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  • Author: A. Bhattacharya, x
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in Eos

Abstract

A 3D model of dispenser cathode with toroid shape heater assembly is simulated using simulation software, ANSYS Multi-physics. The reported design study of cathode heater assembly of 1 MW 120 GHz gyrotron helps to optimize the input heater power with respect to cathode surface temperature. The simulation study shows that the input power depends strongly on the heater dimension as well as material properties including the potting material. The optimum input power helps to achieve desired current density (10 A/cm2) and cathode surface temperature (1000 °C). Further, the thermal and structural analyses are carried out to study the temperature distribution on the cathode assembly due to the heat dissipation and mechanical strength of the assembly.

Summary

Heavy ion Rutherford backscattering spectrometry, with 30 MeV 19F beam, has been used to study the diffusion of cesium in borosilicate glasses of different compositions. The coefficient of cesium diffusion in borosilicate glass was obtained by analyzing the depth profile of cesium in samples annealed at different temperatures in the range of 100-350 °C. The activation energy of diffusion of cesium was found to be in the range of 9-22 kJ/mol, indicating the inter-diffusion between the cesium and sodium as the mechanism of the diffusion process. The activation energy was found to decrease with increasing sodium content of the glass, indicating that the alkali metal ion diffusion is governed by the interaction between the jumping ion and the dipole field around the oxygen ions.

Abstract

Mineralogical-textural analysis of a multi-layered plagioclase (Pl) + orthopyroxene/biotite (Opx/Bt) vermicular microstructure around garnet from the Bolangir anorthosite massif is presented. X-ray images demonstrate the garnet was zoned prior to decomposition, with Mg decreasing toward its rim, and Mn, Ca, and XFe [= Fe/(Fe + Mg)] increasing. On decomposition, the Ca zonations in garnet were truncated by the consuming garnet margin. By implication, the retreat velocity of the garnet margin was faster than the diffusive velocity for Ca within the garnet. By contrast, the pre-decomposition Fe, Mn, and Mg variations in garnet were considerably modified adjacent to Opx + Pl, but were unaffected adjacent to Bt + Pl intergrowths. In the corona, XFe Opx and XFe Bt decrease toward garnet, whereas the An content in associated Pl increases from An50 to An80-83. Thermo-barometry involving nearest neighbor Opx, Grt, Pl, and Qtz indicate that the corona formed at 6.0 ± 1.0 kbar, 750 ± 50 °C. At the isothermal-isobaric condition of decomposition, the compositional variation across the Bt + Pl corona is explained best by the inward retreat of the garnet margin at a velocity faster than the fastest diffusing element (Fe, Mn). In zones where garnet decomposed to Opx + Pl, the velocity of the consuming garnet margin was such that the intra-garnet Fe, Mn redistribution was aborted in transit.

The compositions and volume proportions of vermicular orthopyroxene and plagioclase in the multi-layered corona are best approximated by the NCFMAS reaction Grtss + Pl1 (± Qtz) → Opx + Pl2. The early formed/matrix plagioclase (Pl1) and inward-retreating garnet (Grtss) margin provided the necessary components for successive Opx + Pl2 layers to grow at Grt-Pl1 interfaces. Tie-line topology in ACFN space indicates Pl2 to be An-rich relative to Pl1. The mineralogical relations indicate the CFMAS reaction, Grtss + Qtz → Opx + An, to be an inadequate analogue of the actual decomposition. Consequently, to assume that the texture indicates near-isothermal decompression across the low-dP/dT CFMAS reaction is possibly an oversimplification.

Abstract

The reduction of the dark current without reducing the photocurrent is a considerable challenge in developing far-infrared (FIR)/terahertz detectors. Since quantum dot (QD) based detectors inherently show low dark current, a QD-based structure is an appropriate choice for terahertz detectors. The work reported here discusses multi-band tunnelling quantum dot infrared photo detector (T-QDIP) structures designed for high temperature operation covering the range from mid-to far-infrared. These structures grown by molecular beam epitaxy consist of a QD (InGaAs or InAlAs) placed in a well (GaAs/AlGaAs) with a double-barrier system (AlGaAs/InGaAs/AlGaAs) adjacent to it. The photocurrent, which can be selectively collected by resonant tunnelling, is generated by a transition of carriers from the ground state in the QD to a state in the well coupled with a state in the double-barrier system. The double-barrier system blocks the majority of carriers contributing to the dark current. Several important properties of T-QDIP detectors such as the multi-colour (multi-band) nature of the photoresponse, the selectivity of the operating wavelength by the applied bias, and the polarization sensitivity of the response peaks, are also discussed.