bluuebunny/crossref_metadata_2025 huggingface dataset:
On the formula for the upper limit of photovoltaic solar energy conversion efficiency
Authors: A De Vos  , C C Grosjean  , H Pauwels
Published: July 2002

The formula for the maximum efficiency of photovoltaic energy conversion in tandem solar cells is discussed. This formula is obtained by the use of a thermodynamically founded diode I-V characteristic, and is compared to a previously published formula which was based on the classical diode I-V characteristic within the framework of the theory of Shockley and Queisser (1961). Series expansions of the complicated efficiency formula are presented. They enable an easy and accurate computation of the maximum efficiency and provide more insight in the basic limit of the photovoltaic effect.
A Mossbauer study of the effect of heat treatment on biotite micas
Authors: U Chandra  , S Lokanathan
Published: July 2002

Addison et al. (1962) and Vedder and Wilkins (1969) have proposed two alternative mechanisms to explain the oxidation which occurs in layered silicates. A Mossbauer study of tri-octahedral micas (iron-rich biotites) after heat treatment from 400 degrees C-1200 degrees C under different conditions has been performed. An attempt was made to explain the results using the mechanism of Vedder and Wilkins. It was found that a large amount of oxygen in contact with the mica grains was necessary for the mica to decompose completely.
Measurement of the ionisation and attachment coefficients in SF6and air mixtures
Authors: M Shimozuma  , H Itoh  , H Tagashira
Published: July 2002

The Townsend first ionisation coefficient alpha /p20, the electron attachment coefficient eta /p20, the effective ionisation coefficient alpha /p20(=( alpha /p20)-( eta /p20)) and the secondary coefficient gammaTfor SF6and air mixtures have been measured by the steady-state Townsend method for 51<or approximately=E/p20<or approximately=160 V cm-1Torr-1(155<or approximately=E/N<or approximately=486 Td). The results show that the alpha /p20of the mixtures does not vary linearly with the fractional SF6partial pressure k and has a minimum value at a fixed value of E/p20. It is found that the eta /p20increase approximately linearly with k at each E/p20except at small k values for E/p20>or approximately=120 V cm-1Torr-1and the alpha /p20varies approximately linearly with k at E/p20=160 V cm-1Torr-1, but the linear relationship of alpha /p20with k tends to break down with decreasing E/p20. The results also show that the gammaTdecreases rapidly from about 10-3to the values of gammaTfor SF6with increasing k at fixed E/p20and the limiting E/p20, at which alpha /p20= eta /p20, rapidly increase with increasing k as those of the SF6and nitrogen mixture.
The evolution of two-dimensional effects in fast-electron transport from high-intensity laser-plasma interactions
Authors: F Amiranoff  , K Eidmann  , R Sigel  , R Fedosejevs  , A Maaswinkel  , Yung-lu Teng  , J D Kilkenny  , J D Hares  , D K Bradley  , B J MacGowan  , T J Goldsack
Published: July 2002

Measurements of local and remote energy deposition by fast electrons have been made in 1.3 mu m and 1.05 mu m laser irradiation experiments with plane targets and various pulse lengths. From optical and X-ray streak photography and spatially resolved K alpha yield measurements it is found that up to 30% of the absorbed laser energy spreads laterally to distances of several millimetres from the focal spot with a spreading velocity in excess of 108 cm s-1.
Influence of radiation fluctuations on plasma tube impedance variations of a CO2laser
Authors: R Nowicki  , J Pienkowski
Published: July 2002

A model for the correlation of the lasing action and the ionisation processes of a CO2laser is presented. The model takes into account the step ionisation of N2and CO molecules. Making use of the method of rate equations the expressions of the correlation between the laser output power variations and the discharge impedance changes, as well as spontaneous light intensity changes, are obtained.
Plasma formation in a gas-liner pinch
Authors: K H Finken  , U Ackermann
Published: July 2002

A gas-liner pinch is optimised for Stark broadening investigations. To obtain the desired discharge conditions the gas injection, the preionisation and geometrical aspects of the discharge chamber are analysed. Quantitative agreement is found between the observed plasma compression and a 'snow-plough' model. The density distribution of the compressed plasma column is derived from double exposure holograms. The peak electron density Neapproximately=1018cm-3; the temperature gives kTeapproximately=10 eV.
Schlieren study of the transition to spark of a discharge in air and SF6
Authors: A Gibert  , J Dupuy  , M Bayle  , P Bayle
Published: July 2002

