Anomalies and time reversal invariance in relativistic hydrodynamics: The second order and higher dimensional formulations
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Kharzeev, Dmitri E.↗
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Lower-Hybrid Wave Structures and Interactions With Electrons Observed in Magnetotail Reconnection Diffusion Regions
We investigate waves close to the lower-hybrid frequency in 12 magnetotail reconnection electron diffusion region (EDR) events with guide field levels of near-zero to 30%. In about half of the events, the wave vector has a small component along the current sheet normal, consistent with known lower-hybrid drift wave properties, but the perpendicular magnetic field fluctuations can be comparable or greater than the parallel component, a feature unique to the waves inside and adjacent to EDRs. Another new wave property is that the wave vector has a significant component along the current sheet normal in some events and completely along the normal for one event. In 1/4of the events, the ∇∙𝑷(sub 𝑒) term has a significant contribution to the wave electric field, possibly a feature of lower-hybrid waves more likely to exist in the diffusion region than further away from the X-line. Electron temperature variations are correlated with the wave potential, due to wave electric field acceleration and crossings at the corrugated separatrix region with different amounts of mixing between reconnection inflowing and outflowing populations. The latter also leads to the anti-correlation between parallel and perpendicular temperature components. Using four-spacecraft measurements, the magnetic field line twisting is demonstrated by the correlated fluctuations in (∇×𝑽(sub 𝐸×𝐵))||and (∇×𝐁)||. The lower-hybrid wave in the EDR of weak guide field reconnection may be generated near separatrices and penetrate to the mid-plane or locally generated, and the latter possibility is beyond the prediction of previous reconnection simulations.
Magnetic reconnection↗
Particle energization in 3D magnetic reconnection of relativistic pair plasmas
We present large scale 3D particle-in-cell simulations to examine particle energization in magnetic reconnection of relativistic electron-positron (pair) plasmas. The initial configuration is set up as a relativistic Harris equilibrium without a guide field. These simulations are large enough to accommodate a sufficient number of tearing and kink modes. Contrary to the non-relativistic limit, the linear tearing instability is faster than the linear kink instability, at least in our specific parameters. We find that the magnetic energy dissipation is first facilitated by the tearing instability and followed by the secondary kink instability. Particles are mostly energized inside the magnetic islands during the tearing stage due to the spatially varying electric fields produced by the outflows from reconnection. Secondary kink instability leads to additional particle acceleration. Accelerated particles are, however, observed to be thermalized quickly. The large amplitude of the vertical magnetic field resulting from the tearing modes by the secondary kink modes further help thermalizing the non-thermal particles generated from the secondary kink instability. Implications of these results for astrophysics are briefly discussed.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Guiding center equations for the magnetic dipole
Since the discovery of Van Allen radiation belts in the 1960s, observations of energetic ions trapped in the Earth's dipole magnetic field have illustrated the remarkable confinement properties of this configuration. As such, it has been used for confining a hot plasma for nuclear fusion studies, starting from the pioneering work of Bo Lehnert and Akira Hasegawa, in the Levitated Dipole Experiment (LDX) at MIT until 2011 and in the RT-1 experiment at the University of Tokyo. More recently, the dipole has been subject to a renewed interest for fusion studies by a couple of startups and for smaller applications as a cold plasma source. While the equilibrium and magneto-hydrodynamic stability of the dipole have been investigated quite in detail, neoclassical properties of the dipole are comparatively much less known: the dipole is more known in geophysics than in fusion science. For this reason, in this paper, we propose a set of Hamiltonian, guiding-center equations to describe the motion of electrons and ions in a magnetic dipole configuration. We also developed a code, and we show the main features of particle motion, benchmarking our results with the analytical solutions for the bounce and precession motion, which are well documented in the literature. We also draw some general conclusions for the neoclassical transport in usual toroidal confinement schemes, such as the tokamak and the stellarator, pointing out the unique advantages of the dipole in confining energetic particles.
