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We propose a new scheme for compression of ultrashort laser pulses enabled by Near-Forward Raman Amplification (NFRA) in plasma. In contrast to traditional forward Raman amplification in plasma, the seed pulse in NFRA propagates faster than the pump through manipulation of the spatiotemporal properties of the pump beam. A “π-pulse” solution similar to that of stimulated Raman backscattering amplification is obtained, indicating that NFRA can be used for compression of ultrashort laser pulses. NFRA is numerically demonstrated with a 2D simulation using a front-tilt near-forward pump pulse.

An efficient method to transform a radiation of X-ray plasma lasers into a train of sub-fs pulses, as well as to form sub-fs pulses directly in an active medium of plasma lasers is suggested. This method is based on temporal and spatial modulation of the parameters of a resonant transition in absorbing or amplifying medium by a moderately strong IR field propagating in the same direction as an X-ray radiation.

Coherent intense attosecond x-ray pulses could lead to a fast dynamical imaging of biological macromolecules and other material nanostructures with a unique combination of a record high temporal and spatial resolution. Plasma-based x-ray laser sources are capable of producing high-energy x-ray pulses but with relatively long picosecond duration. The sources based on high-harmonic generation (HHG) of a laser field allow one to produce much shorter pulses but of lower energy. We suggest two different paths towards intense subfemtosecond x-ray sources: (i) via efficient transformation of the picosecond radiation of the x-ray plasma lasers into the trains of subfemtosecond pulses in a resonantly absorbing medium, and (ii) via amplification of HHG radiation in the active medium of the x-ray plasma lasers. We show that essentially the same technique can be used for realization of both paths. This technique is a modulation of the parameters of the resonant transition (accordingly in absorbing or amplifying medium) produced under the action of a sufficiently strong infrared or optical field. We propose experimental realization of the suggested technique in the passive and/or active media of (i) Li iii ions modulated by the mid-IR laser field and (ii) C vi ions modulated by the optical laser radiation.

The main objective of this project was to conduct experimental and theoretical research to find conditions leading to higher, than previously obtained efficiency η of transfer the pump energy into the short seed beam in plasma of Stimulated Raman Back-Scattering (SRBS). We have demonstrated very large amplification and compression in our earlier SRBS plasma. However, the efficiency η was much too low to reach very high intensity of the output beam in the focal spot. Recently, by solving a very difficult technical SRBS’ problem, namely, the creation of very reproducible and much larger diameter plasma channels than in our earlier research, we propose a new approach to obtain higher efficiency η. The crucial new result was a very reproducible, low noise amplified seed in the larger diameter of the plasma channel leading to the higher efficiency. Using this new setup and very encouraging results about increase efficiency continuing this approach in the future the efficiency is expect to reach the range of η ≈15 - 20 % required to develop practical SRBS plasma laser. Intellectual Merit: The model for the present project was created by our earlier SRBS experiments. The main objective of those experiments was to amplify and compressmore » the seed pulses in a plasma . The experiments demonstrated an unprecedented large pulse intensity amplification of 20,000 in system of 2-passes in ~2mm long plasma, and the seed pulse compression from 550fsec down to ~50fsec. The pump and seed beams in the present project have diameters of ~0.2–0.25mm each, propagating in ~0.35 - 0.45mm diameter and ~2-2.5mm long plasma channels (optimal length for our SRBS experiment) with input pump and seed intensities of 2x1014 and 3x1013 W/cm2, respectively. Such an SRBS system design was “prescribed” by computer simulations, which predict elimination of the SRBS “ saturation” for a such relatively short plasma channel. Plasma channels has been created by combining shorter (200psec) and longer (5nsec) laser pulses using an axicon lens for shorter pulse and spherical lens for longer pulse, a unique procedure we have developed for SRBS experiments.« less

It was shown some time ago that when the excitation of an ensemble of two-level atoms is swept in the direction of lasing, so that atoms are prepared in the excited state just as the radiation from previously excited atoms reaches them, the resulting laser amplifier is “highly anomalous” and yields superradiant emission without population inversion. We here show that transient gain in a three-level system has common features with Dicke superradiance and can yield strong extreme ultraviolet lasing in, for example, He atoms (at 58 nm) or He-like ions such as B3+ (at 6.1 nm).

