Low Surface Field Research Articles

Mass–radius relation for magnetized white dwarfs from SDSS

ABSTRACT We present the observational mass–radius (M–R) relation for a sample of 47 magnetized white dwarfs (WDs) with the magnetic field strength (B) ranging from 1 to 773 MG, identified from the SDSS data release 7 (DR7). We derive their effective temperature, surface gravity (log g), luminosity, radius, and mass. While atmospheric parameters are derived using a Virtual Observatory Spectral Energy Distribution Analyzer (VOSA), the mass is derived using their location in the HR diagram in comparison with the evolutionary tracks of different masses. We implement this mass measurement instead of a more traditional method of deriving masses from log g, which is unreliable as is based on SED and generates errors from other physical parameters involved. The main disadvantage of this method is that we need to assume a core composition of WDs. As it is complicated to identify the exact composition of these WDs from low-resolution spectra, we use tracks for the masses 0.2 to 0.4 M⊙ assuming a He-core, 0.5 to 1.0 M⊙ assuming CO core, and above M⊙ assuming O–Ne–Mg core. We compare the observed M–R relation with those predicted by the finite temperature model by considering different B, which are well in agreement considering their relatively low-surface fields, ≲109 G. Currently, there is no direct observational detection of magnetized WDs with B > 109 G. We propose that our model can be further extrapolated to higher B, which may indicate the existence of super-Chandrasekhar mass (M > 1.4 M⊙) WDs at higher B.

Spitzer Light Curves of the Young, Planetary-mass TW Hya Members 2MASS J11193254–1137466AB and WISEA J114724.10–204021.3

Abstract We present Spitzer Space Telescope time-series photometry at 3.6 and 4.5 μm of 2MASS J11193254−1137466AB and WISEA J114724.10−204021.3, two planetary-mass, late-type (∼L7) brown dwarf members of the ∼10 Myr old TW Hya Association. These observations were taken in order to investigate whether or not a tentative trend of increasing variability amplitude with decreasing surface gravity seen for L3–L5.5 dwarfs extends to later-L spectral types and to explore the angular momentum evolution of low-mass objects. We examine each light curve for variability and find a rotation period of 19.39+0.33 −0.28 hr and semi-amplitudes of 0.798+0.081 −0.083% at 3.6 μm and 1.108+0.093 −0.094% at 4.5 μm for WISEA J114724.10−204021.3. For 2MASS J11193254−1137466AB, we find a single period of 3.02+0.04 −0.03 hr with semi-amplitudes of 0.230+0.036 −0.035% at 3.6 μm and 0.453 ± 0.037% at 4.5 μm, which we find is possibly due to the rotation of one component of the binary. Combining our results with 12 other late-type L dwarfs observed with Spitzer from the literature, we find no significant differences between the 3.6 μm amplitudes of low surface gravity and field gravity late-type L brown dwarfs at Spitzer wavelengths, and find tentative evidence (75% confidence) of higher amplitude variability at 4.5 μm for young, late-type Ls. We also find a median rotation period of young brown dwarfs (10–300 Myr) of ∼10 hr, more than twice the value of the median rotation period of field-age brown dwarfs (∼4 hr), a clear signature of brown dwarf rotational evolution.

Strength, Depth, and Geometry of Magnetic Sources in the Crust of the Moon From Localized Power Spectrum Analysis

AbstractSpacecraft observations show that weak magnetic fields of crustal origin are ubiquitous across the surface of the Moon. To investigate the origin of these magnetic anomalies, a model was developed for the magnetic power spectrum that consists of ensembles of randomly magnetized sills or prisms. Localized spectrum analyses constrained how the parameters of this model vary with position, including the size of the sources, a quantity proportional to their mean‐squared dipole moment, and the depth to the top and bottom of the magnetized region. The depth to the top of the magnetized region varies from the surface to about 25 km. The magnetic carriers in the deep crust likely formed at the same time as the crust itself, implying that a core‐generated dynamo field must have existed when the crust was cooling during the first 100 Myr of lunar evolution. The parameter related to the strength of magnetization shows the existence of a prominent region on the nearside hemisphere that is largely unmagnetized and that correlates with a region of extremely low surface field strengths. This region lies entirely within a geological province that is highly enriched in heat‐producing elements (the Procellarum KREEP Terrane), suggesting that this region escaped being magnetized because of prolonged high crustal temperatures. The nearside magnetic low may be representative of the size of that portion of the crust that is highly enriched in heat‐producing elements, which is almost one third the size of the Procellarum KREEP Terrane based on surface thorium abundances.

