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Propagation of a high intensity (∼1019 W/cm2) ultrashort (∼35 fs) laser pulse through a cloud of water spheres (150 nm diameter) results in hot electron driven proton acceleration up to 1 MeV. It is suggested that during the propagation of the short pulse through the low density wing of the cloud, the leading pulse pedestal is reduced owing to absorption by the preplasma created. Then, the high-intensity peak of the pulse propagates through this underdense plasma and interacts with the high-density inner part of the cloud, which has not been transformed into an underdense plasma so that a sheath acceleration process at each individual microsphere can take place. The observed proton spectra show strong modulations, which are interpreted within the framework of a known fluid-expansion model incorporating two hot-electron populations with significantly different densities and temperatures.

The collisionally excited transient inversion scheme is shown to produce exceptionally high gain coefficients and gain-length products. Data are presented for the Ne-like titanium and germanium and Ni-like silver X-ray lasers (XRL's) pumped using a combination of nanosecond and picosecond duration laser pulses. This method leads to a dramatic reduction of the required pump energy and makes down-sizing of XRL's possible, an important prerequisite if they are to become commonly used tools in the long-term.

Small water droplets (20 micron in diameter) have been exposed to intense (∼ 1019 W/cm2) laser pulses in order to study ultrashort (∼ 35 fs) laser pulse driven ion acceleration. Ion emission spectra registered simultaneously in forward and backward direction in respect to the incident laser beam carry similar integral ion energy but show different ion cutoff energies. With simple model estimations on basis of the confined and spherical geometry of the droplet-target, we inferred acceleration field strengths of about (0.7–2) MV/μm. Up to 9% of the incident laser energy is converted to kinetic energy of ions, which have been accelerated to energies above 100 keV and up to 1.5 MeV. A laser pedestal at an intensity of about 10−7 of the peak intensity at 1–2 ns in front of the pulse peak still limits the achievable cutoff energies of emitted protons from the droplet. The observed increase of cutoff energies with an enhanced temporal contrast of the laser pulse is elucidated within a simple acceleration model.

A Ti:Sa 10 Hz laser system delivering femtosecond (t < 50 fs) pulses at the subjoule energy level is discussed.

An efficient scheme of transient inversion col- lisional x-ray laser proposed several years ago1-3 was successfully realized4,5 by use of 3p–3s transitions (J = 0–I, λ = 32.6 nm) in [Ne]-Iike Ti.

The quality of the laser beam wave front is one of the most important parameters limiting the laser intensity available for laser-matter interactions. Application of an adaptive mirror can substantially improve the wave front parameters, thus increasing maximum focusable intensity. Optimizing the laser beam with the adaptive system we succeeded to improve the Strehl ratio of the recompressed pulse from /spl sim/ 0.3 to /spl sim/0.5-0.6, achieving focusable intensity of /spl ges/1019 W/cm/sup 2/ at duration of the laser pulse of 35 fs.

Experimental and theoretical work about energetic ion emission from massive targets obtained with 35 fs laser pulses at an intensity level of 10/sup 18/ W/cm/sup 2/ to 10/sup 19/ W/cm/sup 2/ is reported. Ultrashort laser pulses in combination with a hot electron confining target can drive ions to MeV energies via a direct field acceleration mechanism.

An efficient acceleration of energetic ions is observed when small heavy-water droplets of {approx}20 {mu}m diameter are exposed to ultrafast ({approx}40 fs) Ti:sapphire laser pulses of up to 10{sup 19} W/cm{sup 2} intensity. Quantitative measurements of deuteron and neutron spectra were done, allowing one to analyze the outward and inward directed deuteron acceleration from the droplet. Neutron spectroscopy based on the D(d,n) fusion reaction was accomplished in four different spatial directions. The energy shifts of those fusion neutrons produced inside the exploding droplet reflect a remaining deuteron acceleration inside the irradiated droplet along the axis of the incident laser beam. The overall neutron yield of the microdroplets is relatively small as a result of the dominant outward directed acceleration of the deuterons with 1200 neutrons/shot. Relying on the 'explosionlike' acceleration of such spherical droplet targets we have developed a spray target consisting of heavy-water microspheres with diameters of 150 nm. Both the high deuteron energies of up to 1 MeV resulting from the irradiation intensity of {approx}10{sup 19} W/cm{sup 2} as well as the collisions between the deuterons and the surrounding spray delivered about one order of magnitude more neutrons than the single-droplet system. The {approx}6x10{sup 3} neutrons per lasermore » pulse from the spray can be attributed to an efficient deuteron release from a significantly smaller laser excited volume as from deuterium-cluster targets.« less

Summary form only given. The diffraction gratings are mounted in a vacuum chamber of the compressor. In these conditions the diffraction gratings and other gold coated mirrors illuminated by the laser beam are subjected to carbon contamination, especially on the place of the output beam. This reduces diffraction efficiency of the compressor. The cleaning method used at our 100 TW Ti:sapphire laser system is based on a radio frequency discharge in a mixture of oxygen and argon at low pressure.

