An experimental study of a self-confined flow with ring-vorticity distribution

A new form of self-confined flow was investigated in which a recirculation zone forms away from any solid boundary. An inviscid flow analysis indicated that in a purely meridional axisymmetric flow a stationary, spherical, self-confined region should occur in the center of a streamlined divergent-convergent enlargement zone. The spherical confinement region would be at rest and at constant pressure. Experimental investigations were carried out in a specially built test apparatus to establish the desired confined flow. The streamlined divergent-convergent interior shape of the test section was fabricated according to the theoretical calculation for a particular streamline. The required inlet vorticity distribution was generated by producing a velocity profile with a shaped gauze screen in the straight pipe upstream of the test section. Fluid speed and turbulence intensity were measured with a constant-temperature hot-wire anemometer system. The measured results indicated a very orderly and stable flow field

Towards Long-endurance Flight: Design and Implementation of a Variable-pitch Gasoline-engine Quadrotor

Majority of today's fixed-pitch, electric-power quadrotors have short flight endurance ($<$ 1 hour) which greatly limits their applications. This paper presents a design methodology for the construction of a long-endurance quadrotor using variable-pitch rotors and a gasoline-engine. The methodology consists of three aspects. Firstly, the rotor blades and gasoline engine are selected as a pair, so that sufficient lift can be comfortably provided by the engine. Secondly, drivetrain and airframe are designed. Major challenges include airframe vibration minimization and power transmission from one engine to four rotors while keeping alternate rotors contra-rotating. Lastly, a PD controller is tuned to facilitate preliminary flight tests. The methodology has been verified by the construction and successful flight of our gasoline quadrotor prototype, which is designed to have a flight time of 2 to 3 hours and a maximum take-off weight of 10 kg.Comment: 6 page

An MHD Model For Magnetar Giant Flares

Giant flares on soft gamma-ray repeaters that are thought to take place on magnetars release enormous energy in a short time interval. Their power can be explained by catastrophic instabilities occurring in the magnetic field configuration and the subsequent magnetic reconnection. By analogy with the coronal mass ejection (CME) events on the Sun, we develop a theoretical model via an analytic approach for magnetar giant flares. In this model, the rotation and/or displacement of the crust causes the field to twist and deform, leading to flux rope formation in the magnetosphere and energy accumulation in the related configuration. When the energy and helicity stored in the configuration reach a threshold, the system loses its equilibrium, the flux rope is ejected outward in a catastrophic way, and magnetic reconnection helps the catastrophe develop to a plausible eruption. By taking SGR 1806 - 20 as an example, we calculate the free magnetic energy released in such an eruptive process and find that it is more than $10^{47}$ ergs, which is enough to power a giant flare. The released free magnetic energy is converted into radiative energy, kinetic energy and gravitational energy of the flux rope. We calculated the light curves of the eruptive processes for the giant flares of SGR 1806 - 20, SGR 0526-66 and SGR 1900+14, and compared them with the observational data. The calculated light curves are in good agreement with the observed light curves of giant flares.Comment: Accepted to Ap

Magnetic impurity in the vicinity of a vacancy in bilayer graphene

We use quantum Monte Carlo method to study a magnetic impurity located next to a vacancy in bilayer graphene with Bernal stacking. Due to the broken symmetry between two sublattices in bilayer system, there exist two different types of vacancy induced localized state. We find that the magnetic property of the adatom located on the adjacent site of the vacancy depends on whether the vacancy belongs to A or B sublattice. In general, local moment is more strongly suppressed if the vacancy belongs to the sublattice A when $\mu \sim 0$. We switch the values of the chemical potential and study the basic thermodynamic quantities and the correlation functions between the magnetic adatom and the carbon sites.Comment: 3 pages, 4 figures, conferenc

Calibration and Irradiation Study of the BGO Background Monitor for the BEAST II Experiment

