A capillary gas cell for laser wakefield acceleration was developed with the aid of three-dimensional computational fluid dynamics simulations. The gas cell was specially designed to provide upward density tapering in the longitudinal direction, which is expected to suppress the dephasing problem in laser wakefield acceleration by keeping the accelerated electrons in the acceleration phase of the wake wave. The density-tapered capillary gas cell was fabricated by sapphire plates, and its performance characteristics were tested. The capillary gas cell was filled with a few hundred millibars of hydrogen gas, and a Ti:sapphire laser pulse with a peak power of 3.8 TW and a pulse duration of 40 fs (full width at half maximum) was sent through the capillary hole, which has a length of 7 mm and a square cross section of 350 × 350 µm2. The laser-produced hydrogen plasma in the capillary hole was then diagnosed two-dimensionally by using a transverse Mach–Zehnder interferometer. The capillary gas cell was found to provide an upward plasma density tapering in the range of 1018 cm−3–1019 cm−3, which has a potential to enhance the electron beam energy in laser wakefield acceleration experiments.
Three soil aquifer treatment (SAT) columns were operated for five months to investigate the dependence of the treated water quality upon the depth of unsaturated vadose zone in terms of its organics, nitrogen and phosphorus contents. It was found that DOC removal was independent of the unsaturated vadose zone depth except during the initial flooding period when the effluent was injected into each column. From tests to study the nitrate profile, it was found that the pattern of nitrification was dependent on the depth of the unsaturated zone during the initial periods of SAT flooding. However, no further nitrification occurred at the end of the flooding since the injection of the effluent continued to provide a high loading concentration to the SAT columns. The lack of phosphate removal in this study indicates that its removal is more dependent upon the soils and aquifer characteristics than on the depth of unsaturated zone.
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