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Fourier-transform spectroscopy offers high resolution, wavelength accuracy and broad tunability, but is so far limited to the mid-ultraviolet range, down to wavelengths of 140 nm. Now, based on a wavefront-division scanning interferometer, researchers present a Fourier-transform spectroscopy scheme that covers a broad wavelength range of 40–250 nm with 7% tunability and an extrinsic absolute wavelength accuracy of 10−7.

We report the work done on a transient soft X-ray laser (SXRL) beam to deliver a proper extreme UV irradiation source for applications. The same optical tool was first demonstrated on a quasi stationnary state (QSS) soft X-Ray laser at the PALS Institute in Prague. The problem set by the transient soft X-Ray laser developed by the LIXAM at the LULI installation in Palaiseau is more crucial, first because the beam spatial profile is more irregular secondly because high repetition rate soft X-ray laser facilities in the future are based on this SXRL type. The spots obtained show a 20 micron average diameter and a rather homogeneous and smooth profile that make them a realistic irradiation source to interact with targets requiring relatively high fluence (near 1 J/cm2) or intensity (near 1011 W/cm2) in the extreme UV domain.

The spectral width of a Ni-like silver transient soft X-ray laser (4d-4p λ= 13.9 nm) was experimentally inferred from the measurement of its temporal coherence, performed with a novel wavefront division interferometer. The measured coherence time of 3 ps corresponds to a spectral linewidth of 0.7 mA which is narrow. We compare this experimental result to numerical simulations of the amplified line profile, performed along two different approaches. Both calculations predict a spectral linewidth that is about 3 times larger than the experimental value. We discuss several effects that might explain this discrepancy. We show that, due to the short duration of the X-ray laser pulse, the assumption of a stationary electromagnetic field used in both the experimental analysis and in the calculations need to be reconsidered.

Beam sections of transient collisional excitation (TCE) soft X-ray lasers exhibit highly contrasted intensity modulations forming complex patterns. We present model calculations that reproduce these experimental observations both qualitatively and quantitatively. The low spatial coherence, high longitudinal coherence and short pulse duration of the source are shown to control the visibility of the beam structures. Comparison with other types of laser-based soft X-ray lasers is discussed. We derive simple relationships that relate the spatial and temporal characteristics of the emitting source to the main features of the beam small-scale structures.

La spectrometrie par transformation de Fourier est une technique maintenant bien maitrisee, qui permet d'obtenir des spectres a la fois exacts et a tres haute resolution, de l'infra-rouge lointain a l'ultra-violet, jusque vers 140-120 nm. En dessous, en particulier de 140 a 20 nm environ, la technique est completement absente, essentiellement en raison de la quasi impossibilite de fabriquer des lames separatrices ayant les qualites requises. Or, dans ce domaine, la physique atomique et moleculaire est demandeuse d'une precision et d'une resolution que n'apportent que partiellement les spectrometres a reseaux utilises. Nous presentons les principes et la realisation d'un interferometre a balayage dedie a la spectrometrie TF jusque λ = 58 nm, avec une resolution au moins egale a 400 000 a 80 nm. Le probleme de la separatrice est resolu radicalement par l'utilisation d'un interferometre a division de front d'onde. La precision du balayage en difference de marche est obtenue grâce a un systeme original de controle et de mesure du deplacement du reflecteur mobile. Nous presentons les premieres mesures realisees dans l'air, vers 200 nm (bande d'absorption de O 2 , source synchrotron), avec les parametres de balayage du systeme nominal (500 000 echantillons espaces de 30 nm env., TF en cosinus).

In some applications, microimaging in EUV requires a long working distance, i.e. a long distance between the object plane and the first optical surface. This is in particular the case when imaging hot plasmas, such as those produced by high energy pulsed lasers focused onto solid targets for X-ray laser production or plasma fusion studies. In this context, a working distance of several 10's cm is required, which makes even the usual Schwarzschild configurations not convenient. Other major requirements are a high resolution and phase detection capability, in order to allow a direct detection of the plasma density. The field should be millimetric. A system was build for diffraction limited (resolution 1 pm) interferometric imaging at λ=13.9 nM, using an X-ray laser source. This system was first tested on a synchrotron beamline (LURE, Orsay). Also, the imaging part was used to obtain high resolution images of the lasing emission provided by a Ni-like, picosecond Ag plasma, at λ=13.9 nm. Such a system might be useful in other applications, eg. phase-sensitive micro-inspection of reflecting surfaces in the field of EUV lithography.

