We present a theoretical investigation on the influence of ion temperature with cross-relaxation (CR) effects upon a collisionally pumped, optical-field-ionized (OFI) soft-X-ray laser (SXRL). Our results indicate that ion heating can induce a significant modification of the SXRL energy. However, whereas the CR rate is large in an OFI plasma, its contribution on the SXRL energy is found to be small.

We present what we believe to be the first measurement of the spectral properties of a soft x-ray laser seeded by a high-order harmonic beam. Using an interferometric method, the spectral profile of a seeded Ni-like krypton soft x-ray laser (32.8 nm) generated by optical field ionization has been experimentally determined, and the shortest possible pulse duration has been deduced. The source exhibits a Voigt spectral profile with an FWHM of 3.1+/-0.3 mA, leading to a Fourier-transform pulse duration of 4.7 ps. This value is comparable with the upper limit of the soft x-ray pulse duration determined by experimentally investigating the gain dynamics, from which we conclude that the source has reached the Fourier limit. The measured bandwidth is in good agreement with the predictions of a radiative transfer code, including gain line narrowing and saturation rebroadening.

It has been recently demonstrated experimentally that seeding a high-harmonic pulse into an Optical-Field-Ionized gas can generate a coherent soft x-ray laser beam of up to 1üJ. In order to analyze the physical processes involved in the amplification of the x-ray laser pulse through the plasma amplifier a 3D numerical code named COFIXE_MB has been developed using a Maxwell-Bloch treatment. It brings detailed information about the x-ray pulse evolution, especially regarding the fast evolution of the pulse temporal profile and the spatial filtering of the wave front structure by the amplifier.

We present in this paper the first measurement of the spectral profile of a seeded soft x-ray laser. Using a varying path diference interferometer the temporal coherence of a seeded OFI x-ray laser has been experimentally determined, leading to a coherence length of 5ps of the order of the gain lifetime. The measured bandwidth is of order 3.4 mÅ which is in good agreement with the prediction of a theoretical model presented here.

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.

We report what is to our knowledge the first demonstration of spatial filtering of a high-order harmonic beam into a soft-x-ray laser plasma amplifier at 32.8 nm. After amplification the seed energy is enhanced by a factor of 50, and the beam profile of the amplified beam exhibits an Airy-like shape due to the spatial filtering by the optical field ionized plasma. Moreover, the transverse coherence of the spatially filtered amplified beam is strongly enhanced, resulting in the generation of a peak coherent power of 0.9 x 10(5) to 1.8 x 10(5) W.

The evolution with time of the ion temperature of a xenon gas during and after the irradiation with a high intensity laser beam is determined. The processes taken into account are (i) direct interaction with the laser beam, (ii) binary collisions with hot electrons, (iii) contribution with the electrostatic wakefield, (iv) hydrodynamic expansion and (v) transfer of potential to kinetic energy among the plasma ions. It is shown that, because the ions are strongly coupled, the last process is by far the most efficient one. Using the One Component Plasma (OCP) model, a simple analytical formula for the ion temperature at equilibrium is obtained. Application of this work to the development of soft x‐ray laser sources is stressed out. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)