Experiments demonstrate the ~77× amplification of 0.5 to 3.5-ps pulses of seed light by interaction with Langmuir waves in a low density (1.2 × 1019 cm−3) plasma produced by a 1-ns, 230-J, 1054-nm pump beam with 1.2 × 1014 W/cm2 intensity. The waves are strongly damped (kλD = 0.38, Te = 244 eV) and grow over a ~ 1 mm length, similar to what is experienced by scattered light when it interacts with crossing beams as it exits an ignition target. The amplification reduces when the seed intensity increases above ~1 × 1011 W/cm2, indicating that saturation of the plasma waves on the electron kinetic time scale (

We propose a new mechanism for the self-guiding of ultra-intense sub-picosecond laser pulses in gaseous media. It can be realized via optical field ionization by a laser pulse as it propagates inside an expanding cylindrical shock wave launched into ambient gas by a decayed plasma filament. In experiments, the filament was created in a hydrogen jet by a low energy femtosecond laser pre-pulse line focused with axicon lens. We demonstrated ionization-assisted guiding in structures with diameter as small as 14 μm and up to 3.5 mm long. The intensity reached 5 × 1017 W/cm2 in a single mode propagating for more than 100 Rayleigh lengths.We propose a new mechanism for the self-guiding of ultra-intense sub-picosecond laser pulses in gaseous media. It can be realized via optical field ionization by a laser pulse as it propagates inside an expanding cylindrical shock wave launched into ambient gas by a decayed plasma filament. In experiments, the filament was created in a hydrogen jet by a low energy femtosecond laser pre-pulse line focused with axicon lens. We demonstrated ionization-assisted guiding in structures with diameter as small as 14 μm and up to 3.5 mm long. The intensity reached 5 × 1017 W/cm2 in a single mode propagating for more than 100 Rayleigh lengths.

Experiments on Stimulated Raman Backscattering (SRBS) in plasma have demonstrated significantly higher energy conversion in a double-pass amplifier where the laser pulses go through the plasma twice compared with a single-pass amplifier with double the plasma length of a single pass. In this paper, the improvement in understanding recent experimental results is presented by considering quite in detail the effects of plasma heating on the modeling of SRBS. Our simulation results show that the low efficiency of single-pass amplifiers can be attributed to Landau damping and the frequency shift of Langmuir waves. In double-pass amplifiers, these issues can be avoided, to some degree, because pump-induced heating could be reduced, while the plasma cools down between the passes. Therefore, double-pass amplifiers yield considerably enhanced energy transfer from the pump to the seed, hence the output pulse intensity.

In this review we concentrate our attention on X-ray laser research and development covering the so called soft X-ray region (down to ~ 10 nm) and the ``water window'' region (down to 2.3 – 4.4 nm). We present the development of soft X-ray lasers (SXLs) and their applications using primarily collisional and recombination schemes. We present the paths towards compact systems and major achievements to date. We discuss conditions for using both schemes to reach the ``water window'' region. In this discussion we also present another possible schemes based on photo-pumping, inner shell transitions in atoms and ions and ``Doppler Compression'' of IR laser radiation down to the X-ray region with its intrinsic tunability possibility. A significant part of the review is dedicated to the wide range of applications of SXLs with wavelengths at and above 10 nm. Also discussed is the potential of extending the range of biological applications for the ``water window'' region.

The output power of any laser system is limited by the power that any of its individual components can withstand without being destroyed. This increases the size, complexity and cost of next-generation lasers built using conventional optical-gain media. But by using a plasma, which is impervious to optical damage, as the gain medium, such limitations could be significantly surpassed. Here, we report the use of stimulated Raman backscattering in a millimetre-scale plasma to simultaneously amplify and compress an input pulse to increase its intensity by more than two orders of magnitude. Further amplification and compression of such a pulse was achieved by passing it twice through our system, an approach that could be readily extended in future with a cavity-like multipass design. Such an approach could enable the construction of a new generation of compact, low-cost ultrahigh-intensity laser systems.

