By optimizing the intracavity dispersion compensation in a self mode-locked Ti:sapphire laser, we have generated pulses of less than 11 fs in duration. Dispersion within the laser cavity can be reduced by using a short highly-doped Ti:sapphire crystal and a prism glass which reduces third-order dispersion. Amplifier design which reduce third order dispersion have also been tested, and we have demonstrated stretching an 11 fs pulse by a factor of 1000, followed by recompression of 20 fs pulse duration. Our results demonstrate that the exceptionally broad bandwidth of Ti:sapphire can be utilized to generate pulses shorter than has been possible with any other type of laser material to date.

A Chirped Pulse Amplification (CPA) mode of operation is being developed on the VULCAN high power Nd:glass laser system, at the Rutherford Appleton Laboratory (RAL). Experiments have been carried out using an interim configuration yielding pulses of up to 30 J on target of 2.4 picoseconds length at focused intensities up to 4 X 10¹⁷ W cm^-2, with contrast ratio of 10⁶. In the CPA technique the amplification of a stretched pulse to high energy followed by recompression provides a means of delivering a higher peak power to target than can be propagated through the laser system due to non-linear effects and component damage thresholds. In the system described here a grating pair is used to stretch a transform limited pulse of 2 ps, to 80 ps prior to amplification to high energy (80 J). The linear stretch produced by the gratings enables the pulse to be recompressed without pedestal. Recompression to the 10 TW level by a second grating pair is carried out in a vacuum propagation and reflective focusing system to avoid non-linear effects in air, windows and lenses.

A large aperture KrF laser (SPRITE) based on a 27 cm diameter electron beam pumped amplifier has been developed at the Rutherford Appleton Laboratory and is in operation as a user facility. Recent development of this system has concentrated on the demands of short pulse and high brightness and this has led to two parallel program. One concentrates on efficient multiplexed energy extraction in one or more very low divergence beams using the approach known as 'Raman beam combining'. The second is directed to maximizing the brightness of a single amplified pulse by minimizing the pulse duration and using the technique of chirped pulse amplification (CPA) and recompression.

A two-frequency CO₂ laser beam was used to beat-excite a large amplitude electron plasma wave in a resonant density plasma. The accelerating fields of the relativistic plasma wave were probed with collinear injected 2.1 MeV electrons from an electron linac. Some electrons gained at least 7 MeV in traversing the approximately 1 cm length of the beat wave accelerator, with the measurement limited by the 9.1 MeV high energy cut-off of the detection system. The corresponding average acceleration gradient is > 0.7 GeV/m and the average wave amplitude n₁/n₀ is > 8%. Estimates based on collective Thomson scattering indicate that peak wave amplitudes of 15 - 30% may have been achieved.

In this paper we will present work in progress in our experimental investigation of the coupling of intense, sub-picosecond laser pulses with plasmas preformed on solid targets. (This situation is to be contrasted with the interaction of intense laser fields with solid-density matter, a subject which has generated considerable interest in the last several years.) We will discuss our characterization of the energy distribution of energetic electrons which escape a solid target irradiated by an intense laser. We have also performed experiments to study the excitation of parametric instabilities near the quarter-critical layer and second-harmonic generation near the critical layer in the plasma. We will discuss some preliminary scattered light spectroscopy measurements. Other topics related to this work are discussed.

The defocusing of a 1053 nm, 1-ps laser pulse focused in a cell filled with different noble gases has been studied in the 10 mbar to 1 bar pressure range. The focusing has been studied as a function of the gas pressure and of the incident laser power. The results clearly show that the beam tends to defocus as the incident power or the rare gas pressure are increased. The defocusing is interpreted as being due to the influence of the plasma refractive index: the radial profile of the electron distribution induces a minimum of the refractive index on the laser axis and consequently a refraction of the laser beam. The main and important consequence is that high laser intensities (10¹⁷ - 10¹⁸ W/cm²) and high electron densities (10¹⁹ - 10²⁰ cm^-3) cannot be achieved simultaneously with infrared laser radiation.

We discuss the mechanism responsible for the enhancement of Brillouin backscattered light in the context of recent experimental observations. The scattering takes place from the fluctuations produced during the nonlinear evolution of stimulated Raman scattering. The frequency spectra of Brillouin light shows blue and red shifted components consistent with experimental observations.

We report on the computer simulation results for a new laser plasma interaction regime, concentrating on the collisionless, kinetic nature of the plasma motion. It is found that even for normal incidence, the absorption of the laser light by the plasma electrons near critical can be as high as 60%, and that the temperature of the hot electrons produced can be predicted simply by knowing the associated ponderomotive potential of the incident laser. We also describe a new odd harmonic generation mechanism due to the interaction of the laser with the plasma electrons near critical.