Beam commissioning of the SuperKEKB collider began in 2016. The Beam Exorcism for A STable experiment II (BEAST II) project is particularly designed to measure the beam backgrounds around the interaction point of the SuperKEKB collider for the Belle II experiment. We develop a system using bismuth germanium oxide (BGO) crystals with optical fibers connecting to a multianode photomultiplier tube (MAPMT) and a field-programmable gate array (FPGA) embedded readout board for monitoring the real-time beam backgrounds in BEAST II. The overall radiation sensitivity of this system is estimated to be $(2.20\pm0.26)\times10^{-12}$ Gy/ADU (analog-to-digital unit) with the standard 10 m fibers for transmission and the MAPMT operating at 700 V. Our $\gamma$-ray irradiation study of the BGO system shows that the exposure of BGO crystals to $^{60}$Co $\gamma$-ray doses of 1 krad has led to immediate light output reductions of 25--40%, and the light outputs further drop by 30--45% after the crystals receive doses of 2--4 krad. Our findings agree with those of the previous studies on the radiation hard (RH) BGO crystals grown by the low thermal gradient Czochralski (LTG Cz) technology. The absolute dose from the BGO system is also consistent with the simulation, and is estimated to be about 1.18 times the equivalent dose. These results prove that the BGO system is able to monitor the background dose rate in real time under extreme high radiation conditions. This study concludes that the BGO system is reliable for the beam background study in BEAST II

Pressure induced superconductivity on the border of magnetic order in MnP

We report the discovery of superconductivity on the border of long-range magnetic order in the itinerant-electron helimagnet MnP via the application of high pressure. Superconductivity with Tsc~1 K emerges and exists merely near the critical pressure Pc~8 GPa, where the long-range magnetic order just vanishes. The present finding makes MnP the first Mn-based superconductor. The close proximity of superconductivity to a magnetic instability suggests an unconventional pairing mechanism. Moreover, the detailed analysis of the normal-state transport properties evidenced non-Fermi-liquid behavior and the dramatic enhancement of the quasi-particle effective mass near Pc associated with the magnetic quantum fluctuations.Comment: 5 figure

Josephson Currents in Quantum Hall Devices

We consider a simple model for an SNS Josephson junction in which the "normal metal" is a section of a filling-factor $\nu=2$ integer quantum-Hall edge. We provide analytic expressions for the current/phase relations to all orders in the coupling between the superconductor and the quantum Hall edge modes, and for all temperatures. Our conclusions are consistent with the earlier perturbative study by Ma and Zyuzin [Europhysics Letters {\bf 21} 941-945 (1993)]: The Josephson current is independent of the distance between the superconducting leads, and the upper bound on the maximum Josephson current is inversely proportional to the perimeter of the Hall device.Comment: Revtex4. 22 pages 9 figures. Replaced version has minor typos fixed and one added referenc

A Noisy Monte Carlo Algorithm

We propose a Monte Carlo algorithm to promote Kennedy and Kuti's linear accept/reject algorithm which accommodates unbiased stochastic estimates of the probability to an exact one. This is achieved by adopting the Metropolis accept/reject steps for both the dynamical and noise configurations. We test it on the five state model and obtain desirable results even for the case with large noise. We also discuss its application to lattice QCD with stochastically estimated fermion determinants.Comment: 10 pages, 1 tabl

Single-particle Excitation Spectra of C60_{60} Molecules and Monolayers

In this paper we present calculations of single-particle excitation spectra of neutral and three-electron-doped Hubbard C$_{60}$ molecules and monolayers from large-scale quantum Monte Carlo simulations and cluster perturbation theory. By a comparison to experimental photoemission, inverse photoemission, and angle-resolved photoemission data, we estimate the intermolecular hopping integrals and the C$_{60}$ molecular orientation angle, finding agreement with recent X-ray photoelectron diffraction (XPD) experiments. Our results demonstrate that a simple effective Hubbard model, with intermediate coupling, $U=4t$, provides a reasonable basis for modeling the properties of C$_{60}$ compounds.Comment: 6 page