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At least 991 records · Page 55
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DOE OSTI2014

Comment on “Magnetic geometry and physics of advanced divertors: The X-divertor and the snowflake” [Phys. Plasmas 20, 102507 (2013)]

In the recently published paper “Magnetic geometry and physics of advanced divertors: The X-divertor and the snowflake” [Phys. Plasmas 20, 102507 (2013)], the authors raise interesting and important issues concerning divertor physics and design. However, the paper contains significant errors: (a) The conceptual framework used in it for the evaluation of divertor “quality” is reduced to the assessment of the magnetic field structure in the outer Scrape-Off Layer. This framework is incorrect because processes affecting the pedestal, the private flux region and all of the divertor legs (four, in the case of a snowflake) are an inseparable part of divertor operation. (b) The concept of the divertor index focuses on only one feature of the magnetic field structure and can be quite misleading when applied to divertor design. (c) The suggestion to rename the divertor configurations experimentally realized on NSTX (National Spherical Torus Experiment) and DIII-D (Doublet III-D) from snowflakes to X-divertors is not justified: it is not based on comparison of these configurations with the prototypical X-divertor, and it ignores the fact that the NSTX and DIII-D poloidal magnetic field geometries fit very well into the snowflake “two-null” prescription.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Formation of magnetic discontinuities through viscous relaxation

According to Parker's magnetostatic theorem, tangential discontinuities in magnetic field, or current sheets (CSs), are generally unavoidable in an equilibrium magnetofluid with infinite electrical conductivity and complex magnetic topology. These CSs are due to a failure of a magnetic field in achieving force-balance everywhere and preserving its topology while remaining in a spatially continuous state. A recent work [Kumar, Bhattacharyya, and Smolarkiewicz, Phys. Plasmas 20, 112903 (2013)] demonstrated this CS formation utilizing numerical simulations in terms of the vector magnetic field. The magnetohydrodynamic simulations presented here complement the above work by demonstrating CS formation by employing a novel approach of describing the magnetofluid evolution in terms of magnetic flux surfaces instead of the vector magnetic field. The magnetic flux surfaces being the possible sites on which CSs develop, this approach provides a direct visualization of the CS formation, helpful in understanding the governing dynamics. The simulations confirm development of tangential discontinuities through a favorable contortion of magnetic flux surfaces, as the magnetofluid undergoes a topology-preserving viscous relaxation from an initial non-equilibrium state with twisted magnetic field. A crucial finding of this work is in its demonstration of CS formation at spatial locations away from the magnetic nulls.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

The status of neutralino dark matter

The lightest neutralino in supersymmetry is the most studied dark matter candidate. This writeup reviews the status of neutralino dark matter in minimal and nonminimal supersymmetric models in light of recent null results at the XENON100 experiment and the observation of a 130 GeV gamma ray signal from the Galactic Center by the Fermi LAT.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

The importance of matched poloidal spectra to error field correction in DIII-D

Optimal error field correction (EFC) is thought to be achieved when coupling to the least-stable “dominant” mode of the plasma is nulled at each toroidal mode number (n). The limit of this picture is tested in the DIII-D tokamak by applying superpositions of in- and ex-vessel coil set n = 1 fields calculated to be fully orthogonal to the n = 1 dominant mode. In co-rotating H-mode and low-density Ohmic scenarios, the plasma is found to be, respectively, 7× and 20× less sensitive to the orthogonal field as compared to the in-vessel coil set field. For the scenarios investigated, any geometry of EFC coil can thus recover a strong majority of the detrimental effect introduced by the n = 1 error field. Despite low sensitivity to the orthogonal field, its optimization in H-mode is shown to be consistent with minimizing the neoclassical toroidal viscosity torque and not the higher-order n = 1 mode coupling.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Measurement of spatial and temporal behavior of H{sub α} emission from Aditya tokamak using a diagnostic based on a photomultiplier tube array

A photo multiplier tube (PMT) array based spectroscopic diagnostic with fast time response of 10 μs and spatial resolution ∼3 cm has been developed and installed on Aditya tokamak to study the spatial and temporal behavior of H{sub α} emissions from typical discharges. Collimated light has been collected from the plasma along 16 lines of sight passing through entire plasma poloidal cross section of Aditya and detected by two 8 channels PMT arrays after selecting H{sub α} emission using interference filter. The studies are carried out during plasma formation phase of Aditya by changing vertical field and its delay with respect to loop voltage. It is observed that plasma initiated in the high field side in typical discharges of Aditya. The plasma formation position is matched with null field location estimated through simulation.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Multi-channel Doppler backscattering measurements in the C-2 field reversed configuration

