Synchrotron Radiation From Radiatively Inefficient Accretion Flow Simulations: Applications to Sgr A*

We calculate synchrotron radiation in three-dimensional pseudo-Newtonian magnetohydrodynamic simulations of radiatively inefficient accretion flows. We show that the emission is highly variable at optically thin frequencies, with order of magnitude variability on time-scales as short as the orbital period near the last stable orbit; this emission is linearly polarized at the 20-50 % level due to the coherent toroidal magnetic field in the flow. At optically thick frequencies, both the variability amplitude and polarization fraction decrease significantly with decreasing photon frequency. We argue that these results are broadly consistent with the observed properties of Sgr A* at the Galactic Center, including the rapid infrared flaring.Comment: Accepted for publication in Ap

Numerical simulations of the Fourier transformed Vlasov-Maxwell system in higher dimensions --- Theory and applications

We present a review of recent developments of simulations of the Vlasov-Maxwell system of equations using a Fourier transform method in velocity space. In this method, the distribution functions for electrons and ions are Fourier transformed in velocity space, and the resulting set of equations are solved numerically. In the original Vlasov equation, phase mixing may lead to an oscillatory behavior and sharp gradients of the distribution function in velocity space, which is problematic in simulations where it can lead to unphysical electric fields and instabilities and to the recurrence effect where parts of the initial condition recur in the simulation. The particle distribution function is in general smoother in the Fourier transformed velocity space, which is desirable for the numerical approximations. By designing outflow boundary conditions in the Fourier transformed velocity space, the highest oscillating terms are allowed to propagate out through the boundary and are removed from the calculations, thereby strongly reducing the numerical recurrence effect. The outflow boundary conditions in higher dimensions including electromagnetic effects are discussed. The Fourier transform method is also suitable to solve the Fourier transformed Wigner equation, which is the quantum mechanical analogue of the Vlasov equation for classical particles.Comment: 41 pages, 19 figures. To be published in Transport Theory and Statistical Physics. Proceedings of the VLASOVIA 2009 Workshop, CIRM, Luminy, Marseilles, France, 31 August - 4 September 200

A web-based resource for radiation safety courses

CQU runs regular courses for intending Radiation Safety Officers from a variety of industry sectors. Participants have varying levels of prior knowledge to bring to the intensive three day program. To cater for those students whose prior knowledge of radiation physics is weak the print-based media for the course have been converted into a CD-ROM incorporating animations, worked examples, progress checks and self-assessment items. This allows the students who need extra study time on the basic physics concepts to cover as much as possible of the material before commencement of the course. The first course sessions can then concentrate on students’ individual problems and the required program content. This approach also allows the student with strong prior learning to submit an assessment test and be awarded a partial course exemption. Although the material selection is specifically for Radiation Safety training the CD-ROM has also been used in bridging courses, as reinforcement for first year students, and within our Engineering Technology Instrumentation program. The number of students who have used the CD-ROM is not statistically significant but student feedback on the material is very positive

Comparison between resistive and collisionless double tearing modes for nearby resonant surfaces

The linear instability and nonlinear dynamics of collisional (resistive) and collisionless (due to electron inertia) double tearing modes (DTMs) are compared with the use of a reduced cylindrical model of a tokamak plasma. We focus on cases where two q = 2 resonant surfaces are located a small distance apart. It is found that regardless of the magnetic reconnection mechanism, resistivity or electron inertia, the fastest growing linear eigenmodes may have high poloidal mode numbers m ~ 10. The spectrum of unstable modes tends to be broader in the collisionless case. In the nonlinear regime, it is shown that in both cases fast growing high-m DTMs lead to an annular collapse involving small magnetic island structures. In addition, collisionless DTMs exhibit multiple reconnection cycles due to reversibility of collisionless reconnection and strong ExB flows. Collisionless reconnection leads to a saturated stable state, while in the collisional case resistive decay keeps the system weakly dynamic by driving it back towards the unstable equilibrium maintained by a source term.Comment: 15 pages, 9 figure

A gyro-gauge independent minimal guiding-center reduction by Lie-transforming the velocity vector field