Short-pulse high-intensity laser-plasma interactions are investigated numerically with a fluid code and experimentally with optical and x-ray diagnostics. The emitted x-ray spectrum is characterized in the vicinity of the water window for laser intensities of 4 X 10¹⁷ W/cm² and pulsewidths of 400 fs. Optimization of the x-ray pulsewidth using these results is also discussed.

Experiments are reported on optical ionization of gases produced by 12 ps pulses of focused KrF laser radiation at intensities of 3 X 10¹⁷ W/cm². Simultaneous measurements of Thomson, stimulated Raman and stimulated Brillouin scattering have been used to determine the electron energy distribution and temperature in He and Ne (static and pulsed gas jet targets) at pressures of 10^-3 - 1 bar. The electron velocity distribution evolves from a multiphoton ionization spectrum at low pressure to a thermal electron distribution at higher pressure. The ion determined portion of the Thomson scattered spectrum is highly non-thermal for pressures > a few mbar. At high pressure, significant Raman and Brillouin backscattering are observed. The results show that inverse bremsstrahlung absorption dominates heating for p > 10^-2 bar except at the highest pressures where SRS may also be important.

The interaction of subpicosecond 1.06 micrometers laser light at intensities up to 10¹⁸ W/cm² with dense preformed plasmas is investigated by measurements of the absorption of the laser light in the plasma and by measurements of the production of bremsstrahlung x- rays. Absorption measurements are made by collecting the scattered light in an Ulbricht sphere. Light scattered in the backward and specular directions is collected separately. Measurements are presented for both high and low Z targets. X-ray production is measured using a nine channel filter/scintillator spectrometer.

With a time-resolved pump-probe setup, the dynamical interactions of intense 400-fs laser pulses with a solid target are studied with a time resolution of about 250 fs. The motion of the plasma critical surface is measured by the means of the doppler shift of the probe laser. It is found that when the average electron quiver energy (mv²_os/2) becomes comparable to the electron thermal energy (kT_e), the ponderomotive force of the high-intensity laser significantly reduces the thermal expansion of the laser-plasma.

We have demonstrated a system for subpicosecond, soft-x-ray continuum, pump-probe absorption spectroscopy. Using multiphoton ionization to abruptly change the x-ray absorption spectra of a gas, we have measured the temporal profile of laser-generated x-ray pulses near 90 eV. Although the x-ray pulses from a laser-generated plasma here were only as short as approximately 20 psec, the technique is extendible to higher energy x-rays which will have pulse durations approaching 100 fsec. We also present the absorption spectra of Kr ions produced under conditions of high intensity non-resonant multiphoton ionization. The spectra are identified with a fit to the Cowan code.

The K-shell emission from porous aluminum targets is used to infer the density and temperature of plasmas created with 800 nm and 400 nm, 140 fs laser light. The laser beam is focused to a minimum spot size of 5 micrometers with 800 nm light and 3 micrometers with 400 nm light, producing a normal incidence peak intensity of 10¹⁸ Watts/cm². A new 800 fs x-ray streak camera is used to study the broadband x-ray emission. The time resolved and time integrated x-ray emission implies substantial differences between the porous target and the flat target temperature.

Strong K_(alpha ) emission is observed from a plasma produced by a high intensity contrast, picosecond, p-polarized laser pulse. The K_(alpha ) emission is found to be induced by hot electrons, which have a temperature of around 3 keV and carry up to 20% of the laser energy. Due to the relatively high K_(alpha ) yield, small source size, and short duration of emission, a high-peak spectral brightness x-ray source is obtained.

We present recent results of our effort to develop an efficient, user-friendly, table-top ultrafast X-ray source. The factors affecting the duration and the intensity of the X-ray emission in the keV range are studied. Time-dependent calculation of the atomic physics coupled to a Fokker- Planck code is used for a quantitative analysis of the experimental results.

We propose a demonstration experiment of optical-field-ionized plasma x-ray lasing at 247 angstroms in Nitrogen which can tolerate strong refractive defocussing. The relatively low ionization potential energy of approximately equals 98 eV in Nitrogen should result in correspondingly low residual electron temperature T_e which, in turn, allows large gain at lower values of electron density. The use of low electron density provides less collisional heating as well as reduced refractive defocussing. In addition to using low electron density to mitigate refraction, we also propose the use of a wide UV laser-driver focus to give reduced transverse intensity gradients near the laser axis. Milder transverse intensity gradients lead to longer refraction lengths which effectively define the volume of x-ray lasing. These two techniques for avoiding excessive refraction are used in tandem to identify an experimental regime where x-ray lasing dominates spontaneous noise by an unambiguous margin associated with a gain-length product of five.