Hamiltonian mechanics↗
Magnetohydrodynamic actuation of droplets for millimetric planar fluidic systems
In this work, a magnetohydrodynamic method is proposed for the actuation of droplets in small-scale planar fluidic systems, providing an alternative to commonly used methods such as electrowetting-on-dielectric. Elementary droplet-based operations, including transport, merging, and mixing, are demonstrated. The forces acting on millimetric droplets are carefully investigated, with a primary focus on the magnetic actuation force and on the unbalanced capillary forces that arise due to hysteresis. A super-hydrophobic channel is 3D printed to guide the droplets, with thin wires installed as contact electrodes and permanent magnets providing a static magnetic field. It is shown that droplet motion is enhanced by increasing the droplet size and minimizing the electrode contact surface. The effects of channel geometry on threshold voltage and minimum moveable droplet volume are characterized. Finally, the presence of electrolysis is investigated and mitigating strategies are discussed.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Highlights for DOE ASCR Applied Math Office [Slides]
An efficient Picard-based solver is proposed for a novel energy conserving particle integrator preserving all first-order guiding center drifts and correct gyroradius for large time steps in arbitrary (non-uniform) magnetic fields. This research enables the efficient deployment of the novel asymptotic preserving (AP) particle orbit integrator into modern energy-conserving, implicit particle-in-cell codes, delivering a truly multiscale simulation capability.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Kinetic theory analysis of solar wind interaction with planetary objects
A purely kinetic treatment is proposed for the interaction of the solar wind with any small planetary object. Small refers to those cases where the solar wind proton's thermal gyroradius is arbitrarily taken to be greater than 0.1 radius of the object under investigation. The 'object' may possibly include an ionosphere or magnetosphere. The collisionless Boltzmann equation, neglecting the magnetic field, is used to calculate steady-state profiles of density and velocity around the obstacle. A low density plasma void in the umbral region and a compression in the penumbral region are clearly found. The present technique, despite its neglect of the interplanetary magnetic field, is proposed as an alternative zeroth order approach to the continuum, local magnetic anomaly, and guiding center approaches used by others for the particular case of moon. Some recent, potentially relevant, observations on and in front of the moon are discussed.
Wu, S. T.↗
Hollow-cylinder waveguide isolators for use at millimeter wavelengths
A semiconductor waveguide isolator consisting of a hollow column of a semiconductor mounted coaxially is considered in a circular waveguide in a longitudinal dc magnetic field. An elementary and physical analysis based on the excitation of plane waves in the guide and a more rigorous mode matching analysis are presented. These theoretical predictions are compared with experimental results for an InSb isolator at 94GHz and 75 K.
Kanda, M.↗
Hollow-cylinder waveguide isolators for use at millimeter wavelengths
The device considered in this study is a semiconductor waveguide isolator consisting of a hollow column of a semiconductor mounted coaxially in a circular waveguide in a longitudinal dc magnetic field. An elementary and physical analysis based on the excitation of plane waves in the guide and a more rigorous mode-matching analysis (MMA) are presented. These theoretical predictions are compared with experimental results for an InSb isolator at 94 GHz and 75 K.
Kanda, M.↗
Radiation and Polarization Signatures from Magnetic Reconnection in Relativistic Jets. I. A Systematic Study
Blazars are relativistic magnetized plasma outflows from supermassive black holes that point very close to our line of sight. Their emission is nonthermal-dominated and highly variable across the entire electromagnetic spectrum. Relativistic magnetic reconnection has been proposed as the driver of particle acceleration during blazar flares. While recent particle-in-cell (PIC) simulations have self-consistently studied the evolution of magnetic reconnection and particle acceleration therein, the resulting radiation signatures have not been systematically explored. In particular, the polarization signatures, which directly reflect the characteristic strongly dynamical evolution of magnetic field during reconnection, have not been carefully investigated. In this paper, we present a systematic study of radiation and polarization signatures arising from magnetic reconnection in blazars, based on combined PIC and polarized radiation transfer simulations with various physical parameters. We identify a harder-when-brighter trend in the spectral evolution. Moreover, higher-frequency bands (ultraviolet to X-ray) tend to flare earlier than lower-frequency bands (infrared to optical) in the synchrotron spectral component. Most importantly, polarization signatures appear more variable with higher frequencies. We find that the variation in temporal polarization depends strongly on the guide field strength. Specifically, reconnection with a significant guide field component leads to a very high polarization degree that contradicts typical blazar observations, while large polarization angle rotations are unique signatures of magnetic reconnection between nearly antiparallel magnetic field lines. These rotations are at least 90° and can extend to >180°, and they may be in either direction. Furthermore, these results imply that blazars that have shown large polarization angle rotations intrinsically have more nearly antiparallel magnetic field morphology.
