Using a combination of B-spline-based model potential techniques with R-matrix Floquet theory, we have carried out single-photon and two-photon ionization calculations of Ca. Good quantitative agreement is obtained with experiment for the single-photon ionization cross-section above the first threshold. We investigate the intensity dependence of the photoionization spectra, and also illustrate how laser-induced degenerate states affect it. Inclusion of the spatial and temporal profile of the laser pulse is examined. The two-photon ionization spectra, below the 3d2 De threshold, show a clear perturbation due to the interloping 4p21 Se state.

We describe the first demonstration of a collisionally excited optical-field-ionization laser driven within a waveguide. Lasing on the 4d(9)5d-4d(9)5p transition at 41.8 nm in Xe8+ was observed to be closely correlated to conditions under which the pump laser pulses were guided well by a gas-filled capillary discharge waveguide. Simulations of the propagation of the pump laser radiation show that gain was achieved over essentially the whole 30 mm length of the waveguide.

We present the results of R-matrix Floquet calculations for generalized two- and three-photon ionization cross sections of neon and argon in the XUV region. Photoionization is strongly enhanced by singly- and doubly-excited resonances. We show that interference effects between different Rydberg states can strongly modify the photoionization cross sections. The observation of three- and four-photon resonances in the two- and three-photon spectra, respectively, demonstrates the influence from higher-order processes. The cross sections may be used to estimate the intensity attained by harmonic radiation in the XUV region.

Edge localized mode (ELM) mitigation experiments have been carried out at JET in type-I ELMy H-mode plasmas. ELM mitigation was achieved through a stochastic edge induced by the ex-vessel error field correction coils system and was found to be independent of the distance of the plasma edge to the JET wall. In all cases the detrimental effect of the ELM on pedestal confinement was considerably reduced during the stochastization phase. The new high resolution Thomson scattering (HRTS) diagnostic, which has recently become operational at JET, is used to study the behaviour of electron temperature, density and pressure pedestal profiles during the mitigation phase. The pedestal properties are monitored in a consistent way by applying a hyperbolic tangent fit to the HRTS measurements. Stability modelling with the ELITE code using the fits to the spatially resolved pedestal profile measurements suggests that the observed ELM mitigation can be understood in terms of the peeling–ballooning model. The mitigation correlates with a reduction of the edge pressure gradient (due to both a reduced height and an increased width of the edge pressure transport barrier) which is consistent with the linear ELM stability theory. It is observed that in the case of an n = 1 external field, the barrier position and height are correlated with the toroidal direction of the applied perturbation. Agreement is found between the barrier position measured by HRTS and obtained from plasma equilibrium reconstruction. The external perturbation induces a deformation of the plasma column that is primarily toroidally symmetric with a secondary component that is toroidally asymmetric

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International audienceWe report a significant increase of the output of a 41.8-nm Xe^8+ laser achieved by means of multimode guiding of high-intensity femtosecond laser pulses in a gas-filled dielectric capillary tube. The optimized lasing signal from a 15-mm-long capillary was nearly an order of magnitude higher than that from a gas cell of the same length. Simulations of the propagation of the pump laser pulse in the capillary confirmed that this enhancement is due to reflections from the capillary wall, which increase the length of the Xe^8+ plasma column generated. The influence of gas pressure and focusing position on the lasing is also presented

International audienceWe give on overview of recent advances in collisionally pumped optical field-ionization soft X-ray lasers developed at LOA. Saturated amplification has been achieved on the 5d-5p transition in Xe^8+ at 41.8 nm, and on the 4d-4p transition in Kr^8+ at 32.8 nm. We demonstrate a significant increase of the energy output from the Xe^8+ laser driven within two types of wave-guide. Finally, we present results of a pioneering work aimed to set up and characterize the first true soft X-ray laser chain

International audienceWe present the status of optical field ionization soft X-ray lasers. The amplifying medium is generated by focusing a high-energy circularly polarized 30-fs 10-Hz Ti: sapphire laser system in a gaseous medium. Using xenon or krypton, strong laser emission at 41.8 and 32.8 nm, respectively, has been observed. After presenting the basis of the physics, we present recent characterization of the sources as well as dramatic improvement of their performances using the waveguiding technique