We report on the development of a multi-millijoule, ultra high-brightness Ne-like soft x-ray laser (XRL) at the wavelength of 21.2 nm, recently undertaken at the PALS Centre. The device has been implemented as a routine radiation source delivering about 4 mJ of energy in pulses with a duration of ~100 ps, in a narrowly collimated beam exhibiting high spatial quality and possessing high spatial coherence. The peak power of about 40 MW, generated by this device, is the largest value attained by an XRL to date. The same holds true for the peak spectral brightness, amounting to 1027 photons s−1 mm−2 mrad−2 0.1%BW−1. The active medium consists of a 3 cm long plasma column, sequentially generated from a slab target by an energy of ~500 J delivered by the iodine 1.315 μm driving laser. The population inversion is produced by focusing tightly down to the line ~500 J of energy into a much wider preplasma column, which makes it possible to create a lasing medium with a reduced lateral density gradient and thereby to minimize the lateral refraction of the x-ray beam. The multi-millijoule output is generated in double-pass amplification regime, achieved by a carefully designed half cavity using a flat multilayer Mo : Si mirror positioned a few millimetres near the plasma end. We describe the measured characteristics of the output beam generated by double-pass amplification, and illustrate its spatial beam coherence obtained using a Fresnel wavefront splitting interferometer. Given the available energy, peak power and brightness in a single pulse, the demonstrated XRL constitutes a tool for novel applications in both solid state and plasma physics. Examples of such applications are the generation of high-energy density, low-temperature plasmas relevant to laboratory astrophysics, x-ray interferometry of plasmas and solids with spatial resolution ranging from a micron to a few nanometres according to the specific experimental arrangement, or study of soft x-ray photoionization.

We describe a Fourier transform spectrometer designed to operate down to 60 nm on a synchrotron beam line. As far as we know, there is no such instrument available in the EUV (λ < 140 nm) partly because manufacturing accurate beam splitters remains the major difficulty at these wavelengths. We use a wave front division interferometer instead of an amplitude division one to overcome this difficulty. The interferometer is based on a modified Fresnel bimirror configuration, which is controlled by an original optical system. This system keeps the mirror tilt error to a negligible value during mirror translation, and provides a sensitive interferometric measurement of the mirror translation. The sampling interval is 29 nm (path difference), allowing one to record large band spectra down to λ = 58 nm with spectral resolution δσ = 0.33 cm -1 for 512 K samples (one-sided interferograms). We measured the apparatus function by recording an interferogram from a He–Ne stabilized laser. By studying the white noise in the corresponding spectrum, we found that the sampling error in the interferogram was about 0.4 nm rms, which produces a near-perfect apparatus function. Finally, we recorded the visible/near UV spectrum of an arc mercury lamp for illustration.

Obtaining an X-ray laser emission from plasmas, created and driven by an intense IR laser, has been pursued at the Laboratoire de Spectroscopie Atomique et Ionique (LSAI) for several years. At present, we operate various types of X-ray lasers driven by IR laser pulses of different durations (600 ps, 100 ps, and 600 ps/1 ps). A review of different techniques used at the LSAI to produce a strongly amplified emission using the collisional excitation pumping is presented. In the second part of this paper, to illustrate the potential of the X-ray lasers for applications, we present the main results obtained with an X-ray laser emitting at 21.2 nm in a study of surface defects of a niobium cathode, induced by strong electrical fields. We also describe a novel imaging interferometry device using an X-ray laser as a source and designed as a tool for high-resolution diagnostic of dense plasmas.

