Soft X-ray Amplification Research Articles

Bessel spatial profile of a soft x-ray laser beam

We report far-field profile measurements of an optical-field-ionized high-order harmonic-seeded soft x-ray laser. We show that the beam transverse profile can be controlled between a regular Gaussian shape and a Bessel profile exhibiting several rings via the infrared laser pump intensity. These experimental data are supported by a complete numerical modeling including a two-dimensional plasma amplifier simulation and a two-level soft x-ray amplification using a Maxwell–Bloch treatment. This model takes into account the experimental high-order harmonic wavefront and intensity before it is numerically amplified.

Observation of a plasma waveguide in a preformed plasma pumped by double-pulse laser irradiation for the efficient soft x-ray amplification

A molybdenum slab target was irradiated by double pulses of a line-focused chirped-pulse-amplification Nd:glass laser. A probe beam was injected in the plasma in the longitudinal direction, and we found that the probe beam was guided in the plasma with the plasma length of up to 4.3 mm at the time of 0.8-1.5 ns after the peak of the second pulse. Comparison with hydrodynamics code implied that the electron-density dip or flat region was formed, which was favorable for the efficient soft x-ray lasing under the coupling of the seed x-ray and the x-ray amplifier.

Investigation of a highly saturated soft X-ray amplification in a capillary discharge plasma waveguide

The characterization of the laser beam intensity distribution of a highly saturated 46.9-nm soft X-ray laser excited by capillary discharges is reported. The laser produces a total output energy of 300 μJ/pulse by amplification in plasma channels having lengths up to 0.45 m. A regime of laser amplification, which is almost free from the effect of the refraction defocusing, is experimentally determined. This regime produces a soft X-ray laser beam with an intense sub-milliradiant component. In the longer active medium the laser intensity distribution reaches the divergence of 0.6 mrad, which approaches the limit of diffraction. A comparison of the experimental results with the simulations performed with a ray-tracing code shows that the small divergence of the beam could be attributed to the effect of a weak index waveguiding of the laser beam through the long plasma channels.

Time-resolved pinhole images of a discharge-created capillary plasma dedicated to soft X-ray amplification

Time evolution of the spatial distribution of radiation emitted by an ablative discharge-created plasma in a polyethylene capillary has been investigated to study the feasibility of soft X-ray amplification. Time resolved images of the plasma have been obtained using a pinhole camera device, for different spectral ranges from VUV to soft X-ray. The different phases of the plasma evolution (ionisation, ablation, cooling) were observed and yielded hints to optimise the discharge system for soft X-ray lasing at 18.2 nm.

Investigating the dynamics of fast capillary discharges leads to soft X-ray laser realization at 46.9 nm

The dynamic evolution of plasma columns, generated by a fast capillary discharge setup, was measured by an off-axis imaging apparatus. Appropriate initial conditions were found where stable compression of the plasma column could be obtained. The appearance of a sequence of shock and refraction waves during the plasma column compression is clearly seen. Relative line emission from Ne-like and F-like Ar ions at 25-50A spectral range was used for electron temperature and density measurements, during the column compression. Appropriate conditions for soft x-ray laser operation were found at the time the first shock wave reaches the axis. Indeed, strong soft x-ray amplification at 46.9nm (3s-3p, J=0-1) was measured at this stage. The small signal gain of the measured amplified line is ∼0.9 cm -1 and the beam divergence is less than 5mRad.

Soft X-ray laser amplification through dynamics of capillary discharge Z-pinch plasmas

The optimum conditions for soft x-ray amplification through the dynamics of Ar plasma, created by the capillary discharge Z-pinch, have been investigated. The numerical simulation of plasma dynamics was based on magnetohydrodynamic one-dimensional radiation model at different initial discharge pressures ranging from 240 to 15000 mTorr. The plasma behavior of the implosion-pinch phase was analyzed, taking plasma atomic processes into account. The optimum soft x-ray lasing occurs in the 3s-3p transition in Ne-like Ar plasma. Radiation emission and absorption effects on gain coefficients were also examined.

Plasma Dynamics in Capillary Discharge Soft X-Ray Lasers

The dynamics and stability of collapsing gas columns, generated by a fast capillary discharge setup, are studied for obtaining soft x-ray amplification in highly ionized ions. Electron temperature and density measurements at the peak of the compression stage are used for tuning the discharge parameters. Once the needed conditions were achieved, strong amplification of the 3s-3p transition in Ne-like Ar ions at 469 A is observed. A gain coefficient of >0.75 cm(-1) and a beam divergence of <5 mrad are measured along plasma columns of <150 microm diameter and up to 165 mm length.