This paper is about the recent experimental results on amplification of the CV line in the “water window” at 4.03 nm from resonance transition to the ground level of He-like ions in recombination scheme. The indication of the amplification of the CV line has been observed when an elongated narrow plasma channel was created, where high intensity 100 fs beams, optimal for creating CV ions in high density plasma, was propagated up to 0.5–0.6 mm. Without channeling the effective plasma length was much shorter and there was no indication of amplification.The large interest in gain generation in He-like ions in the transition to ground state is due to the possibility of applying a recently developed theory of Lasing Without Inversion (LWI) in XUV and X-ray regions to largely increase the gain for such transitions. The presented results of the indication of CV line amplifications are being discussed from the point of view of using LWI as a superradiance gain increase, hence to construct a very compact soft X-ray laser in the “water window”.The last part of the paper is related to the application of the ultra-intensive fs plasma laser, which is currently in the process of development by using stimulated Raman backscattering (SRBS) to create a plasma amplifier and compressor, as the pump for compact laser operating in the “water window” and also at shorter wavelengths.

We present a highly temporally and spatially resolved direct measurement of the propagation of ionization and recombination fronts in bulk ZnSe. The fronts were created by two-photon absorption (TPA) of 10 ps, 800 nm 'pump' pulses incident on both sides of a 1 mm thick crystal. The measurement was achieved by the spectral analysis of the frequency shift of a 100 fs 'probe' pulse transversely propagating through the crystal at the time of excitation and recombination. The probe was frequency shifted due to the temporal change in the refractive index of the medium. The delay between the probe and the pumps was finely scanned for time-resolved measurements, while imaging of the probe onto a spectrometer slit provided spatial resolution across the crystal. The spatially and temporally resolved shift agrees well with a model that includes TPA and recombination. Our set-up provides a general technique for measurements of rapid ionization and recombination processes.

A minimally invasive keratorefractive procedure using high-intensity, low-energy femtosecond laser pulses to perform intrastromal ablation is described. Because of the low pulse energy and the ultrashort duration, tissue in the corneal stroma can be ablated with almost no heat or shockwave generation. This technique obviates the need for the laser in situ keratomileusis (LASIK) flap but retains the advantages of the LASIK procedure. In the technique, a series of femtosecond laser pulses create temporary microchannels in the stroma, oriented perpendicular to the eye’s optical axis. After the microchannels are created, a second series of femtosecond pulses directly ablate the desired amount of stromal tissue in a controlled fashion. The ablated material is ejected from the microchannels so the surface layer above the ablated regions collapses, with a consequent change in the refractive power of the cornea. Financial Disclosure No author has a financial or proprietary interest in any material or method mentioned.

General : Our major scientific achievements in Raman Backscattering (RBS) amplification and compression of short laser pulses in plasma. The laser system based on RBS steps in where the current technology of chirped pulse amplification (CPA) (extremely successful in developing ultra-short and ultra-intense laser pulses in last 2 decades) becomes difficult and very expensive to apply. Good base for such RBS laser was created by our recent experiments, which were supported by GPS grants. The main objective of the present grant was: improvement efficiency of energy transfer from pump to seed. The results surpassed our expectations; we improved the efficiency of energy transfer from pump to seed by a factor of 6 compared to the best of our previous results and amplified seed pulse compressed down to about 50 fsec.

Periodic plasma density modulations have been observed in a laser spark created by focusing a 5–20 MW laser pulse into nitrogen at 1 to 2 atmospheres. The formation of such structures has been studied by measuring the period as a function of the wavelength λ and the incident angle γ of the laser pulse. The results show good agreement with a surface-instability model. The intensity of the scattered laser light has the same periodicity and is consistent with the measured plasma density profile.

