10,337 research outputs found
Time resolved pattern evolution in a large aperture laser
We have measured quasi-instantaneous transverse patterns in a broad aperture
laser. Non-ordered patterns yielding to boundary determined regular structures
in progressive time-integrated recording are observed. The linear analysis and
numerical integration of the full Maxwell-Bloch equations allow us to interpret
the features of the experiment. We show that this system being far from
threshold cannot be fully understood with a perturbative model.Comment: 7 pages, 5 GIF figures . To be published in Phys. Rev. Let
Symplectic algorithm for constant-pressure molecular dynamics using a Nose-Poincare thermostat
We present a new algorithm for isothermal-isobaric molecular-dynamics
simulation. The method uses an extended Hamiltonian with an Andersen piston
combined with the Nos'e-Poincar'e thermostat, recently developed by Bond,
Leimkuhler and Laird [J. Comp. Phys., 151, (1999)]. This
Nos'e-Poincar'e-Andersen (NPA) formulation has advantages over the
Nos'e-Hoover-Andersen approach in that the NPA is Hamiltonian and can take
advantage of symplectic integration schemes, which lead to enhanced stability
for long-time simulations. The equations of motion are integrated using a
Generalized Leapfrog Algorithm and the method is easy to implement, symplectic,
explicit and time reversible. To demonstrate the stability of the method we
show results for test simulations using a model for aluminum.Comment: 7 page
New capabilities for Monte Carlo simulation of deuteron transport and secondary products generation
Several important research programs are dedicated to the development of facilities based on deuteron accelerators. In designing these facilities, the definition of a validated computational approach able to simulate deuteron transport and evaluate deuteron interactions and production of secondary particles with acceptable precision is a very important issue. Current Monte Carlo codes, such as MCNPX or PHITS, when applied for deuteron transport calculations use built-in semi-analytical models to describe deuteron interactions. These models are found unreliable in predicting neutron and photon generated by low energy deuterons, typically present in those facilities.
We present a new computational tool, resulting from an extension of the MCNPX code, which improve significantly the treatment of problems where any secondary product (neutrons, photons, tritons, etc.) generated by low energy deuterons reactions could play a major role. Firstly, it handles deuteron evaluated data libraries, which allow describing better low deuteron energy interactions. Secondly, it includes a reduction variance technique for production of secondary particles by charged particle-induced nuclear interactions, which allow reducing drastically the computing time needed in transport and nuclear response calculations. Verification of the computational tool is successfully achieved. This tool can be very helpful in addressing design issues such as selection of the dedicated neutron production target and accelerator radioprotection analysis. It can be also helpful to test the deuteron cross-sections under development in the frame of different international nuclear data program
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