79 ASTRONOMY AND ASTROPHYSICS↗
Guided Self-Assembly and Control of Vortices in Ensembles of Active Magnetic Rollers
Active magnetic colloids are capable of rich collective behavior and complex self-organization. The interplay between short- and long-range interactions taking place away from equilibrium often results in a spontaneous formation of localized dynamic microstructures. Here we report a method for guided self-assembly and control of self-organized colloidal vortices emerging in a ferromagnetic particle ensemble energized by a uniaxial alternating (ac) magnetic field. The structure of a vortex composed of rolling magnetic particles can be stabilized and manipulated by means of an additional strongly localized alternating magnetic field provided by a minicoil. By tuning the parameters of the localized field, we effectively control the dimensions and particle number density in the vortex. We find that the roller vortex self-organization is assisted by field-induced magnetic "steering" rather than magnetic field gradients and is only possible while the system is in the active (magnetic rollers) state. In this work, we demonstrate that parameters of the emergent vortex are efficiently tuned by a phase shift between alternating magnetic fields. Overall, the method for assisted self-organization of rolling magnetic colloids into a vortex with on-demand characteristics suggests a new tool for active matter control and manipulation that may lead to a development of new approaches toward the guided microscopic transport in active particle systems.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
The effect of adiabatic focusing upon charged-particle propagation in random magnetic fields
The charged particles considered are scattered by random fields while they propagate along the diverging lines of force of a spatially inhomogeneous guiding field. Their longitudinal transport is described in terms of the eigenfunctions of a Sturm-Liouville operator which incorporates the effect of adiabatic focusing along with that of scattering. The relaxation times and characteristic velocities which appear in this matrix formulation of the transport problem are graphed and tabulated. Explicit formulas which describe the particle-density profile that results from a localized impulsive injection are derived for two different regimes. In the first regime, where focusing is relatively weak, a diffusive mode of propagation is dominant, but coherent modes are also present, and they become prominent as the intensity of focusing increases. In the second regime, where focusing is strong and where diffusion does not occur, the propagation is purely coherent. The existence of this supercoherent mode of particle transport opens up many possibilities for the interpretation of astrophysical phenomena.
Earl, J. A.↗
A UNIFIED PARTICLE DIFFUSION THEORY FOR CROSS-FIELD SCATTERING: SUBDIFFUSION, RECOVERY OF DIFFUSION, AND DIFFUSION IN THREE-DIMENSIONAL TURBULENCE
A new nonlinear theory for cosmic-ray scattering across the mean magnetic field is derived. This theory can be applied for arbitrary turbulence geometry. Previous theories such as the extended nonlinear guiding center theory are deduced as special limits. Furthermore, the new theory can explain subdiffusive transport for slab turbulence and the recovery of diffusion for slab/two-dimensional and three-dimensional turbulence. The nonlinear standard theory for field line wandering can be obtained as a special limit.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Parametrial Boost Using Midline Shielding Results in an Unpredictable Dose to Tumor and Organs at Risk in Combined External Beam Radiotherapy and Brachytherapy for Locally Advanced Cervical Cancer
Purpose: Midline-blocked boost (MBB) fields are frequently used in the treatment of locally advanced cervical cancer. The purpose of this study was to evaluate the dose contribution from MBBs to tumor and organs at risk. Methods and Materials: Six patients with locally advanced cervical cancer (IIB-IIIB) treated with definitive chemoradiotherapy and magnetic resonance imaging (MRI)-guided brachytherapy were analyzed. A three-phase plan was modeled: 45 Gy (1.8 Gy per fraction) four-field box, 9 Gy (1.8 Gy per fraction) MBB (midline-shielded anteroposterior/posteroanterior fields), and intracavitary MRI-guided brachytherapy boost of 28 Gy (7 Gy per fraction). Midline shields 3, 4, and 5 cm wide were simulated for each patient. Brachytherapy and MBB plans were volumetrically summed. The rectum, sigmoid, and bladder minimum dose in the most exposed 2 cm{sup 3} of an organ at risk (D{sub 2cc}) and high-risk clinical target volume (HR-CTV) and intermediate-risk clinical target volume (IR-CTV) D90 and D100 were evaluated. The intended HR-CTV D90 was 85 Gy or greater, and the intended IR-CTV D90 was greater than 60 Gy. Results: After a 4-cm MBB, HR-CTV D90 remained lower than 85 Gy in all cases (mean, 74 Gy; range, 64-82 Gy). High-risk clinical target volume (85 Gy) coverage increased slightly from 73% (range, 64-82%) to 78% (range, 69-88%). Mean IR-CTV D90 increased from 56 Gy (range, 53-64 Gy) to 62 Gy (range, 59-67 Gy). Intermediate-risk clinical target volume 60-Gy dose coverage increased from 81% (range, 72-96%) to 96% (range, 90-100%). The mean volume irradiated to 85 Gy increased by 14 cm{sup 3} (range, 10-22 cm{sup 3}), whereas the volume irradiated to 60 Gy increased from 276 cm{sup 3} (range, 185-417 cm{sup 3}) to 592 cm{sup 3} (range, 385-807 cm{sup 3}). Bladder, rectum, or sigmoid D{sub 2cc} increased by more than 50% of the boost dose in 4 of 6 patients. Conclusions: Midline-blocked boosts contribute substantial dose to rectum, sigmoid, and bladder D{sub 2cc}. HR-CTV dose and 85-Gy coverage remain compromised in large tumors despite MBB. IR-CTV 60-Gy coverage improved at the expense of a considerable increase in volume of normal tissue irradiated to 60 Gy.
