Deposits of clastic carbonate-dominated (calciclastic) sedimentary slope systems in the rock record have been identified mostly as linearly-consistent carbonate apron deposits, even though most ancient clastic carbonate slope deposits fit the submarine fan systems better. Calciclastic submarine fans are consequently rarely described and are poorly understood. Subsequently, very little is known especially in mud-dominated calciclastic submarine fan systems. Presented in this study are a sedimentological core and petrographic characterisation of samples from eleven boreholes from the Lower Carboniferous of Bowland Basin (Northwest England) that reveals a >250 m thick calciturbidite complex deposited in a calciclastic submarine fan setting. Seven facies are recognised from core and thin section characterisation and are grouped into three carbonate turbidite sequences. They include: 1) Calciturbidites, comprising mostly of highto low-density, wavy-laminated bioclast-rich facies; 2) low-density densite mudstones which are characterised by planar laminated and unlaminated muddominated facies; and 3) Calcidebrites which are muddy or hyper-concentrated debrisflow deposits occurring as poorly-sorted, chaotic, mud-supported floatstones. These

Phd by thesis

We show that perfect absorption can be achieved in a system comprising a single lossy dielectric layer of thickness much smaller than the incident wavelength on an opaque substrate by utilizing the nontrivial phase shifts at interfaces between lossy media. This design is implemented with an ultra-thin (∼λ/65) vanadium dioxide (VO2) layer on sapphire, temperature tuned in the vicinity of the VO2 insulator-to-metal phase transition, leading to 99.75% absorption at λ = 11.6 μm. The structural simplicity and large tuning range (from ∼80% to 0.25% in reflectivity) are promising for thermal emitters, modulators, and bolometers.

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Ultra-thin perfect absorber employing a tunable phase change material

Homeland security and military defense technology considerations have stimulated intense interest in mobile, high power sources of millimeter-wave (mmw) to terahertz (THz) regime electromagnetic radiation, from 0.1 to 10THz. While vacuum electronic sources are a natural choice for high power, the challenges have yet to be completely met for applications including noninvasive sensing of concealed weapons and dangerous agents, high-data-rate communications, high resolution radar, next generation acceleration drivers, and analysis of fluids and condensed matter. The compact size requirements for many of these high frequency sources require miniscule, microfabricated slow wave circuits. This necessitates electron beams with tiny transverse dimensions and potentially very high current densities for adequate gain. Thus, an emerging family of microfabricated, vacuum electronic devices share many of the same plasma physics challenges that are currently confronting “classic” high power microwave (HPM) generators including long-life bright electron beam sources, intense beam transport, parasitic mode excitation, energetic electron interaction with surfaces, and rf air breakdown at output windows. The contemporary plasma physics and other related issues of compact, high power mmw-to-THz sources are compared and contrasted to those of HPM generation, and future research challenges and opportunities are discussed.

Plasma physics and related challenges of millimeter-wave-to-terahertz and high power microwave generationa)

Radar-absorbing materials are used in stealth technologies for concealment of an object from radar detection. Resistive and/or magnetic composite materials are used to reduce the backscattered microwave signals. Inability to control electrical properties of these materials, however, hinders the realization of active camouflage systems. Here, using large-area graphene electrodes, we demonstrate active surfaces that enable electrical control of reflection, transmission and absorption of microwaves. Instead of tuning bulk material property, our strategy relies on electrostatic tuning of the charge density on an atomically thin electrode, which operates as a tunable metal in microwave frequencies. Notably, we report large-area adaptive radar-absorbing surfaces with tunable reflection suppression ratio up to 50 dB with operation voltages <5 V. Using the developed surfaces, we demonstrate various device architectures including pixelated and curved surfaces. Our results provide a significant step in realization of active camouflage systems in microwave frequencies.

/pdf/graphene-enabled-electrically-switchable-radar-absorbing-25srtj182y.pdf

Graphene-enabled electrically switchable radar-absorbing surfaces

/pdf/graphene-enabled-electrically-switchable-radar-absorbing-k7cvid4dx2.pdf

Graphene-enabled electrically switchable radar absorbing surfaces

Multipactor is a discharge phenomenon common to radiofrequency vacuum electronic systems [1] . Transmission lines that are undergoing multipactor, which is driven by secondary electron emission, are prone to serious negative consequences. Multipactor disrupts normal signal transmission, leading to attenuation and degradation of the drive signal [1] , [2] . In extreme cases, multipactor can even lead to localized heating and catastrophic failure of the device. These severe consequences require innovation in suppressing multipactor discharge.

Multipactor Suppression in S-band Coaxial Transmission Lines

A dual recirculating planar crossed-field amplifier (RPCFA) has been designed at the University of Michigan. The Dual RPCFA features two slow-wave structures (SWS) in a single anode housing. Using CST particle studio and ANSYS HFSS, the Dual RPCFA is shown to have an increased effective bandwidth and operating gain when compared to the single-sided RPCFA [1] .

