Search

We propose a new mechanism of high-order harmonic generation during an interaction of a high-intensity laser pulse with underdense plasma. A tightly focused laser pulse creates a cavity in plasma pushing electrons aside and exciting the wake wave and the bow wave. At the joint of the cavity wall and the bow wave boundary, an annular spike of electron density is formed. This spike surrounds the cavity and moves together with the laser pulse. Collective motion of electrons in the spike driven by the laser field generates high-order harmonics. A strong localization of the electron spike, its robustness to oscillations imposed by the laser field and, consequently, its ability to produce high-order harmonics is explained by catastrophe theory. The proposed mechanism explains the experimental observations of high-order harmonics with the 9 TW J-KAREN laser (JAEA, Japan) and the 120 TW Astra Gemini laser (CLF RAL, UK) [A. S. Pirozhkov, et al., arXiv:1004.4514 (2010); A. S. Pirozhkov et al, AIP Proceedings, this volume]. The theory is corroborated by high-resolution two-and three-dimensional particle-in-cell simulations.

We have demonstrated a high gain neon-like argon ion X-ray laser using a gas puff target. The elongated X-ray laser plasma column was produced by irradiating the gas puff target with line-focused double picosecond laser pulses with a total energy of 9 J in a travelling-wave excitation scheme. Strong lasing was observed, and a high gain coefficient of 18.7 cm-1 and a narrow beam divergence of less than 3.7 mrad were measured on the transient collisionally excited 3p1S0-3s1P1 transition for neon-like argon at 46.9 nm with targets up to 0.45 cm long.

A line pair for a resonant photo-pumping x-ray-laser scheme is proposed in which the wavelength matching between the aluminum K{alpha}{sub 2} line ({lambda}= 0.833 95 nm) and the 2p{sup 6}-(2p{sub 1/2},4d{sub 3/2}){sub 1} transition of the neonlike zinc ions ({lambda}= 0.834 00 nm) is used. The population kinetics code of the neonlike zinc ions in plasma under irradiation of the aluminum K{alpha} line shows that substantial amplification gain can be generated in the transition of (2p{sub 1/2},3p{sub 1/2}){sub 0}-(2p{sub 1/2},4d{sub 3/2}){sub 1} at a wavelength of 3.5 nm. We also investigate the experimental arrangement of this scheme, which implies that this scheme is feasible with the present ultra-short-pulse-laser technology.

We have developed a soft x-ray laser (SXRL) interferometer capable of the single-shot imaging of nano-scaled structure dynamics. The interferometer consisted of the reflection optics including double Lloyd's mirrors and focusing optics, and the interference fringes are produced on the detector surface. The depth and lateral resolutions of the interferometer were about 1 nm and 1.8 μm, respectively. By using this interferometer, the initial stage (~50 ps) of the ablation process of the Pt surface pumped by a 70 fs Ti:Sapphire laser pulse was observed. The expansion speed of the surface estimated from the result (34 nm/50 ps) indicated that the nano-bubble structures were formed in the initial stage of the ablation. In order to observe the detailed dynamics, the temporal synchronization between the pump and probe pulses was improved to be 3 ps by adopting a portion of the SXRL and pump beams as the time fiducials, to which the pump and probe timing was adjusted by using the x-ray streak camera.

A versatile soft x-ray flat-field grating spectrograph to be installed to a conventional transmission electron microscope has been developed. A holographic spherical grating of a 1200-lines/mm effective groove density which places emphasis on the low energy region of 50-200 eV is designed by an aspheric wavefront recording system. Laminar and blazed types master (LM and BM) gratings and their respective replica (LR and BR) gratings are fabricated by holographic exposure and ion-beam etching methods. Absolute diffraction efficiencies in the 50-300 eV range at the angle of incidence of 86.0 degrees were measured using a synchrotron radiation. The first order diffraction efficiencies are 6.1-7.5% (or 12%) for LM (or BM) and 7.4-9.6% (or 13%) for LR (or BR) gratings at near 55 eV, and over 5% (or 8%) in the 50-200 eV range for LM and LR (or BM and BR) gratings. The replica gratings show the comparable first-order diffraction efficiencies with their respective laminar and blazed types of master gratings.

