A. Djaoui

Lasing properties of the J = 0–1 and the J = 2–1 lines of a neonlike germanium soft-x-ray laser

Lasing properties of a collisional-excitation Ne-like Ge soft-x-ray laser have been studied with exploding-foil, single-slab, and double-slab targets under identical pumping conditions. Experimental results for the angular intensity distributions and the temporal variations of the lasing intensities are examined with a hydrodynamic code and ray-trace calculations. The observed angular distributions are well reproduced by these analyses, and it is found that the effective gain regions are located on the high-density side of the expected gain regions. It is shown that the observed lasing intensity of the J = 0 to J = 1 line is strongly correlated with the temporal change of the calculated electron temperature for both the slab and the exploding-foil targets.

Improvements to ion-correlation experiments in dense plasmas

Improved measurements of ion-correlation effects in a dense shock-compressed plasma are presented. The extended X-ray-absorption fine-structure (EXAFS) technique on the aluminum K edge is used to observe the short-range order within a dense plasma. Densities of about three times solid density were measured with good accuracy. The experimental measurements of density give results that are in good agreement with the MEDUSA onedimensional fluid code predictions. The improved quality of the data enabled us to calculate the ion coupling parameter during the compression and the subsequent heating of the plasma. An estimation of the temperature is given on the basis of published models, and an approximate agreement is obtained with the MEDUSA code predictions.

Progress with saturated soft X-ray lasers pumped by the Vulcan laser

Ne-like and Ni-like ions have been pumped in the transient gain regime using intense picosecond pump pulses from Vulcan in its CPA mode. High gain coefficients of at least approximately 30/cm are observed for the Ne-like ions Ti XIII, Ge XXIII and Sn XXIII at wavelengths 31.2, 19.6 and 12.0 nm respectively and approximately 20/cm for the Ni-like ion Sm XXXV at 7.3 nm. Saturated output is found in all cases for target lengths shorter than 10 mm and the effect of traveling wave pumping has been studied and unequivocably demonstrated. An experimental campaign to observe four wave mixing using a soft x-ray laser and an optical laser in a sum-difference frequency mixing scheme has been initiated. Preliminary results are described and future directions discussed.

ICF target ignition studies in planar, cylindrical, and spherical geometries

A model for nonlocal a particle transport is implemented in a one-dimensional radiation hydrodynamics code and applied for typical directly and indirectly driven ICF target simulations. Ignition criteria are compared for four configurations; spherical, cylindrical, planar with a central hot spot, and another planar configuration with a hot spot on one side. This last planar configuration is then used for fast ignitor studies, since it is able to simulate hot spot formation by direct laser heating and free expansion towards the incoming beam. Ignition requirements as a function of laser intensity and pulse length, taking into account the relationship between intensity, hot electron temperature, and range, are then determined.

Optical ionization and heating of gases by intense picosecond KrF laser radiation

We report on multiphoton ionization experiments using picosecond (ps) and sub-ps UV-laser radiation at focused intensities up to 10 18 W/cm 2 . The experiments are concerned with determining the electron temperature of optically ionized gases produced by intense KrF lasers. Thomson scattering, stimulated Raman scattering (SRS), and X-ray emission measurements have been made and compared with modeling calculations of heating. A particular objective is to identify the respective roles of above-threshold ionization, nonlinear inverse bremsstrahlung absorption, and SRS in determining the temperature of the electrons. Results for 350-fs pulses are compared with previous measurements for 12-ps pulses (for which strikingly different behavior is observed). The importance of using sub-ps, short-wavelength lasers to minimize electron temperature is confirmed.

Measurement of the photo-pump strength of the 3d-5f transitions in the automatically line matched Ni-like Sm photo-pumped X-ray laser

The photo-pump strengths of both the ((3d4)0(3d6)0)0 - (((3d3)3/2(3d6)0)3/2(5f)5/2)1 and the ((3d4)0(3d6)0)0 - (((3d4)0(3d5)5/2)5/2(5f)7/2)1 transitions in Ni-like Sm34+ have been measured as 2.4*10(-4) and 2.4*10(-4) photons/mode respectively. The implications of the measurement are briefly discussed in a comparison of the merits of automatically line matched photo-pump scheme to those of the collisional excitation Ni-like Sm(+34) scheme.

Effects of self-generated electric and magnetic fields in laser-generated fast electron propagation in solid materials: Electric inhibition and beam pinching

We present some experimental results which demonstrate the presence of electric inhibition in the propagation of relativistic electrons generated by intense laser pulses, depending on target conductivity. The use of transparent targets and shadowgraphic techniques has made it possible to evidence electron jets moving at the speed of light, an indication of the presence of self-generated strong magnetic fields.

Propagation in compressed matter of hot electrons created by short intense lasers

We performed the first experimental study of propagation in compressed matter of hot electrons created by a short pulse intense laser. The experiment has been carried out with the VULCAN laser at Rutherford compressing plastic targets with two ns laser beams at an intensity ⩾1014u2009W/cm2. A CPA beam with an intensity ⩾1016u2009W/cm2 irradiated the rear side of the target and created hot electrons propagating through the compressed matter. K‐α emission was used as diagnostics of hot electron penetration by putting a chloride plastic layer inside the target.We performed the first experimental study of propagation in compressed matter of hot electrons created by a short pulse intense laser. The experiment has been carried out with the VULCAN laser at Rutherford compressing plastic targets with two ns laser beams at an intensity ⩾1014u2009W/cm2. A CPA beam with an intensity ⩾1016u2009W/cm2 irradiated the rear side of the target and created hot electrons propagating through the compressed matter. K‐α emission was used as diagnostics of hot electron penetration by putting a chloride plastic layer inside the target.

Refractive compensation in slab target geometries

Results showing the enhancement effect on soft X‐ray lasing emission intensity with target substrate curvature of up to 1 mrad per mm, which compensated for refraction due to electron density gradients present in the gain region of a Ne‐like Ge expanding plasma are shown. Time integrated relative intensity and pointing data are presented for the Ne‐like Ge 19.6, 23.6, and 28.6 nm 3p‐3s transitions. A proposed simple illumination geometry for compensating for refraction due to lateral electron density gradients is also presented.