23,217 research outputs found
What can we infer about the underlying physics from burst distributions observed in an RMHD simulation ?
We determine that the sizes of bursts in mean-square current density in a
reduced magnetohydrodynamic (RMHD)simulation follow power-law probability
density function (PDF). The PDFs for burst durations and waiting time between
bursts are clearly not exponential and could also be power-law. This suffices
to distinguish their behaviour from the original Bak et al. sandpile model
which had exponential waiting time PDFs. However, it is not sufficient to
distinguish between turbulence, some other SOC-like models, and other red noise
sources.Comment: In press, Planetary and Space Science. Proceedings of a session at
European Geophysical Society General Assembly, Nice, 200
Power law burst and inter-burst interval distributions in the solar wind: turbulence or dissipative SOC ?
We calculate for the first time the probability density functions (PDFs) P of
burst energy e, duration T and inter-burst interval tau for a known turbulent
system in nature. Bursts in the earth-sun component of the Poynting flux at 1
AU in the solar wind were measured using the MFI and SWE experiments on the
NASA WIND spacecraft. We find P(e) and P(T) to be power laws, consistent with
self-organised criticality (SOC). We find also a power law form for P(tau) that
distinguishes this turbulent cascade from the exponential P(tau) of ideal SOC,
but not from some other SOC-like sandpile models. We discuss the implications
for the relation between SOC and turbulence.Comment: 3 pages, 1 figure. Submitted to PRL on 25th February 2000. Revised
version re-submitted on 9th May 2000. Second revised version submitted Phys.
Rev. E on 26th June, 200
Simulator test to study hot-flow problems related to a gas cooled reactor
An advance study of materials, fuel injection, and hot flow problems related to the gas core nuclear rocket is reported. The first task was to test a previously constructed induction heated plasma GCNR simulator above 300 kW. A number of tests are reported operating in the range of 300 kW at 10,000 cps. A second simulator was designed but not constructed for cold-hot visualization studies using louvered walls. A third task was a paper investigation of practical uranium feed systems, including a detailed discussion of related problems. The last assignment resulted in two designs for plasma nozzle test devices that could be operated at 200 atm on hydrogen
PACE and EISCAT radar observations of short-lived flow bursts on the nightside
Concurrent observations from two widely spaced radar experiments of quasi periodic flow bursts in the nightside are presented. The flow bursts closely resemble single radar observations reported by Williams et al. By using the Polar Anglo-American Conjugate Experiment (PACE) HF radar array at Halley Bay in conjunction with the EISCAT Common Program (CP) 2-D experiment, the flow bursts are shown to be a global phenomenon and important information as to their development and propagation can be determined
Nuclear Spins as Quantum Memory in Semiconductor Nanostructures
We theoretically consider solid state nuclear spins in a semiconductor
nanostructure environment as long-lived, high-fidelity quantum memory. In
particular, we calculate, in the limit of a strong applied magnetic field, the
fidelity versus time of P donor nuclear spins in random bath environments of Si
and GaAs, and the lifetime of excited intrinsic spins in polarized Si and GaAs
environments. In the former situation, the nuclear spin dephases due to
spectral diffusion induced by the dipolar interaction among nuclei in the bath.
We calculate the decay of nuclear spin quantum memory in the context of Hahn
and Carr-Purcell-Meiboom-Gill (CPMG) refocused spin echoes using a formally
exact cluster expansion technique which has previously been successful in
dealing with electron spin dephasing in a solid state nuclear spin bath. With
decoherence dominated by transverse dephasing (T2), we find it feasible to
maintain high fidelity (losses of less than 10^{-6}) quantum memory on nuclear
spins for times of the order of 100 microseconds (GaAs:P) and 1 to 2
milliseconds (natural Si:P) using CPMG pulse sequences of just a few (~2-4)
applied pulses. We also consider the complementary situation of a central
flipped intrinsic nuclear spin in a bath of completely polarized nuclear spins
where decoherence is caused by the direct flip-flop of the central spin with
spins in the bath. Exact numerical calculations that include a sufficiently
large neighborhood of surrounding nuclei show lifetimes on the order of 1-5 ms
for both GaAs and natural Si. Our calculated nuclear spin coherence times may
have significance for solid state quantum computer architectures using
localized electron spins in semiconductors where nuclear spins have been
proposed for quantum memory storage
Recommended from our members
The excitation of plasma convection in the high-latitude ionosphere
Recent observations of ionospheric flows by ground-based radars, in particular by the European Incoherent Scatter (EISCAT) facility using the “Polar” experiment, together with previous analyses of the response of geomagnetic disturbance to variations of the interplanetary magnetic field (IMF), suggest that convection in the high-latitude ionosphere should be considered to be the sum of two intrinsically time-dependent patterns, one driven by solar wind-magnetosphere coupling at the dayside magnetopause, the other by the release of energy in the geomagnetic tail (mainly by dayside and nightside reconnection, respectively). The flows driven by dayside coupling are largest on the dayside, where they usually dominate, are associated with an expanding polar cap area, and are excited and decay on ∼10-min time scales following southward and northward turnings of the IMF, respectively. The latter finding indicates that the production of new open flux at the dayside magnetopause excites magnetospheric and ionospheric flow only for a short interval, ∼10 min, such that the flow driven by this source subsequently decays on this time scale unless maintained by the production of more open flux tubes. Correspondingly, the flows excited by the release of energy in the tail, mainly during substorms, are largest on the nightside, are associated with a contracting polar cap boundary, and are excited on ∼1-hour time scales following a southward turn of the IMF. In general, the total ionospheric flow will be the sum of the flows produced by these two sources, such that due to their different response times to changes in the IMF, considerable variations in the flow pattern can occur for a given direction and strength of the IMF. Consequently, the ionospheric electric field cannot generally be regarded as arising from a simple mapping of the solar wind electric field along open flux tubes
Large stellar disks in small elliptical galaxies
We present the rotation velocities V and velocity dispersions sigma along the
principal axes of seven elliptical galaxies less luminous than M_B= -19.5.
These kinematics extend beyond the half-light radii for all systems in this
photometrically selected sample. At large radii the kinematics not only confirm
that rotation and "diskiness" are important in faint ellipticals, as was
previously known, but also demonstrate that in most sample galaxies the stars
at large galactocentric distances have (V/sigma)_max of about 2, similar to the
disks in bona-fide S0 galaxies. Comparing this high degree of ordered stellar
motion in all sample galaxies with numerical simulations of dissipationless
mergers argues against mergers with mass ratios <=3:1 as an important mechanism
in the final shaping of low-luminosity ellipticals, and favors instead the
dissipative formation of a disk.Comment: 11 pages LaTex with 4 Postscript figure
VETA-I x ray test analysis
This interim report presents some definitive results from our analysis of the VETA-I x-ray testing data. It also provides a description of the hardware and software used in the conduct of the VETA-I x-ray test program performed at the MSFC x-ray Calibration Facility (XRCF). These test results also serve to supply data and information to include in the TRW final report required by DPD 692, DR XC04. To provide an authoritative compendium of results, we have taken nine papers as published in the SPIE Symposium, 'Grazing Incidence X-ray/EUV Optics for Astronomy and Projection Lithography' and have reproduced them as the content of this report
- …