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Relativistic surface high harmonics have been considered a unique source for the generation of intense isolated attosecond pulses in the extreme ultra-violet and x-ray spectral ranges. Their practical realization, however, is still a challenging task and requires identification of optimum experimental conditions and parameters. Here, we present measurements and particle-in-cell simulations to determine the optimum values for the most important parameters. In particular, we investigate the dependence of harmonics efficiency, divergence, and beam quality on the pre-plasma scale length as well as identify the optimum conditions for generation of isolated attosecond pulses by measuring the dependence of the harmonics spectrum on the carrier - envelope phase of the driving infrared field. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

International audience; Real-time observation of ultrafast dynamics in the microcosm is a fundamental approach for understanding the internal evolution of physical, chemical and biological systems. Tools for tracing such dynamics are flashes of light with duration comparable to or shorter than the characteristic evolution times of the system under investigation. While femtosecond (fs) pulses are successfully used to investigate vibrational dynamics in molecular systems, real time observation of electron motion in all states of matter requires temporal resolution in the attosecond (1 attosecond (asec) = 10(-18) s) time scale. During the last decades, continuous efforts in ultra-short pulse engineering led to the development of table-top sources which can produce asec pulses. These pulses have been synthesized by using broadband coherent radiation in the extreme ultraviolet (XUV) spectral region generated by the interaction of matter with intense fs pulses. Here, we will review asec pulses generated by the interaction of gas phase media and solid surfaces with intense fs IR laser fields. After a brief overview of the fundamental process underlying the XUV emission form these media, we will review the current technology, specifications and the ongoing developments of such asec sources.

Background. Oxidative stress is a hallmark of CKD and this alteration is strongly implicated in LV hypertrophy and in LV dysfunction.Methods and Patients. We resorted to the strongest genetic biomarker of paraoxonase-1 (PON1) activity, the Q192R variant in the PON1 gene, to unbiasedly assess (Mendelian randomization) the cross-sectional and longitudinal association of this gene-variant with LV mass and function in 206 CKD patients with a 3-year follow-up.Results. The R allele of Q192R polymorphism associated with oxidative stress as assessed by plasma 8-isoPGF2α(P=0.03) and was dose-dependently related in a direct fashion to LVMI (QQ: 131.4 ± 42.6 g/m2; RQ: 147.7 ± 51.1 g/m2; RR: 167.3 ± 41.9 g/m2;P=0.001) and in an inverse fashion to systolic function (LV Ejection Fraction) (QQ: 79 ± 12%; RQ: 69 ± 9%; RR: 65 ± 10%P=0.002). On longitudinal observation, this gene variant associated with the evolution of the same echocardiographic indicators [LVMI: 13.40 g/m2per risk allele,P=0.005; LVEF: −2.96% per risk allele,P=0.001]. Multivariate analyses did not modify these associations.Conclusion. In CKD patients, the R allele of the Q192R variant in the PON1 gene is dose-dependently related to the severity of LVH and LV dysfunction and associates with the longitudinal evolution of these cardiac alterations. These results are compatible with the hypothesis that oxidative stress is implicated in cardiomyopathy in CKD patients.

In this paper we report on an experimental study of high harmonic radiation generated in nanometer-scale foil targets irradiated under normal incidence. The experiments constitute the first unambiguous observation of odd-numbered relativistic harmonics generated by the v × B component of the Lorentz force verifying a long predicted property of solid target harmonics. Simultaneously the observed harmonic spectra allow in-situ extraction of the target density in an experimental scenario which is of utmost interest for applications such as ion acceleration by the radiation pressure of an ultraintense laser.

Studies of ultrafast dynamics along with femtosecond-pulse metrology rely on non-linear processes, induced solely by the exciting/probing pulses or the pulses to be characterized. Extension of these approaches to the extreme-ultraviolet (XUV) spectral region opens up a new, direct route to attosecond scale dynamics. Limitations in available intensities of coherent XUV continua kept this prospect barren. The present work overcomes this barrier. Reaching condition at which simultaneous ejection of two bound electrons by two-XUV-photon absorption becomes more efficient than their one-by-one removal it is succeeded to probe atomic coherences, evolving at the 1fs scale, and determine the XUV-pulse duration. The investigated rich and dense in structure autoionizing manifold ascertains applicability of the approach to complex systems. This initiates the era of XUV-pump-XUV-probe experiments with attosecond resolution., 27 pages