The results of two different series of experiments on generation lasing action in the soft x-ray region are presented. In the first series lasing in hydrogen-like Li III ions at 13.5 nm (2-1 transition to ground state) in a 5 mm long LiF microcapillary at 2 Hz repetition rate is demonstrated. The initial plasma was created in a 0. 3 mm diameter microcapillary by a low power Nd:YAG laser (0. 1 J in 5 ns), whereas the plasma pumping was proceeded with a 50 mJ, 250 fs UV laser. In the second series lasing is demonstrated in hydrogen-like B V ions at 26.2 nm (3-2 transition) using a very small Nd:YAG pumping laser (0.45 J in 8 ns) at a repetition rate of I Hz. The preplasma in a B 2 O 3 microcapillary was created from wall ablation with a 0.2 J (20 ns) KrF laser, and the whole soft x-ray lasing system takes up less space than is available on a 1.2 m × 3 m optical table. In the last part of the paper the development of soft-x-ray lasers (SXLs) for application in lithography and soft x-ray microscopy is discussed.

The main goal of this paper is to present experimental results on comparison of lasing efficiency for Li III 13.5 nm (2-1 transition) line in LiF microcapillary plasmas using 0.25 and 1 /spl mu/m subpicosecond pumping lasers. A formula for soft X-ray laser efficiency calculation is presented and used for such comparisons. The results for discharge created preplasma in L=4 mm and L=14 mm microcapillaries are also presented and compared with the results for laser created preplasma.

Using a very small Nd/YAG pumping laser (0.45 J in 8 nsec), we have demonstrated high gain $(G\ensuremath{\approx}14--19\mathrm{cm}{}^{\ensuremath{-}1})$ and gain-length product $(\mathrm{GL}\ensuremath{\ge}5)$ in hydrogenlike B V ions at a wavelength of 26.2 nm (the 3-2 transition) at a repetition rate of 1 Hz. The preplasma in a ${\mathrm{B}}_{2}{\mathrm{O}}_{3}$ microcapillary was created from wall ablation with a 0.2-J (in 20 nsec) KrF laser. The gain was generated in the microcapillary plasma in a traveling wave regime by longitudinal pumping. This very efficient soft x-ray lasing system takes up less space than is available on a $4\mathrm{ft}\ifmmode\times\else\texttimes\fi{}10\mathrm{ft}$ optical table.

Soft x-ray lasing with a gain length GL{approx_equal}5.5 was demonstrated in hydrogenlike LiIII at 13.5 nm (2-1 transition) in a 5 mm long LiF microcapillary using a 50 mJ, 250 fs UV laser beam at a 2 Hz repetition rate. The initial plasma was created in the 0.3 mm diameter microcapillary by a low power Nd/YAG laser. The strongly nonlinear increase of the 13.5 nm line intensity with increasing microcapillary length was compared with a linear increase of the LiIII 11.4 nm (3-1 transition) line intensity. {copyright} {ital 1996 The American Physical Society.}

Gain measurements have been performed in a recombining carbon plasma generated by a line focussed Nd-glass laser beam on a multi-fin target with the aim of developing a “table-top” soft X-ray laser. With a laser beam of only 4 J (2ns) and a 6 mm long target gain up to 7.1 cm−1 was measured for the C VI 18.2 nm line (gain-length ∼4.3). During these measurements high gain (6.5 cm−1) was also measured for the first time on the C VI 13.5 nm line (4−2 transition). These measurements were done in the process of preparation for two-target experiments. High gain was measured independently for two separate, identical 6 mm targets. Changes in the influx into the plasma of iron ions from “cooling” blades significantly affected the location of the gain region and can be used as a mechanism to control the gain region in two-target experiments.

In our presentation we overview progress we made in developing a new ultrashort and ultraintensive laser system based on Raman backscattering (RBS) amplifier /compressor from time of 10th XRL Conference in Berlin to present time of 11th XRL Conference in Belfast. One of the main objectives of RBS laser system development is to use it for pumping of recombination X-ray laser in transition to ground state of CVI ions at 3.4 nm. Using elaborate computer code the processes of Optical Field Ionization, electron energy distribution, and recombination were calculated. It was shown that in very earlier stage of recombination, when electron energy distribution is strongly non-Maxwellian, high gain in transition from the first excited level n=2 to ground level m=1 can be generated. Adding large amount of hydrogen gas into initial gas containing carbon atoms (e.g. methane, CH4) the calculated gain has reached values up to 150–200 cm−2 Taking into account this very encouraging result, we have proceed with arrangement of experimental setup. We will present the observation of plasma channels and measurements of electron density distribution required for generation of gain at 3.4 nm.

