The development of the short pulse laser capabilities described in the first part of this conference provides opportunities to study new phenomena in plasmas. Some phenomena that require laser pulses there are short Cr (1) and relativistic I C 1) are described here including plasma wakefield excitation, relativistic self-focusing of light, remote guiding of lasers in plasmas (without optical elements), harmonic generation, and photon acceleration (frequency upshifting).

The evolution of density profiles in picosecond laser plasmas have been inferred from frequency shifts of reflected probe laser beams. The frequency shifts are caused by two mechanisms: hydrodynamic expansion and ionization front motion. Both time-resolved expansion velocities and ionization rates are presented for plasmas produced by 1 0 Wcm2, 1 ps, 580 nm laser irradiation.

We present studies of high-intensity laser-plasma interactions using a 1-ps, 1.053-pm laser. It is currenily operated in a single-pulse mode with energies up to -100 mJ and focused intensities up to mid-1015 W/cm2. We have measured the absorption of laser light in plasmas produced during interactions with various moderate targets using an Ulbricht's sphere. Charge collectors were used to study the characteristics of the ion expansion. X-ray and XUV spectroscopy were used to measure the spectral emission from the plasmas. These results are compared to 1-D hydrodynamic code (LILAC) simulations. LILAC has been modified to improve its validity for short-pulse interactions but it is still found that the absorption is underestimated. Potential explanations, including suprathermal electron generation, are discussed.

High Z exploding foil targets are used in many soft x-ray laser schemes. The foils are typically irradiated with a long ('1 ns) optical laser pulse that burns through the foil to produce a large hot plasma with long density scale lengths suitable as an amplifier. While it is expanding, and before the plasma conditions are suitable for stimulated emission, a large fraction of the heating laser energy is lost through radiation and conduction. We report the results of experiments attempting to increase the efficiency of exploding foil amplifiers through the following procedure. A laser beam ('1 2 W/cm2) is used to heat the foil sufficiently to expand it to approximately 200 pm. A high intensity (''1 5 W/cm2) short pulse (1 0 ps) laser beam is then used to raise the plasma to the desired temperature and ion state. Temporally resolved x-ray spectra from Yb foils are presented.

We present in this paper observations of stimulated Briiiouin backscattered light from a plasma irradiated by a 1 0 ps 1 .06 m laser pump, at intensities between 1 012 and 1 016 W cm2. During this short time duration, the observed growth of the instability can be explained only in terms of a purely temporal growth, in contrast to the convective growth normally associated with this instability.

In order to investigate the production of plasma with simultaneous high density and high degree of ionisation and to study laser-plasma energy transport, experiments have been undertaken to measure the plasma electron densities formed in a thin layer of aluminium buried below an overlay of plastic when the plastic overlay is irradiated by focussed lasers of wavelength (and pulse length).53tm (2Ops) ; .35im (2Ops) and .27 pm (5Ops) . For the .53im and .35pni wavelengths, the shorter pulse length (2Ops) results in higher time-averaged electron densities (up to 6 x 1022 cm3) than for the .27tm wavelength experiments with 50 ps pulse length, but the production of hydrogen-like aluminium relative to helium-like is less with the shorter pulse-length.

We present scaling laws for the interaction of femtosecond high power lasers with solid targets. We solve for scale lengths and temperatures of the hot dense plasma in both the classical and flux limited electron thermal conduction regimes, and apply those results to the prediction of radiation output from these targets. The radiative yield and duration are given as functions of both laser and target conditions. Our results compare quite favorably with sophisticated numerical simulations.

Efficient VUV generation near 160 nm has been demonstrated in Mg vapor at pressures between .1 -1. Torr and an estimated laser intensity of 10 W/cm2 The highest efficiency measured is on the order of .1 % and it is found that the vUv generation scales as P35 j the above pressure range. The data indicate that stimulated emission from molecular Mg plays an important role.

The plasma produced by the interaction of a 1-ps, 1-rim, mid-1015 W/cm2 laser pulse with moderate Z targets is investigated spectroscopically using high-resolution, time-integrated x-ray and XUV spectrometers. The x-ray spectra are analyzed and the plasma conditions are inferred using a variety of theoretical models, including a time-dependent atomic physics computer code POPION. The electron temperature is estimated to be 1 keV during the transient plasma associated with the laser pulse. K emission may indicate the presence of a significant population of suprathermal electrons in the plasma.