Nous avons developpe un interferometre base sur le bimiroir de Fresnel pour calibrer une technique de depot d'une couche dephasante de molybdene sur des multicouches Mo-Si. L'epaisseur nominale de la couche dephasante de Mo variait de 10 a 80 nm. Les interferogrammes projetes sur un ecran fluorescent puis enregistres par une CCD refroidie etaient traites par une technique basee sur la transformee de Fourier spatiale de l'interferogramme, pour obtenir le dephasage optique entre une zone de reference et la zone dephasee. Nous avons montre la pertinence de cette technique interferometrique dont l'erreur potentielle sur l'epaisseur optique des plots est estimee a λ/100. La meme technique permet l'evaluation d'un profil de pente d'un front d'onde jusqu'a une frequence spatiale de l'ordre de a 0,5-1 mm -1 . Dans ce cadre, la sensibilite peut atteindre 0,4 μrad rms a 14 nm.

At the Institute of Laser Engineering, Osaka University, we have obtained Ni-like x-ray lasers of various atomic number elements, including many shorter wavelength Ni-like lasing lines around 5nm. The saturated amplification of Ni-like Ag lasing line at the wavelength of 13.9nm have been observed. Using these x-ray lasers, we are preparing the application experiments such as probing a laser-produced plasma with an x-ray laser interferometer.

A dedicated interferometric microimaging system has been designed and build to produce a magnified, diffraction limited image-interferogram of a coherently illuminated object, e.g. a laser plasma illuminated by a soft X-ray laser. The optical system consists of quasi-ellipsoidal, multilayered mirror, illuminated at a 6 deg incidence from normal, associated with a grazing incidence Fresnel-bimirror interferometer. The working distance (optics to object) is 540 mm. The corrected aperture is 8x8 mm square. The magnification can be 10 or more, without loss of resolution. In a first realization, the ellipsoidal mirror shape was approximated by a quasi-ideal torus, making the system not diffraction limited. The expected resolution (3-4 μm) was checked by imaging a metallic grid at 21.2 nm. In a second step, we have produced a better approximation of the ellipsoid, which is diffraction limited at λ=13.9 nm, under incidence 6.08 deg from normal. In this case, the actual corrected field is at least 0.9 mm, and the interferometric field is 0.6 mm perpendicular to fringes (these distances on object side). We discuss the design principles, leading to the choice of the various optical parameters, and the realization techniques.

Since 1998, we are able to produce a saturated lasing emission in Ni-like silver at 13.9 nm, using short multi pulses (100 ps) combined with a low energy prepulse. This source is now very well characterized and ready to be used for the applications. By contrast with the Ne-like ions, we have found that it is not possible to obtain intense X-ray emission from Ni-like ions when using long pump pulses (600 ps). Simulations performed at the CEA confirm these observations. We will present a summary of the results of the simulations and the experiments on the Ni-like silver laser 13.9 nm, comparing in particular the role of the different pumping parameters.

International audience; Interferometric, direct determinations of the f 1 scattering factor near absorption edges in the soft x-ray range is demonstrated. The interferometric system, which is based on wave front division (no beam splitter) with plane mirrors only, produces a linear fringe pattern. The principle consists in direct measuring of the fringe shift occurring upon insertion of a sample into one interferometer arm, by means of a dedicated detection system. This provides the optical thickness, which in turn gives the f 1 factor, knowing either the sample mass per unit surface, or the sample thickness and density. With the sample being probed in transmission under near normal incidence, the determination of f 1 is not perturbed by the absorption part of the complex scattering factor. Therefore, f 1 data obtained here can be said new and independent with respect to those obtained previously, in the sense that they are obtained from a new, purely experimental technique, and are neither deduced from nor perturbed by absorption. The interferometer design used can be implemented in a very large spectral range. For demonstrating the ability of this interferometric system to provide such new f 1 data, a thin free-standing carbon foil near the K edge (from 4.1 to 4.65 nm, 302-267 eV), and a membrane-supported silver layer near an M edge (from 2.6 to 3.8 nm, 477-326 eV) were used as test objects.