A versatile heterodyne Doppler Backscattering (DBS) system is used to measure density fluctuation levels (in the wavenumber range kρ{sub s} ≤ 50), and the toroidal E × B flow velocity in the C-2 Field-Reversed Configuration (FRC). Six tunable frequencies in three waveguide bands (26 GHz ≤ f ≤ 90 GHz) are launched using monostatic beam optics, via a quasi-optical beam combiner/polarizer and an adjustable parabolic focusing mirror (inside the vacuum enclosure) achieving Gaussian beam spot sizes of 3–5.5 cm at the X/O-mode cutoff. The DBS system covers plasma densities of 0.8 × 10{sup 13} ≤ n{sub e} ≤ 1 × 10{sup 14} cm{sup −3}, and provides access to the FRC core (up to the field null) and across the FRC separatrix into the scrape-off layer plasma.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Modelling of three dimensional equilibrium and stability of MAST plasmas with magnetic perturbations using VMEC and COBRA

It is known that magnetic perturbations can mitigate edge localized modes (ELMs) in experiments, for example, MAST [Kirk et al., Nucl. Fusion 53, 043007 (2013)]. One hypothesis is that the magnetic perturbations cause a three dimensional corrugation of the plasma and this corrugated plasma has different stability properties to peeling-ballooning modes compared to an axisymmetric plasma. It has been shown in an up-down symmetric plasma that magnetic perturbations in tokamaks will break the usual axisymmetry of the plasma causing three dimensional displacements [Chapman et al., Plasma Phys. Controlled Fusion 54, 105013 (2012)]. We produce a free boundary three-dimensional equilibrium of a lower single null MAST relevant plasma using VMEC [S. P. Hirshman and J. C. Whitson, Phys. Fluids 26, 3553 (1983)]. The safety factor and pressure profiles used for the modelling are similar to those deduced from axisymmetric analysis of experimental data with ELMs. We focus on the effect of applying n = 3 and n = 6 magnetic perturbations using the resonant magnetic perturbation (RMP) coils. A midplane displacement of over ±1 cm is seen when the full current is applied. The current in the coils is scanned and a linear relationship between coil current and midplane displacement is found. The pressure gradient in real space in different toroidal locations is shown to change when RMPs are applied. This effect should be taken into account when diagnosing plasmas with RMPs applied. The helical Pfirsch-Schlüter currents which arise as a result of the assumption of nested flux surfaces are estimated for this equilibrium. The effect of this non-axisymmetric equilibrium on infinite n ballooning stability is investigated using COBRA [Sanchez et al., J. Comput. Phys. 161, 576–588 (2000)]. The infinite n ballooning stability is analysed for two reasons; it may give an indication of the effect of non-axisymmetry on finite n peeling-ballooning modes, responsible for ELMs; and infinite n ballooning modes are correlated to kinetic ballooning modes which are thought to limit the pressure gradient of the pedestal [Snyder et al., Phys. Plasmas 16, 056118 (2009)]. The ballooning mode growth rate gains a variation in toroidal angle. The equilibria with midplane displacements due to RMP coils have a higher ballooning mode growth rate than the axisymmetric case and the possible implications are discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Comparison of test particle acceleration in torsional spine and fan reconnection regimes

Magnetic reconnection is a common phenomenon taking place in astrophysical and space plasmas, especially in solar flares which are rich sources of highly energetic particles. Torsional spine and fan reconnections are important mechanisms proposed for steady-state three-dimensional null-point reconnection. By using the magnetic and electric fields for these regimes, we numerically investigate the features of test particle acceleration in both regimes with input parameters for the solar corona. By comparison, torsional spine reconnection is found to be more efficient than torsional fan reconnection in an acceleration of a proton to a high kinetic energy. A proton can gain as high as 100 MeV of relativistic kinetic energy within only a few milliseconds. Moreover, in torsional spine reconnection, an accelerated particle can escape either along the spine axis or on the fan plane depending on its injection position. However, in torsional fan reconnection, the particle is only allowed to accelerate along the spine axis. In addition, in both regimes, the particle's trajectory and final kinetic energy depend on the injection position but adopting either spatially uniform or non-uniform localized plasma resistivity does not much influence the features of trajectory.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

On electric field in anti-de Sitter spacetime

In this paper we calculate the electromagnetic field produced using retarded Green's function in Anti-de Sitter spacetime (AdS). Since this spacetime is non-globally hyperbolic and has no Cauchy surface, we only consider the field originated from a charge moving along its geodesic in the region consists of points covered by future null geodesic of the charge.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Classification of constraints and degrees of freedom for quadratic discrete actions