The formation of filaments during the transition to spark in a point-to-plane discharge is analysed in air (250 Torr) and SF6(60 Torr). Two separate analyses of the phenomenon are effected. First, the evolution of the electronic population is studied by means of a current probe and photomultiplier, and secondly, the Schlieren technique is used to follow the changes occurring in the density of the neutrals. An image converter is employed to record the Schlieren pictures which permit the observation of the expansion of the channel containing the neutrals. The average temperature of the neutrals is deduced from the rate of radial expansion of the channel and hence the resultant pressure rise at the core of the channel can be estimated. Evidence is given of the formation of local constrictions where intense heating of the neutrals occurs. At these points thermal ionisation is no longer negligible and a large fall in the resistivity of the discharge takes place, giving zones where the initiation process of the thermalisation of the discharge can occur.
Electrodeposited semiconducting molybdenum selenide films. II. Optical, electrical, electrochemical and photoelectrochemical solar cell studies
Authors: S Chandra  , D P Singh  , P C Srivastava  , S N Sahu
Published: July 2002

For pt.I see ibid., vol.17, p.2115 (1984). Electrodeposited molybdenum selenide films have been characterised by optical, electrical and electrochemical studies. The optical absorption spectra show the material to be an indirect band gap semiconductor with band gap 1.14 eV. Electrical conductivity at different temperatures shows an impurity conduction and the presence of deep level traps. Mott-Schottky plots have been drawn to determine the type of semiconductor and its parameters, ND, Ecand Ev. The J-V characteristics of the MoSe2film in contact with 2I-/I2electrolyte indicate that the charge transfer at the interface is via the valence band and it also involves the surface states. Results from photoelectrochemical solar cells fabricated using various electrodeposited films are also discussed.
Densification of thermal SiO2 due to intrinsic oxidation stressing
Authors: A Fargeix  , G Ghibaudo
Published: July 2002

Deals with densification of the thermally grown SiO2 films resulting from intrinsic oxidation stressing. A quantitative model giving the stress levels within the oxide during and after growth evolves from a viscoelastic relaxation scheme. Amount of stress and related quantities such as wafer curvature, refractive index or density are calculated and compared with different kinds of experimental data. As an example, it is demonstrated that low temperature oxidation processes are basically responsible for high density oxides because of Si-SiO2 intrinsic strain.
A thermally stimulated depolarisation technique for studying the freezing of water dispersed within emulsions
Authors: P Pissis
Published: July 2002

A thermally stimulated depolarisation (TSD) technique is presented, for studying the crystallisation of supercooled water dispersed within emulsions. The technique makes use of the high resolving power and the scanning character of the TSD method and of the phenomenon of evolution with time of the dielectric properties of ice dispersions in oil. It enables the determination of the freezing temperature, when the emulsion is submitted to steady cooling, and of the fraction of frozen water as a function of time, when the emulsion is maintained at a fixed temperature. No models or formulae for the dielectric behaviour of the emulsion are needed to this end. Examples illustrating the efficiency of the technique are also given.
Simultaneous measurement of thermal conductivity and thermal diffusivity of some building materials using the transient hot strip method
Authors: R Singh  , N S Saxena  , D R Chaudhary
Published: July 2002

A transient hot strip method developed by Gustafsson (1979), for simultaneous measurement of thermal conductivity and thermal diffusivity of solids and fluids with low electrical conductivity, is used for the simultaneous measurements of thermal conductivity and thermal diffusivity of some building materials such as dry dune sand, dry brick powder, dry cement, dry mud powder and dry asbestos powder at room temperature and normal pressure. The hot strip works both as a plane heat source and a temperature sensing device. The experimental values of thermal conductivity and thermal diffusivity of these materials are compared with those obtained from thermal probe and hot-wire methods. A good agreement in the values indicates that the transient hot strip (ThS) method can also be used for loose porous materials for obtaining these thermal parameters.
Neutral heating in glow to spark transition in air and nitrogen
Authors: P Bayle  , M Bayle  , G Forn
Published: July 2002