Nous avons etudie les proprietes de coherence spatiale d'un faisceau harmonique a 55nm dans deux experiences distinces. D'une part, nous caracterisons la taille et la divergence du faisceau au foyer d'un miroir spherique. D'autre part, nous mesurons le degre de coherence spatiale dans le faisceau, par une technique d'interferometrie a division de front d'onde. Les deux mesures caracterisent une bonne coherence spatiale du faisceau, parametre M 2 2 et degre de coherence spatiale γ 0.5, qui peut etre utilisee dans les applications.

We present the very first experiment allowing in situ observation of surface evolution during very high electric field application. This has been achieved by soft (/spl lambda/=21.2 nm) X-ray laser Fresnel two-mirror interferometry. The surface under study was a niobium planar cathode opposite to a stainless steel blade-like anode. The applied field range on the cathode was F/spl ap/0 to 50 MV/m. The interferograms were single shots of 80 ps duration, allowing to probe the surface morphology quasi-instantaneously. The delay between two shots was 20 min while the electric field was kept on. The comparison of successive interferograms gives the Nb surface evolution over /spl sim/2 h. Thus, we observed the appearance of defects of /spl sim/3 nm high and their evolution with time. These may be the precursors of the minute breakdown which occurred later on. This technique thus makes it possible to detect the preliminary stages of a discharge.

A wavefront division interferometer has been used for the first time with a soft X-ray laser (λ = 21.2 nm). The experiment aims to demonstrate X-ray laser interferometry in this configuration and to investigate the phase shifting measurement accuracy as well. The X-ray laser is generated in a neon-like zinc plasma in which it makes two passes thanks to a multilayer mirror half-cavity. The X-ray pulse duration is ≈50 ps. The beam has a very high brightness (≈4 × 10 15 W cm −2 sr −1 in 0.01% bandwidth) which allows us to place the interferometer far from the source (2.8 m) and thus to benefit by a large transverse coherence width. The interferometer consists of a Fresnel bi-mirror which adds coherently one part of the X-ray laser beam section to the other one. Single laser-shot interferograms of a reflecting sample provided with a λ 2 dephasing step (51.7 nm height) have then been successfully recorded. The phase shifting accuracy resulting from the smallest observable fringe change is about λ 20 .

X-ray interference microscopy diagnostics of a nanosecond-pulse pumped Al (Z=13) plasma are presented. A transient picosecond Ni-like Ag X-ray laser (13.9 nm) is used as the probe beam and a wavefront division bi-mirror interferometer is adopted as the diagnosis tool. From the fringe shifts, the two-dimensional plasma density profile at 1 ns after the peak of the heating pulse is deduced. Opposite fringe shifts during the plasma development are observed and possible explanations for this behavior are discussed. This study will contribute to the validation of 1D and 2D hydrodynamic codes as well as to the better understanding of the physics during the development of laser produced plasmas.

The development of soft x‐ray interferometry demands high quality optical elements and high brightness sources. However, the balance between these two requirements depends on the exact nature of the interferometer setups. Generally speaking, amplitude division interferometers require beam splitters, the optical quality of which remain a big technological issue. On the contrary, the high brightness sources needed by wavefront division interferometers are readily available, and will progress in the near future. Examples of such interferometers are given for various applications in development or in project.

The visualization of traveling acoustic waves is achieved by means of acoustic stroboscopy. The limitations of the technique, interpretation of results, frequency range, and optical noise are reviewed. Two methods of noise reduction are studied: spatial filtering and holography. Optical holography of acoustic fields is also shown to offer the best advantages among the various methods of visualization outlined here.

We describe a computer generated phase hologram that converts a gaussian laser beam into two uniform collimated beams, and we compare it to another beam-shaper that was previously suggested.

International audience; We report on an innovative two-dimensional imaging extreme ultraviolet (XUV) interferometer operating at 32 nm based on the mutual coherence of two laser high order harmonics (HOH) sources, separately generated in gas. We give the first evidence that the two mutually coherent HOH sources can be produced in two independent spatially separated gas jets, allowing for probing centimeter-sized objects. A magnification factor of 10 leads to a micron resolution associated with a subpicosecond temporal resolution. Single shot interferograms with a fringe visibility better than 30% are routinely produced. As a test of the XUV interferometer, we measure a maximum electronic density of 3×10^20 cm^−3 1.1 ns after the creation of a plasma on aluminum target.

