36 research outputs found
Studies of ablated plasma and shocks produced in a planar target by a sub-nanosecond laser pulse of intensity relevant to shock ignition
No full textThe effect of laser intensity on characteristics of the plasma ablated from a low-Z (CH) planar target irradiated by a 250 ps, 0.438 μm laser pulse with the intensity of up to 1016 W/cm2 as well as on parameters of the laser-driven shock generated in the target for various scale-lengths of preformed plasma was investigated at the kilojoule Prague Asterix Laser System (PALS) laser facility. Characteristics of the plasma were measured with the use of 3-frame interferometry, ion diagnostics, an X-ray spectrometer, and K α imaging. Parameters of the shock generated in a Cl doped CH target by the intense 3ω laser pulse were inferred by numerical hydrodynamic simulations from the measurements of craters produced by the shock in the massive Cu target behind the CH layer. It was found that the pressure of the shock generated in the plastic layer is relatively weakly influenced by the preplasma (the pressure drop due to the preplasma presence is ∼10-20%) and at the pulse intensity of ∼1016 W/cm2 the maximum pressure reaches ∼80-90 Mbar. However, an increase in pressure of the shock with the laser intensity is slower than predicted by theory for a planar shock and the maximum pressure achieved in the experiment is by a factor of ∼2 lower than predicted by the theory. Both at the preplasma absence and presence, the laser-to-hot electrons energy conversion efficiency is small, ∼1% or below, and the influence of hot electrons on the generated shock is expected to be weak
Investigation of laser plasmas relevant to shock ignition at PALS
No full textWe 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. © 2011 SPIE
Generation of high pressure shocks relevant to the shock-ignition intensity regime
No full textAn experiment was performed using the PALS laser to study laser-target coupling and laser-plasma interaction in an intensity regime 1016 W/cm2, relevant for the “shock ignition” approach to Inertial Confinement Fusion. A first beam at low intensity was used to create an extended preformed plasma, and a second one to create a strong shock. Pressures up to 90 Megabars were inferred. Our results show the importance of the details of energy transport in the overdense region
The influence of preformed plasma on a laser-driven shock produced in a planar target at the conditions relevant to shock ignition
No full textThe effect of preformed plasma on a laser-driven shock produced in a planar target at the conditions relevant to the shock ignition scenario of ICF was investigated at the kilojoule PALS laser facility. Characteristics of the preformed plasma were controlled by the delay Δt between the auxiliary beam (1ω, 7×1013 W/cm2) and the main 3ω, 250 ps laser pulse of intensity up to 1016 W/cm2, and measured with the use of 3-frame interferometry, ion diagnostics, an X-ray spectrometer and Kα imaging. Parameters of the shock produced in a CH(Cl) target (25 μm or 40 μm thick) by the intense 3ω laser pulse with energy ranging between 50 J and 200 J were determined by measuring the craters produced by the shock in a massive Cu target behind the layer of plastic. The volume and the shape of these craters was found to depend rather weakly on the preplasma thickness, which implies the same is true for the total energy of shocks and pressure generated by them. From the comparison of the measured crater parameters with those obtained in 2D simulations using the PALE code, it was estimated that for I3ω 1016 W/cm2 the pressure at the rear (non-irradiated) side of the 25-μm plastic layer reaches about 100 Mbar. Copyright © (2013) by the European Physical Society (EPS)
M-L band x-rays (3-3.5 KeV) from palladium coated targets for isochoric radiative heating of thin foil samples
No full textWe describe experiments designed to produce a bright M-L band x-ray source in the 3-3.5 keV region. Palladium targets irradiated with a 10(15) W cm(-2) laser pulse have previously been shown to convert up to similar to 2% of the laser energy into M-L band x-rays with similar pulse duration to that of the incident laser. This x-ray emission is further characterized here, including pulse duration and source size measurements, and a higher conversion efficiency than previously achieved is demonstrated (similar to 4%) using more energetic and longer duration laser pulses (200 ps). The emission near the aluminium K-edge (1.465-1.550 keV) is also reported for similar conditions, along with the successful suppression of such lower band x-rays using a CH coating on the rear side of the target. The possibility of using the source to radiatively heat a thin aluminium foil sample to uniform warm dense matter conditions is discussed
First radiative shock experiments on the SG-II laser