We report on investigations of high-order harmonic generation from solid surfaces in the coherent wake emission regime with relativistically intense few-cycle (8 fs) laser pulses. Significant spectral broadening compared to previous experiments with many-cycle pulses and the appearance of substructures on the harmonics are observed that strongly fluctuate from shot-to-shot. Measurements in which the linear polarization was rotated or ellipticity of the laser pulse was varied exhibit a strong dependence of the harmonic emission on the polarization state of the incident pulse. We show that the observed spectral features are ultimately connected to the sub-cycle electron dynamics in the laser-solid interaction and thus proof of the few-cycle nature of the observed harmonic emission. Using a simple model we have investigated the factors that play an important role in the shape of the emitted spectrum.

Harmonic generation from relativistically oscillating plasma surfaces formed during the interaction of high contrast lasers with solid-density targets has been shown to be an efficient source of extreme ultraviolet (XUV) and X-ray radiation. Recent work has demonstrated that the exceptional coherence properties of the driving laser can be mirrored in the emitted radiation, permitting diffraction limited performance and attosecond phase locking of the harmonic radiation. These unique properties may allow the coherent harmonic focusing (CHF) of high harmonics generated from solid density targets to intensities on the order of the Schwinger limit of 1029 W cm-2 with laser systems available in the near future [Phys. Rev. Lett. 93, 115002 (2004)] and thus pave the way for unique experiments exploring the nonlinear properties of vacuum on ultra-fast timescales. In this paper we investigate experimentally as well as numerically the prospect of focusing high harmonics under realistic experimental conditions and demonstrate, using particle in cell (PIC) simulations, that precise control of the wavefronts and thus the focusability of the generated harmonics is possible with pre-shaped targets.

When a pulse of light reflects from a mirror that is travelling close to the speed of light, Einstein’s theory of relativity predicts that it will be up-shifted to a substantially higher frequency and compressed to a much shorter duration. This scenario is realized by the relativistically oscillating plasma surface generated by an ultraintense laser focused onto a solid target. Until now, it has been unclear whether the conditions necessary to exploit such phenomena can survive such an extreme interaction with increasing laser intensity. Here, we provide the first quantitative evidence to suggest that they can. We show that the occurrence of surface smoothing on the scale of the wavelength of the generated harmonics, and plasma denting of the irradiated surface, enables the production of high-quality X-ray beams focused down to the diffraction limit. These results improve the outlook for generating extreme X-ray fields, which could in principle extend to the Schwinger limit. A systematic demonstration of the generation and focusing of laser-driven high-order harmonics to a near-diffraction-limited spot suggests that scaling this approach to ever higher intensities could be easier than first thought.

An experiment demonstrating the generation of subfemtosecond pulses of light through the interaction of laser light with a solid target underlines the potential of this approach to lead to a new generation of intense sources of attosecond pulses.

We describe and demonstrate a novel experimental technique to record two picosecond-separated images of the evolution of a single-shot laser-produced plasma using a two-color probe beam. In this technique, the probe beam was first partially frequency doubled. The 1ω and 2ω components were then temporally separated in a dispersive medium. The pulses remained intrinsically synchronized, both having a duration of ∼150 fs, but they were separated by 4.1 ps. Using a Nomarskii-type interferometer, this novel technique was used to obtain two time-resolved snapshots of the evolution of the electron density at different time steps during the formation of a relativistic plasma channel.

Continuing efforts in ultrashort pulse engineering have recently led to the breakthroughs of the generation of attosecond (10−18 s) pulse trains1,2,3,4,5,6,7 and isolated pulses8,9,10,11. Although trains of multiple pulses can be generated through the interaction of many-optical-cycle pulses with gases—a process that has led to intense extreme-ultraviolet emission3,4,5—the generation of isolated high-intensity pulses, which requires few-cycle driving pulses, remains a challenge. Here, we report a vital step towards the generation of such pulses, the production of broad continuum extreme-ultraviolet emission using a high-intensity, many-cycle, infrared pulsed laser, through the interferometric modulation of the ellipticity of 50-fs-long driving pulses. The increasing availability of high-power many-cycle lasers and their potential use in the construction of intense attosecond radiation—with either gas or solid-surface targets12—offer exciting opportunities for multiphoton extreme-ultraviolet-pump–extreme-ultraviolet-probe studies of laser–matter and laser–plasma interactions.

