Relativistic high-power laser–matter interactions

Synchrotrons have for decades provided invaluable sources of soft X-rays, the application of which has led to significant progress in many areas of science and technology. But future applications of soft X-rays--in structural biology, for example--anticipate the need for pulses with much shorter duration (femtoseconds) and much higher energy (millijoules) than those delivered by synchrotrons. Soft X-ray free-electron lasers should fulfil these requirements but will be limited in number; the pressure on beamtime is therefore likely to be considerable. Laser-driven soft X-ray sources offer a comparatively inexpensive and widely available alternative, but have encountered practical bottlenecks in the quest for high intensities. Here we establish and characterize a soft X-ray laser chain that shows how these bottlenecks can in principle be overcome. By combining the high optical quality available from high-harmonic laser sources (as a seed beam) with a highly energetic soft X-ray laser plasma amplifier, we produce a tabletop soft X-ray femtosecond laser operating at 10 Hz and exhibiting full saturation, high energy, high coherence and full polarization. This technique should be readily applicable on all existing laser-driven soft X-ray facilities.

A high-intensity highly coherent soft X-ray femtosecond laser seeded by a high harmonic beam

The Prague Asterix Laser System (PALS) is a new international laboratory where research teams are invited to compete for the beam time. The PALS Center runs an iodine photodissociation high-power laser system delivering up to 1.2 kJ of energy in ∼400 ps pulses at the wavelength of 1.315 μm. Optional doubling and tripling of the frequency is assured by large-diameter nonlinear crystals. The ASTERIX IV laser [H. Baumhacker et al., Appl. Phys. B 61, 325 (1995)], transferred from Garching into a new laser hall in Prague, was updated and put into operation on 8 June 2000. These upgrades include new beam delivery options and a twin interaction chamber, which is designed flexibly for a broad spectrum of applications. Results of the first series of experiments are presented and some planned upgrades are briefly described. These include implementation of adaptive optics, replacement of the iodine master oscillator by a more flexible solid state oscillator based on fiber optics, and a femtosecond extension of the laser output to reach the petawatt pulse power region.

The Prague Asterix Laser System

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.

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Table-top soft x-ray lasers

The deeply saturated operation of a double-pass Ne-like zinc soft x-ray laser emitting at a wavelength of 21.2 mm is reported, along with results of a study and optimization of its beam parameters and coherence. The active medium is a 3-cm-long plasma column produced by a separately delivered prepulse and a main pulse with a duration of \ensuremath{\sim}450 ps, supplied by a 1.315-\ensuremath{\mu}m laser. To generate plasma with reduced lateral density gradients, the \ensuremath{\sim}130-\ensuremath{\mu}m-wide main pulse focus, producing a pump irradiance of $\ensuremath{\sim}2.8\ifmmode\times\else\texttimes\fi{}{10}^{13}{\mathrm{W}\mathrm{}\mathrm{cm}}^{\ensuremath{-}2},$ is placed on top of a much broader prepulse focus. The x-ray laser emission emerges as a narrowly collimated beam possessing high spatial quality and coherence. The dependence of the output on the prepulse conditions, the pump power, and the setup of the half-cavity mirror are investigated and discussed. The study of the beam transverse coherence demonstrates its strong dependence on pump energy. The large gain-length product attained through half-cavity operation allows for an efficient extraction of the energy stored in the active medium. With a small-signal gain of 7(\ifmmode\pm\else\textpm\fi{}0.5) ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, the double-pass beam is deeply saturated and provides \ensuremath{\sim}4 mJ of energy in one pulse, corresponding to peak power in excess of 40 MW, and implies that this system is the most powerful x-ray laser yet demonstrated. The inherent engineering conception of the device is compatible with a repetition rate in a hertz domain.

Multimillijoule, highly coherent x-ray laser at 21 nm operating in deep saturation through double-pass amplification

In this paper we review the development of a soft-x-ray laser at 21.2 nm, recently carried out at the Laboratoire de Spectroscopie Atomique et Ionique. Amplification is generated by electron collisional pumping on a ${2\mathrm{s}}^{2}$${2\mathrm{p}}^{5}$3p-${2\mathrm{s}}^{2}$${2\mathrm{p}}^{5}$3s, J=0--1 transition in neonlike zinc (${\mathrm{Zn}}^{20+}$). The lasing medium is a \ensuremath{\sim}150 \ensuremath{\mu}m\ifmmode\times\else\texttimes\fi{}2 cm line plasma produced by irradiating slab Zn targets at a net intensity of \ensuremath{\sim}1.4\ifmmode\times\else\texttimes\fi{}${10}^{13}$ W ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$, using a 1.06-\ensuremath{\mu}m, 600-ps-long pulsed laser delivering \ensuremath{\sim}350 J of net energy on the target. Accompanying the driving pulse by a 100-mJ-level prepulse train through deliberately imperfect isolation of the mode-locked laser oscillator, the J=0--1 gain coefficient \ensuremath{\sim}5 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ is generated. The emission appears in an \ensuremath{\sim}100-ps burst and precedes the lasing on both the much weaker J=2--1 lines and the x-ray continuum peak. The J=0--1 output source region, the beam spatial profile, and its time history have been measured and their relevance to the kinetics of this lasing system is discussed. A half cavity was implemented and a strong double-pass amplification at 21.2 nm with a 2-cm-long plasma obtained. With the gain-length product achieving \ensuremath{\sim}17.3 according to the particular configuration, saturation was demonstrated, which is confirmed through a schematic model of the level kinetics. To our knowledge, it is the first neonlike soft-x-ray laser where saturation on the J=0--1 line was achieved by using a half-cavity arrangement. Absolute energy measurements indicate \ensuremath{\sim}400 \ensuremath{\mu}J in the half-cavity-generated beam and a peak output power of \ensuremath{\sim}5 MW, which makes this laser one of the most efficient soft-x-ray lasing systems demonstrated to date.