Using the x-ray lasers as amplifiers of ultrashort x-ray pulses has been investigated as a scheme for ultimate light sources. For successful implementation, the characteristics of the scheme should be taken into account in designing the amplifiers. In this paper, the basic physics and characteristics of the scheme were analyzed by using the Maxwell-Bloch equations incorporating time-dependent gain, random spontaneous emission, atomic level degeneracy, and radiation polarization. The variation of the pulse parameters such as energy, bandwidth, pulsewidth, and polarization were explained based on a simple mechanism of pulse growth and also compared to that in the conventional x-ray lasers. These results should be the basic information for a practical implementation of the scheme.

Ultra thin gold films having a thickness of 20-30 nm are favorable laser plasma targets for a soft x-ray microscopy, because the ultra thin films emit intense soft x-rays at the wavelength of water window region from the rear side with respect to the surface irradiated with short pulse laser. Using rear side emissions, the distance between the x-ray source and the specimens can be reduced so that the x-ray flux on specimens increases. In addition, the microscope system can be designed to be compact when the specimen holder and x-ray source are combined in one piece. In the present study, the biological specimen holder combined with a gold ultra thin film plasma target has been developed for a contact-type soft x-ray microscope. This x-ray microscope system needs not any x-ray optics such as a condenser and/or an objective optics which causes a decrease in x-ray photons for imaging. Specimen holder equipped with the plasma target keeps biological specimens at wet condition in vacuum. In this study, x-ray images of hydrated living cells (MH-S mouse alveolar macrophage cell line) have been obtained successfully by use of the newly developed specimen holder. These experimental results reveal that the soft x-ray image can be taken safely. Specimen holder combined with plasma x-ray source will be a key component of a compact soft x-ray microscope using in a laboratory.

In order to clarify an interaction between xenon (Xe) clusters and a soft xray laser (wavelength: 13.9 nm, pulse width: ~7 ps, intensity: 2×1010 W/cm2), we measured the energy distribution of photo- and Auger electrons as well as the ion mass spectrum. Since the photon energy of the soft x-ray laser (89.2 eV) is sufficiently high to photoionize Xe 4d inner electrons, inner-shell ionization rather than valence-shell ionization is the dominant ionization process, resulting in the Auger decay. It was found that prominent photo- and Auger electron peaks were not observed, while the continuum energy spectra dominated line peaks. Moreover, the energy distribution obtained implies the production of low temperature plasma.

With use of photon techniques including visible light, soft and hard x-rays, precise fundamental laser welding processes in the repair and maintenance of nuclear plant engineering were reviewed mechanistically. We make discussions centered on the usefulness of an intense soft x-ray beams for evaluations of spatial residual strain distribution and welded metal convection behavior including the surface morphology. Numerical results obtained with a general purpose three-dimensional code SPLICE for the simulation of the welding and solidifying phenomena. Then it is concluded that the x-ray beam would be useful as one of the powerful tools for understanding the mechanisms of various complex phenomena with higher accuracy and higher resolution.

This paper gives an overview of recent progress in the study of laser-driven plasma x-ray lasers in Japan Atomic Energy Agency (JAEA). Fully spatial coherent plasma soft x-ray laser (SXRL) at 13.9 nm with 0.1 Hz repetition rate is now routinely used in the wide variety of the applications: The highlights of these applications are the study of fluctuation in the atomic structure of ferroelectric substances under the phase transition using double SXRL probe technique and the first observation of surface dynamics of laser ablation with 10 ps-time and 1 nm-depth resolution using a single-shot SXRL interferometer.