Our presentation consisted of two parts. In the first part we presented main theoretical results on the plasma and pumping conditions required to generate gain at 3.4 nm in H-like CVI ions in transition from the first excited state to ground state. Transient population inversion is generated during the recombination process. It was shown that high gain (up to G~200 cm-1) can be achieved using currently available compact lasers. In the second part we presented a new type of compact laser generating ultra-short and ultra-intensive pulses via Raman Backscattering (RBS) amplification and compression in plasma. We achieved large (up to 1000) “seed” amplification and its compression from~1 psec down to 150 fsec in only 2 mm long plasma pumped with~1014 W/cm2 pulses. We also presented very recent experiments on amplification in 2 passes setup. Such RBS amplifier and compressor is expected to provide in not too far future intensities~1020 W/cm2 at high repetition rate in compact (university type) system, which would be ideal pump not only for 3.4 nm laser but even for shorter wavelength lasers.

A high power UV laser has been developed as a pump source for short wavelength (down to 1 nm) X-ray lasers. Various schemes are considered and theoretical analysis is discussed. Spectroscopic studies of laser-target interaction have been performed and, in particular, the effect of a prepulse on plasma generation has been investigated. Analysis of the observed spectra indicates that reduction of the prepulse energy results in a higher temperature plasma. Investigation of the interaction using thin layered targets is also presented. These data provide evidence for initially hot plasma conditions generated from target layers ≦150 A. Discussions of proposed laser schemes at 1–5 nm are presented.

Compressing laser pulses to extremely high intensities through backward Raman amplification might be accomplished in a plasma medium. While the theory is relatively straightforward for homogeneous fully ionized plasma, a number of important effects enter when the plasma is not fully ionized. In particular, when a mixture of gases is employed to accomplish the coupling, there can be several thresholds for incremental ionization. The refraction of both the pump and the seed is then strongly affected by the plasma ionization. Moreover, in the case of Raman backscattering in partially ionized plasma, the degree of plasma ionization is particularly sensitive to the counterpropagating geometry. This idea is examined in light of data for a recent experiment on a Raman amplifier.

The formation of plasma channels with nonuniformity of about +- 3.5% has been demonstrated. The channels had a density of 1.2x10^19 cm-3 with a radius of 15 um and with length >= 2.5 mm. The channels were formed by 0.3 J, 100 ps laser pulses in a nonflowing gas, contained in a cylindrical chamber. The laser beam passed through the chamber along its axis via pinholes in the chamber walls. A plasma channel with an electron density on the order of 10^18 - 10^19 cm-3 was formed in pure He, N2, Ar, and Xe. A uniform channel forms at proper time delays and in optimal pressure ranges, which depend on the sort of gas. The influence of the interaction of the laser beam with the gas leaking out of the chamber through the pinholes was found insignificant. However, the formation of an ablative plasma on the walls of the pinholes by the wings of the radial profile of the laser beam plays an important role in the plasma channel formation and its uniformity. A low current glow discharge initiated in the chamber slightly improves the uniformity of the plasma channel, while a high current arc discharge leads to the formation of overdense plasma near the front pinhole and further refraction of the laser beam. The obtained results show the feasibility of creating uniform plasma channels in non-flowing gas targets., 15 pages, 7 figures, submitted to Physics of Plasmas

We present a numerical investigation of the parameters characterizing the recombination gain in the LiIII 2→1 transition (13.5nm). The numerical model includes the initial Optical Field Ionization (OFI) of the plasma by an intense 100fs laser pulse, taking into account ATI heating, particle collisions and spatial effects. Gain is then calculated during the process of recombination as the plasma expands and cools down. We characterize the behavior of the gain under different pumping parameters and initial plasma conditions. In addition we present the latest experimental results on Raman amplification of ultrashort pulses in the pursuit for a better and more compact scheme to enable a ``table-top'' x-ray laser.

Raman amplification of subpicosecond laser pulses up to 95 times is demonstrated at corresponding frequencies in a gas-jet plasma. The larger amplification is accompanied by a broader bandwidth and shorter pulse duration. Theoretical simulations show a qualitative agreement with the measurements, and the effects of the plasma conditions and laser intensities are discussed.