Using a high-intensity 1-ps laser we have demonsirated both second and third harmonic production in a jet of krypton gas. The production of even harmonics is a forbidden process in the dipole approximation. Both intensity and pressure scaling for the harmonic strengths were investigated. The laser was operated at 1.053-pin wavelength and l-ps pulse length resulting in intensities ranging from 1013 1016 W/cm2.

Intense, subpicosecond lasers are capable of producing plasmas at or near solid density with temperatures of down to a few eV. Such a plasma is strongly coupled with respect to particle-particle interactions and the use of standard, Spitzer descriptions of plasma processes is not valid. Here we develop an expression for the electrical resistivity, p, in such a plasma. Since r governs laser light absorption via inverse bremsstrahlung, it must be calculated with some care to ensure that modeling of the plasma is correct.

The preliminary measurement of a 100 eV ultrashort soft x-ray pulsewidth using cross correlation is presented based on the principle that ponderomotive potentials of high intensity visible light shift x-ray absorption lines at femtosecond speeds. We are developing a high resolution technique for the direct measurement of pulse durations of soft x-ray pulses that are emitted from laser produced plasmas driven by subpicosecond lasers. This technique relies on the interaction of a high intensity ultrashort pulse visible laser with the bound levels of an atom through the high intensity A.C. Stark effect. In the presence of the visible laser, all atomic levels undergo some shift in energy1 . Specifically, the final state will shift significantly for a bound-bound inner shell transition in a noble gas in the soft x-ry regime (in which an inner shell electron is excited to an autoionizing Rydberg state). Thus, for a narrow spectral region centered on the unperturbed inner shell transition energy, the absorption of radiation may be 'switched off' when the visible laser is present. This switching of the absorption characteristics forms the basis for our pulsewidth measurement. If the timing between the x-ray pulse and the visible laser pulse is varied, the switch will map out the temporal evolution of the x-ray pulse with a resolution equal to the visible laser pulsewidth. Our experiment uses a high density pulsed krypton gas source as the abosrbing medium, and montior the 3d-5p inner shell transition at 91.2 eV with a 1.5 m grazing incidence monochromator. The plasma is produced on a solid gold plated copper rod by 3 mJ of 308 nm light with a pulsewidth of about 300 fs. The high intensity visible laser has a peak intensity of about iO' W/cm2 in a 100 fs pulse at 616 nm. By delaying the visible laser with respect to the x-ray pulse, we have made a very preliminary measurement of about 1.5 ps for the duration of the x-ray burst. The time to accumulate the data in this experiment was on the order of 2 hours with poor signal to noise. Although the data indicate a change in the absorption due to the visible laser, it is not good enough to make a conclusive measurement. We are currently in the process of redesigning the experiment to incorporate a multilayer coated focussing optic to enhance the x-ray flux by a factor of 1000. This should enable us to obtain good signal to noise and establish this method as a valuable diagnostic for laser produced plasmas.

The advent of compact high peak power short pulse lasers has made tabletop recombination x-ray lasers pumped by these systems a real possibility. In this paper we discuss some of the relevant issues for these lasers and highlight a number of issues that require consideration. Results of simulations designed to estimate potential gains in the quasisteady- state region are also described.

We present improved experimental results of measurements of ion correlation effects in a dense plasma. The EXAFS technique was used to observe the short range order within a dense plasma produced by colliding laser-induced shock waves. Densities about three times solid density have been measured. An estimate of the temperature during compression and subsequent heating of plasma is made using published mcdels and an approximate agreement obtained with NEDUSA on-dimensional fluid code predictions.

Intense, subpicosecond lasers are capable of producing plasmas at or near solid density with temperatures of down to a few eV. Such a plasma is strongly coupled with respect to particle-particle interactions and the use of standard, Spitzer descriptions of plasma processes is not valid. Here we develop an expression for the electrical resistivity, p, in such a plasma. Since r governs laser light absorption via inverse bremsstrahlung, it must be calculated with some care to ensure that modeling of the plasma is correct.