Search Results (8 total)

We present a detailed comparison of the measured characteristics of Thomson backscattered x rays produced at the Picosecond Laser-Electron Interaction for the Dynamic Evaluation of Structures facility at Lawrence Livermore National Laboratory to predicted results from a newly developed, fully three-dimensional time and frequency-domain code. Based on the relativistic differential cross section, this code has the capability to calculate time and space dependent spectra of the x-ray photons produced from linear Thomson scattering for both bandwidth-limited and chirped incident laser pulses. Spectral broadening of the scattered x-ray pulse resulting from the incident laser bandwidth, perpendicular wave vector components in the laser focus, and the transverse and longitudinal phase spaces of the electron beam are included. Electron beam energy, energy spread, and transverse phase space measurements of the electron beam at the interaction point are presented, and the corresponding predicted x-ray characteristics are determined. In addition, time-integrated measurements of the x rays produced from the interaction are presented and shown to agree well with the simulations.

The measurement of emittance in space-charge dominated, high brightness beam systems is investigated from conceptual, computational, and experimental viewpoints. As the self-field-induced collective motion in the low energy, high brightness beams emitted from photoinjector rf guns are more important in determining the macroscopic beam evolution than thermal spreads in transverse velocity; traditional methods for phase space diagnosis fail in these systems. We discuss the role of space charge forces in a traditional measurement of transverse emittance, the quadrupole scan. The mitigation of these effects by use of multislit- or pepper-pot-based techniques is explained. The results of a direct experimental comparison between quadrupole scanning and slit-based determination of the emittance of a 5 MeV high brightness electron beam are presented. These data are interpreted with the aid of both envelope and multiparticle simulation codes. It is shown that the ratio of the beam’s β function to its transverse plasma wavelength plays a central role in the quadrupole scan results. Methods of determining the presence of systematic errors in quadrupole scan data are discussed.

Exploiting the energetic interaction of intense femtosecond laser pulses with deuterium clusters, it is possible to create conditions in which nuclear fusion results from explosions of these clusters. We have conducted high-resolution neutron time-of-flight spectroscopy on these plasmas and show that they yield fast bursts of nearly monochromatic fusion neutrons with temporal duration as short as a few hundred picoseconds. Such a short, nearly pointlike source now opens up the unique possibility of using these bright neutron pulses, either as a pump or a probe, to conduct ultrafast studies with neutrons.

The interaction of high-power, subpicosecond laser pulses with gas targets is expected to produce highly nonequilibrium plasmas whose parameters are controlled by the laser wavelength and polarization. We investigate such plasmas by measuring time-resolved x-ray-emission spectra in highly ionized helium and neon plasmas produced by high-power optical ionization. Electron temperatures are observed to increase with increasing laser wavelength and with variation of the laser polarization from linear to circular. These results are in qualitative agreement with current models for production of tunnel-ionized laser plasmas. Limited quantitative agreement, however, reflects the complexity of the optical ionization process and suggests the important role rapid cooling processes can play in these plasmas. Emission spectra are combined with time-dependent kinetic simulations to assess prospects for x-ray lasers pumped by rapid electron-ion recombination.

We report the first measurements on the distribution of electron energies and associated recombination kinetics in tunnel-ionized plasmas of interest to recent x-ray laser proposals. Subpicosecond laser pulses, focused to an intensity of 1×10¹⁶ W /cm², create a fully ionized helium plasma. Measurement of the resulting emission spectrum allows a determination of the time-dependent electron velocity distribution. The initial distribution is found to be strongly non-Maxwellian and subsequent recombination kinetics proceed more slowly than predicted for a Maxwellian distribution of electrons.

Experimental measurements of the absolute energy yields and conversion efficiencies of high-order harmonic radiation in the spectral range of 31 to 17 nm are presented. We examine energy yields as a function of a number of parameters including drive laser wavelength, target atom, focal geometry, and peak laser intensity, and we have generated individual harmonics with energies as high as 60 nJ at wavelengths as short as 20 nm. Under optimum conditions, we find that conversion efficiencies of as high as 10^-7 into each harmonic are possible.

We employ an alternative technique for determining the Lorentzian linewidth based upon the simultaneous measurement of the vapor transmission and group-velocity delay outside the Doppler core of an optically dense transition. We apply this technique to measure the cross sections for broadening of the intercombination lines of calcium, 4s^{2 1}S₀→4s4p ³P₁, and strontium, 5s^{2 1}S₀→5s5p ³P₁, by collisions with argon, neon and the ground-state neutral vapor. We compare our measured results with calculated values as well as previously determined values by other workers.

Production of sum-frequency radiation and ‘‘even’’ harmonics from the mixed 1ω (1053 nm) and 2ω (527 nm) field is observed and compared to measurements of the relative conversion efficiency of the independent fields. By controlling the relative polarization of the two fields, we can control the mixing efficiency and produce coherent extreme ultraviolet (XUV) radiation polarized orthogonally to the strong driving field. Extension to produce XUV radiation of arbitrary polarization is discussed.