Summary form only given. The Ti:sapphire lasers of a 100 TW-Petawatt power level need active crystals of 30-100 mm diameter. Because of high amplification gain those crystals are subjected to a high parasitic lasing. We have currently upgraded our Ti:sapphire laser system for 30 TW in a 10-Hz mode and 100 TW in a single shot mode. The last power stage of the laser uses a 35 mm diameter and 15 mm thick Ti:sapphire crystal pumped by frequency doubled pulses of the Nd:glass laser. Up to 30 J of the pump energy can be delivered to the Ti:sapphire crystal. Despite the crystal cladding, a parasitic self-lasing started in the Ti:sapphire crystal when the pump energy exceeded 20 J (pump density 2.8 J/cm/sup 2/) thus limiting the output energy to /spl sim/7 J. The further increase in the pump energy leads to destruction of the originally uniform cladding layer due to absorption of radiation by the layer. We have found that the process of the destruction has two stages. In the first stage, after the first laser shot, only a half of the layer was removed in the direction perpendicular to the optical axis of our crystal. In the second stage, after the next successive laser shot, the rest of the layer was removed also. Development of the parasitic lasing in the Ti:sapphire crystals depends strongly on the aspect ratio of the pumped region. The threshold for parasitic lasing depending on the level of crystal scattering on the side surface of the crystal and on the aspect ratio is presented. Assuming that the scattering on the side surfaces of the Ti:sapphire crystal is close to the normal incidence reflection (7%), the parasitic lasing starts at the aspect ratio values around 0.8-1. In our case the pumped area had an aspect ratio of 2 which is well above the lasing threshold.

Summary form only given. Pushing the peak power of Ti:sapphire laser systems toward the 100 TW levels and beyond is connected with several special technological and physical problems. The laser pulse duration after recompression is limited by both the pulse spectral width and residual phase distortions accumulated in the chirped pulse amplification system. They both can lead to the appearance of a pedestal and not-transform-limited recompression of the laser pulse over the beam aperture. The amplitude front can be bent and tilted. Such laser pulses will have a substantially broadened pulse duration for the case of sharp focusing on the target. Using the existing modifications of the stretcher-amplifier-compressor arrangement it is possible to reach a dispersion control up to the fourth and even higher orders of phase dispersion on the optical axis of the laser system.

The absolutely calibrated K-shell spectra emitted form short-living aluminum plasma at laser intensities of 5 by 1015- 4 by 1018 W/cm2 are reported. The experiments performed with the constant energy, variable-length laser pulse are modeled by the 1D hydrodynamics code including non-linear resonance absorption of the laser radiation, fast electron acceleration and energy transfer into the target. The characteristic features of the measured and the post- processed spectra are outlined; the scaling rules for the conversion efficiency of the laser radiation into the line x-ray emission are discussed.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The unique combination of an intense heavy ion beam accelerator and a high energy laser opens the possibility of exploring new physics taking advantage of the synergy of both facilities. A variety of new fields can be addressed with this combination in plasma physics, atomic physics, nuclear- and astro-physics as well as material research. In addition, using CPA-technology, laser pulses with a pulse power of up to a petawatt opens the door to explore the regime of fully relativistic plasmas. Therefore the Gesellschaft fuer Schwerionenforschung is augmenting the current high intensity upgrade of the heavy ion accelerator facility with the construction of PHELIX. Designed with two pulse-generating front ends and send to multiple experimental areas PHELIX will serve as a highly versatile laser system for various applications. In this report, we present the design of the laser system and some key experiments that can be performed with this combination for the first time.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

K. A. Janulewicz, J. J. Rocca, F. Bortolotto, M. P. Kalachnikov, V. N. Shlyaptsev, W. Sandner, and P. V. Nickles Max Born Institute, Max-Born-Strasse 2A, D-12489 Berlin, Germany Department of Electrical and Computer Engineering, Colorado State University, Fort Collins, Colorado 80523 Department of Applied Science, UC Davis-Livermore, California 94550 ~Received 1 September 2000; published 6 February 2001!

