Cross-field gas breakdown in a coaxial cylinder geometry

Sparking potentials in a coaxial cylinder geometry in oxygen and dry air were measured in crossed electric and magnetic fields. From the data effective collision frequencies were calculated using the equivalent pressure concept. It is shown that the equivalent pressure concept holds good for deriving the effective collision frequencies in non-uniform electric fields.

Wire-cylinder corona discharge with the wire at positive potential

A wire-cylinder corona discharge was studied in nitrogen and dry air in crossed electric and magnetic fields for values of magnetic field ranging from 0 to 3000 G with the wire at positive potential. In the absence of a magnetic field pre-onset streamers and pulses were observed in nitrogen. In both nitrogen and dry air breakdown streamers were observed just before spark breakdown of the gap. Furthermore, experiments in dry air at atmospheric pressure in an electric field indicate regular pre-onset streamers appearing at time intervals of 19.5 µs. The appearance of regular pre-onset streamers suggests that it is not possible for negative ions to form a sheath close to the anode as postulated by Hermstein (1960) for the formation of steady or glow corona in a point-plane gap.

Computer simulation of multi-elemental fusion reactor materials

Thermonuclear fusion is a sustainable energy solution, in which energy is produced using similar processes as in the sun. In this technology hydrogen isotopes are fused to gain energy and consequently to produce electricity. In a fusion reactor hydrogen isotopes are confined by magnetic fields as ionized gas, the plasma. Since the core plasma is millions of degrees hot, there are special needs for the plasma-facing materials. Moreover, in the plasma the fusion of hydrogen isotopes leads to the production of high energetic neutrons which sets demanding abilities for the structural materials of the reactor. This thesis investigates the irradiation response of materials to be used in future fusion reactors. Interactions of the plasma with the reactor wall leads to the removal of surface atoms, migration of them, and formation of co-deposited layers such as tungsten carbide. Sputtering of tungsten carbide and deuterium trapping in tungsten carbide was investigated in this thesis. As the second topic the primary interaction of the neutrons in the structural material steel was examined. As model materials for steel iron chromium and iron nickel were used. This study was performed theoretically by the means of computer simulations on the atomic level. In contrast to previous studies in the field, in which simulations were limited to pure elements, in this work more complex materials were used, i.e. they were multi-elemental including two or more atom species. The results of this thesis are in the microscale. One of the results is a catalogue of atom species, which were removed from tungsten carbide by the plasma. Another result is e.g. the atomic distributions of defects in iron chromium caused by the energetic neutrons. These microscopic results are used in data bases for multiscale modelling of fusion reactor materials, which has the aim to explain the macroscopic degradation in the materials. This thesis is therefore a relevant contribution to investigate the connection of microscopic and macroscopic radiation effects, which is one objective in fusion reactor materials research.

Hysteresis in model spin system

The aim of this paper is to construct a nonequilibrium statistical‐mechanics theory to study hysteresis in ferromagnetic systems. We study the hysteretic response of model spin systems to periodic magnetic fields H(t) as a function of the amplitude H0 and frequency Ω. At fixed H0, we find conventional, squarelike hysteresis loops at low Ω, and rounded, roughly elliptical loops at high Ω, in agreement with experiments. For the O(N→∞), d=3, (Φ2)2 model with Langevin dynamics, we find a novel scaling behavior for the area A of the hysteresis loop, of the form A∝H0.660Ω0.33. We carry out a Monte Carlo simulation of the hysteretic response of the two‐dimensional, nearest‐neighbor, ferromagnetic Ising model. These results agree qualitatively with the results obtained for the O(N) model.

Magnetoresistance of FexNi80-xCr20 (50

The authors have measured longitudinal and transverse magnetoresistance (MR) of crystalline pseudo-binary alloys FexNi80-xCr20 (50magnetic phases because the alloys have compositions close to the critical composition regime for ferromagnetism. While the Fe-Ni-Cr alloys have a FCC gamma -phase, the binary Fe-Cr alloy has a BCC alpha -phase. This gives an opportunity to compare magnetoresistances for gamma -phase and alpha -phase Fe alloys when they lie close to the critical composition. The experiments were conducted at 4.2 K with magnetic fields up to 7 T. The authors observed that all the alloys show negative magnetoresistance at 4.2 K in fields up to 7 T. However, for the gamma -phase alloys the typical maximum MR ( Delta rho / rho ) is about 1%, while for the alpha -phase alloy it is 10%. In the gamma -phase alloys there is a small but finite anisotropy of MR in the phases with long-range magnetic order which gradually vanishes near the critical region (x=xc approximately=59-63) when it becomes a spin glass. In the range x approximately=xc, Delta rho / rho varies as Mn (M=magnetization) with n approximately=2.