We provide a comprehensive classification of constraints and degrees of freedom for variational discrete systems governed by quadratic actions. This classification is based on the different types of null vectors of the Lagrangian two-form and employs the canonical formalism developed in Dittrich and Höhn [“Constraint analysis for variational discrete systems,” J. Math. Phys. 54, 093505 (2013); e-print http://arxiv.org/abs/arXiv:1303.4294 [math-ph]] and Höhn [“Quantization of systems with temporally varying discretization I: Evolving Hilbert spaces,” J. Math. Phys. 55, 083508 (2014); e-print http://arxiv.org/abs/arXiv:1401.6062 [gr-qc]]. The analysis is carried out in both the classical and quantum theory and applies to systems with both temporally varying or constant discretization. In particular, it is shown explicitly how changes in the discretization, e.g., resulting from canonical coarse graining or refining operations or an evolving background geometry, change the dynamical content of the system. It is demonstrated how, on a temporally varying discretization, constraints, Dirac observables, symmetries, reduced phase spaces, and physical Hilbert spaces become spacetime region dependent. These results are relevant for free field theory on an evolving lattice and linearized discrete gravity models.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Koszul information geometry and Souriau Lie group thermodynamics

The François Massieu 1869 idea to derive some mechanical and thermal properties of physical systems from 'Characteristic Functions', was developed by Gibbs and Duhem in thermodynamics with the concept of potentials, and introduced by Poincaré in probability. This paper deals with generalization of this Characteristic Function concept by Jean-Louis Koszul in Mathematics and by Jean-Marie Souriau in Statistical Physics. The Koszul-Vinberg Characteristic Function (KVCF) on convex cones will be presented as cornerstone of 'Information Geometry' theory, defining Koszul Entropy as Legendre transform of minus the logarithm of KVCF, and Fisher Information Metrics as hessian of these dual functions, invariant by their automorphisms. In parallel, Souriau has extended the Characteristic Function in Statistical Physics looking for other kinds of invariances through co-adjoint action of a group on its momentum space, defining physical observables like energy, heat and momentum as pure geometrical objects. In covariant Souriau model, Gibbs equilibriums states are indexed by a geometric parameter, the Geometric (Planck) Temperature, with values in the Lie algebra of the dynamical Galileo/Poincaré groups, interpreted as a space-time vector, giving to the metric tensor a null Lie derivative. Fisher Information metric appears as the opposite of the derivative of Mean 'Moment map' by geometric temperature, equivalent to a Geometric Capacity or Specific Heat. These elements has been developed by author in [10][11].

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Investigating the origin of efficiency droop by profiling the temperature across the multi-quantum well of an operating light-emitting diode

Performance degradation resulting from efficiency droop during high-power operation is a critical problem in the development of high-efficiency light-emitting diodes (LEDs). In order to resolve the efficiency droop and increase the external quantum efficiency of LEDs, the droop's origin should be identified first. To experimentally investigate the cause of efficiency droop, we used null-point scanning thermal microscopy to quantitatively profile the temperature distribution on the cross section of the epi-layers of an operating GaN-based vertical LED with nanoscale spatial resolution at four different current densities. The movement of temperature peak towards the p-GaN side as the current density increases suggests that more heat is generated by leakage current than by Auger recombination. We therefore suspect that at higher current densities, current leakage becomes the dominant cause of the droop problem.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Investigation and quantification of nonlinearity using surrogate data in a glow discharge plasma

Detection of nonlinearity has been carried out in periodic and aperiodic floating potential fluctuations of DC glow discharge plasma by generating surrogate data using iterative amplitude adjusted Fourier transform method. We introduce “delay vector variance” analysis (DVV) for the first time, which allows reliable detection of nonlinearity and provides some easy to interpret diagram conveying information about the nature of the experimental floating potential fluctuations (FPF). The method of false nearest neighbourhood is deployed on the FPF's to find a good embedding so as to be acquainted with the precise knowledge of m, which is desirable for carrying out DVV analysis. The emergence of nonlinearity with increase in discharge voltage has been ensured by taking into consideration the total energy present in different band of frequencies excited due to nonlinear processes. Rejection of null hypothesis has been verified by performing the rank test method that confirms the presence of nonlinearity quantitatively.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Evolution of field line helicity during magnetic reconnection