International audienceWe introduce a gyro-gauge independent formulation of a simplified guiding-center reduction, which removes the fast time-scale from particle dynamics by Lie-transforming the velocity vector field. This is close to Krylov-Bogoliubov method of averaging the equations of motion, although more geometric. At leading order, the Lie-transform consists in the generator of Larmor gyration, which can be explicitly inverted, while working with gauge-independent coordinates and operators, by using the physical gyro-angle as a (constrained) coordinate. This brings both the change of coordinates and the reduced dynamics of the minimal guiding-center reduction order by order in a Larmor radius expansion. The procedure is algorithmic and the reduction is systematically derived up to full second order, in a more straightforward way than when Lie-transforming the phase-space Lagrangian or averaging the equations of motion. The results write up some structures in the guiding-center expansion. Extensions and limitations of the method are considered

The Evolution of Low-Level Radioactive Waste (LLW) Disposal Practices at the Savannah River Site Coupled with Vigorous Stakeholder Interaction

Low-level radioactive waste (LLW) disposal practices at SRS evolved from trench disposal with little long-term performance basis to disposal in robust concrete vaults, again without modeling long-term performance. Now, based on an assessment of long-term performance of various waste forms and methods of disposal, the LLW disposal program allows for a ''smorgasbord'' of various disposal techniques and waste forms, all modeled to ensure long-term performance is understood. New disposal techniques include components-in-grout, compaction/volume reduction prior to disposal, and trench disposal of extremely low activity waste. Additionally, factoring partition coefficient (Kd) measurements based on waste forms has been factored into performance models. This paper will trace the development of the different disposal methods, and the extensive public communications effort that resulted in endorsement of the changes by the SRS Citizens Advisory Board

Toroidal plasma rotation in the PLT tokamak with neutral-beam injection

Toroidal plasma rotation in the Princeton Large Torus, PLT, has been measured for various plasma and neutral beam injection conditions. Measurements of the plasma rotational velocities were made from Doppler shifts of appropriate spectral lines and include data from both hydrogen and deuterium beams and co- and counter-injection at several electron densities. Without injection, a small but consistent toroidal rotation exists in a direction opposite to the plasma current (counter-direction) in the plasma center but parallel to the current (co-direction) in the plasma periphery. Using these measured velocities and the plasma density and temperature gradients, radial electron fields can be determined from theory, giving E/sub r / approx. = 40 V/cm near the plasma center and E/sub r/ approx. = 10 V/cm near the plasma edge. Insertion of a local, 2.5 percent magnetic well produced no observable effect on the beam driven rotation. Modeling of the time evolution and radial distribution of the rotation allows one to deduce an effective viscosity of the order of (1 to 5) x 10/sup 4/ cm/sup 2//sec

Plasma Oscillations and Expansion of an Ultracold Neutral Plasma

We report the observation of plasma oscillations in an ultracold neutral plasma. With this collective mode we probe the electron density distribution and study the expansion of the plasma as a function of time. For classical plasma conditions, i.e. weak Coulomb coupling, the expansion is dominated by the pressure of the electron gas and is described by a hydrodynamic model. Discrepancies between the model and observations at low temperature and high density may be due to strong coupling of the electrons.Comment: 4 pages, 4 figures. Accepted Phys. Rev. Let

Analysis of drift effects on the tokamak power scrape-off width using SOLPS-ITER

SOLPS-ITER, a comprehensive 2D scrape-off layer modeling package, is used to examine the physical mechanisms that set the scrape-off width (lambda(q)) for inter-ELM power exhaust. Guided by Goldston\u27s heuristic drift (HD) model, which shows remarkable quantitative agreement with experimental data, this research examines drift effects on lambda(q) in a DIII-D H-mode magnetic equilibrium. As a numerical expedient, a low target recycling coefficient of 0.9 is used in the simulations, resulting in outer target plasma that is sheath limited instead of conduction limited as in the experiment. Scrape-off layer (SOL) particle diffusivity (D-SOL) is scanned from 1 to 0.1 m(2) s(-1). Across this diffusivity range, outer divertor heat flux is dominated by a narrow (similar to 3-4 mm when mapped to the outer midplane) electron convection channel associated with thermoelectric current through the SOL from outer to inner divertor. An order-unity up-down ion pressure asymmetry allows net ion drift flux across the separatrix, facilitated by an artificial mechanism that mimics the anomalous electron transport required for overall ambipolarity in the HD model. At D-SOL = 0.1 m(2) s(-1), the density fall-off length is similar to the electron temperature fall-off length, as predicted by the HD model and as seen experimentally. This research represents a step toward a deeper understanding of the power scrape-off width, and serves as a basis for extending fluid modeling to more experimentally relevant, high-collisionality regimes