Dual Recirculating Planar Crossed-Field Amplifier Design

Summary form only given. The highly enhanced electric field surrounding the microtips on a field emission cathode can induce electron emission by quantum tunneling. Cathodes which generate large currents, such as those used in high power devices, drive a large current through a small microtip. In this paper we examine the Joule-heating of these microtips and their subsequent thermal expansion. Where the base of these heated tips join the cool substrate below, temperature gradients can cause tensile and shearing stresses, which in certain situations may lead to fracture of the microtips. Estimates of the magnitude of these thermomechanical stresses are calculated and compared for examples of micron scale carbon/graphite fiber and carbon nanotube (CNT) field emitters subjected to microsecond pulses of intense electron emission. The work function (phi) of materials utilized in field emission cathodes was investigated computationally using ab-initio quantum mechanical modeling with the Vienna ab-initio simulation package (VASP). This approach enables the detailed and self-consistent treatment of a system based, on quantum mechanical principles, yielding extremely accurate information pertaining to its electronic properties. New insights for the CsI and CsI/graphite system have been obtained and correlated with experimental results obtained from the surface characterization of CsI films thermally evaporated onto graphite substrates. The deposition and growth of thin CsI films on substrates by thermal evaporation comprises a promising first step toward the growth of thin CsI layers on high aspect ratio, highly-dense micropost assemblies fabricated from Si templates

Characterization of material performance of carbon-based field emitters

Summary form only given. Recent experiments have proven that imposition of an azimuthally-varying axial magnetic field to microwave oven magnetrons causes a significant (-30 dB near carrier) decrease in the close-in noise, elimination of sidebands and faster startup. Noise reduction is observed at low, medium and high currents, for fresh and aged, 10-vane, double-strapped magnetrons produced by various manufacturers. This noise reduction technique perturbs the magnetic field by adding a number of permanent magnets to the outside of one of standard circular magnets of the magnetron. The magnitude of the axial magnetic field perturbation was up to 50%, measured at the outside of the cavity structure. Experiments were performed utilizing Panasonic 2M167A-M10 and Toshiba 2M172 J(E) magnetrons, with a low-ripple DC power supply at parameters: voltage about -4 kV, tube current up to 300 mA, and power levels up to 800 W. Several magnetic perturbation arrangements were experimentally and computationally studied. For an N-cavity magnetron operating in the pi-mode, optimal "magnetic priming" for rapid startup employs N/2 azimuthal magnetic variations, as the electrons are kinematically-pre-bunched in N/2 spokes within just one Larmor period. A highly idealized model of magnetic priming revealed a parametric instability, which favors a radial expansion of electrons from the cathode to the anode, in the absence of rf fields and electrostatic effects.

http://ieeexplore.ieee.org/iel5/9291/29525/01339810.pdf

Noise reduction and magnetic priming for kW magnetrons by azimuthally varying axial magnetic fields

Summary form only given. Dielectric window breakdown remains a significant issue for high power microwave systems. Using a particle-in-cell (PIC) model with Monte Carlo collisions, we investigate the transition of dielectric window breakdown from a vacuum multipactor discharge to a collisional microwave discharge in a number of noble gases. At low pressure, the dominant mechanism of electron creation is the single-surface multipactor, and the mean energy of the electron population is hundreds of eV. As pressure increases to 10-50 torr, an intermediate regime is obtained in which electrons generated in volumetric ionization compete with the multipactor electrons generated at the dielectric window surface. In this regime, the mean energy declines significantly, and we observe two distinct electron populations: a surface population participating in the multipactor process, and a detached population shielded from the surface fields by ions. Approaching atmospheric pressure, volumetric ionization dominates, and the multipactor process is extinguished as the electron mean energy drops to a few eV. In this collisional regime, the nearly neutral discharge detaches from the dielectric window surface, and the surface charge and field that drives electrons into the window is also eliminated. The electron energy probability function changes from a bi- Maxwellian at low pressure to a higher order kinetic form at high pressure. The time to achieve breakdown is described across a broad range of pressures for Ne, Ar, and Xe, and a general analytic scaling law was deduced. Using the electron generation rates from the kinetic PIC model, we developed a global model with an enhanced electron energy distribution function form along with an energy equation which matches the kinetic results well across a broad range of pressure. We are now extending that model to air, a broader range of frequency and microwave power levels.

Y.Y. Lau

Papers

Multipactor Suppression in S-band Coaxial Transmission Lines

Dual Recirculating Planar Crossed-Field Amplifier Design

Characterization of material performance of carbon-based field emitters

Noise reduction and magnetic priming for kW magnetrons by azimuthally varying axial magnetic fields

Advances in modeling microwave window breakdown