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A classical molecular dynamics study is applied to simulate the scattering of NO from Pt(111) in the energy range of 0.3–1 eV. The solid consists of a large number of crystal atoms that interact via an anharmonic nearest-neighbor potential. The NO–Pt(111) interaction potential is constructed as a pairwise additive potential with a well depth of 1 eV for the N end of the molecule towards the surface and purely repulsive for the O end. The in-plane scattering results obtained with this model potential are compared with recent experiments for NO–Pt(111). The angular intensity distributions, the final translational energy, as well as the rotational energy distributions with the corresponding alignment are in qualitative agreement with those experimental results. A detailed examination of the collision dynamics shows that multiple collisions with the surface results predominantly in superspecular scattering. The rotational angular momentum of the scattered molecules exhibits a preference for cartwheeling alignment and the rotational energy distributions for specular and normal exit angles can be described with a Boltzmann distribution, whereas for grazing exit angles they are distinctly non-Boltzmann. The latter structure results from a cutoff in the rotational excitation by the attraction of the well. The high rotational excitation clearly originates from molecules that initially are oriented with the O end towards the surface, whereas for the low rotational excitation this orientation preference disappears. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Rotational excitation of NO scattered from Pt(111) has been measured for incoming energies from 0.3 to 1.6 eV. For an initial energy of 0.3 eV a clear rotational rainbow, which we assign to O-end collisions, is visible at superspecular exit angles. This is very surprising, since NO–Pt(111) is a chemisorption system with a binding energy of 1 eV. Sharp, pronounced rainbows are visible in the range of incoming kinetic energies where the initial sticking coefficient is high. For an initial energy of 0.3 eV the initial sticking coefficient is 0.9, and at this energy the clearest rotational rainbow is observed. In contrast, at an initial energy of 1.6 eV the sticking coefficient is 0.5 and no rotational rainbow is observed. At subspecular exit angles the distributions are indistinguishable from a Boltzmann distribution at all the energies investigated, and show a clear energy dependence. Boltzmann-type distributions at high incident energy indicate a thorough redistribution of the available energy, although they cannot be explained in terms of a simple statistical model. The scattering results are interpreted as indicating a competition between direct scattering from the repulsive wall and indirect scattering via the deep potential well. Most of the direct scattering can be assigned to O-end collisions with the surface, where the binding energy is expected to be much smaller. Indirect scattering becomes more important at higher energies as more molecules are able to escape the well promptly. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Recent experiments in the DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] show that the resistive wall mode (RWM) can be stabilized by smaller values of plasma rotation than previously reported. Stable discharges have been observed with beta up to 1.4 times the no-wall kink stability limit and ion rotation velocity (measured from CVI emission) less than 0.3% of the Alfvén speed at all integer rational surfaces, in contrast with previous DIII-D experiments that indicated critical values of 0.7%–2.5% of the local Alfvén speed. Preliminary stability calculations for these discharges, using ideal magnetohydrodynamics with a drift-kinetic dissipation model, are consistent with the new experimental results. A key feature of these experiments is that slow plasma rotation was achieved by reducing the neutral beam torque. Earlier experiments with strong neutral beam torque used “magnetic braking” by applied magnetic perturbations to slow the rotation, and resonant effects of these perturbations may have led to a larger effective rotation threshold. In addition, the edge rotation profile may have a critical role in determining the RWM stability of these low-torque plasmas. [ABSTRACT FROM AUTHOR]

For the situation in which a specific laser beam waist is required at a certain position, we develop a practical method of selecting focal lengths for a combination of two lenses and of calculating their positions. The method provides, in an easy graphical way, the solution of this two-lens focusing problem of Gaussian beams by finding the crossing point of two parametric curves.