A Fourier transform spectrometer dedicated to Vacuum Ultra-Violet [180-40nm] photoabsorption has been developed. The first experimental results, presented here, show unprecedented resolving power (above 700000). The instrument is an endstation on the DESIRS synchrotron beamline.

We describe a Fourier transform (FT) spectrometer designed to operate down to 60 nm (20 eV) on a synchrotron radiation beamline for high resolution absorption spectrometry. As far as we know, such an instrument is not available below 140 nm mainly because manufacturing accurate and efficient beam splitters remains a major problem at these wavelengths, especially if a wide bandwidth operation is desired. In order to overcome this difficulty, we developed an interferometer based on wave front division instead of amplitude division. It relies on a modified Fresnel bimirror configuration that requires only flat mirrors. The instrument provides path difference scanning through the translation of one reflector. During the scanning, the moving reflector is controlled by an optical system that keeps its direction constant within a tolerable value and provides an accurate interferometric measurement of the path difference variation. Therefore, a regular interferogram sampling is obtained, producing a nominal spectral impulse response and an accurate spectral calibration. The first results presented in this paper show a measured spectral resolution of delta(sigma)=0.33 cm-1 (interval between spectral samples). This was obtained with a sampling interval of 29 nm (path difference) and 512 K samples from a one-sided interferogram using a cosine FT. Such a sampling interval should allow the recording of large bandwidth spectra down to lambda=58 nm with an ultimate resolving power of 500,000 at this wavelength. In order to check the instrument performances, we first recorded an interferogram from a He-Ne stabilized laser. This provided the actual spectral impulse function, which was found to be fully satisfactory. The determination of the impulse response distortion and of the noise on the vacuum ultraviolet (VUV) spectral range provided accurate information in the sampling error profile over a typical scan. Finally, the instrument has been moved to the SU5 undulator-based synchrotron radiation beamline (Super-ACO facility, LURE, Orsay, France). A high resolution spectrum of O2 (the Schumann-Runge absorption bands, 185-200 nm) was computed from recorded interferograms using the beamline monochromator at the zeroth order to feed the instrument with an 11% relative bandwidth "white" beam (2003). These UV measurements are very close to those found in the literature, showing nominal performances of the FT spectrometer that should translate into an unprecedented resolving power at shortest VUV wavelengths. A recent upgrade (2007) and future developments will be discussed in light of the current installation of the upgraded FT spectrometer as a permanent endstation for ultrahigh resolution absorption spectrometry on the VUV beamline DESIRS at SOLEIL, the new French third generation synchrotron facility.

Resume. Un prototype de spectrometre VUV par TF avait ete decrit precedemment, en particulier lors de la conference UVX2004. Ce prototype a ete entierement reconstruit en vue de son installation en ultra-vide sur SOLEIL (ligne DESIRS), avec des specifications ameliorees. Le premier test en VUV a ete realise en novembre 2007, sur l’argon autour de 79 nm (15.7eV). Sur ce premier spectre, le pouvoir resolvant (PR) experimental obtenu (hors Doppler) est egal au PR theorique, soit 540000 (FWHM) dans les conditions de l’experience. Depuis, l’instrument a produit les spectres des gaz rares (Ne-Ar-Kr-Xe), entre 50 et 100nm, avec un PR (FWHM) superieur a 700000, en vue d’etudier systematiquement le fonctionnement et ses performances. Nous examinerons particulierement la resolution spectrale, la precision absolue de l’echelle spectrale, et le rapport signal-bruit. Une chambre echantillon dediee est en cours de test. Elle comporte une chambre sans fenetre permettant une pression de plusieurs millibars, et un jet atomique ou moleculaire, pour eliminer l’elargissement Doppler.