Signatures of an anomalous Nernst effect in a mesoscopic two-dimensional electron system

We investigate the Nernst effect in a mesoscopic two-dimensional electron system (2DES) at low magnetic fields, before the onset of Landau level quantization. The overall magnitude of the Nernst signal agrees well with semiclassical predictions. We observe reproducible mesoscopic fluctuations in the signal that diminish significantly with an increase in temperature. We also show that the Nernst effect exhibits an anomalous component that is correlated with an oscillatory Hall effect. This behavior may be able to distinguish between different spin-correlated states in the 2DES.

Magnetically actuated propulsion at low Reynolds numbers: towards nanoscale control

Significant progress has been made in the fabrication of micron and sub-micron structures whose motion can be controlled in liquids under ambient conditions. The aim of many of these engineering endeavors is to be able to build and propel an artificial micro-structure that rivals the versatility of biological swimmers of similar size, e. g. motile bacterial cells. Applications for such artificial ``micro-bots'' are envisioned to range from microrheology to targeted drug delivery and microsurgery, and require full motion-control under ambient conditions. In this Mini-Review we discuss the construction, actuation, and operation of several devices that have recently been reported, especially systems that can be controlled by and propelled with homogenous magnetic fields. We describe the fabrication and associated experimental challenges and discuss potential applications.

Anomalies in free flux-flow hall effect

The low-T-c layered superconductor 2H-NbSe2 shows remarkable results for free flux-flow Hall effect. At low magnetic fields, the Nozieres-Vinen result of a field-independent Hall angle appears to hold. At larger fields, a marked departure occurs leading to an extremely sharp and pronounced minimum slightly below H-c2, unaccounted for in the standard theoretical models. The results suggest the existence of collective dynamics and phase transitions (such as melting) in a clean flux line lattice.

Stochastic fluctuations of the solar dynamo

Attempts in the past to model the irregularities of the solar cycle (such as the Maunder minimum) were based on studies of the nonlinear feedback of magnetic fields on the dynamo source terms. Since the alpha-coefficient is obtained by averaging over the turbulence, it is expected to have stochastic fluctuations, and we show that these fluctuations can explain the irregularities of the solar cycle in a more satisfactory way. We solve the dynamo equations in a slab with a single mode, taking the alpha-coefficient to be constant in space but fluctuating stochastically in time with some given amplitude and given correlation time. The same level of percentile fluctuations (about 10 %) produces no effect on an alpha-omega dynamo, but makes an alpha-2 dynamo completely chaotic. The level of irregularities in an alpha-2-omega dynamo qualitatively agrees with the solar behavior, reinforcing the conclusion of Choudhuri (1990a) that the solar dynamo is of the alpha-2-omega-type. The irregularities are found to increase on increasing either the amplitude or the correlation time of the stochastic fluctuations. The alpha-quenching mechanism tends to make the system stable against the irregularities and hence it is inferred that the alpha-quenching should not be too strong so that the irregularities are not completely suppressed. We also present a simple-minded analysis to understand why the stochastic fluctuations in the alpha-omega, alpha-2-omega and alpha-2 regimes have such different outcomes.

Charge-ordering in manganates

Ordering of Mn3+ and Mn4+ ions occurs in the rare earth manganates of the general composition Ln(1-x)A(x)MnO(3) (Ln rare earth, A = Ca, Sr). Such charge-ordering is associated with antiferromagnetic and insulating properties. This phenomenon is to be contrasted with the ferromagnetic metallic behavior that occurs when double-exchange between the Mn3+ and Mn4+ ions predominates. Two distinct types of charge-ordering can be delineated. In one, a ferromagnetic metallic (FMM) state transforms to the charge-ordered (CO) state on cooling. In the other scenario, the CO state is found in the paramagnetic ground stale and there is no ferromagnetism down to the lowest temperatures. Magnetic fields transform the CO state to the FMM state, when the average radius of the A-site cations is sufficiently large ([r(A)] > 1.17 Angstrom). Chemical melting of the CO state by Cr3+ substitution in the Mn site is also found only when [r(A)] greater than or similar to 1.17 Angstrom. The effect of the size of the A-cations on the Mn-O-Mn angle is not enough to explain the observed variations of the charge-ordering temperature as well as the ferromagnetic Curie temperature T-c. An explanation based on a competition between the Mn and A-cation orbitals for sigma-bonding with the oxygen rho(sigma) orbitals is considered to account for the large changes in T-c and hence the true bandwidth, with [r(A]). Effects of radiation, electric field, and other factors on the CO state are discussed along with charge-ordering in other manganate systems. Complex phase transitions, accompanied by changes in electronic and magnetic properties, occur in manganates with critical values of(rA) Or bandwidth. Charge-ordering is found in layered manganates, BixCa1-xMnO3 and CaMnO3-delta.