No full textWe report on the design and first results from experiments looking at the
formation of radiative shocks on the Shenguang-II (SG-II) laser at the Shanghai
Institute of Optics and Fine Mechanics in China. Laser-heating of a two-layer
CH/CH-Br foil drives a 40 km/s shock inside a gas-cell filled with argon
at an initial pressure of 1 bar. The use of gas-cell targets with large
(several mm) lateral and axial extent allows the shock to propagate freely
without any wall interactions, and permits a large field of view to image
single and colliding counter-propagating shocks with time resolved,
point-projection X-ray backlighting ( m source size, 4.3 keV
photon energy). Single shocks were imaged up to 100 ns after the onset of the
laser drive allowing to probe the growth of spatial non-uniformities in the
shock apex. These results are compared with experiments looking at
counter-propagating shocks, showing a symmetric drive which leads to a
collision and stagnation from 40 ns onward. We present a preliminary
comparison with numerical simulations with the radiation hydrodynamics code
ARWEN, which provides expected plasma parameters for the design of future
experiments in this facility
Single-shot multi-keV x-ray absorption spectroscopy using an ultrashort laser-wakefield accelerator source
No full textExperimental measurements of the collisional absorption of XUV radiation in warm dense aluminium
No full textThe collisional (or free-free) absorption of soft x rays in warm dense aluminium remains an unsolved problem. Competing descriptions of the process exist, two of which we compare to our experimental data here. One of these is based on a weak scattering model, another uses a corrected classical approach. These two models show distinctly different behaviors with temperature. Here we describe experimental evidence for the absorption of 26-eV photons in solid density warm aluminium (Te≈1 eV). Radiative x-ray heating from palladium-coated CH foils was used to create the warm dense aluminium samples and a laser-driven high-harmonic beam from an argon gas jet provided the probe. The results indicate little or no change in absorption upon heating. This behavior is in agreement with the prediction of the corrected classical approach, although there is not agreement in absolute absorption value. Verifying the correct absorption mechanism is decisive in providing a better understanding of the complex behavior of the warm dense state.</p
Experimental results performed in the framework of the HiPER European Project
No full textThis paper presents the goals and some of the results of experiments conducted within the Working Package 10 (Fusion Experimental Programme) of the HiPER Project. These experiments concern the study of the physics connected to "Advanced Ignition Schemes", i.e. the Fast Ignition and the Shock Ignition Approaches to Inertial Fusion. Such schemes are aimed at achieving a higher gain, as compared to the classical approach which is used in NIF, as required for future reactors, and making fusion possible with smaller facilities. In particular, a series of experiments related to Fast Ignition were performed at the RAL (UK) and LULI (France) Laboratories and were addressed to study the propagation of fast electrons (created by a short-pulse ultra-high-intensity beam) in compressed matter, created either by cylindrical implosions or by compression of planar targets by (planar) laser-driven shock waves. A more recent experiment was performed at PALS and investigated the laser-plasma coupling in the 1016 W/cm2 intensity regime of interest for Shock Ignition. © 2011 SPIE
X-ray diagnostics of fast electrons propagation in high density plasmas obtained by cylindrical compression
No full textWe report on X-ray diagnostics results from an experiment on fast electrons propagation in cylindrically compressed targets. It was performed on the
VULCAN TAW laser facility at RAL (UK) using four long pulses (1ns, 70 J each at 2!) to compress a cylindrical polyimide target lled with CH foam at 3 di erent
initial densities. The cylindrical geometry allows us to reach temperatures and densities higher than those obtained in planar geometry compression. 2D
hydrodynamic simulations predicted a core density range from 4 to 8 g=cm3 and a core temperature from 30 eV up to 175 eV at maximum compression. An
additional short laser pulse (10 ps, 160 J at !) was focused on a Ni foil at one of the cylinder edges in order to generate a fast electrons current propagating along
the compressed target. A X-ray radiography diagnostic was implemented in order to estimate the core plasma conditions of the compressed cylinder. Moreover
two Bragg X-ray spectrometers collected the K fuorescence from the target so as to determine the variations of fast electrons population during the compression