A large, UV preionized excimer krypton-fluoride (KrF) laser amplifier was designed and fabricated. This laser amplifier produced an on-target laser power density of 2*10/sup 18/ W/cm/sup 2/. Maximum output energy was determined to be 5 J in a 20-ns-long pulse, and maximum instantaneous power was achieved in a 250-350-mJ, 300-fs pulse with an injection (seed) pulse of approximately=3-4 mJ in a triple pass configuration. The output wavelength is 248 nm. The amplifier has a volume of approximately 5 liters. UV preionization was accomplished with a traveling spark board. Fast, low inductance switching of the high voltage was accomplished through the use of an electrically triggered rail-gap. Some of the design and tradeoffs of the electrical pulse discharge circuitry, considerations related to the materials used, the optics, gas handling, timing requirements, and some experimental results are discussed. >

We present numerical calculation of recombination gain in LiIII transition to ground state (2 → 1). The model includes the initial ionization of the plasma by an intense fs laser pulse, and continues through the expansion and cooling of the plasma simultaneously with the recombination process. We show that although initial estimations of the energy absorption by the plasma from the ionization laser does not allow for recombination gain, the expansion and cooling processes that take place immediately after ionization, in addition to the non‐Maxwellian distribution of electrons in the plasma, give rise to high gain under feasible experimental conditions.

We present results related to the search for optimum conditions for lasing to ground state of H‐like LiIII ions at 13.5 nm. These conditions are being considered from the point of view of the development of a prototype of a very compact 13.5 nm laser for metrology of soft x‐ray (EUV) lithography. Theoretical simulations are discussed in relation to experimental data. Experiments on channeling of ultrashort high intensity pumping laser beam in microcapillary plasma are presented for conditions appropriate for lasing at 13.5 nm. We are also discussing Raman amplification of ultrashort pulses in microcapillary plasma as a possibility for future use in X‐ray lasers.

The optimum conditions for gain generation for the transition to the ground state at 13.5 nm in Liiii ions were investigated in discharge created pre‐plasma. The spatial and temporal evolution of plasma density was measured and a hollow density profile was observed. Waveguiding of high power laser pulses has been demonstrated in discharge created plasma channels, which can be helpful in increasing the interaction length, thus improving the gain‐length product.

Increasing the power of ultra-high-intensity lasers requires crystal amplifiers and metre-scale optical compression gratings that are ever more difficult to build. Simulations suggest that Raman amplification in a plasma could permit the generation of laser intensities many orders of magnitude higher than currently possible.

The experimental results on generation of high gain-length product for 13.5 nm radiation from 2 - 1 transition in hydrogen-like Li III ions are presented for 1 micrometers subpicosecond pumping laser. The comparison with earlier results, obtained with 0.25 micrometers subpicosecond pumping laser, is discussed in terms of gain generation efficiency. The results for discharge created pre-plasma in L equals 4 mm and L equals 14 mm microcapillaries are also presented.

The aim of this work was to investigate the conditions for waveguiding of nanosecond laser pulses in plasmas in microcapillary and gas jet for the development of a compact soft x-ray laser (SXL). A detailed study of He- and H-like carbon and nitrogen ion spectra, as a function of gas (plasma) densities, and plasma length was performed. The majority of the presented data were obtained in plasmas created in nitrogen injected into the vacuum chamber by pulsed electromagnetic valve. The waveguide conditions for the pump laser pulse (1.054 micrometer, 2 - 7 J, 1.5 ns) were achieved by line optical breakdown (up to 6 mm long) produced in gas by focusing a pre-pulse laser beam with an axicon lens. Intensive strong lines of H- and He-like ions were observed and the possibilities of obtaining the lasing conditions for SXL are discussed.

The formation of an elongated laser spark in a high density gas puff target has been studied experimentally using 10 J 1.5 ns Nd:glass laser pulses. The laser sparks were created by focusing the laser beam on the gas puff targets with a spherical lens or an axicon. The spark was characterized using an imaging system with CCD camera and a grazing incidence grating x-ray spectrometer. Propagation of the second laser pulse of 1.5 ns time duration along the spark plasma was also studied.