Dotted-Array Plasma Production by Using a Line-Focusing Lens System with Segmented Prism Array for Compact X-ray Laser Experiments

A long narrow plasma originated from an array of microdots has been produced using a new line-focusing lens system. The lens system consists of a five-segmented prism array, a beam expander and a cylindrical lens assembly that was designed for compact X-ray laser experiments using a small Nd:YAG (yttrium aluminum garnet) laser. The irradiation pattern on a slab target has a small-scale intensity modulation due to interference among the segmented beams. The focused line has an array of microdots with a 50 µm diameter, a 140 µm spacing and an 11 mm length. Spectroscopic observation has been carried out in the expanding plasma produced by irradiating the YAG laser via this lens system onto an Al target. Near the target surface, highly ionized plasma was produced to have a spatial distribution corresponding to the microdot array in the irradiation pattern. It has been demonstrated that soft X-ray amplification takes place in the plasma produced by this irradiation method with very low input energy.

Table-top soft x-ray lasers

This article reviews the progress in the development of practical table-top sources of soft x-ray laser radiation. The field is rapidly approaching the stage at which soft x-ray lasers sufficiently compact to fit onto a normal optical table will be routinely utilized in science and technology. This is the result of recent advances in the amplification of soft x-ray radiation in both compact laser-pumped and discharge-pumped devices. The use of excitation mechanisms that take full advantage of new ultrafast high power optical laser drivers and multiple pulse excitation schemes has resulted in the demonstration of saturated soft x-ray amplification at wavelengths as short as 14 nm using several Joule of laser-pump energy. Moreover, several schemes have demonstrated significant gain with only a fraction of a Joule of laser-pump energy. In addition, the demonstration of saturated table-top soft x-ray lasers pumped by very compact capillary discharges has shattered the notion that discharge-created plasmas are insufficiently uniform to allow for soft x-ray amplification, opening a route for the development of efficient, high average power soft x-ray lasers. Recently, a table-top capillary discharge laser operating at 46.9 nm has produced millijoule-level laser pulses at a repetition rate of several Hz, with a corresponding spatially coherent average power per unit bandwidth comparable to that of a beam line at a third generation synchrotron facility. This review summarizes fundamental and technical aspects of table-top soft x-ray lasers based on the generation of population inversions in plasmas, and discusses the present status of development of specific laser systems.

Soft X-Ray Amplification at 26.2 nm with 1-Hz Repetition Rate in a Table-Top System

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.

Progress in the Development of Table-Top Discharge-Pumped Soft X-Ray Lasers

The demonstration of large soft x-ray amplification in a dischargc-created plasma has opened a new patle to the development of compact and practical soft x-ray lasers. We review our progress in the development and study of these ultrashort wavelength lasers. The field has advanced from the first observation of large amplification in a discharge-created plasma in Ne-like Ar [J.J. Rocca. V.N. ShlyapLsev, F.G. Tomasel, O.D. Cortazar, D. Hartshorn, and J.L.A. Chilla, Phys. Rev. Lett. 73, 2192 (1994)]. to the denionstration of an extremely compact saturated laser at 46. 9 nm. In this paper we give an overview of these and other selected results. They include the observation of large amplification (gain-length product of 7.5) in Ne-like at 60.8 nm in material ablated from a solid target by a discharge, and preliminary results of the search for gain at a shorter wavelength in Ne-like Ca. A recent study of the spatial coherence of the capillary discharge 46.9 nm Ne-like Ar lasers which provides the first experimental measurement of a monotonic increase of the spatial coherence with length in a soft x-ray amplifier. is also summarized.

Demonstration of Soft X-Ray Lasing with Neonlike Argon and Nickel-like Xenon Ions Using a Laser-Irradiated Gas Puff Target

We report the first demonstration of soft x-ray amplification in an elongated plasma column produced by laser irradiation of a gas puff target. The gas puff target is formed by the pulsed injection of gas from a high-pressure solenoid valve through a nozzle in the form of a narrow slit. The plasma column to be used as an x-ray laser active medium is produced by irradiating the gas puff target with a high-power laser beam focused to a line. Lasing in neonlike argon at 46.9 nm and strong indications of lasing in nickel-like xenon at 10.0 nm were observed.

Collisional excitation x-ray lasers: Toward a compact x-ray source

Soft x-ray amplification has been observed over the wavelengths of 14.3 nm — 4.6 nm in nickel-like ions of various atoms with the atomic numbers of Z=47 (Ag) to 72 (Hf). These results were obtained by irradiation of a preformed plasma with a short duration laser pulse. Observation of polarization of the amplified soft x-ray radiation infers that the population inversion is strongly affected by the trapping of the resonance radiation. Conditions for increasing the population inversion density by pumping with shorter-duration laser pulses due to transient effects are examined.

Dynamics of the plasma column of a capillary discharge soft X-ray laser

A sequence of time-resolved soft X-ray pinhole images shows the rapid plasma compression in a fast high-power capillary discharge. The compressed plasma column has a length-to-diameter ratio >500 and a good axial uniformity, characteristics that have allowed for the first demonstration of large soft X-ray amplification in a discharge-created plasma.

