1,809 research outputs found
Non-perturbative statistical theory of intermittency in ITG drift wave turbulence with zonal flows
The probability distribution functions (PDFs) of momentum flux and zonal flow formation in ion-temperature-gradient (ITG) turbulence are investigated in two different models. The first is a general five-field model (ni, , Ti, Te, vi) where a reductive perturbation method is used to derive dynamical equations for drift waves and a zonal flow. The second is a reduced two-field model (, Ti) that has an exact non-linear solution (bipolar vortex soliton). In both models the exponential tails of the zonal flow PDFs are found with the same scaling ( ), but with different coefficients cZF. The PDFs of momentum flux is, however, found to be qualitatively different with the scaling (PDF ~ exp{âcMRs}), where s = 2 and s = 3/2 in the five and two-field models, respectively
The Operation of the Tokamak Fusion Test Reactor Tritium Facility
The TFTR tritium operations staff has successfully received, stored, handled, and processed over five hundred thousand curies of tritium for the purpose of supporting D-T (Deuterium-Tritium) operations at TFTR. Tritium operations personnel nominally provide continuous round the clock coverage (24 hours/day, 7 days/week) in shift complements consisting of I supervisor and 3 operators. Tritium Shift Supervisors and operators are required to have 5 years of operational experience in either the nuclear or chemical industry and to become certified for their positions. The certification program provides formal instruction, as well as on the job training. The certification process requires 4 to 6 months to complete, which includes an oral board lasting up to 4 hours at which time the candidate is tested on their knowledge of Tritium Technology and TFTR Tritium systems. Once an operator is certified, the training process continues with scheduled training weeks occurring once every 5 weeks. During D-T operations at TFTR the operators must evacuate the tritium area due to direct radiation from TFTR D-T pulses. During `` time operators maintain cognizance over tritium systems via a real time TV camera system. Operators are able to gain access to the Tritium area between TFTR D-T pulses, but have been excluded from die tritium area during D-T pulsing for periods up to 30 minutes. Tritium operators are responsible for delivering tritium gas to TFRR as well as processing plasma exhaust gases which lead to the deposition of tritium oxide on disposable molecular sieve beds (DMSB). Once a DMSB is loaded, the operations staff remove the expended DMSB, and replace it with a new DMSB container. The TFIR tritium system is operated via detailed procedures which require operator sign off for system manipulation. There are >300 procedures controlling the operation of the tritium systems
Spectroscopic biomedical imaging with the Medipix2 detector
This study confirms that the Medipix2 x-ray detector enables spectroscopic bio-medical plain radiography. We show that
the detector has the potential to provide new, useful information beyond the limited spectroscopic information of modern
dual-energy computed tomography (CT) scanners. Full spectroscopic 3D-imaging is likely to be the next major
technological advance in computed tomography, moving the modality towards molecular imaging applications. This
paper focuses on the enabling technology which allows spectroscopic data collection and why this information is useful.
In this preliminary study we acquired the first spectroscopic images of human tissue and other biological samples
obtained using the Medipix2 detector. The images presented here include the clear resolution of the 1.4mm long distal
phalanx of a 20 week old miscarried foetus, showing clear energy-dependent variations. The opportunities for further
research using the forthcoming Medipix3 detector are discussed and a prototype spectroscopic CT scanner (MARS,
Medipix All Resolution System) is briefly described
Charge occupancy of two interacting electrons on artificial molecules - exact results
We present exact solutions for two interacting electrons on an artificial
atom and on an artificial molecule made by one and two (single level) quantum
dots connected by ideal leads. Specifically, we calculate the accumulated
charge on the dots as function of the gate voltage, for various strengths of
the electron-electron interaction and of the hybridization between the dots and
the (one-dimensional) leads. With increasing of the (negative) gate voltage,
the accumulated charge in the two-electron ground state increases in gradual
steps from 0 to 1 and then to 2. The value 0 represents an "insulating" state,
where both electrons are bound to shallow states on the impurities. The value
of 1 corresponds to a "metal", with one electron localized on the dots and the
other extended on the leads. The value of 2 corresponds to another "insulator",
with both electrons strongly localized. The width of the "metallic" regime
diverges with strength of the electron-electron interaction for the single dot,
but remains very narrow for the double dot. These results are contrasted with
the simple Coulomb blockade picture.Comment: 12 pages, 7 figure
Velocity autocorrelation function of a Brownian particle
In this article, we present molecular dynamics study of the velocity
autocorrelation function (VACF) of a Brownian particle. We compare the results
of the simulation with the exact analytic predictions for a compressible fluid
from [6] and an approximate result combining the predictions from hydrodynamics
at short and long times. The physical quantities which determine the decay were
determined from separate bulk simulations of the Lennard-Jones fluid at the
same thermodynamic state point.We observe that the long-time regime of the VACF
compares well the predictions from the macroscopic hydrodynamics, but the
intermediate decay is sensitive to the viscoelastic nature of the solvent.Comment: 7 pages, 6 figure
Non-LTE Treatment of Fe II in Astrophysical Plasmas
We describe our implementation of an extremely detailed model atom of singly
ionized iron for NLTE computations in static and moving astrophysical plasmas.