We present recent work on the development of the compact ('table top') soft x-ray lasers (SXLs) using two approaches. In one approach we use low energy Nd/Glass laser (pulse duration, (tau) equals 1.5 nsec, beam energy E equals 3 - 5 J), and in the second approach we use powerful subpicosecond (PSP) laser ((tau) equals 200 fsec, E equals 0.1 - 0.3 J, maximum power density P approximately equal to 2 multiplied by 10 18 W/cm 2 ). We discus generation high gain at 18.2 nm and 13.5 nm in CVI line focussed Nd/Glass laser on a carbon target and the unexplained difficulty to obtain gain-length GL greater than 4.5 in such a configuration. The time evolution of intensities of spectral lines and their correlation with time averaged spectra are analyzed. We also present measurement of plasma refraction as a possible explanation of the limitation of GL. More recently we changed the configuration of this experiment using as a target polyethylene microcapillary with focussing Nd/Glass laser at the entrance of the microcapillary. Very encouraging results were obtained for CVI 18.2 nm line and the possibilities of generation high GL in the near future are discussed.

X-ray laser experiments involving the creation of fast recombining plasmas by optical field ionization of preformed targets were conducted. A nonlinear increase in the intensity of the 13.5nm Lyman-{alpha} line in Li III with the length of the target plasma was observed but only for distances less than the laser confocal parameter and for low plasma electron temperatures. Multiphoton pumping of resonant atomic transitions was also examined and the process of multiphoton ionization of FIII was found to be more probable than multiphoton excitation.

We present recent results on the development of a small scale soft X-ray laser with low pumping laser energy using a multi-fin target in a two-target chamber. With only 4 J (2 ns) laser beam energy a maximum gain of 7.1 cm-1 was measured for the CVI 18.2 nm line for a single 6 mm long target. Similar gain (6.5 cm-1) was also measured for the CVI 13.5 nm line. Control of the gain region in the plasma was demonstrated by changing the influx of iron into the plasma. We also present results from our attempt to generate gain on the 2-1 transition in LiIII at 13.5 nm using a powerful sub-picosecond KrF laser system following work by the RIKEN (Japan) group.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

To optimize the coupling to indirect drive targets in the National Ignition Campaign (NIC) at the National Ignition Facility [E. Moses et al., Phys. Plasmas 16, 041006 (2009)], a model of stimulated scattering produced by multiple laser beams is used. The model has shown that scatter of the 351 nm beams can be significantly enhanced over single beam predictions in ignition relevant targets by the interaction of the multiple crossing beams with a millimeter scale length, 2.5 keV, 0.02−0.05 × critical density, plasma. The model uses a suite of simulation capabilities and its key aspects are benchmarked with experiments at smaller laser facilities. The model has also influenced the design of the initial targets used for NIC by showing that both the stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) can be reduced by the reduction of the plasma density in the beam intersection volume that is caused by an increase in the diameter of the laser entrance hole (LEH). In this model, a linea...

Propagation of 800 nm, 120 fs laser pulses with intensities of 4×1016 W/cm2 in supersonic gas jets of N2 and H2 is studied using a shear-type interferometer. The plasma density distribution resulting from photoionization is resolved in space and time with simultaneously measured initial neutral density distribution. A distinct difference in laser beam propagation distance is observed when comparing propagation in jets of H2 and N2. This is interpreted in terms of ionization induced refraction, which is stronger when electrons are produced from states of higher ionization potential. Three dimensional particle-in-cell simulations, based on directly solving the Maxwell–Lorentz system of equations, show the roles played by the forward Raman and ionization scattering instabilities, which further affect the propagation distance.