The development of continuous pumping to the target system is an important issue for realizing an x-ray laser (XRL) with the high repetition rate. We have developed a 13.9 nm XRL using a silver tape target and demonstrated a highly coherent XRL with an oscillator-amplifier configuration using two tape target systems and the TOPAZ laser system with a 10-J and a 0.1-Hz repetition rate. The output energy is comparable to the x-ray laser generated with a silver-deposited slab target, and the pointing stability using the new tape target system is better than conventional slab target.

A novel wavelength dispersive soft X-ray emission spectrometer (WD-SXES) has been developed. It covers nominally the X-ray energy range between 50 and 210 eV [1, 2]. One of the characteristic features of the WD-SXES is parallel detection of the signal so that it can be used like a conventional energy dispersive spectrometer. One other feature is a high energy resolution, which is about 0.2 eV. This resolution is comparable to the one acquired with X-ray photoelectron spectroscopy or electron energy loss spectroscopy. This feature enables us to obtain various information about chemical bonding in bulk samples from observed spectra with a high energy resolution.

The paper is devoted to experimental and theoretical studies of ablation of condensed matter by optical (OL),extreme ultraviolet (EUV) and X-ray lasers (XRL). Re sults obtained at two dierent XRL are compared. The“rst XRL is collision Ag-plasma laser with pulse duration L = 7 ps and energy of quanta h =89.3 eV, whilethe second one is EUV free electron l aser (EUV-FEL) and has parameters L =0.3 ps and energy of quanta 20.2eV. It is shown that ablation thresholds for these XRL at LiF dielectric are approximately the same. A theory ispresented which explains slow growth of ablated mass w ith ”uence in case of XRL as a result of transition fromspallative ablation near threshold to evaporative ablation at high ”uencies. It is found that the metal irradiatedby short pulse of OL remains in elastic state even in hig h shear stresses. Material strength of aluminum at veryhigh deformation rates V/V 10 9 s 1 is de“ned.Keywords: Short pulse laser ablation, EUV and X-ray laser ablation of LiF, material strength de“ned in laserexperiment

Using highly coherent radiation at a wavelength of 13.9 nm from a Ag-plasma soft X-ray laser, we constructed a pump-and-probe interferometer based on a double Lloyd's mirror system. The spatial resolutions are evaluated with a test pattern, showing 1.8-mum lateral resolution, and 1-nm depth sensitivity. This instrument enables a single-shot observation of the surface morphology with a 7-ps time-resolution. We succeeded in observing a nanometer scale surface dilation of Pt films at the early stage of the ablation process initiated by a 70 fs near infrared pump pulse.

Observation of soft x-ray emissions from laser produced plasmas using ultra thin film targets has been carried out. Au ultra thin films deposited on silicon nitride membranes were irradiated with a high contrast Nd:glass laser pulses. The spectral properties of emitted soft x-rays were monitored with an x-ray spectrograph from the membrane side. The observed emission intensities had a clear dependence on the Au film thickness. The results suggest that most of the laser energy irradiated is absorbed by the Au films and few of the energy goes into the silicon nitride membranes, which means an efficient laser energy deposition to the ultra thin Au film target and a high energy conversion rate from laser to x-rays.

Linear polarization measurements were carried out using a newly developed soft x‐ray polarimeter and ellipsometer (SXPE) for complete polarization analysis at a soft x‐ray beamline (BL‐11) of the SR Center, Ritsumeikan University, Japan. A Mo/Si multilayer mirror (period: 10.36 nm, ratio of Mo layer to period: 0.64, and 23 bilayers with topmost Si layer) was deposited on the surface of a commercial Si(111) substrate by an ion beam sputtering method. It was cut into two pieces which were then used as reflection‐type polarizers. When the incident wavelength was scanned from 12.4 nm to 14.8 nm by a Monk‐Gillieson type varied‐line‐spacing grating monochromator of the BL‐11, the angles of incidence of both the Mo/Si multilayer polarizers in the SXPE were also varied from 37.5° to 52.3°. The polarizances depended strongly on the wavelength, and the best performance of over 99% was obtained in the vicinity of 14 nm. Using these polarizers, we assessed that the degree of linear polarization of the BL‐11 was almost constant at 85–88% in the measured wavelength range.