Efficient, high-brightness soft-x-ray laser at 21.2 nm

The efficiency and pulse distribution characterization of an Si(Li) detector (SLD) were obtained using the monochromatized Super-ACO (LURE, Orsay, France) synchrotron beam. The selected energies ranged from 700 to 7000 eV and the radiation intensity was successively measured with a proportional counter, used as a reference, then with the detector to be calibrated. More than 100 experimental points were recorded; steps as small as 1 eV were used to examine carefully the silicon and gold absorption edges. The SLD spectra were processed using the COLEGRAM deconvolution software, which fits different theoretical functions to experimental data. The processing includes a least-squares fitting with the Marquart-Levenberg algorithm and the use of a Hypermet-type function, the sum of a main Gaussian function with a low-energy tail. The proportional counter efficiency curve is calculated using attenuation coefficients for the polypropylene window and argon-methane counter gas. The complete SLD peak shape calibration was obtained: the variation of each fitting function parameter with energy is shown. The tailing parameters show sharp discontinuities that can be linked to different interaction points in the detector front end. This detector pulse distribution calibration obtained with monochromatized radiation could be used to process accurately usual X-ray spectra including several peaks and satellites. The uncertainty in the SLD efficiency calibration points is about 3%. The experimental values were fitted with a theoretical function describing the transmission through different absorbing layers. It is seen that the so-called silicon 'dead layer' acts as a partially active layer, the corresponding events being recorded in the peak tail.

Si(Li) Detector Efficiency and Peak Shape Calibration in the Low‐Energy Range Using Synchrotron Radiation

We report results of polarization experiments on the collisionally excited Ne-like Ge soft-x-ray laser where we have used an injector-amplifier multistage geometry. The polarization state of the x-ray beam was analyzed by two crossed 45{degree} angle of incidence multilayer mirrors which act as linear polarizers. Results were evaluated by comparing intensities of time-integrated beam patterns behind each polarizer. The polarization state of the amplified spontaneous emission (ASE) output at 23.2 and 23.6 nm from the injector plasma alone was systematically studied and, as expected, revealed no macroscopic degree of polarization observable within the precision of the experiment. When the injector output beam was linearly polarized by using a third 45{degree} angle of incidence multilayer mirror and coupled into the amplifier plasma, the degree of polarization of the amplifier output was {similar_to}0.98, the total gain-length product attained for the polarized beam was {approx}12, and the final beam energy was {approx}20 nJ. The results obtained are discussed with relevance to processes determining the polarization properties of unsaturated ASE systems.

Demonstration of amplification of a polarized soft-x-ray laser beam in a neonlike germanium plasma.

In this article we report a new wavefront sensor, developed at the Laboratoire de Spectroscopie Atomique et Ionique, for a full characterization of soft X-ray beams. The Shack-Hartmann sensor has a theoretical accuracy in the order of at a wavelength around 13 nm. A cartography of the wave-vectors pointing of laser-pumped soft X-ray laser has been achieved. It has shown the presence of many ripples probably coming from plasma instabilities. Capillary discharge soft X-ray laser has been also investigated. For all the pumping configurations, the wavefront is spherical, divergent with a radius of about 6.5 m at 2.5 m from the plasma end. The best wavefront exhibits an error to a perfect wave of 3λ rms. Assuming to focus the beam with a f = 50 mm diffraction-limited mirror, a theoretical focal spot size of 0.5  μ m in diameter have been estimated containing 70% of the incident energy. In that case an intensity of 4 × 10 13  W cm −2 should be achieved.

Wavefront measurements in the soft X-ray range

Description of a new experimental set-up developed in Palaiseau for X-ray laser studies is given. This appliance includes a spherical multilayer mirror put at the rear of the 6 cm long laser generated aluminium plasma and two original devices. Those consist of a grazing incidence spectrometer where detector is on the focal circle and moves linearly with the wavelength range and a divergence analyzer. Both devices are extensively described.

P. Dhez

Papers

Efficient, high-brightness soft-x-ray laser at 21.2 nm

Si(Li) Detector Efficiency and Peak Shape Calibration in the Low‐Energy Range Using Synchrotron Radiation

Demonstration of amplification of a polarized soft-x-ray laser beam in a neonlike germanium plasma.

Wavefront measurements in the soft X-ray range

Experimental Device For XUV. Experiments, Especially Adapted To The Directional Emission From A Plasma Column.