Caracterisation spatio-temporelle d’un laser XUV injecte J.-P. Goddet1, S. Sebban1, O. Guilbaud2, G. Maynard3, B. Cros3, J. Gautier1, Ph. Zeitoun1, C. Valentin1, F. Tissandier1, T. Marchenko1, G. Lambert1, D. Benredjem2, A. Boudaa3, A. Klisnick2, D. Ros2, S. Kazamias2, K. Cassou2,3, J. Habib2, G. Jamelot2, J.-C. Lagron2, D. Joyeux4, S. De Rossi4, D. Phalippou4, F. Delmotte4, M.F. Ravet4, A. Calisti5, T. Mocek6, M. Kozlova6 et K. Jakubczak6

We present in this paper theoretical and experimental investigations of temporal and spectral properties of seeded soft x-ray lasers. Bloch-Maxwell simulations of the harmonic pulse propagation in a soft x-ray laser plasma have been performed. Results show a growing wake of coherent radiation formed after the harmonic pulse.We describe the first measurement of the spectral bandwidth of a seeded soft x-ray laser. Using a varying path difference interferometer the spectral profile of a seeded OFI x-ray laser has been experimentally determined, leading to a Fourier-transform pulse duration of 5ps. The measured bandwidth is in good agreement with simulations. Finally we present the progress toward the implementation of a seeded soft x-ray laser at 18.9 nm at the new LASERIX facility.

Since the first seeding of an OFI soft x-ray laser in 2004, we progressed towards the full characterization of the output beam. The final is to be able to deliver to users well-known beam. Temporal as well as spatial parameters have been measured for different conditions of amplification. We observed a strong enhancement of the spatial coherence due to the amplification process with a far-field pattern exhibiting an airy-like shape. The gain zone having strong discontinuity behaves like a hard pinhole. Spatial filtering has been also observed on the wave front (δ/5 root-mean-square, rms, before seeding and δ/20 rms after amplification). Temporal coherence has been studied thanks to the use of a Fourier- Transform spectrometer. Spectral widths, δδ/δ, around 10-5 have been measured for different plasma lengths or gas pressures. Departure from Gaussian shape has been clearly observed on the spectral line for some cases.

A VUV Fourier transform spectrometer based on a wavefront division interferometer has been built. Our ultimate goal is to provide a high resolution absorption spectrometer in the 140 – 40 nm range using the new third generation French synchrotron source Soleil as the background continuum. Here, we present the design and latest performance of the instrument scanning control system. It is based on multiple reflections of a monomode, frequency‐stabilized HeNe laser between two plane mirrors allowing the required sensitivity on the displacement of the interferometer mobile arm. The experimental results on the sampling precision show an rms error below 5 nm for a travel length of 7.5 mm.

Beam sections of transient collisional excitation (TCE) soft X-ray lasers exhibit highly contrasted intensity modulations forming complex patterns. We present model calculations that reproduce these experimental observations both qualitatively and quantitatively. The low spatial coherence, high longitudinal coherence and short pulse duration of the source are shown to control the visibility of the beam structures. Comparison with other types of laser-based soft X-ray lasers is discussed. We derive simple relationships that relate the spatial and temporal characteristics of the emitting source to the main features of the beam small-scale structures.

Transient perturbed niobium surface states are observed for the first time during a long continued action of the applied DC-field owing to surface imaging by the means of an X–UV laser interferometry technique.

We present a detailed analysis of an experiment carried out recently in which the temporal coherence of the Ni-like silver transient X-laser at 13.9 nm was measured. Two main consequences of this measurement will be discussed and interpreted with numerical calculations. First we show that the high temporal coherence length measured corresponds to an extremely narrow spectral width of the X-ray laser line. Second we show that the high temporal coherence helps to explain the presence of small-scale structures observed in the cross-section of all transient X-ray laser beams.

International audience; We present measurements of in situ nanometric-resolution topographical modifications of thin niobium layers subjected to strong electric fields. The Nb layers, deposited on a fused silica substrate, are interferometrically flash probed using soft x-ray laser (XRL) at the wavelength of 21.2 nm. Its pulses are reflected by the probed sample under grazing incidence angle, and the information about surface deformation is obtained by a Fresnel wave-front-division interferometer. It was experimentally established that the probing pulses at the soft x-ray wavelength do neither produce any measurable photoelectric-field emission, nor alter the topographical features of the probed surface. The examined Nb electrodes were periodically probed while the electric field was increased up to 80 MV/m, and alterations of their topographical characteristics with a resolution of ∼2 nm in the relief elevation were obtained. It was found that behavior of the Nb layer strongly depends on the polarity of the applied voltage. Only small modifications are observed with the Nb surface at the positive potential, whereas the negative potential induces significant transient surface perturbations, with peak-to-valley elevation differences ∼50 nm. The qualitative nature of these modifications was observed to be unrelated to intermittent parasitic breakdowns.