We demonstrate a tool for quantitative measurements in the extreme ultraviolet (EUV) spectral region measuring spatially resolved atomic ionization products at the focus of an EUV beam. The ionizing radiation is a comb of the 11th–15th harmonics of a Ti:Sapphire femtosecond laser beam produced in a Xenon gas jet. The spatial ion distribution at the focus of the harmonics is recorded using an ion microscope. Spatially resolved single- and two-photon ionization products of Argon and Helium are observed. From such ion distributions single- and two-photon generalized cross sections can be extracted by a self-calibrating method. The observation of spatially resolved two-EUV-photon ionization constitutes an initial step towards future single-shot temporal characterization of attosecond pulses.

We report on simultaneous efficiency and divergence measurements for harmonics from solid targets generated by the relativistic oscillating mirror mechanism. For a value of the normalized vector potential of \(a_{\mathrm{L}}\simeq 1.5\), we demonstrate the generation of 30 μJ high-harmonic radiation in a \(17\pm 3\) mrad divergence cone. This corresponds to a conversion efficiency of \(\gtrsim\)10−4 in the 17–80 nm range into a well-confined beam. Presuming phase-locked harmonics, our results predict unprecedented levels of average power for a single attosecond pulse in the generated pulse train. Results of PIC simulations raise the prospect of attaining efficiencies of a few percent at higher laser intensities.

In this contribution, we address laser-driven electron transport in fast ignition research and the need for ultra-fast (

Recent developments in the generation, metrology and potential applications of XUV-attosecond (as) pulses, formed by the coherent superposition of harmonics of an infrared femtosecond (fs) laser pulse, are reviewed. Particular emphasis is given to the recently achieved second-order autocorrelation (AC) measurement of a sub-femtosecond (sub-fs) pulse train. The method is a non-trivial extension in the XUV spectral range of the well-established technique routinely used in ‘fs’ pulse metrology. Besides the direct visualization of a periodic attosecond structure and the quantitative information it supplies, the approach introduces a unique technique for the implementation of XUV-pump XUV-probe type of experimental studies of ultra-fast phenomena. It further relates to the metrology and time domain application interests of other advanced XUV radiation sources, such as the XFEL-based installations. While the implementation of the method and its refined versions with laser harmonics is a challenge at the limits of the realizable, the profoundly high scheduled intensities of XFEL sources are expected to sufficiently enhance its robustness and range of applicability.

The unequivocal demonstration of attosecond XUV pulse train generation based on the synthesis of a comb of harmonics of an infrared femtosecond laser pulse is inevitably linked to a reliable method of pulse characterization in this temporal regime. The most commonly used method to date relies on a cross-correlation technique between the fundamental laser frequency and the XUV light from which a measurement of the relative phases of the consecutive harmonics is obtained and the attosecond temporal structure in a train of XUV pulses is inferred. Here we report on an alternative method that renders possible the direct observation of a train of attosecond pulses in form of a second-order autocorrelation trace. The method is an extension in the XUV spectral range of the well-established technique commonly used for the characterization of femtosecond laser pulses. Besides being the first direct visualization of a periodic attosecond structure, the approach supplies quantitative information on the overall pulse ...

We report on the progress towards a 2nd-order autocorrelation measurement of a coherent superposition of the 7th to the 15th harmonic of the fundamental frequency of the Ti : Sapphire laser. The two-photon ionization of He atoms was investigated and found to be an appropriate non-linear detector. In addition, the technique of volume autocorrelation based on a split mirror wavefront divider was analysed and shown to exhibit adequate degree of modulation. Moreover, the approach presented opens up the path to XUV-pump XUV-probe type of temporal studies of ultrafast processes with a sub-fs temporal resolution.

We present an experimental and theoretical study of the influence of the spatial beam quality (fluence and phase distributions) on the second-harmonic generation in KDP crystals pumped by 180-fs pulses at 790 nm. Conversion efficiency and beam focusability are investigated experimentally and theoretically by the numerical analysis of the second harmonic, considering effects due to the cubic nonlinearity, beam diffraction, group-velocity walk-off, and dispersion of the pulses. It was found that the uniform intensity and phase distributions of the fundamental beam are essential to obtain a high focal intensity of the second-harmonic beam.