A highly coherent 13.9 nm x-ray laser (XRL) is generated under an oscillator-amplifier configuration using a new tape target system and a driver laser system with a 0.1 Hz repetition rate. The output energy is comparable to the XRL generated with a silver-deposited slab target, and the pointing stability using the new tape target system is better than conventional slab targets.

A new apparatus for polarimetric and ellipsometric measurements based on the rotating-analyzer method in the soft x-ray region has been designed, constructed, and installed in the soft x-ray beamline (BL-11) at the SR Center of Ritsumeikan University, Shiga, Japan. It can realize the optical configurations for the complete polarization analysis by using six independently movable drive shafts. A demonstration of the capabilities of the apparatus has been performed using Mo/Si multilayer polarizers deposited by an ion beam sputtering method. It is for the first time shown that the degree of linear polarization of monochromatized light from the BL-11 is approximately 87% at 92 eV since the beamline has been constructed.

This paper gives an overview of recent progress of laser-driven plasma x-ray lasers in Japan Atomic Energy Agency (JAEA). Fully spatial coherent plasma x-ray laser (XRL) at 13.9 nm with 0.1 Hz repetition rate has been developed using new driver laser system TOPAZ, and the succeeding optimization of the pumping condition has realized more efficient generation of the coherent x-ray pulse. The 0.1 Hz XRL is now routinely used in the wide variety of the application experiments: The highlights of these applications are the study of fluctuation in the atomic structure of ferroelectric substances under the phase transition using the double XRL probe beam technique and the construction of new x-ray laser interferometer to observe nano-scale dynamics of materials.

Electromagnetic wave generation in the extreme ultraviolet (XUV) and infrared (IR) wavelength range occurs during the interaction of intense short laser pulses with underdense plasmas. XUV pulses are generated through laser light reflection from relativistically moving electron dense shells (flying mirrors). A proof-of-principle and an advanced experiment on flying mirrors are presented. Both of the experiments demonstrated light reflection and frequency upshift to the XUV wavelength range (14-20 nm). The advanced experiment with a head-on collision of two laser pulses exhibited the high reflected photon number. IR radiation, which is observed in the forward direction, has the wavelength of 5 μm and dominantly the same polarization as the driving laser. The source of the IR radiation is attributed to emission from relativistic solitons formed in the underdense plasma.

Laser light reflection by a relativistically moving electron density modulation (flying mirror) in a wake wave generated in a plasma by a high intensity laser pulse is investigated experimentally. A counterpropagating laser pulse is reflected and upshifted in frequency with a multiplication factor of 37--66, corresponding to the extreme ultraviolet wavelength. The demonstrated flying mirror reflectivity (from $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ to $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$, and from $1.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ to $0.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$, for the photon number and pulse energy, respectively) is close to the theoretical estimate for the parameters of the experiment.

We report the soft x-ray Fourier transform holography using a high brilliance and highly coherent x-ray laser at 13.9nm. A Fresnel zone plate generates a point reference source and works as a beam splitter. The object is placed close to the first order focal spot and the transmitted zeroth order beam illuminates the object directly. A Fourier transform hologram is generated by the interference between the object wave at a large distance from the object and the spherical reference wave. The Fourier transform hologram of a grid pattern was successfully recorded with only one XRL pulse and an x-ray CCD camera and successfully numerically reconstructed. The spatial resolution is estimated to be about 1 μm.