The heating of the neutral molecules in the glow to spark transition in air and nitrogen has been theoretically studied. A hydrodynamic first-order model has been used for electrons (density, momentum) and a second-order model (density, momentum, temperature) for neutral molecules. The interaction between the electron dynamics and the neutral dynamics has been compared in two kinds of discharges: a fast discharge (170 ns) in an overvolted plane parallel gap in nitrogen, and a discharge of longer duration (1.15 mu s) in a positive point-to-plane gap in air. The kinetics of the discharge, which is linked to the possible feedback of the neutral gas on the electron cloud, is mainly a function of the duration of the energy injection. In the case of the fast parallel gap discharge, the background gas plays a passive role; it is an energy absorber, so its temperature increases continuously. In the case of the point-to-plane discharge, lasting longer, the dynamics of the interaction between the electron cloud and the background gas constitutes the fundamental process of the discharge. The gas heating gives rise to a neutral depopulation in the core of the discharge. This leads to an increase of the reduced electric field E/N, thus to an increase of the electron production and thus of the current. In this kind of discharge, the heterogeneity of the neutral gas leads to the breakdown, owing to a constriction of the current core induced by the depopulation mechanisms.
Blast wave propagation in glow to spark transition in air
Authors: P Bayle  , M Bayle  , G Forn
Published: July 2002

The formation and propagation of post-discharge blast waves are studied theoretically and the minimum rate of energy transfer between electrons and neutral particles is deduced in the case of a positive point-to-plane discharge in air, described and numerically analysed in a previous paper by the authors. In this type of discharge, the injection of energy in the discharge is spread in time. The blast waves generated by the discharge constitute a set of waves with a complex structure. They first propagate separately, then merge in a single wave. The core of the discharge remains perturbed for a long time (i.e. low neutral densities and high temperatures) after the discharge crossing and thus constitutes a privileged path for possible recurring discharges. The classical analysis of blast waves was adapted to the experimental situation in which the energy injection is spread in time and space. The influence of the injected energy on the expansion radius of the wave has been studied and the authors show that the minimum rate of energy transfer fminbetween electrons and neutral particles in the glow to spark transition can be deduced from the Schlieren records. In the experiments analysed, the rate of energy transfer f is greater than 20+or-6%.
Electron energy distribution functions in electron-beam-sustained discharges: application to magnetic multicusp hydrogen discharges
Authors: J Bretagne  , G Delouya  , C Gorse  , M Capitelli  , M Bacal
Published: July 2002

Electron energy distribution functions (EDF) in magnetic multicusp H2discharges have been calculated by numerically solving the Boltzmann equation. Results for a 90 V, 1-10 A discharge at 40 m Torr are presented and discussed in connection with the importance of e-e Coulomb collisions, of wall losses and of the degree of dissociation affecting the EDF. Scaling laws with the discharge current, the pressure and the applied discharge voltage are presented. A satisfactory agreement is found between the calculated electron temperatures of low energy electrons ( epsilon <5 eV) and the corresponding experimental values obtained by probe techniques.
Propagation of the normal zone in composite superconductors with high contact resistance
Authors: A A Akhmetov  , R G Mints
Published: July 2002

The propagation of the normal zone in composite superconductors with high thermal resistance and high electrical contact resistance between the superconducting and the normal components of the conductor is analysed. It is shown that in certain conditions propagation can result from successive multiple splittings of the resistive domain generated. The minimum initiation current of this process has been found and has been analysed as a function of the composite parameters and external conditions.
Optical diagnostics of SF6high-current discharges
Authors: K -P Hinz
Published: July 2002

Results of optical investigations of SF6high-current arc plasmas are presented. The experimental arrangement allowed measurements at arc currents up to 2.6 kA and at discharge pressures up to 100 kPa. The dependence of the electron density of the arc plasma on these parameters and the laser wavelength were determined using a Michelson interferometer and a Mach-Zehnder interferometer. Besides these measurements, the temperature in the arc core was determined using spectroscopic methods and the diameter of the arc was measured using an image converter camera. The results of the measurements were compared with calculated data supposing local thermodynamic equilibrium. It can be shown that there is a good agreement between the experimental and theoretical results and those of other authors.
Experimental investigation of radially resolved plasma parameters in a low-pressure helium arc
Authors: H Adler  , A Piel
Published: July 2002

The radial variation of plasma parameters in a pulsed low-pressure arc (100-150 Pa) in helium is studied by various methods. For an on-axis electron density of Ne=1.2 and 3.3*1021 m-3 the density profiles are measured by interferometric and spectroscopic methods. The gas temperature is derived from the Doppler broadening of the helium line He I at 388.9 nm. Relative line intensities of He II at 468.6 nm and He I at 471.3 nm yield the electron temperature. Collisional-radiative (CR) models put forward by Mewe and Drawin and Emard take deviations from local thermodynamic equilibrium into account. The necessity of introducing a 'channel model' for this discharge is supported by measurements of the plasma conductivity.
The d.c. properties of an alkali plasma diode
Authors: M T C Fang  , D A Fraser  , J E Allen
Published: July 2002

The theoretical d.c. current-voltage characteristics of a `collisionless' alkali plasma diode are derived. It is assumed that electrons and ions are created only at the two incandescent plane electrodes, the latter being at the same temperature. The electrons and ions are assumed to leave the electrodes with half-Maxwellian distributions of velocities. Some ions are trapped in the potential well between the electrodes.