X-ray lasers with pulse duration shorter than 20 ps allow the possibility of imaging laser produced plasmas with micrometers resolution. In addition, the high peak brightness of these new sources will allow us to study nonlinear optics in the xuv region. In this paper we will describe our efforts to produce collisionally pumped short pulse x-ray lasers. Initial results, which have produced approximately 45 ps (FWHM) x-ray lasers, using a double pulse irradiation technique are presented along with a discussion of the prospects for reducing the pulse width.

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.

The technique of laser-induced ablation of thin films from glass slide substrates has been investigated as a candidate vapor target production method for studies of both tunneling-driven x-ray/xuv recombination lasers and relativistic propagation using intense, ultra-short laser pulses. It is shown by simultaneous two-wavelength interferometry that particle densities of order 10¹⁹/cm³ are readily achieved and that some intrinsic ionization accompanies the plume formation. Absorption measurements with both 100 picosecond and 125 femtosecond pulses are consistent with observed edge velocities near 10⁶ cm/sec. The level of ionization drive by the intense 125 femtosecond laser pulse has been coarsely estimated. Averaged estimates from spectral blue shifting of spectra transmitted through the plume are consistently lower than those obtained from evaluation of saturation intensity thresholds based on the sequential nonresonant optical field ionization (OFI) process.

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.

Recent progress in the understanding of high-order harmonic conversion from atoms and ions exposed to high-intensity, short-pulse optical lasers is reviewed. We find that ions can produce harmonics comparable in strength to those obtained from neutral atoms, and that the emission extends to much higher order. Simple scaling laws for the strength of the harmonic emission and the maximum observable harmonic are suggested. These results imply that the photoemission observed in recent experiments in helium and neon contains contributions from ions as well as neutrals.

Multiply charged ion production in rare gases (He, Ne, Ar and Xe) has been investigated using a 1-ps Nd-glass laser pulse at (lambda) equals 1053 nm with intensities in the 10¹³ - 10¹⁸ W/cm² range, for both linearly and circularly polarized light. At the maximum laser intensity, all electrons of the outer shell are removed, except in Ne for which charge states up to 7+ are observed. Comparison of experimental data with Ammosov et al tunneling model shows a very good agreement for both polarizations, indicating that ionization with a 1 ps pulse in the near infrared light mainly occur in tunneling regime, while for longer pulses multiphoton processes dominate. The ion production curves obtained with linear and circular polarization light show that (1) ionization threshold intensities are at least 40% higher for circular polarized light, and (2) ionization by direct processes are never observed in circular polarized light.

We have measured the ion yields for helium and neon ionized by 120 femtosecond, 614 nanometer laser pulses with intensities up to 10¹⁶ watts per square centimeter. We have found that the He II and Ne II data exhibit features incompatible with standard nonresonant sequential ionization. These features reduce the usefulness of optical field ionization for monitoring laser intensity. For the experiment, we expect dynamic resonances to have little effect on the ionization, and we attribute the features to nonsequential ionization based on the simultaneous saturation of the features and the singly ionized charge states.

The high- intensity soft X-ray beams forma.tion and control are important for a large number of basic and appl ied researches. The ma.in types of soft X-ray sources are the synchrotron (SR) sources and high temperature plasma. sources. The specialized SR sources [1] are the most convenient and advantageous for technical appl ications, X-ray l ithography, for example. Whereas SR sources are very complexly bui lt and expensive apparatus. The high temperature plasma sources (laser plasma., pinch, etc.) are reliable and of moderate cost apparatus. The laser plasma sources have high efficiency and high spatial stabi lity of emission poi nt and wide waverange of soft X-ray emission [2-4]. But soft X-ray emission of al1 plasma sources is accompanying by intensive flows of plasma expansion products. To get rid of the plasma. expansion products various types of soft X-ray optics are used. The la;er irrad iation conversion into soft X-ravs prov ided by -6 focusing multi layer mirrors is about 10 [ 5, 6J . We have proposed to use wide-range wavegui de focusing soft X-ray optics with a laser plasma source to increase the conversion [ 7].

Based on the Schrodinger equation model , this paper presents a nume­ rj_ cal analysis of the time evolution of the populations of the levels implied in the fifth order harmonic generation , taking into account the d amping effects, which emphasize that the choice of the parameters (decay rate, detunings) which characterize the interaction tetween the atomic system and the strong laser field is very important.