Effect of the front surface field (a-Si:H) on the spectral response of thin films heterojunctions solar cells

The present paper accounts for a simulation study carried out to determine and optimize the effect of the high---low junction emitter front surface field (FSF) of thin heterojunction solar cell $$\text {n}^{+}$$n+(a-Si: H)/n(SiGe)/p(Si)). A theoretical model describing the behavior of heterojunction $$\text {n}^{+}$$n+---n---p type solar cells has been proposed. Besides, the expression of the photovoltaic equations allowing for the obtention of the optimal quantum efficiency of the solar cells based on the suggested model has been given. The effects of the a-Si:H-layer thickness and Ge fraction are discussed, using the computed results. The main role of an FSF layer is to reduce the effect of front surface recombination and the enhancement of light-generated free carriers' collection. This is primarily responsible for the increase of the spectral response compared with the conventional solar cell. This is achieved by the drastic reduction of the effective recombination at the emitter upper boundary and the optimum value of $$\text {n}^{+}$$n+ layer thicknesses (a-Si:H) is about 2 nm.

Implications of long-range wakefields on multi-bunch beam dynamics in the ILC with a new low surface field superconducting cavity

This article focusses on a beam dynamics study for the linacs of the ILC. In particular, the impact of long-range transverse wakefields on the beam quality is studied for the case in which the ILC would be built using the new low surface field (NLSF) superconducting cavities. This presents an alternative design to the baseline TESLA-style cavities. The progress of the beam down ~10km of each linac is simulated using the tracking computer code PLACET. In addition, the results of an analytical matrix method, in which the beam is subjected to identical wakefields from each cavity, are also presented. Both systematic and random errors, arising as a natural process during fabrication, are implemented in the beam tracking study. The latter source of error is found to be beneficial, as emittance dilution is reduced due to the beam receiving non-coherent kicks.

Design evolution and properties of superconducting parallel-bar rf-dipole deflecting and crabbing cavities

Deflecting/crabbing cavities serve a variety of purposes in different accelerator applications, primarily in separating a single beam into multiple beams and in rotating bunches for head-on collisions at the interaction point in particle colliders. Deflecting/crabbing cavities are also used for transverse and longitudinal emittance exchange in beams, x-ray pulse compression, and for beam diagnostics. Compact superconducting deflecting/crabbing cavities are under development due to strict dimensional constraints and requirements for higher field gradients with low surface losses. The TEM-like superconducting parallel-bar cavity supports low operating frequencies, thus making the design favorable for many of the deflecting/crabbing cavity applications. The design of the parallel-bar cavity based on cylindrical straight loading elements and rectangular outer conductors has evolved and been adapted to improve the design properties by modifying the design geometry. The improved design with trapezoidal-shaped loading elements and cylindrical outer conductor has attractive properties such as low and well-balanced peak surface fields and high transverse shunt impedance. Additionally, the wide separation of modes in the higher-order mode spectrum and the absence of lower-order mode are advantageous in high current applications. The evolution of the parallel-bar geometry into an rf-dipole geometry is presented with a detailed analysis of the properties for each design.

Optimisation of the new low surface field accelerating structure for the ILC

The main superconducting radio frequency (SRF) linacs of the international linear collider (ILC) operate at a frequency of 1.3GHz with a π phase advance per cell in the standing wave mode. An option being considered to reduce the overall footprint and project cost is to enhance the cavity gradient. The present baseline design for the main linacs of ILC demands the cavities to be able to reach a gradient of 35MV/m—although during commissioning and operation the gradient will be 31.5MV/m. This research concerns itself with the new cavity design with a view to reaching higher gradients. This design is focussed on minimising the surface electromagnetic fields and maximising the bandwidth of the accelerating mode. This new shape, which is referred to as the New Low Surface Field (NLSF) design. A design of a complete nine-cell cavity, including power couplers and higher order mode damping couplers is presented.

Design and simulation of a new type of 500 MHz single-cell superconducting RF cavity

This paper illustrates the design and simulation of a unique 500 MHz single-cell superconducting radio frequency cavity with a fluted beam pipe and a coaxial-type fundamental power coupler. The simulation results show that the cavity has a high r/Q value, a low peak surface field and a large beam aperture, so it can be a candidate cavity for high current accelerators. With the help of a fluted beam tube, almost all the higher order modes can propagate out of the cavity, especially the first two dipole modes, TE111 and TM110, and the first higher monopole mode, TM011. The external quality factor of the coaxial fundamental power coupler is optimized to 1.2×105, which will be useful when it is applied in the light source storage ring.

