P. Koester

Prepulse effect on intense femtosecond laser pulse propagation in gas

The propagation of an ultrashort laser pulse can be affected by the light reaching the medium before the pulse. This can cause a serious drawback to possible applications. The propagation in He of an intense 60-fs pulse delivered by a Ti:sapphire laser in the chirped pulse amplification (CPA) mode has been investigated in conditions of interest for laser-plasma acceleration of electrons. The effects of both nanosecond amplified spontaneous emission and picosecond pedestals have been clearly identified. There is evidence that such effects are basically of refractive nature and that they are not detrimental for the propagation of a CPA pulse focused to moderately relativistic intensity. The observations are fully consistent with numerical simulations and can contribute to the search of a stable regime for laser acceleration.

Electron radiography using a table-top laser-cluster plasma accelerator

We explore the use of a laser-based electron gun for applications in transmission electron radiography and microscopy at electron energies up to 2 MeV. This new approach holds the promise to overcome some limitations of existing conventional electron guns at high beam energies especially for ultrafast applications. Our laser-electron gun is based on titanium-sapphire, ultrashort pulse lasers to drive electron acceleration in a plasma. The focused laser pulse travels in a tailored Ar gas target and accelerates electrons to MeV energy in less than a millimetre. As a first application, we use this electron beam to perform contact transmission electron radiography of cm-scale thin and thick samples. We obtain transmission electron radiography of organic and inorganic dense objects over a field of view more than 50 mm wide. The images are well exposed and show details of both thick and thin samples. The spatial resolution for the current geometrical configuration was found to be approximately 60 µm and was limited by geometrical effects combined with the intrinsic detector resolution and diffusion in the sample.

Thomson Scattering Imaging From Ultrashort Ultraintense Laser Interaction With Gas

Laser-plasma acceleration can provide acceleration gradients which are thousands of times stronger than conventional electron accelerators. The laser propagation length is a crucial parameter that must be extended to achieve high-energy electrons. Here, we show that color images of the laser-plasma interaction region taken from the direction perpendicular to the polarization plane are a powerful tool to discriminate between Thomson scattering and plasma self-emission, leading to a precise measurement of the propagation length.

X-ray conversion of ultra-short laser pulses on a solid sample: Role of electron waves excited in the pre-plasma

Flat silicon samples were irradiated with 40 fs, 800 nm laser pulses at an intensity at the best focus of 2·1018 Wcm−2, in the presence of a pre-plasma on the sample surface. X-ray emission in the spectral range from 2 to 30 keV was detected inside and outside the plane of incidence, while varying pre-plasma scale length, laser intensity, and polarization. The simultaneous detection of 2ω and 3ω/2 emission allowed the contributions to the X-ray yield to be identified as originating from laser interaction with either the near-critical density (nc) region or with the nc/4 region. In the presence of a moderate pre-plasma, our measurements reveal that, provided the pre-plasma reaches a scale-length of a few laser wavelengths, X-ray emission is dominated by the contribution from the interaction with the under dense plasma, where electron plasma waves can grow, via laser stimulated instabilities, and, in turn, accelerate free electrons to high energies. This mechanism leads also to a clear anisotropy in the ang...

Laboratory measurements of resistivity in warm dense plasmas relevant to the microphysics of brown dwarfs.

Since the observation of the first brown dwarf in 1995, numerous studies have led to a better understanding of the structures of these objects. Here we present a method for studying material resistivity in warm dense plasmas in the laboratory, which we relate to the microphysics of brown dwarfs through viscosity and electron collisions. Here we use X-ray polarimetry to determine the resistivity of a sulphur-doped plastic target heated to Brown Dwarf conditions by an ultra-intense laser. The resistivity is determined by matching the plasma physics model to the atomic physics calculations of the measured large, positive, polarization. The inferred resistivity is larger than predicted using standard resistivity models, suggesting that these commonly used models will not adequately describe the resistivity of warm dense plasma related to the viscosity of brown dwarfs.

