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The requirements for the production of high power, subplcosecond laser pulses are reviewed. Use of
the chirped-pulse amplification technique makes solid-state materials competitive with dyes and
excimers for the production of subplcosecond pulses. In cases where both high power and short pulses
are required, solid-state materials are shown to be the material of choice. As an example, the design and
performance of a 3.2 Terawatt Nd:Glass laser system based on chirped-pulse amplification is presented.
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We describe our work on the amplification of short pulses in tunable solid state materials; specifically alexandrite and Ti:sapphire.
Our goal is to amplify femtosecond range pulses to the joule level in a table top size laser. We will describe our results which show
that such a laser is now feasible.
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Regenerative amplification of dark pulses in pulsed oscillators is described as
injection seeding by a two-component signal: the first component is a carrier wave
originating from a quasi-continuous single-mode laser; the second is a short pulse
out of phase by ir. An iterative model predicts switching of the pulsed oscillator
output from a train of dark pulses to a train of bright pulses as the resonance
frequency of the seeded cavity is detuned from the frequency of the carrier wave.
Experimental results obtained with a high-pressure CO2 laser system confirm the
predictions.
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A high power, 300 fsec pulse duration, ultraviolet KrF laser system has been developed as a pump laser for short
wavelength X-ray lasers. Additions and improvements to the laser system will be described. Attempts to reduce the effect of
amplified spontaneous emission (ASE) through the use of spatial filters, saturable absorbers and target design will be
discussed. Improvements to the optical system have been made in order to provide a 3 jtm focal spot size with care being
taken not to introduce significant broadening of the pulse duration. It is estimated that focal spot intensities in excess of
1018 W/cm2 have been obtained on target. Soft X-ray spectra resulting from various laser-target experiments will be
presented. Theoretical schemes for the development of X-ray lasing in the wavelength region of 1-5 nm will be discussed.
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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.
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A Matrix formalism is proposed to treat the propagation of ultrashort light pulses in laser resonators. The existence of
"Gaussian temporal eigenmodes" for laser resonators is discussed, with emphasis on the dependence of the time duration of
such eigenpulses on various parameters of the resonator. Results of computer simulations based on the formalism are
presented.
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Measurement of the characteristics of high energy, high peak power laser pulses for plasma physics experiments
requires detection of prepulses of intensity
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Work in progress at M.I.T. toward construction of a tabletop x-ray laser with a repetition
rate on the order of 0.1 Hz is reviewed. Potential collisionally pumped laser schemes in Nilike
Mo and Nd-like U are described. The use of whisper gallery mirrors as an oscillator
cavity is discussed. The design and construction of a zig-zag slab laser producing 5J in 100
psec pulses with a repetition rate of 0.1-1 Hz is reviewed.
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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).
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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