High-gradient two-beam accelerator structure

A new accelerating structure, which is aimed to provide gradient >150 MV/m for next generation of multi-TeV linear colliders, is suggested [1-3]. The structure is based on periodic system of quasi-optical cavities, which are not coupled with each other. Each of these cavities is excited in several equidistantly-spaced eigen modes by the spatially bunched drive beam in such a way that the RF fields reach peak values only during the short time intervals when an accelerating bunch is resident in a cavity, thus exposing the cavity surfaces to strongest fields for only a small fraction of time. This feature is expected to raise the breakdown and pulse heating thresholds. The proposed structure has smaller ratio α of maximal surface field to accelerating gradient (1<α<2) in comparison with usual single-frequency structure, where this ratio is close to factor 2. Due to all cavities of new accelerating structure are uncoupled, the structure is very reliable, i.e. possible breakdown in a separate cavity does not spoil the whole accelerator. High efficiency and transformer ratio of drive beam power to accelerating beam power are expected to be provided by means of a so-called idea of frequency detuning. In accordance with this idea high-current drive beam leaves its power in a distributed way (at long distance along accelerator). This is achievable due to detuning of eigen frequencies of a structure cavity out of drive bunch frequency. Calculations of a new two-beam accelerating structure consisted of multi-mode rectangular cavities with the parallel driving and accelerated beams, show that high gradient (~150 MV/m), low surface field (~190 MV/m), and high efficiency (~30%) are achievable under beam parameters close to those projected for CLIC (CERN). This structure embodies most of additional attractive properties: the cavity is an all metallic structure, no transfer or coupling structures are needed between the drive and acceleration channels, the cavity fields are symmetric around the axes of the drive beam and the accelerated beam.

Drop spreading on heterogeneous substrates via Monte Carlo simulations

We study the dynamics of liquid drops in the partial wetting regime first on pure surfaces and then on heterogeneous substrates. We model the spreading of a drop by a 3-dimensional Ising model (3D IM). The initial nonequilibrium configuration is a parallelepiped of occupied sites with appropriately chosen boundary conditions from which we let the system evolve towards its equilibrium state via a particle-conserving dynamics. We find that the time behaviors of the base radius R1 (t) and the cosine of the contact angle cos theta; (t) are well described by exponential decay functions with relaxation times tau(r) and tau(cos theta;) correspondingly. Thus it follows that the molecular kinetic theory gives a valid description of the initial stage of drop spreading in the partial wetting regime for times smaller than both relaxation times tau(r) and tau(cos theta;) . Also, our MC results for the 3D IM with regularly distributed single-site impurities at low temperatures and low surface fields are compatible with Cassie's and Israelachvili's equations, the description of the data with Israelachvili's equation being slightly better. At high temperatures and high surface fields there is a deviation from the purely linear behavior of the cosine of the contact angle on the concentration known as Cassie's law for chemically heterogeneous surfaces.

The Hanle Effect as a Diagnostic of Magnetic Fields in Stellar Envelopes. IV. Application to Polarized P Cygni Wind Lines

The Hanle effect has been proposed as a new diagnostic of circumstellar magnetic fields for early-type stars, for which it is sensitive to field strengths in the 1‐300 G range. In this paper we compute the polarized P Cygni line profiles that result from the Hanle effect. For modeling the polarization, we employ a variant of the ‘‘last scattering approximation.’’ For cases in which the Sobolev optical depths are greater than unity, the emergent line intensity is assumed to be unpolarized, while for smaller optical depths, the Stokes source functions for the Hanle effect with optically thin line scattering are used. For a typical P Cygni line, the polarized emission forms in the outer wind, because the Sobolev optical depth is large at the inner wind. For low surface field strengths, weak P Cygni lines are needed to measure the circumstellar field. For high values of the surface fields, both the Zeeman and Hanle diagnostics can be used, with the Zeeman effect probing the photospheric magnetic fields and the Hanle effect measuring the magnetic field in the wind flow. Polarized line profiles are calculated for a selfconsistent structure of the flow and the magnetic geometry based on the WCFields model, which is applicable to slowly rotating stellar winds with magnetic fields drawn out by the gas flow. For surface fields of a few hundred gauss, we find that the Hanle effect can produce line polarizations in the range of a few tenths of a percent up to about 2%.