A strongly nonlinear wake wave driven by an intense laser pulse can act as a partially reflecting relativistic mirror (the flying mirror) [S. V. Bulanov, et al., Bulletin of the Lebedev Physics Institute, No. 6, 9 (1991); S. V. Bulanov, et al., Phys. Rev. Lett. 91, 085001 (2003)]. Upon reflection from such mirror, a counter‐propagating optical‐frequency laser pulse is directly converted into high‐frequency radiation, with a frequency multiplication factor ∼4γ2 (the double Doppler effect). We present the results of recent experiment in which the photon number in the reflected radiation was at least several thousand times larger than in our proof‐of‐principle experiment [M. Kando, et al., Phys. Rev. Lett. 99, 135001 (2007); A. S. Pirozhkov, et al., Phys. Plasmas 14, 123106 (2007)]. The flying mirror holds promise of generating intense coherent ultrashort XUV and x‐ray pulses that inherit their temporal shape and polarization from the original optical‐frequency (laser) pulses. Furthermore, the reflected radi...

The photoionization process in xenon clusters irradiated with a soft x-ray laser pulse (hν­=89.2 eV, intensity: ~1010 W/cm2) was examined using a time-of-flight mass spectrometer, where the laser photon energy was high enough to ionize Xe 4d inner electrons (threshold: 67.55 eV for 4d5/2 and 69.54 eV for 4d3/2). The dominant ion yield resulting from Auger decay of 4d hole was Xe3+ ion. This showed that the double Auger transition probability would be enhanced in the cluster environments. In order to clarify the decay dynamics of 4d inner holes, the electron energy spectrum was measured. It was found that the distinct photo- and Auger electron lines were not observed, while the energy distribution implies the production of a low temperature cluster plasma.

We proposed the method of generation of the circularly polarized x-ray laser using the Zeeman splitting. External magnetic field of 20 T was applied to the gain medium plasma to separate the degenerated lines of nickel-like molybdenum x-ray laser. The splitting of the x-ray laser line was clearly obtained, and the strength of the magnetic field estimated from the quantity of the x-ray laser line splitting was quite higher compared with that of the external magnetic field. It implies that there might be alternative mechanism for enhancement of the magnetic field in the gain medium plasma.

A short laser pulse in wide range of wavelengths, from infrared to X-ray, disturbs electron–ion equilibrium and increases pressure in a heated layer. The case where the pulse duration τ L is shorter than acoustic relaxation time t s is considered in the paper. It is shown that this short pulse may cause thermomechanical phenomena such as spallative ablation regardless of wavelength. While the physics of electron–ion relaxation strongly depends on wavelength and various electron spectra of substances: there are spectra with an energy gap in semiconductors and dielectrics opposed to gapless continuous spectra in metals. The paper describes entire sequence of thermomechanical processes from expansion, nucleation, foaming, and nanostructuring to spallation with particular attention to spallation by X-ray pulse.

Several X-ray laser schemes which have possibilities for extending the present X-ray lasing wavelengths to shorter regions are reviewed. Some of these schemes may be worth reinvestigation using the ultrashort pulse, high intensity lasers that are available at present or in near future.

The time-resolved fluorescence spectrum of wide-gap semiconductors, single crystals of zinc oxide and gallium nitride, excited by nickel-like silver x-ray laser with the wavelength of 13.9 nm were observed. The streak image of fluorescence at around 380 nm and 370 nm for each samples were obtained by one-shot excitation of the x-ray laser. The fluorescence lifetimes were compared with the 351 nm excitation cases and evaluated the samples for scintillation materials of EUV lithography.

The authors describe firstly the lasers for high intensity physics experiments at JAEA including J-KAREN and JLITE-X lasers which can deliver 00TW and 0TW laser power, respectively. Secondly the authors describe demonstration of flying mirror technique which will become a new technique to make a coherent tunable x-ray source. Thirdly a femto-second laser driven incoherent soft x-ray source and its imaging applications for nano-structures are described. The th topic includes femto-second laser driven multiple quantum beam generation and its applications using a laser driven thin foil target.

We have developed a fully coherent x-ray laser at 13.9 nm and the application research has been started. The generation of submicron x-ray beam is important for the application of high intensity x-ray beam, such as the non-linear optics, the material science, and the biology. The submicron x-ray bee am is generated by the soft x-ray laser with using a Fresnel zone plate. The spot diameter is estimated about 680 nm (290 nm at FWHM) by the theoretical calculation. In this experiment, the diameter of the x-ray beam is measured by the knife-edge scan. The diameter and the intensity are estimated 730 nm (310 nm at FWHM) and 3x1011 W/cm2, respectively.

