Modelling and Control of an Annular Momentum Control Device

The results of a modelling and control study for an advanced momentum storage device supported on magnetic bearings are documented. The control challenge posed by this device lies in its dynamics being such a strong function of flywheel rotational speed. At high rotational speed, this can lead to open loop instabilities, resulting in requirements for minimum and maximum control bandwidths and gains for the stabilizing controllers. Using recently developed analysis tools for systems described by complex coefficient differential equations, the closed properties of the controllers were analyzed and stability properties established. Various feedback controllers are investigated and discussed. Both translational and angular dynamics compensators are developed, and measures of system stability and robustness to plant and operational speed variations are presented

Synchronous response modelling and control of an annular momentum control device

Research on the synchronous response modelling and control of an advanced Annular Momentun Control Device (AMCD) used to control the attitude of a spacecraft is described. For the flexible rotor AMCD, two sources of synchronous vibrations were identified. One source, which corresponds to the mass unbalance problem of rigid rotors suspended in conventional bearings, is caused by measurement errors of the rotor center of mass position. The other sources of synchronous vibrations is misalignment between the hub and flywheel masses of the AMCD. Four different control algorithms were examined. These were lead-lag compensators that mimic conventional bearing dynamics, tracking notch filters used in the feedback loop, tracking differential-notch filters, and model-based compensators. The tracking differential-notch filters were shown to have a number of advantages over more conventional approaches for both rigid-body rotor applications and flexible rotor applications such as the AMCD. Hardware implementation schemes for the tracking differential-notch filter were investigated. A simple design was developed that can be implemented with analog multipliers and low bandwidth, digital hardware

Alaskan Submarine Cables: A Struggle with a Harsh Environment

Reviews the laying, repair and maintenance of telegraph and cable systems, at one time 86 submarine cables, between Puget Sound and Alaska 1901-60. Failures due to entanglement by whales, bruising and mauling by anchors and fishing trawls and various destructive effects of geologic agents on the sea floor are noted. In straits on the continental shelf and upper continental slope, cable failures are attributed primarily to chafe by bottom currents and, off the mouths of major streams, to turbidity currents and gravitational slides. Cable repair data for 1903-58 are tabulated, showing reported causes. Some cables at depths of 1000-1500 fathoms on the continental margin cross at least 40 major canyons, but have never failed because of turbidity currents; this indicates a lapse of 2000, possibly 5000 yr since such currents occurred in this region.Les câbles sous-marins en Alaska, une lutte contre un milieu difficile. A un moment donné, le système de communications de l'Alaska comprenait 86 câbles télégraphiques sous-marins, s'étendant du Puget Sound vers le nord à travers l'Alaska côtier, puis vers l'ouest jusqu'à l'arc des Aléoutiennes. Les pannes de ces câbles sont révélatrices des forces dynamiques naturelles qui affectent le fond des mers. Dans les détroits du plateau continental et de la partie supérieure du talus continental, les bris s'expliquent par le frottement dû aux courants profonds : à l'embouchure des grands cours d'eau, ils sont provoqués par les courants de turbidité et les glissements sous-marins

The HI Chronicles of LITTLE THINGS BCDs II: The Origin of IC 10's HI Structure

In this paper we analyze Very Large Array (VLA) telescope and Green Bank Telescope (GBT) atomic hydrogen (HI) data for the LITTLE THINGS(1) blue compact dwarf galaxy IC 10. The VLA data allow us to study the detailed HI kinematics and morphology of IC 10 at high resolution while the GBT data allow us to search the surrounding area at high sensitivity for tenuous HI. IC 10's HI appears highly disturbed in both the VLA and GBT HI maps with a kinematically distinct northern HI extension, a kinematically distinct southern plume, and several spurs in the VLA data that do not follow the general kinematics of the main disk. We discuss three possible origins of its HI structure and kinematics in detail: a current interaction with a nearby companion, an advanced merger, and accretion of intergalactic medium. We find that IC 10 is most likely an advanced merger or a galaxy undergoing accretion. 1:Local Irregulars That Trace Luminosity Extremes, The HI Nearby Galaxy Survey; https://science.nrao.edu/science/surveys/littlethingsComment: 36 pages, 17 figures, accepted for publication in The Astronomical Journa

Measurements of the effect of collisions on transverse beam halo diffusion in the Tevatron and in the LHC

Beam-beam forces and collision optics can strongly affect beam lifetime, dynamic aperture, and halo formation in particle colliders. Extensive analytical and numerical simulations are carried out in the design and operational stage of a machine to quantify these effects, but experimental data is scarce. The technique of small-step collimator scans was applied to the Fermilab Tevatron collider and to the CERN Large Hadron Collider to study the effect of collisions on transverse beam halo dynamics. We describe the technique and present a summary of the first results on the dependence of the halo diffusion coefficient on betatron amplitude in the Tevatron and in the LHC.Comment: 4 pages, 2 figures. Submitted to the Proceedings of the ICFA Mini-Workshop on Beam-beam Effects in Hadron Colliders (BB2013), Geneva, Switzerland, 18-22 March 201