It is found that the potential distribution is associated with a double sheath in front of the negative electrode, a plasma region of constant potential and an electron sheath at the positive electrode. The theoretical (I, V) characteristics are compared with experimental results obtained with a caesium plasma. At low applied voltages good agreement is obtained between theory and experiment; at higher voltages (where oscillations occur) the d.c. current falls to a lower value. Possible mechanisms for this drop are described.
Energy deficit measurements with a liquid gallium ion source
Authors: G L R Mair
Published: July 2002

The author reports on an extensive study of energy deficit measurements of the energy distribution of the ion beam emitted by a gallium liquid metal ion source in an attempt to resolve the controversy arising from the unusually wide divergence of findings by different workers. The controversy can be conceivably resolved in terms of misalignment of the liquid cone, whereby different parts of the beam enter the energy analyser at different times. Coupled to this is an alteration of the work function of the retarding electrode of the energy analyser by contaminants, such as the liquid metal itself, or its oxides, altering in turn the value of the critical energy deficit for surface field ionisation. A detailed investigation is also made for the region of emission current below 1 mu A-a region rarely investigated by the vast majority of researchers, but a most important one from the point of view of the emission mechanism, as well as from an applications' standpoint. Finally, the reasons for the scatter observed in energy spread measurements are considered, and only a tentative explanation can be given, related to ion source misalignment; this misalignment arises from the fact that the liquid cone does not always form at the centre of the apex region of the supporting needle.
Ion flow characteristics from laser-produced line plasmas
Authors: J A Waltham  , P F Cunningham  , M Notcutt  , M M Michaelis
Published: July 2002

A Q-switched ruby laser, focused by means of a spherical-cylindrical lens combination, was used to irradiate massive planar carbon targets. Intensity on target was maintained at 4*109W cm-2as the line focus length-to-width aspect ratio was varied. Measurements made with charge-collecting Faraday cups indicate a collimation of both integrated ion number flux and total ion energy in a plane orthogonal to the laser line focus length, the extent of collimation being dependent upon the line focus aspect ratio. Results are compared with a simple axisymmetric potential fluid flow model.
A collisional-radiative model applicable to argon discharges over a wide range of conditions. III. Application to atmospheric and subatmospheric pressure arcs
Authors: J Vicek  , V Pelikan
Published: July 2002

For pt.II see ibid., vol.22, p.632 (1989). The extensive collisional-radiative model for an argon atom plasma is applied to atmospheric and subatmospheric pressure wall-stabilised arcs in order to clarify the mechanisms by which the excited levels are populated in the axial region of these discharges. Computations are carried out for various sets of input parameters, such as the electron kinetic temperature Te, the atom temperature Ta, the ion temperature Ti, the electron number density ne, the ground-state atom population n1, the plasma column radius R and the escape factors Lambdamnand Lambdam, characterising the non-equilibrium plasmas under consideration. The predicted values of the populations in the excited levels and the qualities derived from them, as well as the values of the critical electron densities for establishing the local thermodynamic equilibrium in the axial region of the arcs investigated, are in good agreement with the corresponding experimental results. The effect of the changes in the discharge parameters Te, Ta, ne, R and Lambda1non the population mechanism, together with the role played by the recombination flow of electrons from a continuum, the deviations of the actual electron distribution function from the corresponding Maxwellian form and by the atom-atom inelastic collisions, are also shown.
The elastic constants of sodium chloride and rubidium chloride in the range 140-300 K
Authors: M Gluyas  , F D Hughes  , B W James
Published: July 2002