We investigate the evolution of field line helicity for magnetic fields that connect two boundaries without null points, with emphasis on localized finite-B magnetic reconnection. Total (relative) magnetic helicity is already recognized as an important topological constraint on magnetohydrodynamic processes. Field line helicity offers further advantages because it preserves all topological information and can distinguish between different magnetic fields with the same total helicity. Magnetic reconnection changes field connectivity and field line helicity reflects these changes; the goal of this paper is to characterize that evolution. We start by deriving the evolution equation for field line helicity and examining its terms, also obtaining a simplified form for cases where dynamics are localized within the domain. The main result, which we support using kinematic examples, is that during localized reconnection in a complex magnetic field, the evolution of field line helicity is dominated by a work-like term that is evaluated at the field line endpoints, namely, the scalar product of the generalized field line velocity and the vector potential. Furthermore, the flux integral of this term over certain areas is very small compared to the integral of the unsigned quantity, which indicates that changes of field line helicity happen in a well-organized pairwise manner. It follows that reconnection is very efficient at redistributing helicity in complex magnetic fields despite having little effect on the total helicity.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Quantifying three dimensional reconnection in fragmented current layers

There is growing evidence that when magnetic reconnection occurs in high Lundquist number plasmas such as in the Solar Corona or the Earth's Magnetosphere it does so within a fragmented, rather than a smooth current layer. Within the extent of these fragmented current regions, the associated magnetic flux transfer and energy release occur simultaneously in many different places. This investigation focusses on how best to quantify the rate at which reconnection occurs in such layers. An analytical theory is developed which describes the manner in which new connections form within fragmented current layers in the absence of magnetic nulls. It is shown that the collective rate at which new connections form can be characterized by two measures; a total rate which measures the true rate at which new connections are formed and a net rate which measures the net change of connection associated with the largest value of the integral of E{sub ||} through all of the non-ideal regions. Two simple analytical models are presented which demonstrate how each should be applied and what they quantify.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

X-point position dependence of edge intrinsic toroidal rotation on the Tokamak à Configuration Variable

Recent theoretical work predicts intrinsic toroidal rotation in the tokamak edge to depend strongly on the normalized major radial position of the X-point. With this motivation, we conducted a series of Ohmic L-mode shots on the Tokamak à Configuration Variable, moving the X-point from the inboard to the outboard edge of the last closed flux surface in both lower and upper single null configurations. The edge toroidal rotation evolved from strongly co-current for an inboard X-point to either vanishing or counter-current for an outboard X-point, in agreement with the theoretical expectations. The whole rotation profile shifted roughly rigidly with the edge rotation, resulting in variation of the peak core rotation by more than a factor of two. Core rotation reversals had little effect on the edge rotation. Edge rotation was slightly more counter-current for unfavorable than favorable ∇B drift discharges.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Thin Fresnel zone plate lenses for focusing underwater sound

A Fresnel zone plate (FZP) lens of the Soret type creates a focus by constructive interference of waves diffracted through open annular zones in an opaque screen. For underwater sound below MHz frequencies, a large FZP that blocks sound using high-impedance, dense materials would have practical disadvantages. We experimentally and numerically investigate an alternative approach of creating a FZP with thin (0.4λ) acoustically opaque zones made of soft silicone rubber foam attached to a thin (0.1λ) transparent rubber substrate. An ultra-thin (0.0068λ) FZP that achieves higher gain is also proposed and simulated which uses low-volume fraction, bubble-like resonant air ring cavities to construct opaque zones. Laboratory measurements at 200 kHz indicate that the rubber foam can be accurately modeled as a lossy fluid with an acoustic impedance approximately 1/10 that of water. Measured focal gains up to 20 dB agree with theoretical predictions for normal and oblique incidence. The measured focal radius of 0.68λ (peak-to-null) agrees with the Rayleigh diffraction limit prediction of 0.61 λ/NA (NA = 0.88) for a low-aberration lens.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

On the role of topological complexity in spontaneous development of current sheets

The computations presented in this work aim to asses the importance of field line interlacing on spontaneous development of current sheets. From Parker's magnetostatic theorem, such development of current sheets is inevitable in a topologically complex magnetofluid, with infinite electrical conductivity, at equilibrium. Relevant initial value problems are constructed by superposition of two untwisted component fields, each component field being represented by a pair of global magnetic flux surface. The intensity of field line interlacing is then specified by the relative amplitude of the two superposed fields. The computations are performed by varying this relative amplitude. Also to have a direct visualization of current sheet formation, we follow the evolution of flux surfaces instead of the vector magnetic field. An important finding of this paper is in the demonstration that initial field lines having intense interlacing tend to develop current sheets which are distributed throughout the computational domain with no preference for topologically favorable sites like magnetic nulls or field reversal layers. The onsets of these current sheets are attributed to favorable contortions of magnetic flux surfaces where two oppositely directed parts of the same field line or different field lines come to close proximity. However, for less intensely interlaced field lines, the simulations indicate development of current sheets at sites only where the magnetic topology is favorable. These current sheets originate as two sets of anti-parallel complimentary field lines press onto each other.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