Demonstration of a Discharge Pumped Table-Top Soft-X-Ray Laser

We report the first observation of large soft X-ray amplification (g*1=7.2) in a discharge created plasma. A fast, /spl sim/40 kA current pulse from a compact discharge was used to excite plasma columns up to 12 cm in length in 4 mm channels, producing population inversion in the J=0-1 line of Ne-like Ar and resulting in a gain of 0.6 cm/sup -1/ at 46.9 nm. The capillary discharge excitation scheme has the potential for increasing the wall-plug efficiency of ultrashort wavelength lasers by two orders of magnitude.

An optical field-induced ionization X-ray laser using a preformed plasma scheme

Soft X-ray amplification by optical field-induced ionization (OFI) of a preformed plasma is investigated. A small-signal gain of 20 cm/sup -1/ and a gain-length product of 4 at 13.5 nm on the Lyman-/spl alpha/ transition in hydrogen-like lithium have been obtained using the modified OFI scheme. Experimental and numerical results ensure that we have produced a plasma with a significantly lower electron temperature than what is expected by an above-threshold ionization model. To explain the results, a two component plasma model in which relatively high temperature electrons are produced by OFI in a cold electron bath of the preformed plasma is presented. The model indicates that the average electron temperature of the OFI plasma rapidly decreases since a high-temperature part of the electrons escapes from the focal volume without interaction. The initial electrons produced prior to the field ionization, which survive after the OEI, also significantly contribute to the rapid three-body recombination. Based on the ionization-induced refractive index change, the pulse propagation of a high-intensity pump laser during the OFI is also discussed. >

Recent Progress in Soft X-ray Lasers.

Outstanding progress in soft x-ray lasers. has been made during these 10 years since thefirst demonstration of soft x-ray amplification in 1984. This paper reviews current efforts andprogress in extensive approaches to realize soft x-ray lasers as a laboratory tool. The use ofoptical-field-induced ionization by an ultrashort high-intensity laser is a promising approach. Two types of a soft x-ray laser based on the recombination and the electron collision excitationfollowing the optical-field-induced ionization are discussed. The significant progress in the reductionof the necessary pump energy of a conventional collisional scheme and the demonstration of a large gain-length product in a conventional recombination scheme are also described.In addition to these laser-based approaches, the first demonstration of a capillary discharge XUV laser based on an electron collision excitation is presented.

Space- and time-resolved investigation of short wavelength x-ray laser in Li-like Ca ions

We have demonstrated the soft x-ray amplification for lithium-like Ca17+ 4f-3d transition at 57.7 Å with 900 ps, 1.05 μm drive laser pulse. The spatial distribution of the gain coefficient and temporal history of the lasing line emissions were also obtained.

Collisional excitation X-ray laser experiments in plasmas produced by 0.53-?m and 0.35-?m laser light

Recent Ne- and Ni-like X-ray laser experiments carried out at the Centre d'Etudes de Limeil-Valenton (CEL-V) are reviewed. A variety of experiments in Ne-like X-ray lasers were performed; here we discuss measurements of soft X-ray amplification in Ge (Z=32) and Sr (Z=38) plasmas. In Ge plasmas produced by 0.53-μm laser light at an irradiance of 6.0×1013 W/cm2, gains between 2.2–2.5 cm−1 on the 232.2 and 236.2 A J=2−1 lines and a gain of 1.0 cm−1 on the 196.1 A J=0−1 line were measured. In addition, gains of 4.4 cm−1 and 4.0 cm−1 have been demonstrated on the J=2−1 transitions at 164.1 and 166.5 A in Nelike Sr at laser intensities of 1.3×1014 W/cm2. The effects of pumping the Ne-like Se X-ray laser with 0.35-μm laser light have also been investigated; the Se lasing spectra is similar to that obtained with 0.53-μm light. Experiments have also been carried out to optimize the gain of the 50.3 A Ni-like Yb (Z=70) J=0−1 line. For Yb, no significant increase in gain over that previously reported was seen, but the time history of the Ni-like Yb X-ray laser was measured for the first time. Finally, attempts to extrapolate the Ni-like results to shorter wavelength were made using Ta (Z=73), W (Z=74), and Re (Z=75). No definitive observation of the Ni-like J=0−1 lasing lines was made in these experiments.

Characterisation of soft X-ray amplification observed in Ne-like germanium

Soft X-ray amplification has been observed on five 3p → 3s transitions of Ne-like germanium at 19.6, 23,2, 23.6, 24.7 and 28.6 nm using flat, massive targets irradiated in 50 and 150 μm wide line foci up to 3.2 cm long at intensities of 0.3 → 14 × 10 13W/cm 2 by 1.06 μm laser light in 1 ns and 0.5 ns pulses. Gain lengths up to gl ≈ 12 were measured for the J=2 → 1 transition at 23.6 nm and the temporal behaviour of four of the Ne-like germanium lasing lines has been investigated for the first time. The duration of the gain was 60 → 80% of the fwhm of the laser pulse, with the gain turning off at the trailing half-maximum of the incident laser intensity. Output powers in excess of 10 kW per line have been observed.