Temporal probing of a number of fundamental dynamical processes requires intense pulses at femtosecond or even attosecond (1 as = 10-18 s) timescales. A frequency ‘comb’ of extreme-ultraviolet odd harmonics can easily be generated in the interaction of subpicosecond laser pulses with rare gases: if the spectral components within this comb possess an appropriate phase relationship to one another, their Fourier synthesis results in an attosecond pulse train1,2. Laser pulses spanning many optical cycles have been used for the production of such light bunching3,4, but in the limit of few-cycle pulses the same process produces isolated attosecond bursts5,6. If these bursts are intense enough to induce a nonlinear process in a target system7,8,9, they can be used for subfemtosecond pump–probe studies of ultrafast processes. To date, all methods for the quantitative investigation of attosecond light localization4,6,10 and ultrafast dynamics11 rely on modelling of the cross-correlation process between the extreme-ultraviolet pulses and the fundamental laser field used in their generation. Here we report the direct determination of the temporal characteristics of pulses in the subfemtosecond regime, by measuring the second-order autocorrelation trace of a train of attosecond pulses. The method exhibits distinct capabilities for the characterization and utilization of attosecond pulses for a host of applications in attoscience.

We present experimental results in which a second-order effect, namely two-photon ionization of atomic He induced by a superposition of harmonics, is observed. The harmonics are generated in a Xe gas-jet using a 790-nm 10-Hz femtosecond Ti:sapphire laser and are subsequently focused into a He gas-jet with a Kirkpatrick–Baez arrangement. The superposition is formed by using a thin In filter and it comprises the 7th to 13th harmonics. Solving the time-dependent Schrodinger equation for He in a polychromatic laser field, the He+ ion yield is calculated as a function of the total XUV intensity. Using the calculated yield and taking into account the focusing and transmission properties of the arrangement, the number of He+ ions produced per laser pulse is estimated and is found to be in reasonable agreement with its measured value. The total number of ions produced non-resonantly follows a nearly quadratic dependence on the harmonic intensity, thus establishing the feasibility of a second-order auto-correlation measurement of the superposition of harmonics, i.e., of a direct temporal characterization of attosecond pulse trains.

A new Michelson interferometer utilizing a transmission grating as a beamsplitter is presented. The general features of three different interferometer geometries, as they result from ray tracing analysis, and the application areas of their corresponding operational modes are summarized. The interferometer exhibits an overall ultra-low dispersion, wavelength selectivity and flat response over a broad spectral region in the UV-XUV. Its pulse distortion level is found to be of the order of few attoseconds for higher harmonics. Thus, the prospect of nearly dispersionless auto-and cross-correlation measurements of individual harmonics or harmonic superposition, aiming at their temporal characterization appears to be realizable. Experimental tests of the interferometer, in which the interferometric and intensity autocorrelation spectra of the fundamental and third harmonic of a 45 fs Ti:Sapphire laser were measured, are described. Its performance in the fs regime is experimentally evaluated by comparin...

A novel application of a free-standing transmission grating as a beam splitter in a Michelson-type interferometer is described. The arrangement can operate in the XUV and soft X-ray spectral region and, therefore, it is well suited for the characterization of attosecond pulses. Using ray-tracing codes, we have analyzed three different setups in which spherical mirrors are employed in conjunction with the transmission grating and have investigated in detail their dispersive characteristics. It is shown that such an arrangement can be made to exhibit group-delay dispersion of ∼1 as2 while it provides two co-propagating and converging beams.

International audience

The tunnelling of an electron through a suppressed atomic potential followed by its motion in the continuum, is the fundamental mechanism underlying strong-field laser-atom/molecule interactions. Due to its quantum nature, the interaction is governed by the phase of the released electron wave-packets. Thus, detailed mapping of the electron wave-packet interferences provides essential insight into the physics underlying the interaction. A process which is providing access to the intricacies of the interaction is the generation of high order harmonics of the laser frequency. The phase-amplitude distribution of the emitted extreme-ultraviolet (EUV), carries the complete information about the harmonic generation process and vice-versa. Thus, the visualization of the EUV-spatial-amplitude-distribution, as it results from interfering electron wave-packet contributions, is of crucial importance. Restrictions to this accomplishment are due to the spatially integrating measurement approaches applied so far, that average out the phase effects in the generation process. In this work, we demonstrate a method which overcomes this obstacle. An EUV-spatial-amplitude-distribution-image is induced from the imprint on the measured spatial distribution of ions, produced through EUV-photon ionization of atoms. Interference extremes in the image carry phase information about the interfering electron wave-packets. The present approach provides detailed insight on the strong-field laser-atom interaction mechanism, while establishing the era of phase selective interaction studies. Furthermore, it paves the way for substantial enhancement of the spectral and temporal precision of measurements by in-situ controlling the phase distribution of the emitted radiation and/or spatially selecting the EUV-radiation-atom interaction products., Comment: 13 pages main text with 4 figures, 7 pages supporting material with 3 figures

