Gasanalyse

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Gasanalyse, Untersuchung der Edelgasse

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Superconductivity of Annealed and Consolidated Amorphous YNi2B2C Powders

Amorphous YNi₂B₂C powders have been produced by mechanical attrition. Superconductivity is absent in the amorphous state but recovers upon crystallization. However, the superconducting transition temperature TC is lower than that of the melted analogue. TC can be increased by increasing the annealing temperature. High temperature compaction leads to dense bulk superconducting samples. TC of these bulk samples can be optimised by annealing. Variations in TC are attributed to deviations from stoichiometry. Contamination of the powders with WC wear debris upon annealing may deteriorate the superconducting properties.     

Spectroscopic and laser properties of Cr3+:Yb3+:Er3+: fluoride phosphate glass

Sensitization of the fluorescence of Er³⁺ in fluoride phosphate glass containing up to 20 mol% phosphates by codoping with Cr³⁺ and Yb³⁺ is shown. The low order of ligand field strength for Cr³⁺ (Dq/B=2.04) results in broad Cr³⁺ fluorescence overlapping the Yb³⁺ absorption. The electronic energy transfer efficiency approaches 100%. Deviations of donor decay from the Förster law are interpreted in terms of the inhomogeneously acceptor distribution. The electronic energy transfer efficiency of Yb³⁺ → Er³⁺ reaches a maximum value of 75% for glasses containing 20 mol% phosphates. The transfer is shown to be migrationally accelerated by means of GAF-LAF-FB theory. From Judd-Ofelt parameters a stimulated emission cross-section for the transition⁴I_13/2 →⁴I_15/2 of Er³⁺ of 6.2×10⁻²⁰ cm² is derived. The c.w. laser action of Er³⁺ by Cr³⁺ excitation and double-step energy transfer is shown. The output is tuned continuously from 1536 to 1596 nm. Flashlamp pumping is also shown.

     

Time resolved excited state absorption of Mn

Excited state absorption (ESA) of Mn²⁺ in spinel MgAl₂O₄ is investigated in detail. For excitation ps-pulses of 351 nm wavelength from a tripled Nd phosphate glass laser are used. During relaxation down to the lowest excited state ESA is measured for various delay times between exciting pulse and probe pulse. The resulting absorption spectra are interpreted in terms of the Mn²⁺ configurational model.     

Experimental and theoretical study of density,potential, and current structures of a helium plasma in front of an radio frequency antenna tilted with respect to the magnetic field lines

Potential and density structures in the vicinity of an radio frequency (RF) electrode/antenna in a magnetized plasma are investigated using an RF-compensated cylindrical Langmuir probe. These measurements were performed in the ALINE plasma device in which only electrons can be considered well magnetized. Very precise 2-D maps of the plasma parameters are drawn thanks to a 3-D automatic manipulator on which the probe is mounted. The effect of the tilted magnetic angle between the RF-biased surface and the magnetic lines is also studied thanks to a tilting electrode. Comparison of several simplistic models with the experiments proved the reliability of simple Langmuir probe measurements in such an RF and magnetized environment (space potential vs. tilting angle of the antenna with respect to magnetic field lines and recovery of the floating potential structure using measured currents). A fluid model based on total current density and ion diffusion equations over the biased flux tube provides the same density structures in front of the electrode as the measurements. Those density structures display a “bunny ears” shape and can be explained using transverse RF and collisional current behaviour: In front of the antenna, the transverse ion currents deplete the magnetized flux tube, while at the edge of the biased flux tube, the same currents increase the density.     

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