Investigations on the quality of polysilicon film-gate dielectric interface in polysilicon thin film transistors

The effective electron mobility was measured as a function of surface field in polysilicon thin film transistors having the following three types of gate dielectrics; silicon dioxide deposited by low temperature (350 °C) plasma-enhanced chemical vapor deposition (PECVD), low temperature (400 °C) nitrogen-rich PECVD silicon nitride and high temperature (1050 °C) thermally grown silicon dioxide. At low surface fields, the maximum true effective electron mobility was 40 ± 3 cm 2 V −1 s −1 in all devices independent of the type of gate dielectric, indicating that the quality of the interface is the same. However, at high surface fields a stronger degradation of the mobility was observed in devices having the thermally grown silicon dioxide as gate dielectric, indicating the presence of surface roughness within the interfacial region. The polysilicon structure was studied by transmission electron microscopy in order to ensure that our electrical results were not influenced by variations in polysilicon grain size. It was found that the grain size was the same in all devices including those that were oxidized at high temperature.

Unified charge control model and subthreshold current in heterostructure field-effect transistors

A unified analytical charge control model covering the entire range of gate voltages from below and above threshold is developed for heterojunction field-effect transistors (HFETs). This model is based on a new interpretation of the quantized energy levels for the two-dimensional electron gas. It reduces to a classical charge sheet model in the limit of low surface field. The model is used to interpret the experimental data for the subthreshold regime of HFETs. The results indicate wide range variation of the effective acceptor concentration after device fabrication processing in the unintentionally doped GaAs buffer layer.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Cyclotron emission from AM Herculis binaries - A variable magnetic field, variable temperature model

This paper examines cyclotron emission from a hot plasma flowing to the surface of a magnetic white dwarf. The two-temperature plasma free-falls along the axis of a dipole magnetic field to the white dwarf surface and cools through bremsstrahlung and optically thick cyclotron emission. It is shown that the ion temperature is constant until the magnetic field is of order of 10 MG. This result is weakly dependent on the accretion rate and the strength of the surface magnetic field. Spectra and polarization fractions are calculated for a variety of model parameters. The spectra have a broad peak at optical wavelengths for surface field strengths above about 20 MG. The emission is highly polarized at all frequencies. Cyclotron harmonic structure is often produced for low surface field strengths. This model produces the observed variation of polarization with orbital phase at both optical and infrared wavelengths. It is argued that the observed magnetic field strengths are a consequence of the plasma cooling in a 30-MG region, and that the surface field strength for some of these systems may actually be larger than the observed values. 26 refs.

Astronomy: Neutron stars in Nanjing

Astronomy: Neutron stars in Nanjing

Electron energy deposition in the middle atmosphere

Spectra of locally precipitating 36‐ to 317‐keV electrons obtained by instrumentation on the S3‐2 satellite are used to calculate energy deposition profiles as a function of latitude, longitude, and altitude. In the 70‐ to 90‐km altitude, mid‐latitude ionization due to these precipitating energetic electrons can be comparable to that due to direct solar H Lyman α. At night, the electrons produce ionization more than an order of magnitude greater than that expected from scattered H Lyman α. Maximum precipitation rates in the region of the South Atlantic Anomaly are of the order of 10−2 erg/cm² s with a spectrum of the form j(E) = 1.34 × 105 E−2.27 (keV). Southern hemisphere precipitation dominates that in the north for 1.1 &lt; L &lt; 6 except for regions of low local surface field in the northern hemisphere. Above L = 6, local time effects dominate; i.e., longitudinal effects due to the asymmetric magnetic field which are strong features below L = 6 disappear and are replaced by high‐latitude precipitation events which are local time features.

The planar junction etch for high voltage and low surface fields in planar devices

A modification of the moat etch type of surface contouring is described which can increase the avalanche breakdown voltage of planar p-n junctions and greatly reduce peak surface electric fields. A properly located moat etch or bevel is used to achieve what is effectively a positive bevel intersection angle between the junction and the surface. Qualitative arguments based on charge balance, exact computer solutions, and experimental results show that higher breakdown voltage and much lower surface fields can be achieved as some, but not all, of the deleterious effects of the junction curvature are eliminated. Optimum design and sensitivity to process variations are also considered.