Measurements of the adiabatic elastic constants of NaCl from 190 to 300 K and of RbCl from 140 to 300 K have been made using the `sing-around' system to an absolute accuracy of ±0·2% and with a relative accuracy of ±0·05%. The elastic constants of NaCl agree with other recent measurements. The measurements of the elastic constants of RbCl agree with other work forC11andC44but the variation ofC12with temperature disagrees with the other measurements. The decrease inC12of RbCl with decreasing temperature is in accordance with the variation ofC12in other alkali chlorides. The reasons for the disagreement of the variation ofC12are examined and it is shown that the observed decrease is real.
Effect of coolant channel diameter on homogeneous and heterogeneous methods of reactor analysis in reflected subcritical assembly
Authors: M R Haroon
Published: July 2002

The systematic variation of neutron flux distribution and buckling measurements as a function of fuel loading in a subcritical assembly (graphite, natural uranium) for two different coolant channel diameters have been investigated. The void ratio (volume of voids/lattice volume) in the two cases was 19% and 24%. The experimental results have been compared with theoretical predictions based on diffusion theory. For full assembly cases the results are in good agreement (as might be expected) with the predictions of Syrett's model for graphite moderated reactors. The correlation of 10 partially filled assembly cases was carried out in three ways:

Syrett method lattice parameters with reflector savings calculated from one- and two-group theory for an infinite-slab system.

Numerical solution of the two-group, two-region diffusion equations.

Two-group heterogeneous (source sink) theory for finite systems.

The results show that method (i) breaks down for all cases except those with the full assembly, method (ii) breaks down in nearly all cases and method (iii) holds well down to 36 fuel channels. The breakdown in method (i) and (ii) is most probably due to inaccurate streaming corrections to reactor systems of smaller size. In method (ii), excessive computer time is an additional complicating factor. If the experiments are analysed by heterogeneous methods it is possible to obtain accurate reactor para meters with a partially loaded exponential assembly in cases where only a limited amount of fuel is available.
Simultaneous measurements of the brightness and the energy distribution of electrons emitted from a triode gun
Authors: W H J Andersen  , A Mol
Published: July 2002

The energy distribution of electrons in a part of the electron beam selected by two apertures situated on the axis was measured using a wienfilter-type energy analyser. These measurements gave brightness values for electrons in specific energy intervals of the distribution which fitted the theory extremely well. The influence of the measuring system on the energy distribution was also demonstrated and it indicated that the discrepancy between experiment and theory observed by Nottingham and Hutson was due to a crude approximation in the electron optics of the theory rather than a large electron reflection effect.

The anomalous broadening of the energy distribution (Boersch effect) was found to be independent of the measuring system and had no influence on the brightness. Energy gains were also observed in the broadening process.
Magnetic anisotropy in single-crystal nickel films
Authors: E Secemski  , J C Anderson
Published: July 2002

Single-crystal films of nickel have been deposited in ultra high vacuum under closely controlled conditions, with a resulting high standard of lattice perfection as determined by electron microscopy. Anisotropy measurements made by torque magnetometry yielded the following conclusions:

K1has the same value as for bulk nickel (−5·0 × 104erg cm−3at room temperature). The magnetostrictive behaviour is predictable from bulk properties. An estimate of the order ofK3is also obtained (−3 × 103erg cm−3).

Kucan be reduced to extremely low levels by taking steps to exclude all known mechanisms.

The effect of stress on bothK1andKuhave been investigated and correlated. A constraint temperature (about 110°C), well below the substrate temperature during deposition, has been established.

No contribution toKufrom mechanisms involving oxygen has been observed.

The effects of lattice imperfections are quite marked, and can be the predominantKusource.
Optical constants of thin films of nickel, copper and germanium
Authors: A Nag  , L Ward
Published: July 2002

This article describes an experimental technique for measuring the photometric parametersRp,Rs,TpandTsof thin films on a non-absorbing substrate. Films of germanium, nickel and copper were investigated and the results obtained processed by a hill-climbing iteration technique to calculate the optical constantsnandkand the thicknessd. The variation ofnandkwithdis exhibited graphically and comparisons are drawn with previous results for thin films and bulk specimens of these three materials.
The effect of the neutron energy spectrum on neutron displacement damage in niobium
Authors: S Blow
Published: July 2002

This paper presents an analysis of displacement damage produced in niobium, which has potential use as the first wall material of a fusion reactor.

Lindhard's formalism was used together with cross sections taken from the UK nuclear data file on niobium. Data sets describing damage energy created at various neutron incident energies were generated. It was calculated that:

The ratio of damage density (ie mean damage energy × cross section) at 14 MeV to that at 1 MeV is 3·5 to 1·00.