We are developing plasma x-ray lasers pumped by ultra-short pulse lasers and have concentrated our efforts on developing a silver x-ray laser (13.9 nm) with spatially full coherence by means of a double target method. Recently, we have succeeded in developing a unique new pumping laser system, which is a CPA laser consisting of OPCPA and zigzag-slab type glass amplifiers. The OPCPA was adopted for improving the controllability of pumping pulses instead of a regenerative amplifier and the zigzag-slab amplifiers were adopted for increasing the repetition rate.

The interaction of large xenon clusters with an x-ray laser pulse having a wavelength of 13.9 nm and intensity of up to 2x1010 W/cm2 was investigated using a time-of-flight ion mass spectrometer. The photon energy is high enough to photoionize the inner-shell electron of the Xe atom. In contrast to the experiment at synchrotron radiation, the enhancement of double Auger decay probability with increasing cluster size and x-ray laser intensity was observed for the first time.

We developed new power amplifier using a zigzag slab Nd:glass to generate the TCE x-ray lasers more frequently. The chirped laser light at the wavelength of 1053 nm was amplified to > 10 J and then compressed to be ~1.6 ps at repetition rate of 0.1 Hz. By using the amplified laser light, gain saturated x-ray laser with the Ni-like silver at wavelength of 13.9 nm was obtained.

We report the observation of electron density dip in a laser-produced molybdenum plasma pumped by double laser pulses which are linearly focused on the molybdenum slab target. Interferometer based on Wollaston prism was employed to diagnose the plasma, and we obtained interference pattern together with backlighting image of the plasma. The electron density dip was clearly formed at the time of 1~1.5 ns after the second pulse, and the electron density of the dip was estimated to be around 2x1018 cm-3 4x1018 cm-3. The existence of the density dip was also verified by the backlighting image, which showed that the probe beam was guided in the plasma column with the plasma length of up to 4.3 mm. The experimental result implies that the present plasma can be used as the pre-formed plasma for the axial pumping x-ray laser.

We are developing plasma x‐ray lasers pumped by ultra‐short pulse lasers and have concentrated our efforts on developing a silver x‐ray laser (13.9 nm) with spatially full coherence and application research using a highly coherent x‐ray laser. Recently, we have developed a new pumping laser system, which is a CPA laser consisting of an OPCPA preamplifier and zigzag‐slab type glass amplifiers. For the purpose of further application experiments, 0.1‐Hz operation of the driver laser and the x‐ray laser at a wavelength of 13.9 nm were succeeded.

The temporal profile of the amplification gain of the neonlike manganese x-ray laser in the transient collisional excitation scheme was measured by the use of the 29th-order harmonic light at a wavelength of 26.9 nm as a probe beam. The raise-up time of the gain was delayed by 20 ps from the peak of the pumping laser pulse, and the duration of the gain was 50 ps. This method is a promising powerful tool to realize the highly coherent x-ray laser beam by using a combination of an x-ray seeder and an x-ray amplifier.

To widen the field of application of the x-ray laser, higher repetition rate of the x-ray laser is needed. Moreover, the study on effective generation of x-ray laser by optimizing pumping conditions is very important in order to achieve the high repetition rate. At Japan Atomic Energy Agency (JAEA), we have been developing a new type of high repetition rate x-ray laser driver using zigzag slab amplifier for high repetitive x-ray lasing and an OPCPA (Optical Parametric Chirped Pulse Amplification) preamplifier for optimizing x-ray lasing conditions instead of a regenerative amplifier. We are now studying the optimization of pumping conditions for efficient x-ray lasing by the use of pump laser with very high pulse contrast from the OPCPA preamplifier. And we are planning to perform applications using the high repetition rate x-ray laser such as x-ray holography.