Summary form only given. At present, for basic research in many nonfusion related areas, ultra-high field strength and high repetition rate are of more concern than high pulse energy. In this case, Ti:sapphire (Ti:Sa) appears to be the most attractive and advanced laser medium for laser systems of 100 TW-petawatt level. The peak power of Ti:Sa lasers is still considerably below the limit, although several groups have succeeded in the development of Ti:Sa lasers with peak power exceeding tens of terawatts. We have recently developed a Ti:Sa laser capable of producing >10 TW, 50 fs laser pulses at 10-Hz repetition rate. The system is currently being upgraded to 100 TW for laser-matter interaction at a level of focused intensity I/spl sim/10/sup 19/-10/sup 20/ W/cm/sup 2/.

Summary form only given. Two X-ray laser (XRL) pumping schemes have recently shown significant progress in increasing the gain available and reducing the pump energy required. Both proved to be robust and reliable pumping methods for obtaining lasing in the XUV region of the spectrum. The first one is the transient inversion scheme that includes in its standard version a combination of two laser pulses (long/short) to create and subsequently heat the plasma. The second pumping method uses a fast capillary discharge to create and pump an abundance of Ne-like ions. However, the latter works in the quasisteady-state (QSS) regime giving relatively long output pulses of /spl sime/0.5-1 ns and the former is plagued by refraction. We report on the first, to our knowledge, successful experiment on combining advantages of both systems in one, hybridly pumped Ne-like S/sup +/ x-ray laser. Laser pulse guiding through a capillary discharge plasma and evidence of strong enhancement of the laser line following excitation by the laser pulse are presented.

Summary form only given. Pushing the peak power of Ti:sapphire laser systems to 100 TW level and beyond is connected with several special technological and physical problems. One of them is associated with residual phase distortions arising in chirped pulse amplification systems after pulse recompression. They can lead to the appearance of a pedestal and a lack of full recompression of the laser pulse over the beam aperture. The amplitude front becomes bent and tilted. This problem is especially important for the case of sharp focusing on the target, because the spatially varying residual phase distortions can result in the appearance of a strong pedestal to the pulse and substantial lengthening of the pulse duration in the focal plane. Here we present results of a detailed three dimensional dispersive ray tracing analysis of Ti:sapphire 100 TW laser systems with several arrangements most commonly used in multiterawatt laser stretcher-compressor aberrational schemes.

Results of measurements on a plasma created by a discharge in a sulfur capillary and irradiated by a picosecond laser pulse are presented. Time-resolved transmission measurements and the corresponding XUV spectra are discussed. It is found that the transmission of the laser through the capillary decreases abruptly after the peak of the discharge current, and that the profile of the output laser beam is symmetrized before the onset of the opaque period. The spectra are evidently affected by the laser pulse and show significant intensification on the 3p-3s transition in Ne-like sulfur at 60.84 nm. This observation, combined with the fact that the emission of this line appears spatially displaced from all other lines in the detector and presents a lower divergence, suggest that lasing is occurring.

Novel scheme providing for 7 J Nd-glass subpicosecond MultiTeraWatt laser pulse shortening down to 200 fs with intensity rise time of 100 fs for 2 order of magnitude and prepulse suppression down to quantum noise level are proposed. Non-linear optical two-stage converter consists of sequential second harmonic generator and stimulated raman scattering oscillator-amplifier.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Very efficient production of hot electrons in a dense plasma subject to superstrong laser fields has been reported recently [1-3].

Experimental ultra-intense light-matter interaction has now reached the level of petawatt powers using large-aperture Nd.glass laser systems [1], even if only at high costs per shot and very low repetition rates. If, on the other hand, ultra-high field strength and repetition rate is of more concern than maximum pulse energy, as is typical for basic research in many non-fusion user facilities, titanium-sapphire (Ti:Sa) appears to be the most advanced laser medium.

The development of X-ray lasers is marked by the progression toward shorter wavelengths using less drive energy. A recent X-ray laser experiment at the Rutherford Appleton Laboratory (RAL) demonstrated saturated X-ray laser operation in the Ne-like Ti and Ge X-ray laser schemes at 32.6 and 19.6 nm respectively, with a drive energy of only a few joules. In this paper we describe the laser development to generate the laser drive pulses and travelling wave line-focus target irradiation geometry required for this experiment.

Simulation are presented for the collisionally pumped germanium X-ray laser working in the transient gain scheme. The results obtained are compared to the experimental observables registered during the joint european experiment conducted at RAL. Strong refraction influence on the output has been confirmed by a 3-D raytracing. Reliability of the hydrodynamics code and preliminary results of the pump conditions optimisation are discussed as well.

