisation until 1 keV. In addition to the High Gain Harmonic Generation (HGHG) experiment seeded with High Harmonics in Gases (HHG), allowing radiation down to 0.8 nm to be produced, two beam loops are foreseen to increase the beam current in using the energy recovery technique. They will accommodate fs synchrotron radiation sources in the IR, VUV and X ray ranges together with a FEL oscillator in the 10 nm range. Moreover, an important synergy is expected between accelerator and laser communities. Indeed, electron plasma acceleration will be tested and hard X ray femtosecond radiations will be produced by Thomson Scattering. The first phase of the project, ARC-EN-CIEL phase 1, is now under study. A general overview will be given.
In this paper we discuss some result achieved at Laboratoire d'Optique Appliquee that may improve the capabilities of the laser-produced plasma x-ray source for applications in the study of ultrafast transient structures.
We report recent investigations on collisional Optical-Field Ionization soft x-ray lasers. The amplifying medium is generated by focusing a high energy circularly polarized, 35 fs 10 Hz Ti: sapphire laser system in a few mm cell filled with gas (xenon or krypton). Using xenon, a gain of 67 cm-1 on the 4d95p-4d95d transition at 41.8 nm in Pd-like xenon and a gain-length product of 15 have been inferred at saturation. This source delivers about 5 109 photons per pulse. Using krypton, a large amplification of the 3d94d-3d94p line at 32.8 nm has been observed for the first time. The influence of the pumping energy and the laser polarization on the lasing output are also presented.
Non-thermal melting of semiconductor crystals, phase transitions on a sub-picosecond time scale can be studied by optical pump x-ray probe experiments. Powerful femtosecond lasers deliver brilliant ultrashort K(alpha ) pulses on a time scale from 100 fs to 1 ps that can be optimized for these pump-probe experiments. These experiments consist of two diffracting elements: (i) a bent crystal imaging the flash x-ray source in a narrow spectral window; and (ii) the sample crystal diffracting the ultrashort x-ray pulse. As penetration depths of optical pump beam are usually much shorter than x-ray extinction depths, best sensitivity to ultrafast structural changes is obtained for minimum x-ray extinction depths. This can be achieved by selecting samples containing heavy elements, thin crystalline film samples and by using asymmetric Bragg reflections, respectively. Several theoretical codes have been developed to optimize design of the instruments. X-ray topographic cameras and diffractometers were modified for fabrication and characterization of 2D bent crystals. Best practical results were obtained when structurally perfect wafers of Si, Ge, and quartz crystals were prepared while monitored by x-ray topography and diffractometry. After a final check of x-ray imaging and reflection properties of the toroidal crystals, monochromatic x-ray beam and laser pump beam are adjusted spatially to coincide on the sample crystal. Because converging x-rays impinge on the sample crystal, its rocking curve can be registered as a spatial distribution on the detector. In comparison to synchrotron experiments where about 104 pulses must be integrated, in these experiments rocking curves can be recorded in a single or in a few laser shots. Ultrafast processes are studied in Langmuir Blodgett films containing Cd, in bulk semiconductors, such as InSb, and in CdTe semiconductor films. Focused, pulsed monochromatic x-rays have been transmitted through biological samples to register many reflections, which opens the way to ultrafast studies in structural biology.
We have characterized the ultrafast solid-liquid transition of InSb and CdTe semiconductors by time resolved x-ray diffraction in the femtosecond timescale. Visible spectroscopic data were obtained together with x-ray measurements to characterize the dense electron-ho9le plasma at the origin of the phase transition following the IR excitation.
The emission from plasmas created with fs-lasers provides sub-picosecond x-ray pulses in the keV-range. Intense emission of K(alpha) lines as well as quasi continuum x-rays can be used for time-resolved diffraction and spectroscopy, i.e. to study lattice or atomic dynamics with sub-picosecond resolution by using a laser pump x-ray probe technique. The x-ray yield and x-ray pulse duration of the laser plasma source depend on the laser parameters and the target design, such as intensity, laser wavelength, pulse duration and prepulse level. To accumulate as many photons as possible of the isotropic source an efficient large aperture optic has to be used to select an x-ray line or a wavelength range and focus the radiation onto the sample. It is shown that the use of toroidally bent crystals provides the possibility to refocus 10-4 of the photons emitted in the whole solid angel to spot size of around 80 micrometers with a temporal broadening below 100 fs. Combinations of bent focusing crystals with a flat sample crystal for fast x-ray diffraction application are discussed. Experiments showing the temporal response of laser heated crystals are presented and compared with theoretical simulations based on Takagi-Taupin theory.
Optical pump, x-ray diffraction probe experiments have been used to study the lattice dynamics of organic materials using a laser-produced plasma x-ray source. The x-ray source is generated from a 10 Hz, 26 mJ, 120 fs laser beam focused on a silicon wafer target. The emitted K(alpha ) x-ray radiation is used to probe a cadmium arachidate Langmuir-Blodgett film and a TlAP crystal optically perturbed at laser fluences from 1.8 J/cm2 to 27 J/cm2. Ultrafast disordering inside the lattice -- within a time scale below 600 fs to few tens of picoseconds -- is clearly observed and produce a drop of the probe x-ray diffracted signal.
Short-pulse laser-produced plasmas look very promising for the generation of sub-picosecond X-rays. By combining several experimental techniques, we have significantly progressed towards a better understanding of ultrafast laser-matter interaction. The X-ray yield is a sensitive function of the electron density gradient scale length of the target plasma. In this work, the scale length has been changed by varying the temporal separation between the main laser pulse and a lower intensity prepulse. X-ray spectroscopic diagnostics of the plasma parameters have been used from the analysis of resonance and dielectronic satellite lines. The angular and energy distribution of suprathermal electrons emitted during the ultrafast laser- plasma interaction have been measured as a function of laser polarization and prepulse delay. Frequency-domain interferometry and optical measurements of the reflected probe pulse have been used to study the velocity and the gradient scale length of the expanding plasma. The Kα emission yield peaks for a scale length where resonant absorption is optimized. Hydrodynamic simulations have been performed to investigate the plasma dynamics and the basic processes which control the X-ray emission duration and intensity. Applications of ultrashort Kα X-rays to the diagnostic of solid plasma conditions and as a source for time-resolved diffraction and spectroscopy of transient chemical, biological or physical phenomena are underway.
To simulate the interaction of high laser intensity with solid targets, we have used the 1D code FILM in which the collisional plasma absorption is calculated by solving the linear electromagnetic field for p and s polarization. For p-polarized light the collision frequency is adjusted so that the field in the critical region of the plasma never exceeds the maximum field allowed by the wave breaking limit. Energy transport by thermal conduction is described with the help of the delocalized heat flux theory. The ponderomotive force resulting from the huge filed is taken into account. The calculated temperatures and ion densities are used as an input to a time-dependent atomic physics code. Non-stationary ionization dynamics is demonstrated.
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