The damage energy data sets were integrated over two different neutron spectra in order to compare the total effects. The spectra were taken from: (a) the first wall of a model fusion reactor, with a total flux of 2·8 × 1015neutrons/cm2s; (b) the core centre of the Dounreay Fast Reactor (DFR), total flux 2·5 × 1015neutrons/cm2s.

Analysis showed that the number of atomic displacements per atom per year is

165 displacements for spectrum (a);

78 displacements for spectrum (b).
Cathodic contact erosion due to short-duration gas breakdown arcs
Authors: E W Gray  , J R Pharney  , J A Augis
Published: July 2002

The erosion of palladium contacts due to gas breakdown arcs in air and in an activating environment (naphthalene vapour) has been studied. The rate of erosion in air was not constant with arc energy but increased almost linearly from about 0·1 to 1·5 cm3erg−1over an energy range of about 100 to 500 erg per discharge. Beyond this energy range a slower increase was observed. Under the activated conditions the erosion rate was much lower than in air (eg at 2000 erg the erosion rate in air was 1·65×10−14cm3erg−1and with naphthalene activation the rate was 1·1×10−14cm3erg−1); however, a variation in the erosion rate with energy was also observed. The variation with energy is considered to be caused by part of the available erosion energy being used up in surface film decomposition.
The thermoelectric potential of molten salt thermocells
Authors: K Cornwell
Published: July 2002

The thermoelectric potential of a molten salt thermocell is not only dependent on the molten salt but also on the types of electrodes used for measurement. The thermo-electric potential of a thermocell with reversible electrodes constructed of the metal of the salt is determined by the entropy changes of the cation and is termed the `cationic thermoelectric potential' by the author. Similarly, electrodes of the anionic component of the salt lead to an anionic thermoelectric potential. This paper is concerned with the thermoelectric potential of cationic thermocells containing pure salts, and binary mixtures of pure salts with a common anion or cation. Previous work has been reviewed and the theory of the thermoelectric potential and the entropies of transfer of ions in mixtures of salts has been extended. Experimental studies of cells containing argentous and cuprous halide mixtures between pure metal and alloy electrodes have generally confirmed this theory.

The possibility of using the thermoelectric potential of a molten salt thermocell for electrical generation was examined experimentally by the construction of a 13 V trial generator. This form of direct generation was shown to be possible but the expected efficiency of no more than 10% rather limited the field of application.
The fracture toughness and crack propagation properties of polyester resin casts and laminates
Authors: M J Owen  , R G Rose
Published: July 2002

Fracture toughness tests have been conducted on polyester resin casts and laminates containing a resin flexibilizing additive. In spite of a large change in the resin strain to failure there was little change in the critical stress intensity factor of either the resin casts or laminates. Crack growth rates for the resin casts under cyclic loading could be represented in terms of stress intensity factor range ΔKthrough the power law relationship da/dN=A(ΔK)m, whereAandmare constants;mwas approximately 5 for all resin flexibilities. The crack growth rate was constant provided that general yielding did not occur. Fracture surfaces were examined by scanning electron microscopy, where distinct ridge structures were observed in the crack growth specimens.
Effects of clumps and ion bombardment on electrical breakdown in vacuum
Authors: G P Beukema
Published: July 2002

The formation of emitting sites on highly polished stainless steel and titanium electrodes in vacuum is investigated. It is shown that during the first application of an electric field between broad-area electrodes clumps damage the cathode surface. These clumps originate both from the cathode and the anode. If a spark-conditioned cathode with a field enhancement factor β=40 is placed opposite a new anode, an electric field of 6·5×106V m−1is enough to damage the cathode, resulting in a field enhancement factor of 100. It is demonstrated that conditioning of electrodes by sparks results from the detachment of more or less loosely bound particles from the electrodes. For small electrode separations (d<1 mm) clumps initiate a breakdown if the microscopic field strength βV/dafter the impact on the cathode is higher than a critical field strengthFb, which equals similar 7×109V m−1.