FTS can be performed in the VUV-EUV by using wavefront division interferometers. The first tests of a dedicated instrument, for high resolution absorption FTS down to 60 nm, shows nominal performances at 200 nm.

The X-UV interferometry is a promising tools for plasma electron density measurement. We report on the results of such a plasma interferometry experiment using a soft x-ray laser as a probing beam (λ = 13.9 nm) and a new X-UV interference microscope. This device is a wavefront division interferometer coupled to a high resolution aspherical imaging mirror. Laser produced plasma electron density profiles were investigated at density as high as 10 20 cm -3 . The expanding plasma has been probed at different times of its evolution. Several nanosecond after its production, the plasma exhibits an index of refraction that is no more below one as expected for a pure plasma. The contribution of the bound electrons of plasma ions to the refractive index is discussed for that situation.

We review our recent progress in the development of transient x-ray lasers and of their application to plasma diagnostic. The first observation of C-ray laser emission at the new PHELIX-GSI facility is reported. This TCE X-ray laser will be a promising tool for heavy-ion spectroscopy. We then present the main results obtained at the LULU-CPA facility with a compact high-resolution X-UV imaging device. This device was used to investigate the spatial source structure of the Ni-like silver transient X-ray laser under different pumping conditions. The key-role of the width of the background laser pulse on the shape of the emitting aperture is demonstrated. Finally the imaging device was used as an interference microscope for interferometry probing of a laser-produced plasma. We describe this experiment performed at APRC-JAERI.

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.

We present the very first experiment allowing in-situ observation of surface evolution during very high electric field application. This has been achieved by soft X-ray laser (/spl lambda/=21.2 nm) Fresnel bi-mirror interferometry. The surface under study was a niobium planar cathode opposite a stainless steel bladelike anode. The applied field range on the cathode was E/spl ap/9-50 MV/m. The interferograms where single shot recorded of 80 ps duration allowing to quasi-instantaneously probe the surface morphology. The delay between two shots was 20 min while electric field has been kept on. The comparison of successive interferograms gives the Nb surface evolution over several hours. Thus, we observed the appearance of defects a few nanometers high and their evolution with time. Such a technique makes it possible to detect the preliminary stages of a discharge.

This paper summarises our recent progress achieved in the characterisation and understanding of the Ni‐like Ag transient X‐ray laser pumped under traveling wave irradiation. We carried out two experiments at the LULI CPA laser facility. Several diagnostics of the plasma emission at the XRL wavelength or in the keV range indicate the presence of small‐scale spatial structures in the emitting XRL source. Single‐shot Fresnel interference patterns at 13.9 nm were successfully obtained with a good fringe visibility. For the first time we obtained plasma images with a high spatial resolution about 1 μm, showing the effects of pumping parameters on the X‐ray laser far‐field.

We report on interferometric investigation of perturbed surfaces under high electric field by using the 21.2 nm zinc soft X‐ray laser operating in a half cavity configuration. DC electric fields of up to 100 MV/m were applied to the surface of a niobium sample. Both positive and negative polarizations of the electric field were examined. Single shot, high‐quality X‐ray interferograms were recorded with a double‐mirror Fresnel interferometer and surface maps were obtained from the experimental data by holographic reconstruction.

