Estimates of nonequilibrium ionization phenomena in the inviscid Apollo plasma sheath

Nonequilibrium ionization in asymmetric plasma sheath determined for Apollo spacecraft at superorbital velocity reentr

Time-division SQUID multiplexers with reduced sensitivity to external magnetic fields

Time-division SQUID multiplexers are used in many applications that require exquisite control of systematic error. One potential source of systematic error is the pickup of external magnetic fields in the multiplexer. We present measurements of the field sensitivity figure of merit, effective area, for both the first stage and second stage SQUID amplifiers in three NIST SQUID multiplexer designs. These designs include a new variety with improved gradiometry that significantly reduces the effective area of both the first and second stage SQUID amplifiers.Comment: 4 pages, 7 figures. Submitted for publication in the IEEE Transactions on Applied Superconductivity, August 201

Strongly quadrature-dependent noise in superconducting micro-resonators measured at the vacuum-noise limit

We measure frequency- and dissipation-quadrature noise in superconducting lithographed microwave resonators with sensitivity near the vacuum noise level using a Josephson parametric amplifier. At an excitation power of 100~nW, these resonators show significant frequency noise caused by two-level systems. No excess dissipation-quadrature noise (above the vacuum noise) is observed to our measurement sensitivity. These measurements demonstrate that the excess dissipation-quadrature noise is negligible compared to vacuum fluctuations, at typical readout powers used in micro-resonator applications. Our results have important implications for resonant readout of various devices such as detectors, qubits and nano-mechanical oscillators.Comment: 13 pages, 4 figure

Advanced code-division multiplexers for superconducting detector arrays

Multiplexers based on the modulation of superconducting quantum interference devices are now regularly used in multi-kilopixel arrays of superconducting detectors for astrophysics, cosmology, and materials analysis. Over the next decade, much larger arrays will be needed. These larger arrays require new modulation techniques and compact multiplexer elements that fit within each pixel. We present a new in-focal-plane code-division multiplexer that provides multiplexing elements with the required scalability. This code-division multiplexer uses compact lithographic modulation elements that simultaneously multiplex both signal outputs and superconducting transition-edge sensor (TES) detector bias voltages. It eliminates the shunt resistor used to voltage bias TES detectors, greatly reduces power dissipation, allows different dc bias voltages for each TES, and makes all elements sufficiently compact to fit inside the detector pixel area. These in-focal-plane code-division multiplexers can be combined with multi-gigahertz readout based on superconducting microresonators to scale to even larger arrays.Comment: 8 pages, 3 figures, presented at the 14th International Workshop on Low Temperature Detectors, Heidelberg University, August 1-5, 2011, proceedings to be published in the Journal of Low Temperature Physic

High speed imaging and Fourier analysis of the melt plume during close coupled gas atomisation

A high speed digital analysis technique has been used to study the atomisation plume of a superheated sample of Ni–Al in a close coupled gas atomiser. The atomisation, incorporating a generic melt nozzle and die design was captured using a Kodak high speed digital analyser at a frame rate of 18 k frames per second. The resulting 65 536 frames were then analysed using a specially designed routine, which calculates values of optical brightness and position of the intensity maximum for all frames and performs Fourier analysis on the sequence. The data produced from this analysis show that the plume, pulses at low frequencies (<25 Hz) and precesses at higher frequencies (∼360 Hz) around the atomiser's centreline. To aid investigation into the origins of this precession and other phenomena it was decided to conduct further experiments using an analogue system. The analogue atomiser reproduces the important features of the full atomiser but instead of atomising molten metal, the analogue system atomises water, providing a quick and easy way of testing the effects of changing parameters. Using this system it was found that the precession of the melt plume is independent of the atomiser's gas inlet pressure but strongly dependent on both the die and melt nozzle's geometry

Molecular-beam epitaxy of p-type m-plane GaN

We report on the plasma-assisted molecular-beam epitaxy of Mg-doped (10 (1) over bar0) GaN on (10 (1) over bar0) 6H-SiC. Secondary ion mass spectroscopy measurements show the incorporation of Mg into the GaN films with an enhanced Mg incorporation under N-rich conditions relative to Ga-rich growth. Transport measurements of Mg-doped layers grown under Ga-rich conditions show hole concentrations in the range of p=1x10(18) to p=7x10(18) cm(-3) and a dependence between hole concentration and Mg beam equivalent pressure. An anisotropy in in-plane hole mobilities was observed, with the hole mobility parallel to [11 (2) over bar0] being higher than that parallel to [0001] for the same hole concentration. Mobilities parallel to [11 (2) over bar0] were as high as similar to 11.5 cm(2)/Vs (at p similar to 1.8 x 10(18) cm(-3)). (c) 2005 American Institute of Physics

Growth of p-type and n-type m-plane GaN by molecular beam epitaxy

Plasma-assisted molecular beam epitaxial growth of Mg-doped, p-type and Si-doped, n-type m-plane GaN on 6H m-plane SiC is demonstrated. Phase-pure, m-plane GaN films exhibiting a large anisotropy in film mosaic (similar to 0.2 degrees full width at half maximum, x-ray rocking curve scan taken parallel to [112 ($) over bar0] versus similar to 2 degrees parallel to [0001]) were grown on m-plane SiC substrates. Maximum hole concentrations of similar to 7x10(18) cm(-3) were achieved with p-type conductivities as high as similar to 5 Omega(-1) cm(-1) without the presence of Mg-rich inclusions or inversion domains as viewed by cross-section transmission electron microscopy. Temperature dependent Hall effect measurements indicate that the Mg-related acceptor state in m-plane GaN is the same as that exhibited in c-plane GaN. Free electron concentrations as high as similar to 4x10(18) cm(-3) were measured in the Si-doped m-plane GaN with corresponding mobilities of similar to 500 cm(2)/V s measured parallel to the [112 ($) over bar0] direction. (c) 2006 American Institute of Physics

Numerical and Experimental Investigations of the Effect of Melt Delivery Nozzle Design on the Open- to Closed-Wake Transition in Closed-Coupled Gas Atomization

The single-phase gas-flow behavior of a closed-coupled gas atomization was investigated with four different melt nozzle tip designs with two types of gas die. Particular attention was paid to the open- to closed-wake transition. Experimental Schlieren imaging and numerical modeling techniques were employed, with good agreement between the two being found in relation to the wake closure pressure. It was found that the melt nozzle tip design had a significant impact on the WCP, as did the type of die used, with a convergent–divergent gas die giving significantly high WCPs