The interaction of large Xe clusters with a soft x-ray laser pulse having a wavelength of 13.9 nm and an intensity of up to 2 X 10 10 W/cm 2 was investigated using a time-of-flight ion mass spectrometer. The corresponding laser photon energy was sufficiently high to photoionize Xe 4d innershell electrons. It was found that Xe 3+ ions (which result from double Auger decay of 4d vacancies) became the dominant final ionic product with increasing cluster size and x-ray intensity. This is in contrast to the results of synchrotron radiation experiments involving free Xe atoms, in which Xe 2+ is the dominant resultant ion species. Possible mechanisms responsible for the enhancement of the double Auger transition probability in x-ray laser and cluster interaction are discussed.

A chirped pulse amplification laser with zigzag slab Nd:glass amplifiers dedicated to x-ray laser research is described. The laser provides a 1.6 ps duration pulse with approximately 7 J energy at a repetition rate of 0.1 Hz. In the power amplifier system, laser light is amplified in a two-step manner: The first step is image-relayed multipass amplification up to approximately 1 J with a 10 mm x 10 mm beam. The second step is double-pass amplification up to10 J with a 10 mm x 90 mm beam. By using this laser system, the saturated amplification of the Ni-like Ag laser at a wavelength of 13.9 nm has been successfully demonstrated.

This paper gives an overview of recent progress of laser‐driven plasma x‐ray lasers. For the recombining plasma lasers, the mechanism of generating the population inversion is explained, and the difficulties which we face are pointed out. In the collisional‐excitation lasers, substantial reduction of the pumping energy is successfully achieved for the wavelength up to 12 nm, and the x‐ray lasers are applied to wide variety of research fields such as material science, plasma diagnostics, atomic physics and x‐ray imaging. We also remark the future perspective of the x‐ray lasers, especially for the shorter wavelength x‐ray lasers.

To characterize the temporal evolution of ultrashort X-ray pulses emitted by laser plasmas using a pump-probe method, a magnetic bottle time-of-flight electron spectrometer is constructed. The design is determined by numerical calculations of a mirror magnetic field and of the electron trajectory in a flight tube. The performance of the spectrometer is characterized by measuring the electron spectra of xenon atoms irradiated with a laser-driven plasma X-ray pulse. In addition, two-color above-threshold ionization (ATI) experiment is conducted for measurement of the X-ray laser pulse duration, in which xenon atoms are simultaneously irradiated with an X-ray laser pump and an IR laser probe. The correlation in the intensity of the sideband spectra of the 4d inner-shell photoelectrons and in the time delay of the two laser pulses yields an X-ray pulse width of 5.7 ps, in good agreement with the value obtained using an X-ray streak camera., This work was supported by a Grant-in-Aid for Scientific Research B (No. 26286078) from the Japanese Society for the Promotion of Science. We thank the JAEA X-ray laser research group for laser operations. This work was performed under the shared use program of JAEA Facilities.

We have characterized the spectral structure and the temporal history of the laser-produced high-Z multi-charged ion plasmas for the efficient water window soft x-ray sources. Strong unresolved transition array emission was observed due to 4d–4f and 4f–5g transitions from Au, Pb, and Bi plasmas in the 280–700 eV photon energy region. The temporal behavior of the emission was essentially similar of that of the laser pulse with a slight delay between different transitions. These results provide feedback for accurate modeling of the atomic processes with the radiative hydrodynamic simulations.

We have developed a fully coherent x‐ray laser (XRL) at 13.9 nm by a double‐target configuration. In the experiment, a seed XRL beam from the first target is injected into a plasma amplifier at the second target. The amplified XRL beam has full spatial coherence, 0.2 mrad of the nearly diffraction limited divergence, and 25 nJ of the output energy. For various application researches, we improve the output energy and the beam property of the x‐ray laser. The gain‐length product was gL ∼ 8.2, and the maximum output energy of about 1 μJ is achieved. The divergence of the XRL is affected by the refraction influence in the amplifier medium and/or the diffraction by the small size of the gain medium. We made the 2‐D radiation hydrodynamics simulation code in order to investigate the generation of the gain medium plasma and the refraction influence.

