291,780 research outputs found
Observation of Terahertz Radiation via the Two-Color Laser Scheme with Uncommon Frequency Ratios
In the widely-studied two-color laser scheme for terahertz (THz) radiation
from a gas, the frequency ratio of the two lasers is usually fixed at
1:2. We investigate THz generation with uncommon frequency
ratios. Our experiments show, for the first time, efficient THz generation with
new ratios of 1:4 and 2:3. We observe that the THz
polarization can be adjusted by rotating the longer-wavelength laser
polarization and the polarization adjustment becomes inefficient by rotating
the other laser polarization; the THz energy shows similar scaling laws with
different frequency ratios. These observations are inconsistent with multi-wave
mixing theory, but support the gas-ionization model. This study pushes the
development of the two-color scheme and provides a new dimension to explore the
long-standing problem of the THz generation mechanism.Comment: 6 pages, 3 figure
A Conservative Discontinuous Galerkin Scheme With O(N-2) Operations In Computing Boltzmann Collision Weight Matrix
In the present work, we propose a deterministic numerical solver for the homogeneous Boltzmann equation based on Discontinuous Galerkin (DG) methods. The weak form of the collision operator is approximated by a quadratic form in linear algebra setting. We employ the property of >shifting symmetry> in the weight matrix to reduce the computing complexity from theoretical O(N-3) down to O(N-2), with N the total number of freedom for d-dimensional velocity space. In addition, the sparsity is also explored to further reduce the storage complexity. To apply lower order polynomials and resolve loss of conserved quantities, we invoke the conservation routine at every time step to enforce the conservation of desired moments (mass, momentum and/or energy), with only linear complexity. Due to the locality of the DG schemes, the whole computing process is well parallelized using hybrid OpetiMP and MPI. The current work only considers integrable angular cross-sections under elastic and/or inelastic interaction laws. Numerical results on 2-D and 3-D problems are shown.Mathematic
Cosmic age, Statefinder and diagnostics in the decaying vacuum cosmology
As an extension of CDM, the decaying vacuum model (DV) describes the
dark energy as a varying vacuum whose energy density decays linearly with the
Hubble parameter in the late-times, , and
produces the matter component. We examine the high- cosmic age problem in
the DV model, and compare it with CDM and the Yang-Mills condensate
(YMC) dark energy model. Without employing a dynamical scalar field for dark
energy, these three models share a similar behavior of late-time evolution. It
is found that the DV model, like YMC, can accommodate the high- quasar APM
08279+5255, thus greatly alleviates the high- cosmic age problem. We also
calculate the Statefinder and the {\it Om} diagnostics in the model. It
is found that the evolutionary trajectories of and in the DV
model are similar to those in the kinessence model, but are distinguished from
those in CDM and YMC. The in DV has a negative slope and
its height depends on the matter fraction, while YMC has a rather flat , whose magnitude depends sensitively on the coupling.Comment: 12 pages, 4 figures, with some correction
A finite-strain hyperviscoplastic model and undrained triaxial tests of peat
This paper presents a finite-strain hyperviscoplastic constitutive model
within a thermodynamically consistent framework for peat which was categorised
as a material with both rate-dependent and thermodynamic equilibrium hysteresis
based on the data reported in the literature. The model was implemented
numerically using implicit time integration and verified against analytical
solutions under simplified conditions. Experimental studies on the undrained
relaxation and loading-unloading-reloading behaviour of an undisturbed fibrous
peat were carried out to define the thermodynamic equilibrium state during
deviatoric loading as a prerequisite for further modelling, to fit particularly
those model parameters related to solid matrix properties, and to validate the
proposed model under undrained conditions. This validation performed by
comparison to experimental results showed that the hyperviscoplastic model
could simulate undrained triaxial compression tests carried out at five
different strain rates with loading/unloading relaxation steps.Comment: 30 pages, 16 figures, 4 tables. This is a pre-peer reviewed version
of manuscript submitted to the International Journal of Numerical and
Analytical Methods in Geomechanic
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