62 RADIOLOGY AND NUCLEAR MEDICINE↗
Particle Acceleration in Kinetic Simulations of Nonrelativistic Magnetic Reconnection with Different Ion–Electron Mass Ratios
By means of fully kinetic particle-in-cell simulations, we study whether the proton-to-electron mass ratio m i /m e influences the energy spectrum and underlying acceleration mechanism during magnetic reconnection. While kinetic simulations are critical to studying particle acceleration during magnetic reconnection, a reduced m i /m e is often used to alleviate the demanding computing resources, which leads to artificial scale separation between electron and proton scales. Recent kinetic simulations with high mass ratios have indicated new regimes of reconnection, as electron pressure anisotropy develops in the exhaust region and supports extended current layers. Here, we study whether different m i /m e changes the particle acceleration processes by performing a series of simulations with different mass ratio (m i /m e = 25–400) and guide field strength in a low-β plasma. We find that mass ratio does not strongly influence reconnection rate, magnetic energy conversion, ion internal energy gain, plasma energization processes, ion energy spectra, and the acceleration mechanisms for high-energy ions. Simulations with different mass ratios are different in electron acceleration processes, including electron internal energy gain, electron energy spectrum, and the acceleration efficiencies for high-energy electrons. We find that high-energy electron acceleration becomes less efficient when the mass ratio gets larger because the Fermi -like mechanism associated with particle curvature drift becomes less efficient. These results indicate that when particle curvature drift dominates high-energy particle acceleration, the further the particle kinetic scales are from the magnetic field curvature scales (~d i ), the weaker the acceleration will be.
79 ASTRONOMY AND ASTROPHYSICS↗
Propagation of attosecond electron bunches along the cone-and-channel target
Generation and propagation of attosecond electron bunches along a cone-and-channel target are investigated by particle-in-cell simulation. The target electrons are pulled out by the oscillating electric field of an intense laser pulse irradiating a cone target and accelerated forward along the cone walls. It is shown that the energetic electrons can be further guided and confined by a channel attached to the cone tip. The propagation of these electrons along the channel induces a strong quasistatic magnetic field as well as a sheath electric field since a part of the energetic electrons expands into the surrounding vacuum. The electromagnetic field in turn confines the surface currents. With the cone-and-channel target the energetic electrons can be much better collimated and propagate much farther than that from the classical cone target.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Magnetic Field Mapping of the CLAS12 Torus - A Comparative Study between the Engineering Model and Measurements at JLab
This study provides an overview of the magnetic field measurement and subsequent electromagnetic re-modeling of the CLAS12 torus during the commissioning of the magnet in the fall of 2016. The CLAS12 detector in Hall B is part of the 12 GeV Accelerator Upgrade project at Jefferson Lab. The torus magnet allows precise determination of particle momenta in the forward direction (~ 5 0 x 4 0 ) in the forward direction. The ability to do this requires we know the fB.dl of the torus to within an accuracy of 0.5 % or better. To achieve this, an accurate model of the field along the particle paths is required. The TOSCA code is used to generate a full 3D simulation of the magnetic envelope of the magnet as designed. Experimentally the actual magnetic field within the magnet was surveyed to confirm the model design and to measure the deviation from the ideal case. The magnetic field deviations are attributed to manufacturing variability and assembly tolerances. A final model was created with allowances for these deviations guided by the survey data to create a more precise field integral model which greatly improves momentum resolution capability allowing it to deliver the required specifications.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Direct prediction of nonlinear tearing mode saturation using a variational principle
It is shown that the variational principle of multi-region relaxed magnetohydrodynamics (MRxMHD) can be used to predict the stability and nonlinear saturation of tearing modes in strong guide field configurations without resolving the dynamics and without explicit dependence on the plasma resistivity. While the magnetic helicity is not a good invariant for tearing modes, we show that the saturated tearing mode can be obtained as an MRxMHD state of a priori unknown helicity by appropriately constraining the current profile. The predicted saturated island width in a tearing-unstable force-free slab equilibrium is shown to reproduce the theoretical scaling at small values of Δ' and the scaling obtained from resistive magnetohydrodynamics simulations at large Δ'.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