Recent progress in x-ray laser (XRL) research in Japan Atomic Energy Agency (JAEA) is reviewed. The repetition-rate of the x-ray laser has been improved from each0 minutes to0 seconds (0.1 Hz) by installing new driver laser, TOPAZ, which allows us to promote the applications of fully spatial coherent3.9 nm laser in the wide variety of research fields such as material science, single-shot x-ray holography and atomic physics. In order to improve the present performance of the x-ray lasers, we have investigated the possibilities of the enhancement of the peak brilliance using v-groove target and the generation of circularly polarized x-ray laser under a strong magnetic field. Towards shorter wavelength x-ray lasers, we have investigated several schemes. One is the use of reflection of the light by relativistic plasma mirror driven by laser-wake-field, and the other is photo-pumping scheme using Kα emission from a solid target.

The emission of fundamental and harmonic radiation from the rear side of thin foils in the thickness range $50--460\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ irradiated by intense frequency doubled Ti:sapphire laser pulses of the duration of $150\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ and intensities up to a few ${10}^{18}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$ was investigated. Following up a previous study of the rear side harmonic emission [Teubner et al., Phys. Rev. Lett. 92, 185001 (2004)], we measured the emission efficiencies, polarization properties, and the spectral shapes of the fundamental frequency and the second harmonic. Rear side emission is only observed when the obliquely incident laser light is $p$-polarized. Particle-in-cell (PIC) simulations indicate that the foils remain strongly overdense during the interaction with the laser pulse and that the rear side emission is caused by energetic electron bunches which are generated at the front side by resonance absorption. They are accelerated into the foil and drive strong plasma oscillations at the fundamental and higher harmonic frequencies.

We present measurements of electron densities of plasmas with femtosecond (fs) temporal resolution. The plasmas are generated by laser pulses with different intensities at different time delays. Such plasmas are of great interest as preplasmas for transient, collisionally excited x-ray lasers. The laser pulses producing the plasmas are generated by stretching part of a $130\text{\ensuremath{-}}\mathrm{fs}$ laser pulse of the ATLAS titanium-sapphire laser of our institute and focusing this radiation to a line on molybdenum and silver slab targets. The electron density is measured as a function of distance from the target by interferometry using a Wollaston prism. Using an ultrashort probe pulse allows one to obtain data extremely close, about $10\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$, to the target surface. Experimental data are compared with simulations using the MULTI hydrocode. The results allow comparison of the ablation from a hard (Mo) and a soft (Ag) material, optimization of prepulse-main pulse delay times, and selection of the best pump geometry allowing for propagation of the pump and x-ray beams. These points are key elements for the development of a high-repetition-rate soft-x-ray laser.

We have observed strong amplification with a gain of $14.5\phantom{\rule{0.3em}{0ex}}{\text{cm}}^{\ensuremath{-}1}$ for the $4d\ensuremath{\rightarrow}4p$ transition of the Ni-like La ions at a wavelength of $8.8\phantom{\rule{0.3em}{0ex}}\text{nm}$ pumped by a chirped pulse amplification Nd:glass laser with an energy of $18\phantom{\rule{0.3em}{0ex}}\mathrm{J}$. In this experiment, the laser pulses consist of a prepulse and a main pulse with durations of $200\phantom{\rule{0.3em}{0ex}}\text{ps}$ and $7\phantom{\rule{0.3em}{0ex}}\text{ps}$, separated by $250\phantom{\rule{0.3em}{0ex}}\text{ps}$. The keV x-ray spectroscopy and theoretical calculation indicate that the plasma temperature increased effectively by reducing the prepulse intensity, and that the substantial gain is generated in a small overcritical region heated by the electron heat transport.