An overview of the X-ray Laser project at Princeton University will be given. Emphasis will be on improvements being made to the small scale soft x-ray laser (SXL). New target designs to enhance cooling and others to reduce losses due to beam refraction have been introduced though results are stilt preliminary. Proof of principle experiments for the application of the SXL to both transmission and reflection microscopy have been performed. To generate shorter wavelength x-ray lasers on a reasonable scale-size, a high power, 300 fsec pulse duration, ultraviolet KrF laser system (the PSP-laser) has been developed. Of the several theoretical schemes which exist, the two-laser approach and a newly proposed recombination approach will be described. The new approach proposes to scale the existing 18.2 nm recombination x-ray laser to shorter wavelength (

We present recent experimental data related to the resonant photopumping of Mo VII by Mo XII as a method of generating a VUV source near 600 A. The experiment is based on a scheme proposed by Feldman and Reader (1989), in which the excitation of the 4p5 6s - 4p6 transition in Mo VII by the 5s 2Ssub 1/2 - 4p 2Psub 1/2 transition in Mo XII produces a population inversion on one of several 6s-5p transitions in Mo VII. The generation of plasmas suitable for photopumping experiments is established from the presentation of measurements of line emission from Mo plasmas as a function of laser and target parameters. Results of the laser produced plasma experiments show the successful enhancement of the population of the Mo VII 4p5 6s upper lasing level when pumped by an adjacent Mo XII plasma. We also present results where improvements of the intensity of the Mo XII pump source, achieved using an additional pump laser, lead to the generation of a population inversion for a VUV transition.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A program aimed at the development of a table-top soft X-ray laser on the CVI 18.22 nm transition is reported. A gain-length product of GL = 2.5-3.5 has been observed for a variety of target geometries using a 3-ns 6-15 J Nd:glass driving laser emitting at 1.05 micron. Plans for future experiments are outlined.

A plasma-based resonant backward Raman amplifier/compressor for high power amplification of short laser pulses might, under ideal conditions, convert as much as 90% of the pump energy to the seed pulse. While the theoretical highest possible efficiency of this scheme has not yet been achieved, larger efficiencies than ever before obtained experimentally (6.4%) are now being reported, and these efficiencies are accompanied by strong pulse compression. Based on these recent extensive experiments, it is now possible to deduce that the experimentally realized efficiency of the amplifier is likely constrained by two factors, namely the pump chirp and the plasma wavebreaking, and that these experimental observations may likely involve favorable compensation between the chirp of the laser and the density variation of the mediating plasma. Several methods for further improvement of the amplifier efficiency in current experiments are suggested.

The enhancement of stimulated Raman backscattering (SRBS) amplification was demonstrated by introducing a plasma density gradient along the pump and the seed interaction path and by a novel double-pass design. The energy transfer efficiency was significantly improved to a level of 6.4%. The seed pulse was amplified by a factor of more than 20 000 from the input in a 2mm long plasma, which also exceeded the intensity of the pump pulse by 2 orders of magnitude. This was accompanied by very effective pulse compression, from 500fsto90fs in the first pass measurements and in the second pass down to approximately 50fs, as it is indicated by the energy-pulse duration relation. Further improvements to the energy transfer efficiency and the SRBS performance by extending the region of resonance is also discussed where a uniform ∼4mm long plasma channel for SRBS was generated by using two subsequent laser pulses in an ethane gas jet.

In the 1980's many exciting advances in the field of soft x-ray lasers took place. A novel experimental set-up was developed at Princeton to improve the performance and extend the wavelength range of existing soft x-ray lasers. In th1s paper a new target manipulation system capable of remotely aligning multiple targets in a vacuum chamber within a magnetic field is described Alignment to the magnetic field orientation, incident pump laser and the two axial diagnostics: a high resolution spectrometer equipped with a multichannel array and a time resolved imaging spectrometer is presented. We include a brief explanation of the Macintosh- Camac based data acquisition system. In addition we discuss the possibifity to increase in the near future the brightness of the x-ray laser beam by several orders of magnitude by developing a cavity with multilayer mirrors.

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.© (1990) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The emerging technology of soft x-ray lasers has novel applications to microscopy, lithography, and other fields. This article describes the status of soft x-ray laser research with the aim of bringing the rapid developments in this field to the attention of potential users in other disciplines. The different techniques for generating a population inversion and producing a soft x-ray laser are reviewed. The status of current research in the field and the near-term prospects are described. It is expected that the range of potential applications of soft x-ray lasers will increase as their performance improves. Work aimed at increasing the output power and progressing to shorter wavelengths with these devices is also reviewed.