Kinetics of lithium peroxide oxidation by redox mediators and consequences for the lithium–oxygen cell

Lithium–oxygen cells in which lithium peroxide forms in solution rather than on the electrode surface, can sustain relatively high cycling rates but require redox mediators to charge. The mediators are oxidised at the electrode surface and then oxidise lithium peroxide stored in the cathode. The kinetics of lithium peroxide oxidation has received almost no attention and yet is crucial for operation of the lithium–oxygen cell. It is essential that the molecules oxidise lithium peroxide sufficiently rapidly to sustain fast charging. Here we investigate the kinetics of lithium peroxide oxidation by several different classes of redox mediators. We show that the reaction is not a simple outer–sphere electron transfer and that the steric structure of the mediator molecule plays an important role. The fastest mediator studied here could sustain charging current of up to 1.9 A cm–2, based on a model for a porous electrode described here. Lithium-oxygen cells in which the cathode reaction of lithium peroxide formation takes place in solution rather than on the electrode surface, can sustain relatively high cycling rates but require redox mediators to oxidise it. The mediators are oxidised at the electrode surface and then oxidise lithium peroxide particles in the pores of the cathode that are disconnected from the surface. The kinetics of lithium peroxide oxidation has received almost no attention and yet is crucial for operation of the lithium-oxygen cell. While molecules with fast electron transfer at the electrode surface are common, it is also essential that the molecules oxidise lithium peroxide sufficiently rapidly to sustain relatively fast charging. Here we investigate the kinetics of lithium peroxide oxidation by several classes of redox mediators, with varying electrochemical and structural properties (amines, nitroxy and thiol compounds). The rates range from 0.025 to 7.9 x10—3 cm s—1 with the nitroxy compounds exhibiting the highest rates. We show that the reaction is not a simple outer sphere electron transfer and that the nature of the mediator molecule plays an important role for example the steric hindrance around the active redox centre of the mediator. The fastest mediator studied here could sustain an areal current density on charging of up to 1.9 A cm—2, based on a model for a porous electrode described in the paper

Development of magnetostrictive active members for control of space structures

The goal of this Phase 2 Small Business Innovative Research (SBIR) project was to determine the technical feasibility of developing magnetostrictive active members for use as truss elements in space structures. Active members control elastic vibrations of truss-based space structures and integrate the functions of truss structure element, actively controlled actuator, and sensor. The active members must control structural motion to the sub-micron level and, for many proposed space applications, work at cryogenic temperatures. Under this program both room temperature and cryogenic temperature magnetostrictive active members were designed, fabricated, and tested. The results of these performance tests indicated that room temperature magnetostrictive actuators feature higher strain, stiffness, and force capability with lower amplifier requirements than similarly sized piezoelectric or electrostrictive active members, at the cost of higher mass. Two different cryogenic temperature magnetostrictive materials were tested at liquid nitrogen temperatures, both with larger strain capability than the room temperature magnetostrictive materials. The cryogenic active member development included the design and fabrication of a cryostat that allows operation of the cryogenic active member in a space structure testbed

Beam halo dynamics and control with hollow electron beams

Experimental measurements of beam halo diffusion dynamics with collimator scans are reviewed. The concept of halo control with a hollow electron beam collimator, its demonstration at the Tevatron, and its possible applications at the LHC are discussed.Comment: 5 pages, 4 figures, in Proceedings of the 52nd ICFA Advanced Beam Dynamics Workshop on High-Intensity and High-Brightness Hadron Beams (HB2012), Beijing, China, 17-21 September 201

A rechargeable lithium–oxygen battery with dual mediators stabilizing the carbon cathode

At the cathode of a Li–O2 battery, O2 is reduced to Li2O2 on discharge, the process being reversed on charge. Li2O2 is an insulating and insoluble solid, leading ultimately to low rates, low capacities and early cell death if formed on the cathode surface. Here we show that when using dual mediators, 2,5-Di-tert-butyl-1,4-benzoquinone [DBBQ] on discharge and 2,2,6,6-tetramethyl-1-piperidinyloxy [TEMPO] on charge, the electrochemistry at the cathode surface is decoupled from Li2O2 formation/decomposition in solution. Capacities of 2 mAh cmareal−2 at 1 mA cmareal−2 with low polarization on charge/discharge are demonstrated, and up to 40 mAh cmareal−2 at rates ≫1 mA cmareal−2 are anticipated if suitable gas diffusion electrodes can be devised. One of the major barriers to the progress of Li–O2 cells is decomposition of the carbon cathode. By forming/decomposing Li2O2 in solution and avoiding high charge potentials, the carbon instability is significantly mitigated ( < 0.008% decomposition per cycle compared with 0.12% without mediators)