Space- and time-resolved observation of extreme laser frequency upshifting during ultrafast-ionization

A 65-fs, 800-nm, 2-TW laser pulse propagating through a nitrogen gas jet has been experimentally studied by 90° Thomson scattering. Time-integrated spectra of scattered light show unprecedented broadening towards the blue which exceeds 300 nm. Images of the scattering region provide for the first time a space- and time-resolved description of the process leading quite regularly to such a large upshift. The mean shifting rate was as high as δλ/δt ≈ 3 A/fs, never observed before. Interferometry shows that it occurs after partial laser defocusing. Numerical simulations prove that such an upshift is consistent with a laser-gas late interaction, when laser intensity has decreased well below relativistic values (a0 ≪ 1) and ionization process involves most of the laser pulse. This kind of interaction makes spectral tuning of ultrashort intense laser pulses possible in a large spectral range.

High-charge divergent electron beam generation from high-intensity laser interaction with a gas-cluster target

We report on an experimental study on the interaction of a high-contrast 40 fs duration 2.5 TW laser pulse with an argon cluster target. A high-charge, homogeneous, large divergence electron beam with moderate kinetic energy (~2 MeV) is observed in the forward direction. The results show, that an electron beam with a charge as high as 10 nC can be obtained using a table-top laser system. The accelerated electron beam is suitable for a variety of applications such as radiography of thin samples with a spatial resolution better than 100 micron.

Laser-Plasma Particle Sources for Biology and Medicine

Ultrashort, intense laser pulses can drive in plasmas small sized linear accelerators (Laser-Linac’s) of high energy elementary particles. These novel devices are facing a continuous, fast progress making them suitable alternatives to conventional linacs in many applications. Among them, cancer therapy may have by far the highest social impact at a global level. This paper is aimed at giving an updated overview of the scientific and technological effort devoted worldwide to the optimization of the laser acceleration technology in order to fulfill the clinical requirements. Here we discuss both ion and electron acceleration considering the different, challenging problems to be solved in each case. Current studies on radiobiology already in progress in many labs with the existing laser-based sources of particles are also described. The overall scenario in the field appears extremely exciting, and promises rapid, effective development.

Investigation of laser plasmas relevant to shock ignition at PALS

We present the results of an experiment concerning laser-plasma interaction in the regime relevant to shock ignition. The interaction of high-intensity frequency tripled laser pulse with CH plasma preformed by lower intensity pre-pulse on fundamental wavelength of the kJ-class iodine laser was investigated in the planar geometry in order to estimate the coupling of the laser energy to the shock wave or parametric instabilities such as stimulated Raman or Brillouin scattering, or to the fast electrons. First the complete characterization of the hydrodynamic parameters of preformed plasma was made using crystal spectrometer to estimate the electron temperature and XUV probe to resolve the electron density profile close to the critical density region. The other part of the experiment consisted of the shock chronometry, calorimetry of the back-scattered light and hard X-ray spectrometry to evaluate the coupling to different processes. The preliminary analysis of the measurements showed rather low energy transfer of the high-intensity pulse to back-scattered light (< 5%) and no traces of any significant hot electron production were found in the X-ray spectra.

Parametric Instabilities Study in a Shock Ignition Relevant Regime

Inertial Confinement Fusion with Shock Ignition relies on a very strong shock created by a laser pulse at an intensity of the order of 1016W/cm2. In this context, an experimental campaign at the Prague Asterix Laser System (PALS) has been carried out within the frame of the HiPER project. Two beams have been used, the first to create an extended preformed plasma (scale length of the order of hundreds of micrometers) on a planar target, the second to generate a strong shock wave. Different diagnostics were used to study both the shock breakout at the rear surface of the target and the laserplasma coupling and parametric instabilities. This paper is focused on back-scattering analysis to measure the backreflected energy and to characterize parametric instabilities such as stimulated Brillouin and Raman scattering. Our experimental data show that parametric instabilities do not play a strong role in the laser plasma coupling. Moreover, preliminary analysis of the back reflected light from the interaction region shows that less than 5% of the total incident laser energy was back-reflected, with only a small fraction of that light was originating from parametric instabilities.