12,478 research outputs found
Forbidden transitions in the helium atom
Nonrelativistically forbidden, single-photon transition rates between low
lying states of the helium atom are rigorously derived within quantum
electrodynamics theory. Equivalence of velocity and length gauges, including
relativistic corrections is explicitly demonstrated. Numerical calculations of
matrix elements are performed with the use of high precision variational wave
functions and compared to former results.Comment: 11 pages, 1 figure, submitted to Phys. Rev.
QED calculation of the n=1 and n=2 energy levels in He-like ions
We perform ab initio QED calculations of energy levels for the and
states of He-like ions with the nuclear charge in the range -100.
The complete set of two-electron QED corrections is evaluated to all orders in
the parameter \aZ. Uncalculated contributions to energy levels come through
orders \alpha^3 (\aZ)^2, \alpha^2 (\aZ)^7, and higher. The calculation
presented is the first treatment for excited states of He-like ions complete
through order \alpha^2 (\aZ)^4. A significant improvement in accuracy of
theoretical predictions is achieved, especially in the high- region.Comment: 23 pages, 5 figure
Hysteresis and competition between disorder and crystallization in sheared and vibrated granular flow
Experiments on spherical particles in a 3D Couette cell vibrated from below
and sheared from above show a hysteretic freezing/melting transition. Under
sufficient vibration a crystallized state is observed, which can be melted by
sufficient shear. The critical line for this transition coincides with equal
kinetic energies for vibration and shear. The force distribution is
double-peaked in the crystalline state and single-peaked with an approximately
exponential tail in the disordered state. A linear relation between pressure
and volume () exists for a continuum of partially and/or
intermittently melted states over a range of parameters
Architectures for Human Exploration of Near Earth Asteroids
The presentation explores human exploration of Near Earth Asteroid (NEA) key factors including challenges of supporting humans for long-durations in deep-space, incorporation of advanced technologies, mission design constraints, and how many launches are required to conduct a round trip human mission to a NEA. Topics include applied methodology, all chemical NEA mission operations, all nuclear thermal propulsion NEA mission operations, SEP only for deep space mission operations, and SEP/chemical hybrid mission operations. Examples of mass trends between datasets are provided as well as example sensitivity of delta-v and trip home, sensitivity of number of launches and trip home, and expected targets for various transportation architectures
The Twist of the Draped Interstellar Magnetic Field Ahead of the Heliopause: A Magnetic Reconnection Driven Rotational Discontinuity
Based on the difference between the orientation of the interstellar
and the solar magnetic fields, there was an expectation that the magnetic field
direction would rotate dramatically across the heliopause (HP). However, the
Voyager 1 spacecraft measured very little rotation across the HP. Previously we
showed that the twists as it approaches the HP and acquires a strong
T component (East-West). Here we establish that reconnection in the eastern
flank of the heliosphere is responsible for the twist. On the eastern flank the
solar magnetic field has twisted into the positive N direction and reconnects
with the Southward pointing component of the . Reconnection drives a
rotational discontinuity (RD) that twists the into the -T direction
and propagates upstream in the interstellar medium towards the nose. The
consequence is that the N component of is reduced in a finite width
band upstream of the HP. Voyager 1 currently measures angles
() close to solar values. We present MHD simulations
to support this scenario, suppressing reconnection in the nose region while
allowing it in the flanks, consistent with recent ideas about reconnection
suppression from diamagnetic drifts. The jump in plasma (the plasma to
magnetic pressure) across the nose of HP is much greater than in the flanks
because the heliosheath is greater there than in the flanks.
Large-scale reconnection is therefore suppressed in the nose but not at the
flanks. Simulation data suggest that will return to its pristine
value past the HP.Comment: 19 pages, 5 figures, submitte
Simulating radiative shocks in nozzle shock tubes
We use the recently developed Center for Radiative Shock Hydrodynamics
(CRASH) code to numerically simulate laser-driven radiative shock experiments.
These shocks are launched by an ablated beryllium disk and are driven down
xenon-filled plastic tubes. The simulations are initialized by the
two-dimensional version of the Lagrangian Hyades code which is used to evaluate
the laser energy deposition during the first 1.1ns. The later times are
calculated with the CRASH code. This code solves for the multi-material
hydrodynamics with separate electron and ion temperatures on an Eulerian
block-adaptive-mesh and includes a multi-group flux-limited radiation diffusion
and electron thermal heat conduction. The goal of the present paper is to
demonstrate the capability to simulate radiative shocks of essentially
three-dimensional experimental configurations, such as circular and elliptical
nozzles. We show that the compound shock structure of the primary and wall
shock is captured and verify that the shock properties are consistent with
order-of-magnitude estimates. The produced synthetic radiographs can be used
for comparison with future nozzle experiments at high-energy-density laser
facilities.Comment: submitted to High Energy Density Physic
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