Search Results (6 total)

At the recently built FLASH x-ray free-electron laser, we studied the reflectivity of Si/C multilayers with fluxes up to 3×10¹⁴  W/cm². Even though the nanostructures were ultimately completely destroyed, we found that they maintained their integrity and reflectance characteristics during the 25-fs-long pulse, with no evidence for any structural changes over lengths greater than 3 Å. This experiment demonstrates that with intense ultrafast pulses, structural damage does not occur during the pulse, giving credence to the concept of diffraction imaging of single macromolecules.

This paper reports on the modeling of the Ni-like silver transient x-ray laser at the wavelength of 13.9 nm. Time-dependent populations and gain are calculated consistently with the output intensity. Two-dimensional refraction, i.e., in the direction of the driving laser and parallel to the slab target surface, is modeled by a ray trace code which is a postprocessor of a hydrodynamic code. Temperatures and electron-density variations are given by the hydrocode. Our calculations show that interaction of the x-ray laser field with the amplifying medium, and refraction, affect the output intensity and reduce the gain values by a large factor: from many hundreds per cm, as predicted by collisional-radiative models ignoring the above interaction, to one hundred per cm, at most.

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.

We model an experiment in which an iron slab target is irradiated by superimposing the six beams of the LULI facility onto a 22 mm×100 μm focal line. The driving laser is composed of two Gaussian pulses of 130-ps duration (full width at half maximum), preceded by a prepulse. A strong enhancement of the 3p-3s 0-1 x-ray line at 25.5 nm in neonlike iron was observed. Saturation is attained for plasma lengths near 1 cm. A ray-trace code working as a postprocessor to a hydroatomic code is used to model the x-ray laser beam refraction due to the variation of the electron density versus the distance to the target surface. Knowing the hydrodynamic conditions, i.e., electron density, and electron and ion temperatures, along the ray paths, the radiative transfer equation, and the population equations are solved self-consistently by using a Maxwell-Bloch approach. A more tractable approach, requiring also the intensity of saturation as a function of time and space, is also presented. In this case, calculation time is much less than in the Maxwell-Bloch approach.

A time-resolved measurement of the output from the Ni-like Ag transient-collisional-excitation x-ray laser is described. An ultrafast x-ray streak camera was used to diagnose the output of the J=0→1 4d-4p lasing line at 13.9 nm. The full width at half maximum duration of the x-ray pulse is measured to be of 1.9±0.7 ps at optimum conditions of pump laser irradiation. This is the shortest x-ray laser duration directly demonstrated to date and illustrates the great potential of transient x-ray lasers as a high brightness, picosecond x-ray source for applications.

We report on progress in the optimization and understanding of the collisional pumping of x-ray lasers in the transient regime using an ultrashort subpicosecond heating pulse. An irradiation scheme using a frequency-doubled 600-ps laser pulse to preform the plasma is tested. The effect of traveling-wave irradiation on the time-integrated and time-resolved lasing signal at the 4d ¹S₀→4p ¹P₁ Ni-like Ag line is studied in detail. Under specific irradiation conditions, strong lasing is also obtained on another spectral line at 16.05 nm that is identified as the 4f ¹P₁→4d ¹P₁ transition in Ni-like Ag.