Search

The solution of Poisson's equation and the equation of motion by numerical methods is discussed. By considering the truncation error due to the use of difference equations in the one-dimensional case, it is shown, on the basis of a perturbation theory, that the error in the solution is proportional to the mesh size h . This theory is shown to be in good agreement with numerical solutions obtained on a computer. A correction formula is then derived which makes it possible, by obtaining sointions for two different mesh sizes h a and h b , to form a space-charge-flow solution accurate to second order in h . It is also shown that more accurate forms of the difference equation may be obtained to represent the steady space-charge flow. By these various means, it is possible to increase the accuracy of numerical solutions by an order of magnitude or more. Thus, by using a coarse mesh, but better accuracy, the speed of computation may be considerably increased.

Computer techniques for analyzing an axially symmetric or two-dimensional electrode system with an emitting surface can be used to obtain cathode current density distribution, beam minimum radius and its location, as well as possible electrode current interception. Here, comparison is made of available experimental data from two axially symmetric Pierce-type guns. The choice of the two axially symmetric Pierce-type guns was made for their different value of PV/T ratio, an invariant for transverse scaling. One of them has a low ratio of PV/T (\approx 0.1 \times 10^{-6}) , which signifies considerable beam spread due to the effects of the Maxwellian velocity distribution of the thermal electrons leaving the cathode. The other gun has a high perveance of 2 \times 10^{-6} , and thus relatively high PV/T ratio where beam spreading is mainly due to space-charge forces alone. Good agreement with experimental data is shown. Computer techniques of this type can be a valuable tool for diagnostic purposes of any type of space-charge-flow devices (e.g., electron guns, ion guns for space propulsion, linear accelerators, etc.) and enable the engineer-designer to arrive quickly and cheaply at an optimum configuration.

Convergence and accuracy criteria have been determined for computer techniques analyzing an axially symmetric or two-dimensional electrode system with an emitting surface. The convergence criterion has been drawn from analytical studies of the one-dimensional model and verified numerically by the computer program for an analytically designed, axially symmetric Pierce-type electrode system. The accuracy of the computer program is illustrated with two electron gun configurations of known analytical solutions, and comparison of cathode current emission, beam minimum radius, and its location is made with experimental data of a high perveance Pierce-type gun.

Gold with a nanoporous sponge-like morphology, generated by leaching of AuAg alloys is presented as a new unsupported material system for catalytic applications. The role of residual silver for catalytic activity towards CO oxidation in the temperature range from −20 to 50°C has been investigated by comparison with Au and Au/Ag zeolite catalysts. As revealed by a systematic variation of the silver content in the zeolite catalysts, bimetallic systems exhibit a significantly higher activity than pure gold, probably due to activation/dissociation of molecular oxygen by silver. By STEM tomography we can unambiguously prove that at least some of the particles form inside the zeolite lattice.

The National Ignition Facility point design uses a five-layer capsule to modify the X-ray absorption in order to achieve optimized shock timing. A stepwise copper dopant design defines the layer st...

We discuss progress towards fusion ignition on the NIF. We cover some of the setbacks encountered during the progress of the research at the National Ignition Facility (NIF), but also cover the advances that have been made. The research strategy for the future will also be discussed.

The initial activation probability s0 for reactive dissociation of n‐butane on Ir(110)–(1×2) exhibits at least two reaction channels. For one the direct collisional activation probability is at least 0.21 independent of incident kinetic energy, increasing to near 0.7 with decreasing surface temperature and kinetic energy of the incident beam of butane as a result of trapping of the species in the molecule–surface potential well. At kinetic energies above 120 kJ/mol the reaction probability again increases due to the onset of an activated route. Comparison to the results obtained with ethane suggests that energy transfer to the surface dissipates energy in the collision so that less energy is available to penetrate the longer range repulsive barrier in order to form carbon–metal and hydrogen–metal bonds.

The adsorption of NO on the diamond C(111)-(2x1) surface was studied in the presence of 226nm laser radiation and in the absence of such excitation. Without laser radiation, sticking was not observed on either the diamond C(111)-(1x1) or the reconstructed (2x1) surfaces for ambient (background) NO exposure in an ultra-high vacuum apparatus. The adsorption probability was estimated to be less than 10-8 based on the detection limit of oxygen in Auger electron spectroscopy of 0.3% of a monolayer. Diamond surfaces exposed to ambient NO (purified at 150 K) and 600 μJ/cm2 laser pulses significantly increased the adsorption of NO (see figure 1). Exposures were performed on and off the A2Σ1/2 (v'=0, J'=13/2) ← X2∏3/2(v"=0,J"=13/2) resonance; no significant differences were found. 226 nm photons were of sufficient energy to excite electrons across the ~5.5 eV band gap in diamond. However, the reaction saturated at an oxygen adatom coverage of less than 2% of a monolayer which suggested the adsorption proceeded on "defect sites" on the surface. The natural type IIa diamond was cleaved in air to expose the (111) surface and mechanically polished with 0.25 μm diamond abrasive in olive oil to produce a hydrogen-saturated surface with a mirror finish. Heating the sample to 800 K gave sharp (1x1) LEED patterns. Heating the sample to above 1275 K to remove the hydrogen produced three rotated domains of the (2x1) reconstructed surface.

The adsorption and reaction of acetonitrile (CH3CN) on clean and oxygen covered Ag(110) surfaces has been studied using temperature programmed reaction spectroscopy (TPRS), isotope exchange, chemical displacement reactions and high resolution electron energy loss spectroscopy (EELS). On the clean Ag(110) surface, CH3CN was reversibly adsorbed, desorbing with an activation energy of 10 kcal mol-1 at 166 K from a monolayer state and at 158 K from a multilayer state. Vibrational spectra of multilayer, monolayer and sub-monolayer CH3CN were in excellent agreement with that of gas phase CH3CN indicating that CH3CN is only weakly bonded to the clean Ag(110) surface. On the partially oxidized surface CH3CN reacts with atomic oxygen to form adsorbed CH2CN, OH and H2O in addition to forming another molecular adsorption state with a desorption peak at 240 K. This molecular state shows a CN stretching frequency of 1840 cm-1, which is indicative of substantial rehybridization of the CN bond and is associated with side-on coordination via the π system. The CH2CN species is stable up to 430 K, where C-H bond breaking and reformation begins, leading to the formation of CH3CN at 480 K and HCN at 510 K and leaving only carbon on the surface. In the presence of excess oxygen atoms C-H bond breaking and reformation is more facile leading to additional desorption peaks for CH3CN and H2O at 420 K. This destabilizing effect of O(a) on Ch2CN(a) is explained in terms of an anionic (CH2CN-1) species. Comparison of the vibrational spectra from CH2CN(a) and CD2CN(a) supports the following assignment for the modes of adsorbed CH2CN: ν(Ag-C) 215: δ(CCN) 545; ϱt(CH2) 695; ϱw(CH2) 850; ν(C-C) 960; ϱr(CH2) 1060; δ(CH2) 1375; ν(CN) 2075; and ν(CH2) 2940 cm-1. These results serve to further indicate the wide applicability of the acid-base reaction concept for reactions between gas phase Bronsted acids and adsorbed oxygen atoms on solver surfaces.