We report on the x-ray absorption of Warm Dense Matter experiment at the FLASH Free Electron Laser (FEL) facility at DESY. The FEL beam is used to produce Warm Dense Matter with soft x-ray absorption as the probe of electronic structure. A multilayer-coated parabolic mirror focuses the FEL radiation, to spot sizes as small as 0.3μm in a ~15fs pulse of containing >1012 photons at 13.5 nm wavelength, onto a thin sample. Silicon photodiodes measure the transmitted and reflected beams, while spectroscopy provides detailed measurement of the temperature of the sample. The goal is to measure over a range of intensities approaching 1018 W/cm2. Experimental results will be presented along with theoretical calculations. A brief report on future FEL efforts will be given.
An ultrafast x-ray streak camera is under development at LBNL for application primarily to studies of ultrafast magnetization dynamics. In initial work, a temporal resolution of 900fs in accumulative mode at 5 KHz has been achieved. These results and methods currently being developed to improve the resolution and repetition rate are resented. One of the primary limits to temporal resolution is caused by the finite energy width of the electron distribution from the photocathode. The positive time of flight dispersion with energy in the accelerating region of the camera can be countered by introduction of downstream optics that give negative time of flight dispersion with energy, leading to an approximate overall cancellation of this temporal aberration. Initial results of an end-to-end simulation model using the full photoelectron distribution are presented.
The performance of CsI photocathodes has been characterized for use with grazing incidence soft x-rays. The total electron yield and pulsed quantum efficiency from a CsI photocathode has been measured in a reflection geometry as a function of photon energy (100 eV to 1 keV), angle of incidence and the electric field between the anode and photocathode. The total electron yield and pulsed quantum efficiency increase as the x-ray penetration depth approaches the secondary electron escape depth. Unit quantum efficiency in a grazing incidence geometry is demonstrated. A weak electric field dependence is observed for the total yield measurements; whilst no significant dependence is found for the pulsed quantum efficiency. Theoretical predictions agree accurately with experiment.
Although the realisation of femtosecond X-ray free electron laser (FEL) X-ray pulses is still some time away, X-ray diffraction experiments within the sub-picosecond domain are already being performed using both synchrotron and laser- plasma based X-ray sources. Within this paper we summarise the current status of some of these experiments which, to date, have mainly concentrated on observing non-thermal melt and coherent phonons in laser-irradiated semiconductors. Furthermore, with the advent of FEL sources, X-ray pulse lengths may soon be sufficiently short that the finite response time of monochromators may themselves place fundamental limits on achievable temporal resolution. A brief review of time-dependent X-ray diffraction relevant to such effects is presented.
Time-dependent x-ray diffraction has been measured from laser-irradiated semiconductor crystals. Laser pulses with 100 fs duration and 800 nm wavelength excite the sample inducing phase transitions. 5 keV x-rays from the Advanced Light Source are diffracted by a sagittally-focusing Si (111) crystal and then by the sample crystal, InSb (111), onto an avalanche photodiode. By detecting individual pulses of synchrotron radiation, which have a duration of 70 ps, the diffracted intensity is observed to decrease because of photoabsorption in a disordered surfaced layer. Rocking curves measured after the laser irradiation show a tail, which results from a strained region caused by expansion of the crystal lattice.
We have studied theoretically and experimentally the x-ray production above 1 keV from femtosecond laser plasmas generated on periodically modulated surface targets. Laser energy coupling to plasma surface waves has been modeled using a numerical differential method. Almost total absorption of incident laser radiation is predicted for optimized interaction conditions. Silicon gratings have been irradiated by a 120 fs Ti: sapphire laser at irradiances in excess of 1016W/cm2. X-ray intensities above 1.5 keV (K-shell lines) have been measured as a function of the incidence angle. Results show a distinct x-ray emission maximum for the first order diffraction angle and are in good qualitative agreement with our theoretical predictions.
A multi-terawatt laser was used to generate and characterize plasmas appropriate for recombination pumped x-ray lasers. Thomson scattering was used to determine the electron and ion temperatures of a laser-produced, high-density helium plasma on a subpicosecond time-scale. A recombination pumped x-ray laser on the Lyman-alpha transition of hydrogen- like lithium was also studied.
Laser pulses with high intensity (up to 1018 W/cm2) and short duration (100 fs) were focused on gases and solids. The result was ionized material, and emission of short pulse x-rays and unicycle electromagnetic pulses with subpicosecond duration.
Current x-ray lasers operate in the 35 to 500 angstroms wavelength regimes with 21.5 angstroms being the limit for the successful collisional excitation approach using Nickel-like ions. In this paper, we discuss an x-ray laser scheme in the 5 to 15 angstroms regime. The scheme uses an ultra-short (100 fsec FWHM) intense (1017 Watts/cm2) laser pulse to produce a hot plasma at a laser line focus. This plasma serves as an x-ray source that photo- ionizes the K shell of a nearby concentration of lasant atoms. This produces a population inversion, and resulting positive gain for an allowed 2p-1s radiative transition in the singly charged ion.
We report preliminary results from the analysis of streaked soft x-ray neon spectra obtained from the interaction of a picosecond Nd:glass laser with a gas jet target. In these experiments streaked spectra show prompt harmonic emission followed by longer time duration soft x-ray line emission. The majority of the line emission observed was found to originate from Li- and Be-like Ne and the major transitions in the observed spectra have been identified. Li-like emission lines were observed to decay faster in time than Be-like transitions, suggesting that recombination is taking place. Line ratios of n equals 4 - 2 and n equals 3 - 2 transitions supported the view that these lines were optically thin and thick, respectively. The time history of Li-like Ne 2p-4d and 2p-3d lines is in good agreement with a simple adiabatic expansion model coupled to a time dependent collisional-radiative code. Further x-ray spectroscopic analysis is underway which is aimed at diagnosing plasma conditions and assessing the potential of this recombining neon plasma as a quasi-steady-state recombination x-ray laser medium.
Progress on developing a multiterrawatt source of pulses with duration <100 fsec is described. The laser system is based
on chirped-pulse amplification in titanium-doped sapphire. Experimental results include the development of a stable oscillator
running at 812 nm, using hybrid mode-locking and compressed pulse pumping to produce <60 fsec pulses. Two methods of
active stabilization are used to produce reliable output. In addition, high-gain preamplification in Ti:sapphire has been
demonstrated, using a high-quality pump beam to produce gains of 130 per pass, and -10,000 in a simple double-pass gain
configuration, while avoiding crystal damage problems. Finally, grating pulse stretching and recompression from 60 fsec to
..3øø p and back down to 140 fsec has been demonstrated at low power.