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 nrn. Under optimum conditions, we find that conversion efficiencies of as high as 10 into each harmonic are possible.PACS number(s): 42.65. Ky, 32.80.Rm The generation of high-order harmonic radiation by an intense laser pulse is now a well documented phenomenon [1,2] and is one of the principal new sources of bright XUV radiation being developed for a variety of applications that rely on the interaction of short-wavelength radiation with matter. To date, wavelengths as short as 7 nm have been generated by harmonic generation [3,4]. When compared to other XUV sources such as synchrotrons, FELs (freeelectron lasers), and x-ray lasers, harmonic generation exhibits many advantages such as short pulse duration (40 fs -100 ps), high peak brightness, and tunability. Harmonic generation has been extensively studied by a number of groups using drive lasers with a range of wavelengths and pulse widths [3 -8] and much work has been done to understand the short-wavelength limit and related scaling formulas.In spite of these early studies, the work of characterizing and optimizing high-order harmonic generation as a general purpose XUV source is incomplete. A number of groups have examined the angular divergence and spatial coherence of the harmonics [9 -11]. Previous experiments have also characterized the linewidths of the harmonics [12,13]. The utilization of high-order harmonics in applications has been recently demonstrated by Balcou et aI. , who have used photon energies in excess of 100 eV in photoionization spectroscopy [14].Though there have been preliminary studies [7], a complete measurement of the photon yields achievable with high-order harmonics has not yet been undertaken. In this Rapid Communication we report on measurementsof the absolute energy yields and conversion efficiencies of harmonics generated in the 31 -17-nm wavelength range under a variety of conditions. These experiments were performed using a Nd:glass laser that produced 650-fs pulses at 1052.7 nm with energies up to 8 J, and pulses of its second harmonic at 526 nm with energies up to 4 J [15].The laser was focused into the plume of a pulsed, supersonic nozzle, gas jet. This jet produces localized atomic densities from 10 to 2 X 10 atoms/cm and exhibits a linear density dependence with gas jet backing pressure, verified by backward stimulated Raman scattering measurements [16]. The interaction length through the gas jet is 0.8 mm. Our measurements were conducted with an atom density of 2X10' atoms/cm~30%. The harmonic radiation is sampled by an astigmatic compensated, grazing incidence, XUV spectrometer. A calibrated alum...

Details of schemes for two tabletop size x-ray lasers that require a high-intensity short-pulse driving laser are discussed. The first is based on rapid recombination following optical-field ionization. Analytical and numerical calculations of the output properties are presented. Propagation in the confocal geometry is discussed and a solution for x-ray lasing in Li-like N at 247 A is described. Since the calculated gain coefficient depends strongly on the electron temperature, the methods of calculating electron heating following field ionization are discussed. Recent experiments aimed at demonstrating lasing in H-like Li at 135 A are discussed along with modeling results. The second x-ray laser scheme is based on the population inversion obtained during inner-shell photoionization by hard x rays. This approach has significantly higher-energy requirements, but lasing occurs at very short wavelengths (..1.<15 A). Experiments that are possible with existing lasers are discussed.

Laser plasma interactions in a relativistic parameter regime have been intensively investigated for studying the possibility of fast ignition in inertial confinement fusion ͑ICF͒. Using ultra-intense laser systems and particle-in-cell ͑PIC͒ simulation codes, relativistic laser light self-focusing, super hot electrons, ions, and neutron production, are studied. The experiments are performed with ultra-intense laser with 50 J energy, 0.5-1 ps pulse at 1053 nm laser wavelength at a laser intensity of 10 19 W/cm 2. Most of the laser shots are studied under preformed plasma conditions with a 100 m plasma scale length condition. In the study of laser pulse behavior in the preformed plasmas, a special mode has been observed which penetrated the preformed plasma all the way very close to the original planar target surface. On these shots, super hot electrons have been observed with its energy peak exceeding 1 MeV. The energy transport of the hot electrons has been studied with making use of K␣ emissions from a seeded metal layer in planar targets. The details of ion acceleration followed by beam fusion reaction have been studied with neutron spectrometers. Laser ponderomotive force self-focusing and hot electron generation have been applied to a compressed core to see the effect of heating by injecting 12 beams of 100 ps, 1 TW pulses.

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A soft x-ray laser (wavelength lambda = 15.5 nanometers) was used to create a moiré deflectogram of a high-density, laser-produced plasma. The use of deflectometry at this short wavelength permits measurement of the density spatial profile in a long-scalelength (3 millimeters), high-density plasma. A peak density of 3.2 x 10(21) per cubic centimeter was recorded.