10,351 research outputs found
Magnetostrictive behaviour of thin superconducting disks
Flux-pinning-induced stress and strain distributions in a thin disk
superconductor in a perpendicular magnetic field is analyzed. We calculate the
body forces, solve the magneto-elastic problem and derive formulas for all
stress and strain components, including the magnetostriction . The
flux and current density profiles in the disk are assumed to follow the Bean
model. During a cycle of the applied field the maximum tensile stress is found
to occur approximately midway between the maximum field and the remanent state.
An effective relationship between this overall maximum stress and the peak
field is found.Comment: 8 pages, 6 figures, submitted to Supercond. Sci. Technol., Proceed.
of MEM03 in Kyot
Dust Evolution and the Formation of Planetesimals
The solid content of circumstellar disks is inherited from the interstellar
medium: dust particles of at most a micrometer in size. Protoplanetary disks
are the environment where these dust grains need to grow at least 13 orders of
magnitude in size. Our understanding of this growth process is far from
complete, with different physics seemingly posing obstacles to this growth at
various stages. Yet, the ubiquity of planets in our galaxy suggests that planet
formation is a robust mechanism. This chapter focuses on the earliest stages of
planet formation, the growth of small dust grains towards the gravitationally
bound "planetesimals", the building blocks of planets. We will introduce some
of the key physics involved in the growth processes and discuss how they are
expected to shape the global behavior of the solid content of disks. We will
consider possible pathways towards the formation of larger bodies and conclude
by reviewing some of the recent observational advances in the field.Comment: 43 pages, 6 figures. Chapter in International Space Science Institute
(ISSI) Book on "The Disk in Relation to the Formation of Planets and their
Proto-atmospheres", published in Space Science Reviews by Springe
Large N limit of orbifold field theories
We consider certain orbifoldization of the field theories that
leads to field theories in 4 dimensions. These theories were
recently analyzed using the string theory perturbation technique. It was found
that in the large limit all correlation functions of the orbifold theories
coincide with those of , modulo the rescaling of the gauge coupling
constant. In this paper we repeat the same analysis using the field theoretical
language.Comment: 12 pages, 3 figures, harvmac. Minor change
The Saturation Limit of the Magnetorotational Instability
Simulations of the magnetorotational instability (MRI) in a homogeneous
shearing box have shown that the asymptotic strength of the magnetic field
declines steeply with increasing resolution. Here I model the MRI driven dynamo
as a large scale dynamo driven by the vertical magnetic helicity flux. This
growth is balanced by large scale mixing driven by a secondary instability. The
saturated magnetic energy density depends almost linearly on the vertical
height of the typical eddies. The MRI can drive eddies with arbitrarily large
vertical wavenumber, so the eddy thickness is either set by diffusive effects,
by the magnetic tension of a large scale vertical field component, or by
magnetic buoyancy effects. In homogeneous, zero magnetic flux, simulations only
the first effect applies and the saturated limit of the dynamo is determined by
explicit or numerical diffusion. The exact result depends on the numerical
details, but is consistent with previous work, including the claim that the
saturated field energy scales as the gas pressure to the one quarter power
(which we interpret as an artifact of numerical dissipation). The magnetic
energy density in a homogeneous shearing box will tend to zero as the
resolution of the simulation increases, but this has no consequences for the
dynamo or for angular momentum transport in real accretion disks. The claim
that the saturated state depends on the magnetic Prandtl number may also be an
artifact of simulations in which microphysical transport coefficients set the
MRI eddy thickness. Finally, the efficiency of the MRI dynamo is a function of
the ratio of the Alfv\'en velocity to the product of the pressure scale height
and the local shear. As this approaches unity from below the dynamo reaches
maximum efficiency.Comment: Accepted by The Astrophysical Journa
Oscillatory regimes of the thermomagnetic instability in superconducting films
The stability of superconducting films with respect to oscillatory precursor
modes for thermomag- netic avalanches is investigated theoretically. The
results for the onset threshold show that previous treatments of
non-oscillatory modes have predicted much higher thresholds. Thus, in film
supercon- ductors, oscillatory modes are far more likely to cause
thermomagnetic breakdown. This explains the experimental fact that flux
avalanches in film superconductors can occur even at very small ramping rates
of the applied magnetic field. Closed expressions for the threshold magnetic
field and temperature, as well oscillation frequency, are derived for different
regimes of the oscillatory thermomagnetic instability.Comment: 5 pages, 5 figure
Dendritic flux avalanches in rectangular superconducting films -- numerical simulations
Dendritic flux avalanches is a frequently encountered instability in the
vortex matter of type II superconducting films at low temperatures. Previously,
linear stability analysis has shown that such avalanches should be nucleated
where the flux penetration is deepest. To check this prediction we do numerical
simulations on a superconducting rectangle. We find that at low substrate
temperature the first avalanches appear exactly in the middle of the long
edges, in agreement with the predictions. At higher substrate temperature,
where there are no clear predictions from the theory, we find that the location
of the first avalanche is decided by fluctuations due to the randomly
distributed disorder.Comment: 3 pages, 2 figure
Protoplanetary Disk Turbulence Driven by the Streaming Instability: Non-Linear Saturation and Particle Concentration
We present simulations of the non-linear evolution of streaming instabilities
in protoplanetary disks. The two components of the disk, gas treated with grid
hydrodynamics and solids treated as superparticles, are mutually coupled by
drag forces. We find that the initially laminar equilibrium flow spontaneously
develops into turbulence in our unstratified local model. Marginally coupled
solids (that couple to the gas on a Keplerian time-scale) trigger an upward
cascade to large particle clumps with peak overdensities above 100. The clumps
evolve dynamically by losing material downstream to the radial drift flow while
receiving recycled material from upstream. Smaller, more tightly coupled solids
produce weaker turbulence with more transient overdensities on smaller length
scales. The net inward radial drift is decreased for marginally coupled
particles, whereas the tightly coupled particles migrate faster in the
saturated turbulent state. The turbulent diffusion of solid particles, measured
by their random walk, depends strongly on their stopping time and on the
solids-to-gas ratio of the background state, but diffusion is generally modest,
particularly for tightly coupled solids. Angular momentum transport is too weak
and of the wrong sign to influence stellar accretion. Self-gravity and
collisions will be needed to determine the relevance of particle overdensities
for planetesimal formation.Comment: Accepted for publication in ApJ (17 pages). Movies of the simulations
can be downloaded at http://www.mpia.de/~johansen/research_en.ph
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