Results of a fully relativistic three-dimensional (3-D) single particle code, supported by a theoretical model, on direct laser acceleration of electrons in radial electric and azimuthal magnetic static fields are presented. The ponderomotive force and the longitudinal components of the laser field are taken into account in the code. The electron motion in the static fields is similar to the motion in a magnetic wiggler. At resonance, when the bounce frequency of the wiggling motion is within a few percent of the Doppler shifted laser frequency, the amplitude of transverse oscillation shows a rapid increase accompanied by a fast rise in energy and parallel momentum. For this situation, a theoretical model of energy exchange between the electrons and the laser provides reasonable estimate of energy gain. The single particle code is used in Monte Carlo simulations to study the energy distribution and angular spread of the accelerated electrons in a self-focused high intensity laser pulse interaction.

We have developed a magnetic spectrometer to characterize the hot electrons generated in high-intensity (>1018 W/cm2) laser-plasma interaction experiments. It comprises a dispersive element consisting of a permanent dipole magnet and an electron detector incorporating 45 scintillating/light-guiding plastic fibers connected to a cooled charged-coupled device camera. The main features of this instrument are high spectral resolution, low sensitivity to x and γ rays, and versatility due to its compact design. Performance and operational capabilities are illustrated based on experimental results in which electron energy–spectra in the range of 520 keV to 12.6 MeV were obtained with an energy resolution of 10% and a detection threshold of 106 electrons per MeV.

We have measured angularly resolved and absolutely calibrated spectra of the multi-MeV electrons produced by relativistic self-channeling in a high-density gas jet. Using 200 fs laser pulses with ${P}_{L}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.2\mathrm{TW}$, we have investigated the electron spectrum dependence on the plasma electron density in the range of $3\ifmmode\times\else\texttimes\fi{}{10}^{19}--4\ifmmode\times\else\texttimes\fi{}{10}^{20}{\mathrm{cm}}^{\ensuremath{-}3}$. The experimentally obtained results are closely reproduced by three-dimensional particle-in-cell simulations. A detailed analysis shows that the self-modulated laser wake field, although active, cannot explain the experimental energy spectrum. The bulk of the fast electrons are produced by direct laser acceleration at the channel betatron resonance.

The fast-ignition scheme in ICF has instigated research in a new area of laser-plasma interaction physics. This is due to the high laser-pulse intensities involved. At power densities of 1018 W cm-2, the physics becomes essentially kinetic in nature and highly nonlinear and hence requires special tools for its adequate description. Particle-in-cell (PIC) codes meet this challenge. Simulations obtained with the first three-dimensional PIC code VLPL (virtual laser plasma laboratory) developed at MPQ are presented for a broad variety of irradiation and plasma conditions. Particular attention is paid to a detailed comparison of numerical and experimental results and to the mechanism which accelerates the electrons to relativistic energies.

Background. Patients who die within 90 days of commencing renal replacement therapy (RRT) may be recorded by some centres and not others, and hence data on mortality and survival may not be comparable. However, it is essential to compare like with like when analysing differences between modalities, centres and registries. It was decided, therefore, to look at the incidence of deaths within 90 days in the ERA-EDTA Registry, and to try to define the characteristics of this group of patients. Methods. Between 1 January 1990 and 31 December 1992, 78 534 new patients started RRT in 28 countries affiliated to the ERA-EDTA Registry. Their mean age was 54 years and 31% were over 65 years old. Eighty-two per cent of the patients received haemodialysis (HD), 16% peritoneal dialysis (PD) and 2% had pre-emptive transplantation as first mode of treatment. Results. From January 1990 to March 1993 the overall incidence of deaths was 19% and 4% of all patients died within 90 days from the start of RRT. Among those dying within 90 days 59% were over 65 years compared to 53% over 65 years in those dying beyond this time (P

Two-colour phase control of gas harmonic generation has been demonstrated using a 200-fs Ti:sapphire laser and its second harmonic. In the detection range covering the 7th to the 20th harmonic order, a modulation of up to 27% in the harmonic emission signal was observed by precisely varying the relative phase between the two colours. Simulations based on the single-particle response predict enhancement of the high harmonic efficiency by orders of magnitude for a given relative phase between the two colours.