The field enhancement factor β can be decreased by ion bombardment, but the breakdown voltage is the same as after spark conditioning. It is shown that after conditioning by ion bombardment clumps are responsible for the onset of a spark.
Brillouin spectra of ethyl ether and ethyl alcohol
Authors: Silas E Gustafsson  , H E Gunilla Knape  , Lena M Torell
Published: July 2002

Measurements of Brillouin scattering in ethyl ether and ethyl alcohol were made for scattering angles of 42-150°. The spectra were excited by 4416 Å radiation from an He-Cd laser and analysed with a pressure-scanned Fabry-Perot spectrometer. The measured frequency shifts and linewidths of the Brillouin components were used to determine the velocities and the lifetimes of thermal waves. The frequency range of the hypersonic waves was 2·6-5·9 GHz for ethyl ether and 2·6-6·8 GHz for ethyl alcohol. The measurements do not indicate any dispersion of the sonic velocity within the investigated frequency range and good agreement is obtained with earlier ultrasonic measurements.
A theoretical model for the energy dependence of electron channelling patterns in scanning electron microscopy
Authors: Rolf Sandström  , Jonathan F Spencer  , Colin J Humphreys
Published: July 2002

Equations given by Hirsch and Humphreys (1970) and by Spenceret al(1972) for describing the contrast in electron-channelling patterns (ECP) are generalized by taking into account the energy losses which the fast electrons experience in the specimen. In this way a theory is established which considersdiffraction, backscatteringandenergy losses.This theory is used and calculations are made of the dependence of the contrast in ECP on the energy of the backscattered electrons. Unlike earlier theories for ECP the present work gives the correct behaviour of the backscattering coefficient for large foil thicknesses. It is shown that the visibility can be strongly enhanced by using an energy-analysing detector, in agreement with experimental results. The signal-to-noise ratio is evaluated to find the energy window which gives optimum contrast. For Si, Cu and Au these windows are approximatelyE0>E>E0− 700eV,E0>E>E0− 250 eV andE0>E>E0− 400 eV respectively. Calculations of the mean escape depth as a function of energy show that the electrons are emitted from a thin surface layer for small energy losses. The resolution can therefore be significantly improved in the backscattered mode by using an energy-analysing detector.
Electric-field-induced nucleation: a phenomenon used for determining the boundary of stability of liquids
Authors: D S Parmar  , A K Jalaluddin
Published: July 2002

The limit of absolute thermodynamic stability and tensile strength of liquids (n-hexane, n-pentane and iso-pentane) have been determined using the phenomenon of electric-field-induced nucleation. An extrapolation of the curves of critical voltage applied to the liquid against temperature to the points of zero critical voltage gives the limiting value of the superheat temperature at a given pressure. The locus of these limiting values in the p-t plane, terminating at the critical point, gives the boundary of absolute thermodynamic stability (spinodal) of the liquid. The electrical forces at the interface corresponding to various critical voltages have been proposed to give the limiting negative pressures (tensile strengths). The energy contributed by the electric field over the volume of a vapour nucleus of critical size is in agreement with the value of the thermal energy required for the formation of the nucleus of the same size de novo in a liquid.
Effects of linear fringe fields on the focusing of a parallel electron beam in a symmetric magnetic spectrometer
Authors: A A Kosiara
Published: July 2002

A uniform-field magnetic spectrometer, of pole boundaries designed to focus monoenergetic electrons emitted by a point source, without aberrations in a median plane is considered. Electron trajectories in the median plane of the spectrometer are calculated. Electron equations of motion are solved numerically on a computer using the Runge-Kutta method. The dependence of aberrations on the width and strength of the fringe fields and on the equivalent angular divergence of the electron beam is determined. The aberrations place a lower limit on the energy resolution of the spectrometer, which is in the range of 0.005-0.113 eV for the spectrometer geometries investigated at 100 keV and changes very little with beam energy. The structure of dynamic focus is obtained and compared with the previously investigated point source case.
Ionization, attachment and negative ion reactions in carbon dioxide
Authors: S R Alger  , J A Rees
Published: July 2002

From measurements of the relative abundances of O-and CO3-ions produced in carbon dioxide by electron attachment and subsequent ion-molecule reactions, and from current-growth data, values of the primary ionization coefficient, attachment coefficient and rate coefficient for the reaction O-+2CO2to CO3-+CO2have been obtained for 60*10-17<E/N<152*10-17V cm2at gas number densities N between 0.2 and 5.0*1016cm-3.
For numerical differentiation, dimensionality can be a blessing!
Authors: Anderssen Robert , Hegland Markus
Published: July 2002