Chopped nonlinear magneto-optic rotation: A technique for precision measurements

We have developed a technique for precise measurement of small magnetic fields using nonlinear magneto-optic rotation (NMOR). The technique relies on the resonant laser beam being chopped. During the on time, the atoms are optically pumped into an aligned ground state (Delta m=2 coherence). During the off time, they freely precess around the magnetic field at the Larmor frequency. If the on-off modulation frequency matches (twice) the Larmor precession frequency, the rotation is resonantly enhanced in every cycle, thereby making the process like a repeated Ramsey measurement of the Larmor frequency. We study chopped-NMOR in a paraffin-coated Cs vapor cell. The out-of-phase demodulated rotation shows a Lorentzian peak of linewidth 85 mu G, corresponding to a sensitivity of 0.15nG/root Hz. We discuss the potential of this technique for the measurement of an atomic electric-dipole moment. Copyright (C) EPLA, 2011

Giant radio sources

We present multifrequency Very Large Array (VLA) observations of two giant quasars, 0437-244 and 1025-229, from the Molonglo Complete Sample. These sources have well-defined FR II radio structure, possible one-sided jets, no significant depolarization between 1365 and 4935 MHz and low rotation measure (\ RM \ < 20 rad m(-2)). The giant sources are defined to be those with overall projected size greater than or equal to 1 Mpc. We have compiled a sample of about 50 known giant radio sources from the literature, and have compared some of their properties with a complete sample of 3CR radio sources of smaller sizes to investigate the evolution of giant sources, and test their consistency with the unified scheme for radio galaxies and quasars. We find an inverse correlation between the degree of core prominence and total radio luminosity, and show that the giant radio sources have similar core strengths to smaller sources of similar total luminosity. Hence their large sizes are unlikely to be caused by stronger nuclear activity. The degree of collinearity of the giant sources is also similar to that of the sample of smaller sources. The luminosity-size diagram shows that the giant sources are less luminous than our sample of smaller sized 3CR sources, consistent with evolutionary scenarios in which the giants have evolved from the smaller sources, losing energy as they expand to these large dimensions. For the smaller sources, radiative losses resulting from synchrotron radiation are more significant while for the giant sources the equipartition magnetic fields are smaller and inverse Compton lass owing to microwave background radiation is the dominant process. The radio properties of the giant radio galaxies and quasars are consistent with the unified scheme.

The dynamo effect - a dynamic renormalisation group approach

The dynamo effect is used to describe the generation of magnetic fields in astrophysical objects. However, no rigorous derivation of the dynamo equation is available. We justify the form of the equation using an Operator Product Expansion (OPE) of the relevant fields. We also calculate the coefficients of the OPE series using a dynamic renormalisation group approach and discuss the time evolution of the initial conditions on the initial seed magnetic field.

Propagating MHD Waves in Coronal Holes

Coronal holes are the coolest and darkest regions of the upper solar atmosphere, as observed both on the solar disk and above the solar limb. Coronal holes are associated with rapidly expanding open magnetic fields and the acceleration of the high-speed solar wind. During the years of the solar minima, coronal holes are generally confined to the Sun's polar regions, while at solar maxima they can also be found at lower latitudes. Waves, observed via remote sensing and detected in-situ in the wind streams, are most likely responsible for the wind and several theoretical models describe the role of MHD waves in the acceleration of the fast solar wind. This paper reviews the observational evidences of detection of propagating waves in these regions. The characteristics of the waves, like periodicities, amplitude, speed provide input parameters and also act as constraints on theoretical models of coronal heating and solar wind acceleration.