We report on measurements of the equation-of-state of copper and gold in the multi-ten Megabar range, opacity measurements based on K-shell absorption in aluminium, and X-ray laser studies on a large number of neon-like ions applying the prepulse technique. For these investigations, the one-beam iodine laser facility ASTERIX emitting pulses with energies of up to 1000 J at 1315 nm and of up to 420 J at 438 nm was used. We also give a brief account of experimental and theoretical results referring to the propagation of an ultrashort pulse through underdense hydrogen or nitrogen plasmas and X-ray spectra from an optically field-ionized nitrogen plasma generated either by linearly or elliptically polarized light. For these investigations, 150-fs/200-mJ/800-nm pulses emitted from our titanium:sapphire laser ATLAS were employed.

Backward and forward Raman scattering has been experimentally investigated in a high density hydrogen gas jet target at electron densities up to 0.2 n c , well above the density of previously reported studies. Stokes shifted radiation from both backward and forward scattering is observed with a peak wavelength which is consistent with the frequency matching condition for a cold plasma. Strong up-shifted spectra are observed in the forward direction which cannot be simply attributed to anti-Stokes shifted Raman light. Hard X-ray emission is also observed and its onset together with a saturation in the backscatter signal coincide with the predicted threshold for relativistic self-focusing of the laser in the gas. The results also show a correlation between the generation of hard X-rays and the forward Raman scattering as the laser power is varied.

Optical investigations are reported of the interaction of $0.3$ TW, 250 fs Ti:sapphire laser pulses with underdense plasmas created from high density gas jet targets. Time resolved shadowgraphy using a $2\ensuremath{\omega}$ probe pulse, images of the transmitted radiation and images of $1\ensuremath{\omega}$ and $2\ensuremath{\omega}$ side radiation are presented for various gases. The experimental results and their analysis based on a simple numerical Gaussian beam model show that ionization-induced refraction dominates the interaction process for all gases except hydrogen. The numerical modeling also shows that for a given laser power there exists only a narrow density range in which self-focusing can be expected to occur. In the case of hydrogen for electron densities greater than $\ensuremath{\sim}{10}^{20}{\mathrm{cm}}^{\ensuremath{-}3}$, the onset of channeling expected at the critical power for relativistic self-focusing is experimentally observed. X-ray and forward stimulated Raman scattering measurements were also conducted to verify the onset of high intensity relativistic interactions and the onset of fast electron generation.

Beam divergences of high-order extreme ultraviolet harmonics from intense laser interactions with steep plasma density gradients are studied through experiment and Fourier analysis of the harmonic spatial phase. We show that while emission due to the relativistically oscillating mirror mechanism can be explained by ponderomotive surface denting, in agreement with previous results, the divergence of the emission due to the coherent wake emission mechanism requires a combination of the dent phase and an intrinsic emission phase. The temporal dependence of the divergences for both mechanisms is highlighted while it is also shown that the coherent wake emission divergence can be small in circumstances where the phase terms compensate each other.

Objective. Survival is the ultimate outcome measure in renal replacement therapy (RRT) and may be used to compare performance among centres. Such comparison, however, is meaningless if the influences of comorbidity, age and early deaths are not considered. We therefore studied survival rates on RRT in seven centres in Europe after taking into account the influence of age, early deaths, primary renal diagnoses, and comorbidity. Design. A retrospective survival analysis was carried out on 1407 patients who commenced RRT in seven centres across five European countries during a 7-year period. Patients were stratified into low-, medium- and high-risk groups based mainly on comorbidity and to a lesser extent on age at commencement of RRT. Kaplan-Meier survival and Cox's proportional hazards model were used to compare survival. Results. Before risk stratification overall 2-year survival across the seven centres ranged from 60.2 to 85.3% (69.3-89.9% after excluding early deaths) masking a range of survivals of 27.4% for the high-risk group with the worst survival to 100% in the low-risk group with the best survival. After excluding early deaths 2-year survival in the low risk groups (n = 596) was greater than 90% in all centres. Multivariate analysis showed that the mortality risk increased four fold from low- to medium- and a further 1.6-fold from medium- to high-risk group. However, despite this adjustment for comorbidity and age there still remained a significant difference in survival among some centres, i.e. a 'centre effect' which ranked the centres. Conclusion. Risk stratification diminishes the variance in survival between centres but a centre effect remains despite adjusting for age and comorbidity. Multicentre Correspondence and offprint requests to: Dr I. H. Khan Lecturer, Department of Medicine and Therapeutics, Polwarth Building, Foresterhill, Aberdeen, AB9 2ZD, UK. prospective studies are urgently required to identify the reasons for this apparent centre effect.

