The dynamics of ionization-induced electron injection in the high density (~ 1:2 × 1019cm-3) regime of Laser Wakefield Acceleration (LWFA) was investigated by analyzing betatron X-ray emission inside dielectric capillary tubes. A comparative study of the electron and betatron X-ray properties was performed for both self-injection and ionization-induced injection. Direct experimental evidence of early onset of ionization-induced injection into the plasma wave was obtained by mapping the X-ray emission zone inside the plasma. Particle-In-Cell (PIC) simulations showed that the early onset of ionization-induced injection, due to its lower trapping threshold, suppresses self-injection of electrons. An increase of X-ray fluence by at least a factor of two was observed in the case of ionization-induced injection due to an increased trapped charge compared to self-injection mechanism.
Ph. Martin, G. Doumy, M. Servol, M. Bougeard, H. Stabile, S. Dobosz, P. Monot, F. Quéré, F. Réau, P. d'Oliveira, H. Lagadec, T. Ceccotti, P. Audebert, J. Geindre, S. Hüller
By focusing an intense femtosecond, high temporal contrast, laser on ultra-thin foils (100 nm) in the 1018W/cm2 intensities range, we demonstrate that we create instantaneously a hot solid-density plasma. The use of highorder harmonics generated in a gas jet, providing a probe beam of sufficiently short wavelengths to penetrate in such media, enables to study the dynamics of this plasma on the picosecond time-scale. The comparison of the transmission of two successive harmonics permits to determine the electronic density and the temperature with an accuracy better than 15% never achieved up to date in relativistic regimes.
We investigate the harmonics generation from a pure dielectric target when submitted to laser intensities in the 1018W/cm2. We demonstrate the negative influence of the prepulses and ASE by addressing the direct comparison of the harmonic spectra with and without the introduction of a perfectly controlled plasma mirror system. Harmonics up to the 20th of the fundamental of the Ti-Sa laser are clearly visible in a situation free of any plasma expansion.
Tatiana Pikuz, Anatoly Faenov, A. Magunov, Igor Skobelev, F. Blasco, C. Stenz, Francois Salin, P. Monot, T. Auguste, Sandrine Dobosz, P. D' Oliveira, S. Hulin, Michel Bougeard
Results of fs laser-produced plasma diagnostics, which were provided using Focusing Spectrometer with Spatial Resolution having as an x-ray detector Princeton x-ray CD or Hamamatsu x-ray MCP, are presented. A portable, high-luminosity spherically bent crystal spectrometers were designed for the purposes of measuring very low emissivity x-ray spectra of different targets, heated by fs laser radiation, with simultaneously high spectral and space resolution. Large open aperture mica spherically bent crystals are used as dispersive elements of spectrometers. High tunability allowed to receive high-resolved spectra of clusters, heated by 35 fs Ti:Sa laser pulses with energy only 15 mJ in spectral ranges: 15-17 angstrom - for H- and He-like ions of Oxygen, 5-5.7 angstrom for Ne-like ions spectra of Kr, 3.0- 3.4 and 3.7-4.4 angstrom for H- and He-like spectra of Ar without any realignment of x-ray CCD spectrometer using one set up. Using another alignment ste up of spectrometer with x-ray MCP has been received spectra of solid targets, heated by 60 fs Ti:Sa laser pulses in spectral ranges: 15.2-17.5 angstrom near resonance lines of He-like ion of fluorine, 7.6-8.75 angstrom for spectra between He(alpha ) and K(alpha ) lines of Al and near Rydberg lines of Ne-like Cu, 3.04 - 3.5 angstrom for spectra around K(alpha ) lines of Ca, 1.38-1.59 angstrom for spectra around K(alpha ) lines of Cu. Some results of plasma diagnostics for both cases of clusters and solid targets, heated by fs laser radiation are presented and discussed.
Review of systematic investigations of x-ray radiation properties of different clusters heated by short-pulse high- intensive TI:Sa laser radiation is presented. The cluster targets were formed by the adiabatic expansion in vacuum of an Ar or CO2 gas puff produced by a pulsed valve with a Laval or conical nozzles. The gas jet pressure is varied form 15 up to 100 bar. Detailed theoretical modeling of cluster parameters have been done and compared with experimental measurements. High spectrally and spatially resolved x-ray spectra near resonance lines of H- and He- like ions of oxygen and Ar have been obtained and detailed spectroscopic analysis was consistent with a theoretical two-temperature collisional-radiative model of irradiated atomic clusters incorporating with an effects of highly energetic electronics. The role of laser prepulse for x-ray intensity emission and its spatial distribution were investigated in details. X-ray spectra radiation from plasma with electron density more than 1022 cm-3 was at first time observed. Big effect of fast electrons influence on the x-ray emission of He-like Ar spectra was demonstrated. Comparison with data under various experimental conditions clearly demonstrated that for increasing x-ray output form plasma the most essential to increase size of clusters and has reasonable value of ps prepulse.
Ne-like and Ni-like ions have been pumped in the transient gain regime using intense picosecond pump pulses from Vulcan in its CPA mode. High gain coefficients of at least approximately 30/cm are observed for the Ne-like ions Ti XIII, Ge XXIII and Sn XXIII at wavelengths 31.2, 19.6 and 12.0 nm respectively and approximately 20/cm for the Ni-like ion Sm XXXV at 7.3 nm. Saturated output is found in all cases for target lengths shorter than 10 mm and the effect of traveling wave pumping has been studied and unequivocably demonstrated. An experimental campaign to observe four wave mixing using a soft x-ray laser and an optical laser in a sum-difference frequency mixing scheme has been initiated. Preliminary results are described and future directions discussed.
Saturated operation of an X-ray laser is desirable as a high output irradiance is obtained with reduced shot-to-short variation. The potential of saturated X-ray laser output in probing plasma samples is first investigated. The laser pumping requirements to scale Ni-like saturated X-ray laser output to shorter wavelengths is then analyzed using published atomic physics data and a simple 4-level laser model for gain. A model of amplified spontaneous emission has been modified to accurately predict experimentally observed saturation behavior obtained in different experiments at the Rutherford Appleton Laboratory. In particular, the effects of traveling wave pumping with short duration (approximately 1 ps) laser pulses are investigated. Simulations of Ne-like Ge resonance line emission are compared to experimentally measured spectra.
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