Numerical modeling of transient inversion and capillary discharge x-ray lasers were performed for the purposes of understanding of recent experiments, improving the laser efficiency and planing their further development. The first demonstration of saturation of a low pump energy x-ray laser utilizing a transient inversion mechanism on the 3p - 3s transition at 32.63 nm in Ne-like Ti has been confirmed. A close to saturation amplification was simultaneously achieved for the 3d - 3p, J equals 1 - 1 transition at 30.15 nm. Small signal effective transient gain coefficients of g approximately equals 46 cm-1 and approximately 35 cm-1 and effective gain-length products of 16.7 and 16.9 for these lines were obtained. Experiments and calculations demonstrate that it is possible to achieve saturated laser action in a transient regime with Ne-like Ti for a pump energy as low as approximately 5 J. With such energies just recently the lasing on Ni-like Pd at 14.7 nm has been achieved. MHD simulations of capillary discharges were performed for several elements with shorter lasing wavelengths. A new approach which intends to combine advantages of electrical discharge plasma preparation with CPA-laser excitation was investigated. It will enable a kind of x-ray lasers to be built with ultrashort pulse duration and the high efficiency. In this case the well preformed cylindrical shell of discharge plasma and the transient population inversion combined with the traveling-wave excitation will enable to achieve lasing at much shorter wavelengths.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The use of multi-pulse irradiation of neon-like ions has been shown to produce orders-of-magnitude enhancement of x-ray laser output. Recent results obtained at the Rutherford Appleton Laboratory are reviewed with an emphasis on understanding the reasons for the enhancement. Simulations with the fluid and atomic physics code EHYBRID are used to show that enhancement occurs because of a spatial enlargement of the gain region and consequent better propagation of the x- ray laser beam along the gain region.

Low pump energy transient gain x-ray lasers in Ti at 32.6 nm, 30.15 nm, in V at 30.4 nm and Ge at 19.6 nm using picosecond pulse heating of a long pulse preformed plasma of neonlike ions has been realized for the first time. Gain saturation was demonstrated in Ti and Ge XRL. Results of pump consumption, x- ray divergence and output energy are given.

Hot electron production and related Bremsstrahlung emission known in long pulse (tens of ps up to ns FWHM) becomes more important in short and ultra short (ps down to tens of fs FWHM) laser plasma interaction because some ten percent of the laser energy are being transferred to hot electrons, as theoretical (Wilks et al 1992, Gibbon 1994) and experimental (Schnürer et al 1995) studies have indicated. Measured hot electron distributions are compared to model predictions in order to gain more knowledge about the laser plasma absorption mechanism. A control of the hot electron production with the driving optical laser parameters is important for the Bremsstrahlung emission, if one wants to optimize short pulse hard X-ray sources. At relativistic driving laser intensities above 1019 W/cm2 hot electron production is the key part in the fast ignitor concept for the inertial confinement fusion (Tabak et al 1994).

Collisionally excited X-ray lasing in [Ne]-like Ti with a gain value of 19 cm-1 at a drastic reduced pump level of only a few Joules was demonstrated recently [1]. This achievement is based on a novel two step excitation scheme, which was proposed some years ago [2]. The scheme comprises at first a long (ns) heating pulse to establish a [Ne]-like ionic stage of desired target atoms in a plasma and second a short (ps) excitation pulse to produce efficiently a transient population inversion in the [Ne]-like ions. Due to the short pulse produced fast heating of the preformed plasma, a transient inversion [2] occurs, which is related to the different population rates of the levels via collision and not determined by the slower relaxation rates of the excited levels relevant in the quasi-steady state (QSS) regime. The transient inversion yields favourably much higher gain values (several tens of cm-1) as well shorter X-pulse output as obtained in the QSS regime (several cm-1) with long pulse pumping.

Three approaches in the development of efficient small-scale X-ray lasers are considered in this paper. The first is based on electrical discharge in the relatively small capillary during fast current rise-time. The second utilizes modern compact and powerful CPA-lasers for excitation of very large transient inversions. Both of these types of X-ray lasers have recently been implemented in table-top configurations. The third approach is intended to combine advantages of electrical discharge plasma preparation with CPA-laser excitation. It will enable a new kind of X-ray lasers to be built with the ultrashort pulse duration and the high efficiency. The transient population inversion combined with the traveling-wave excitation will also enable to achieve much shorter wavelengths.