The proportion of centres returning the ERA-EDTA Registry questionnaires has decreased considerably in recent years. Demographic information, based on the response rate of centres in 1994 (44%), does not allow reasonable projections for management of renal failure in Europe. To encourage the participation of non-responding centres, the timing was right to show the powerful impact of the ERA-EDTA Registry as a supra-national registry, by studying patients in renal replacement therapy (RRT) suffering from rare diseases. Four such diseases, Fabry's disease, nephropathy due to cyclosporin (CsA), nephropathy due to cisplatin and scleroderma, were studied using the records of 440665 patients on file up to 31 December 1993. There were 83 patients with Fabry's disease (0.0188%), 85 patients with CsA nephropathy (0.0193%), 120 patients with cisplatin nephropathy (0.0272%) and 625 patients with scleroderma (0.142%). Scleroderma was introduced as a primary renal disease (PRD) in the ERA-EDTA Registry in 1977. Seven patients were accepted for RRT in that year, whereas the number increased to over 50 new patients per year after 1986. More than half of the patients were aged over 55 years, and 68% of them were women. Survival rate of dialysis patients suffering from scleroderma was 22% at 5 years, compared to 51% in patients with standard primary renal diseases. The main causes of death were cardiovascular complications (41%), cachexia (15%) and infection (10%). Survival of first graft in a small number of 28 patients was 44% at 3 years, compared to 60% in standard PRD. Patient survival after first transplant, however, was higher by 32% at 3 years compared to that of dialysis patients. Cisplatin nephropathy was introduced as a PRD in the ERA-EDTA Registry in 1985, and since then six to 19 new patients have been accepted for RRT each year. The main reason for undergoing cisplatin treatment was ovarian (32%) and testicular cancer (21%), and the mean interval from treatment to RRT was 21.5 months, ranging widely from 0.1 to 131 months. Patient survival on dialysis was 22% at 5 years, compared to 51% in patients with standard PRD. Malignancy and cachexia accounted for over 60% of the total number of deaths. CsA nephropathy was introduced as a PRD in the ERA-EDTA Registry in 1985 and, despite its rarity, is of particular interest as a new iatrogenic entity resulting from CsA administration, mainly in solid organ transplantation. In 1985, two new patients commenced RRT in Europe, and the number increased to 59 in 1991-93. The main reason for undergoing CsA treatment was heart (68%) and liver transplant (22%), and the mean interval from treatment to RRT was 50.2 months, ranging from 5 to 90 months. Patient survival on dialysis was 46% at 4 years, compared to 58% in patients with standard primary nephropathies. Cardiovascular causes (48%) and infection (17%) were the main causes of death. Fabry's disease was introduced as a PRD in the ERA-EDTA Registry in 1985, and since the four to 13 new patients per year have commenced RRT in Europe. It is a sex-linked recessive disorder primarily affecting males (87%), and the mean age at start of RRT was 38 years. Proteinuria, skin lesions and painful paresthesiae were the most common presenting symptoms, and over 70% of the patients were hypertensive and had significant cardiovascular problems at RRT. Patient survival on dialysis was 41% at 5 years, compared to 68% in patients with standard primary nephropathies. Cardiovascular complications (48%) and cachexia (17%) were the main causes of death. Graft survival at 3 years in 33 patients was not inferior to that of patients with standard nephropathies (72% vs 69%), and patient survival after transplantation was comparable to that of patients under 55 years of age with standard PRD. (ABSTRACT TRUNCATED)

Observations of harmonic generation from solid Al targets irradiated at intensities of 1017 W/cm2 using a tabletop Ti:sapphire terawatt laser system are reported. For irradiances with laser light at a wavelength of [lambda] = 794nm (the fundamental frequency), harmonics up to the 7th, while for irradiances with frequency doubled laser light ([lambda] = 397) nm harmonics up to the 4th of the respective incident radiation were detected.