A recently developed double‐pass, deeply saturated Ne‐like Zn X‐ray laser at 21.2 nm, is presented. The system consists of a 3‐cm long plasma and a half cavity constituted by a flat multilayer mirror located at 8.5 mm. The pump sequence includes a weak prepulse and a main heating pulse, delivering ∼500 J of net energy at 1.315 μm, in pulses with a length of ∼450 ps. A novel prepulse illumination geometry is employed, in which the prepulse beam generates a wide preplasma buffer into which subsequently a much more tightly focused main beam is coupled. The emitted X‐ray beam is narrowly collimated and possesses a remarkably high spatial quality. In the half‐cavity regime, the system provides an output energy of about 4 mJ, corresponding to peak power of 40 MW. A fully routine operation of the half‐cavity has been achieved, making it possible to perform, without any realignment or manual intervention, about 100 shots with one multilayer mirror.

We present an overview of our research activity achieved since the last X‐ray laser Conference in Saint‐Malo. Our research program involves the development of laser‐pumped collisional X‐ray lasers under different regimes of irradiation, and the use of these sources for applications. The work presented involves a number of French and international collaborations and was carried out at different pump laser facilities: LULI (Ecole Polytechnique), LOA (ESNTA) in France; Rutherford Laboratory in U.K; PALS in Czech Republic.

We present a review of new progress performed in several laboratories (Laboratoire pour l'Utilisation des lasers Intenses, Rutherford Appleton Laboratory, Prague Asterix Laser System, Institute of Laser Engineering, Laboratoire d'Optique Appliquee). Concerning the realization of x-ray lasers sources, using different laser pumping techniques (600 ps, 100 ps, ns/ps, OFI) and the optimization of their optical properties, using curved and plane half-cavity mirrors. In parallel of these developments, we present the main results obtained with x-ray laser in interferometry applications. These studies concern on the one hand the Michelson interferometry with an x-ray laser emitting at 13.9 nm (recently realized at LULI), and on the other hand the Fresnel bi-mirror with an x-ray laser emitting at 21.2 nm (recently realized at PALS).© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

This paper summarizes our recent progress achieved in the characterization and understanding of the Ni-like Ag transient x-ray laser pumped under traveling wave irradiation. At the Rutherford Laboratory CPA laser facility, we measured the temporal history of the 13.9 nm laser pulse with a high-resolution streak camera. A very short, approximately 2 ps x-ray laser pulse was directly demonstrated for the first time. More recently we carried out an experiment at the LULI CPA laser facility. Several diagnostics that recorded the plasma emission at the XRL wavelength or in the keV range indicate the presence of small-scale spatial structures in the emitting XRL source. Single-shot Fresnel interferograms at 13.9 nm were successfully obtained with a good fringe visibility. Strong lasing was also observed on the Ni-like 4f-4d line at 16 nm.

Unlike transmission X-ray microscopy, scanning transmission X-ray microscopy (STXM), till now, does not allow phase contrast. Several method have been suggested but no proof of practical feasibility has been yet given. Here we analyze the methods based on the detection of the small beam deflection induced by the object phase gradient, by a segmented detector. It is shown that structuring the zone plate illumination potentially improves the detection. A diffractive beam profiler has been constructed to condition the beam of the NSLS X1A STXM. Recent images are shown, which, compared to numerical simulations, indicate the presence of phase contrast.

The probing of dense (1021–1022 electrons/cm3) laser plasmas produced by high energy pulsed lasers usually makes use of X-ray emission imaging, or, more recently, of microradiography, i.e. of absorption imaging. However, it is well known that probing the index of refraction would bring much valuable informations. As far as index is concerned, interferometry is the choice method. However, the plasma context makes necessary to use for probing a very large brilliance, single pulse source, delivering energetic radiation, such as a soft X-ray laser. We report the design of a dedicated interferometric microimaging system, based on a Fresnel bimirror interferometer, associated with an off-axis spherical mirror, aspherized for diffraction limited (DL) imaging. In the first realization, the ideal mirror shape was approximated by a torus, making the system not diffraction limited. In a second step (currently in progress), a nearly perfect system will be realized, for diffraction limited resolution. The design and t...

We review the recent progress in theoretical and experimental understanding of harmonic generation by intense laser pulses. We present investigations on the spatial and temporal coherence properties of the harmonic emission, showing that they can be controlled. Finally, we give some examples of current applications of this XUV source, in particular in the diagnostic of dense plasmas.