A collisionally excited transient population inversion XUV-laser on [Ne]-like titanium has been realized using a table top terawatt laser. At the lasing wavelength of 32.6 nm a high gain value of 19 plus or minus 1.4 cm -1 has been obtained using plasma columns of 2 to 5 mm length.

Laser energy requirements for the creation of active plasma media with Ne- and Ni-like ions with a `long' subnanosecond prepulse (100 - 1000 ps) and a `short' picosecond main pulse (0.1 - 10 ps) have been determined for foil and low density targets (foams, gases, vapors, etc.). RADEX hydrodynamics and atomic kinetics calculations show that X-ray laser action in the 200 - 400 A can be obtained with 1 - 2 J in 100 - 1000 ps prepulse followed of an approximately 1 ps pulse of similar energy. Transient gains of approximately 100 cm-1 level have been calculated. For the wavelengths 38 - 46 A (`red' edge of `water window' spectral band) a short pulse of only approximately 5 - 10 J is required.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

X-ray emission and laser absorption have been studied with pulses of 1.5 ps duration irradiating solid targets at intensities of 1018 W/cm2. Absorption as a function of the incident laser flux and the irradiation angle in respect to the polarization reaches maximal values of 50% and is discussed in comparison to theoretical works published recently. This gives basic guidelines for efficient x-ray production which we have investigated in the hard (Ex > 50 keV, Bremsstrahlung) and soft (Ex < 1 keV, line emission of ions) spectral region. The feasibility of a short pulse and a plasma source with an extension of only some microns is proofed with an experiment using a spherically bent Mica crystal for collimation of the x-ray beam. The brightness and divergence of that x-ray beam being relevant for applications are given.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Small fluctuations in the acceleration sheath change the pointing of a proton beam accelerated from the rear side of a laser irradiated thin aluminum foil. The proton acceleration was produced with 40fs pulses of a Ti:sapphire laser at an intensity of approximately 1019W∕cm2. This observation has been made with a high spatial resolution Thomson spectrometer. The proton beam pointing has appeared stable in the energy range between the high energy cutoff (3MeV) and 50% of this value. Deviations of the beam position at lower energies changes in a range of 0–3mrad. The recorded pictures show wiggled and continuous proton traces which imply a release of the proton beam from the acceleration zone with a velocity chirp.

We have implemented an adjustable traveling-wave method of excitation for the transverse pumping of laser media based on tilting one of the gratings of the compressor of a chirped pulse amplification [Opt. Commun.56, 219 (1985)] (CPA) laser chain. We show that the only requirement to obtain the best locally compressed pulse is to slightly tilt and translate one of the gratings from its original position. Exact and approximate expressions of these two motions are derived, allowing an easy reconfiguration of any CPA laser. An experimental validation is performed with the first observation of a unidirectional soft-x-ray lasing when subpicosecond pumping pulses are used.

The transient-excitation pumping scheme, in which a picosecond duration pulse rapidly heats the plasma preformed by a low-intensity nanosecond pulse, was used to pump the Ne-like germanium, J=0–1 transition at 19.6 nm. A small-signal gain coefficient of 30 cm-1 was measured for targets ⩽5 mm long.

A 14 TW Ti:Sa laser furnishing pulses with a duration of 30 fs (full width at half maximum) at a repetition rate of 10 Hz was used to expose solid targets to intensities of up to 3x10(18) W/cm(2). Dosimetric techniques were employed to study the total x-ray yield, the spectral and angular distribution of the x-ray photons and the energy distribution of high-energy electrons injected into the solid target and emitted into the vacuum. Scans of laser pulse energy and duration were carried out to study the dependence of the x-ray generation efficiency on these parameters. The radiation transport processes in the target were modeled using the Monte Carlo method. The results of these calculations were used to interpret the measurement results and to critically discuss the applicability of dosimetric methods to the investigation of photon and electron emission from laser-produced plasmas.

Saturation of a low pump energy x-ray laser utilizing a transient inversion mechanism on the $3p\ensuremath{-}3s$ transition at 32.63 nm in Ne-like Ti has been demonstrated. A close to saturation amplification was simultaneously achieved for the $3d\ensuremath{-}3p,$ $J=\stackrel{\ensuremath{\rightarrow}}{1}1$ transition at 30.15 nm. Small signal effective transient gain coefficients of $g\ensuremath{\sim}46$ and $\ensuremath{\sim}35{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and gain-length products of 16.7 and 16.9 for these lines were obtained. Experiments demonstrate that it is possible to achieve saturated laser action in a transient regime with Ne-like Ti for a pump energy as low as $\ensuremath{\sim}5\mathrm{J}.$