A semiempirical method of calibrating a thinned, backside illuminated charge coupled device (CCD) chip in the soft x‐ray region is presented. It is based on determining the thickness of the dead layer self‐consistently using the continuum emission from laser produced plasmas. The CCD camera system was coupled to a transmission grating spectrometer and recorded the spectrally resolved continuum emission from laser irradiated tungsten targets. The thickness of the dead layer was then determined by comparing the experimental spectra with the calculated quantum efficiency for a thinned CCD using a simplified model. In this way the CCD chip was semiempirically calibrated. The accuracy of the calibration in the soft x‐ray range was assessed by comparing the CCD recorded spectra with those recorded by a spectrometer using the absolutely calibrated Kodak 101 photographic plates and a similar transmission grating. Based on this calibration, the CCD sensitivity is deduced to be about two orders of magnitude higher than that of the Kodak plates in this wavelength range.

The conditions required for the production of isolated attosecond pulses from relativistically oscillating mirrors (ROM) are investigated numerically and experimentally. In simulations, carrier-envelope-phase-stabilized three-cycle pulses are found to be sufficient to produce isolated attosecond pulses, while two-cycle pulses will predominantly lead to isolated attosecond pulses even in the absence of carrier-envelope stabilization. Using a state-of-the-art laser system delivering three-cycle pulses at multiple-terawatt level, we have generated higher harmonics up to 70 eV photon energy via the ROM mechanism. The observed spectra are in agreement with theoretical expectations and highlight the potential of few-cycle-driven ROM harmonics for intense isolated attosecond pulse generation for performing extreme ultraviolet-pump extreme ultraviolet-probe experiments.

Attosecond pulses today are generated at pulse energies leading to intensities sufficient to induce two-photon transitions in the extreme ultraviolet (XUV) spectral region. Recently, ultra-broadband coherent XUV continua fulfill also the requirements in inducing such processes. Two-XUV-photon ionization is a pivotal tool in attosecond pulse metrology, as well as for XUV-pump-XUV-probe applications targeting the tracking of ultrafast dynamics, providing at the same time spatial selectivity.Based on these developments, this chapter (a) reviews approaches leading to high intensities of attosecond pulse trains and coherent XUV continua; (b) reviews metrology approaches based on two-XUV-photon ionization, showing their importance through comparative studies with existing XUV-IR cross-correlation approaches; and (c) reports the feasibility of XUV-pump-XUV-probe applications at the 1fs resolution level, in an experiment, where atomic coherence is induced in a rich manifold of doubly excited and inner-shell excited autoionizing states, the evolution of which is tracked through double ionization.

X-ray emission and burn through of radiatively heated plastic foils have been investigated in a gold cavity heated by eight beams of the Gekko XII laser up to a radiation temperature of 140 eV. We measured the temporally resolved reemission spectra of carbon. The time and frequency integrated reemission was 23% of the gold reemission. The measurements are in good agreement with numerical simulations.

Intense thermal x-rays with brightness temperatures in the range 50–100 eV have been generated in laser-irradiated high- Z targets such as plane thin converter foils or spherical micro-cavities. These x-ray sources were used to irradiate various absorber foils consisting of low- Z matter (Be, B, C and Al). The radiative heating of these materials has been studied by time-resolved spectroscopy at sub-keV photon energies. The radiative burnthrough occurring as a consequence of the transition from the cold solid state to an ionized gas has been observed. The measurements agree well with calculations of the radiation transport, for which electron impact line broadening was found to be essential. Of interest is also smoothing of the K -edge of Be and the L -edge of Al observed in the beginning of the heating when the foil is still dense and in a partially degenerate state. We also present first results on the reemission of carbon foils heated by x-rays generated in a gold micro-cavity.

We report the generation of few-cycle multiterawatt light pulses with a temporal contrast of 10(10), when measured as close as 2 ps to the pulse's peak. Tens of picoseconds before the main pulse, the contrast value is expected to spread much beyond the measurement limit. Separate measurements of contrast improvement factors at different stages of the laser system indicate that real contrast values may reach 10(19) and 10(14), when measured 50 and 25 ps before the pulse's peak, respectively. The combination of the shortest pulse duration and the highest contrast renders our system a promising front-end architecture for future multipetawatt laser facilities.