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Impact Ionization (impact + ionization)

Distribution by Scientific Domains

Chemistry	50%

Selected Abstracts

Parallel bandwidth characteristics calculations for thin avalanche photodiodes on a SGI Origin 2000 supercomputer

CONCURRENCY AND COMPUTATION: PRACTICE & EXPERIENCE, Issue 12 2004
Yi Pan
Abstract An important factor for high-speed optical communication is the availability of ultrafast and low-noise photodetectors. Among the semiconductor photodetectors that are commonly used in today's long-haul and metro-area fiber-optic systems, avalanche photodiodes (APDs) are often preferred over p - i - n photodiodes due to their internal gain, which significantly improves the receiver sensitivity and alleviates the need for optical pre-amplification. Unfortunately, the random nature of the very process of carrier impact ionization, which generates the gain, is inherently noisy and results in fluctuations not only in the gain but also in the time response. Recently, a theory characterizing the autocorrelation function of APDs has been developed by us which incorporates the dead-space effect, an effect that is very significant in thin, high-performance APDs. The research extends the time-domain analysis of the dead-space multiplication model to compute the autocorrelation function of the APD impulse response. However, the computation requires a large amount of memory space and is very time consuming. In this research, we describe our experiences in parallelizing the code in MPI and OpenMP using CAPTools. Several array partitioning schemes and scheduling policies are implemented and tested. Our results show that the code is scalable up to 64 processors on a SGI Origin 2000 machine and has small average errors. Copyright © 2004 John Wiley & Sons, Ltd. [source]

Turbulent Dynamics of Beryllium Seeded Plasmas at the Edge of Tokamaks

CONTRIBUTIONS TO PLASMA PHYSICS, Issue 3-5 2010
R.V. Shurygin
Abstract Numerical simulation of turbulent MHD dynamics of beryllium seeded plasmas at the edge of tokamaks is performed. The model is based on the 4-fluid {,, n, pe, pi } reduced nonlinear Braginsky's MHD equations. Neutral hydrogen flow from the wall is described with a diffusion model. Beryllium line radiation is taken into consideration. The Be ion distribution over ionization states is calculated using the reduced model. Electron impact ionization, three body, photo- and dielectronic recombination and charge-exchange with neutral hydrogen are taken into account. Coronal equilibrium is not supposed. Simulations are performed for T-10 parameters. Radial distributions of averaged temperatures and their fluctuation levels, species flows, impurity radiation power, and impurity ions concentrations are obtained as functions of the Be concentration at the wall. The impurity radiation is shown to act on the turbulent oscillation level significantly if the total Be concentration at the wall exceeds 3 · 1011cm,3. The impurity turbulent transversal flow is directed inward and exceeds neoclassical flow significantly. The parallel conductivity and, as a consequence, turbulent transport are increased significantly by impurity radiation. The radiation loss dependence on the neutral Hydrogen concentration at the wall is also examined. The hydrogen concentration increasing the plasma density also rises. The relative beryllium concentration decreases. In total, these two effects are compensated, and the level of radiation losses is changed insignificantly (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Extreme multielectron ionization of elemental clusters in ultraintense laser fields

ISRAEL JOURNAL OF CHEMISTRY, Issue 2 2007
Andreas Heidenreich
In this paper we present computational and theoretical studies of extreme multielectron ionization in Xen clusters (n = 55-2171, initial cluster radii R0 = 8.7-31.0 Å) driven by ultraintense Gaussian infrared laser fields (peak intensity IM = 1015 -1020 W cm,2, temporal pulse length , = 10-100 fs, and frequency v = 0.35fs,1). The microscopic approach, which rests on three sequential-parallel processes of inner ionization, nanoplasma formation, and outer ionization, properly describes the high ionization levels (with the formation of {Xeq+}n with q = 5-36), the inner/outer cluster ionization mechanisms, and the nanoplasma response. The cluster size and laser intensity dependence of the inner ionization levels are determined by a complex superposition of laser-induced barrier suppression ionization (BSI), with the contributions of the inner field BSI manifesting ignition enhancement and screening retardation effects, together with electron impact ionization. The positively charged nanoplasma produced by inner ionization reveals intensity-dependent spatial inhomogeneity and spatial anisotropy, and can be either persistent (at lower intensities) or transient (at higher intensities). The nanoplasma is depleted by outer ionization that was semiquantitatively described by the cluster barrier suppression electrostatic model, which accounts for the cluster size, laser intensity, and pulse length dependence of the outer ionization yield. [source]

Metastable ion study of organosilicon compounds.

JOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 7 2001
(CH3)2Si(OCH3), CH3SiH(OCH3), Part XIII: dimethoxydimethylsilane, dimethoxymethylsilane
Abstract Unimolecular metastable fragmentations of dimethoxydimethylsilane, (CH3)2Si(OCH3)2 (MW 120, 1), and dimethoxymethylsilane, CH3SiH(OCH3)2 (MW 106, 2), upon electron impact ionization have been studied by means of mass-analyzed ion kinetic energy (MIKE) spectrometry and the D-labeling technique in conjunction with thermochemistry. The results have been compared with those of the corresponding carbon analogues, 2,2-dimethoxypropane, (CH3)2C(OCH3)2 (MW 104, 3) and 1,1-dimethoxyethane, CH3CH(OCH3)2 (MW 90, 4). In analogy with the cases of 3 and 4, both molecular ions from 1 and 2 are formed at very low abundance at 70 eV, and begin to decompose by the expulsion of the substituents (H, CH3 or OCH3) on the central silicon atom. These decompositions are followed by the loss of a formaldehyde molecule (CH2O), as commonly observed in the mass spectra of methoxysilanes. Further, an ethylene (C2H4) or a dimethyl ether (CH3OCH3) molecule loss is observed in the fragmentation of some intermediate ions generated from 1+· and 2+·, but the mechanisms are different than those in the cases of 3 and 4. Some of these fragmentations are also different than those reported previously. The relative abundance of the ions in many MIKE spectra is explained by the extension of the Stevenson,Audier rule. The reaction, which is in contrast to the rule, however, is rationalized by the energy of the transition state for the reaction, estimated by semi-empirical molecular orbital calculation. The peak at m/z 59 from 2+· consists only of CH3OSi+ ion, whereas the peak from 1+· consists of two different ions, CH3OSi+ and (CH3)2Si+H. The ions CH3OSi+ from 1+· and 2+· are generated by at least two and three separate routes respectively. Copyright © 2001 John Wiley & Sons, Ltd. [source]