Our model atom includes 617 levels, 13675 primary permitted transitions and up
to 1.2 million secondary transitions. Our approach guarantees that the total
iron opacity is included at the correct wavelength with reasonable memory and
CPU requirements. We find that the lines saturate the wavelength space, such
that special wavelength points inserted along the detailed profile functions
may be replaced with a statistical sampling method. We describe the results of
various test calculations for novae and supernovae.Comment: 17 pages, latex, aip style, no figures included, full text with
figures available at ftp://brian.la.asu.edu/pub/preprint/FeII-NLTE.ps.Z or at
http://brian.la.asu.edu
The Landau Pole and decays in the 331 bilepton model
We calculate the decay widths and branching ratios of the extra neutral boson
predicted by the 331 bilepton model in the framework of two
different particle contents. These calculations are performed taken into
account oblique radiative corrections, and Flavor Changing Neutral Currents
(FCNC) under the ansatz of Matsuda as a texture for the quark mass matrices.
Contributions of the order of are obtained in the branching
ratios, and partial widths about one order of magnitude bigger in relation with
other non- and bilepton models are also obtained. A Landau-like pole arise at
3.5 TeV considering the full particle content of the minimal model (MM), where
the exotic sector is considered as a degenerated spectrum at 3 TeV scale. The
Landau pole problem can be avoid at the TeV scales if a new leptonic content
running below the threshold at TeV is implemented as suggested by other
authors.Comment: 20 pages, 5 figures, LaTeX2
Ground state properties of the 2D disordered Hubbard model
We study the ground state of the two-dimensional (2D) disordered Hubbard
model by means of the projector quantum Monte Carlo (PQMC) method. This
approach allows us to investigate the ground state properties of this model for
lattice sizes up to , at quarter filling, for a broad range of
interaction and disorder strengths. Our results show that the ground state of
this system of spin-1/2 fermions remains localised in the presence of the
short-ranged Hubbard interaction.Comment: 7 pages, 9 figure
Renormalization Group Evolution of Dirac Neutrino Masses
There are good reasons why neutrinos could be Majorana particles, but there
exist also a number of very good reasons why neutrinos could have Dirac masses.
The latter option deserves more attention and we derive therefore analytic
expressions describing the renormalization group evolution of mixing angles and
of the CP phase for Dirac neutrinos. Radiative corrections to leptonic mixings
are in this case enhanced compared to the quark mixings because the hierarchy
of neutrino masses is milder and because the mixing angles are larger. The
renormalization group effects are compared to the precision of current and
future neutrino experiments. We find that, in the MSSM framework, radiative
corrections of the mixing angles are for large \tan\beta comparable to the
precision of future experiments.Comment: 19 pages, 5 figures; error in eq. 8 corrected, references adde
Kosterlitz Thouless Universality in Dimer Models
Using the monomer-dimer representation of strongly coupled U(N) lattice gauge
theories with staggered fermions, we study finite temperature chiral phase
transitions in (2+1) dimensions. A new cluster algorithm allows us to compute
monomer-monomer and dimer-dimer correlations at zero monomer density (chiral
limit) accurately on large lattices. This makes it possible to show
convincingly, for the first time, that these models undergo a finite
temperature phase transition which belongs to the Kosterlitz-Thouless
universality class. We find that this universality class is unaffected even in
the large N limit. This shows that the mean field analysis often used in this
limit breaks down in the critical region.Comment: 4 pages, 4 figure
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