Finite difference methods, such as the mid-point rule, have been applied successfully to the numerical solution of ordinary and partial differential equations. If such formulas are applied to observational data, in order to determine derivatives, the results can be disastrous. The reason for this is that measurement errors, and even rounding errors in computer approximations, are strongly amplified in the differentiation process, especially if small step-sizes are chosen and higher derivatives are required. A number of authors have examined the use of various forms of averaging which allows the stable computation of low order derivatives from observational data. The size of the averaging set acts like a regularization parameter and has to be chosen as a function of the grid sizehh. In this paper, it is initially shown how first (and higher) order single-variate numerical differentiation of higher dimensional observational data can be stabilized with a reduced loss of accuracy than occurs for the corresponding differentiation of one-dimensional data. The result is then extended to the multivariate differentiation of higher dimensional data. The nature of the trade-off between convergence and stability is explicitly characterized, and the complexity of various implementations is examined.
Finite-dimensional representations of quantum affine algebras at roots of unity
Authors: Beck Jonathan , Kac Victor
Published: July 2002

We describe explicitly the canonical map

χ
:

\chi :

Spec

U

ε

(

g

~

)
→

U_{\varepsilon }(\tilde {\mathfrak {g}}) \rightarrow

Spec

Z

ε

Z_{\varepsilon }

, where

U

ε

(

g

~

)

U_{\varepsilon } (\tilde {\mathfrak {g}})

is a quantum loop algebra at an odd root of unity

ε
\varepsilon

. Here

Z

ε

Z_{\varepsilon }

is the center of

U

ε

(

g

~

)

U_{\varepsilon }(\tilde {\mathfrak {g}})

and Spec

R
R

stands for the set of all finite–dimensional irreducible representations of an algebra

R
R

. We show that Spec

Z

ε

Z_{\varepsilon }

is a Poisson proalgebraic group which is essentially the group of points of

G
G

over the regular adeles concentrated at

0
0

and

∞
\infty

. Our main result is that the image under

χ
\chi

of Spec

U

ε

(

g

~

)

U_{\varepsilon }(\tilde {\mathfrak {g}})

is the subgroup of principal adeles.
Empirical estimation of the reliability of ribosomal RNA alignments.
Authors: O'Brien E A , Higgins D G
Published: July 2002

MOTIVATION: The automatic alignment of rRNA sequences can reproduce manual expert alignments with high, but not perfect, fidelity. We examine the use of empirical methods for the identification of regions of an alignment of a new sequence with an existing large alignment which can confidently be predicted to be correctly aligned. RESULTS: We show how to use a simple jack-knife procedure to derive an estimate of the reliability that is to be expected at each position of a large alignment of eukaryotic rRNA sequences. These reliabilities are then improved using measures that are specific to the input sequence. Regions where the sequence-specific reliability method performs particularly well are identified and seen to correspond with elements in the structure of the rRNA molecules that vary between species in the alignment. We also compare these reliability measures to an algorithmic alignment stability measure. AVAILABILITY: The software is available free of charge by sending an e-mail message to emmet@chah.ucc.ie. CONTACT: emmet@chah.ucc.ie
Family pairwise search with embedded motif models.
Authors: Grundy W N , Bailey T L
Published: July 2002

MOTIVATION: Statistical models of protein families, such as position-specific scoring matrices, profiles and hidden Markov models, have been used effectively to find remote homologs when given a set of known protein family members. Unfortunately, training these models typically requires a relatively large set of training sequences. Recent work (Grundy, J. Comput. Biol., 5,<479-492, 1998) has shown that, when only a few family members are known, several theoretically justified statistical modeling techniques fail to provide homology detection performance on a par with Family Pairwise Search (FPS), an algorithm that combines scores from a pairwise sequence similarity algorithm such as BLAST. RESULTS: The present paper provides a model-based algorithm that improves FPS by incorporating hybrid motif-based models of the form generated by Cobbler (Henikoff and Henikoff, Protein Sci., 6, 698-705, 1997). For the 73 protein families investigated here, this cobbled FPS algorithm provides better homology detection performance than either Cobbler or FPS alone. This improvement is maintained when BLAST is replaced with the full Smith-Waterman algorithm. AVAILABILITY: http://fps.sdsc.edu
Stars and Stories: How Films Became Branded Products
Authors: Bakker Gerben
Published: July 2002

Between 1890 and 1940, motion pictures changed from technological novelties into heavily branded consumer products. The high sunk costs and short “shelf-life” of movies led film producers to borrow branding techniques from other consumer goods industries. They tried to build audience loyalty around a number of characteristics, but eventually learned that stars and stories were the most effective “promotion machines,” able swiftly to generate massive brand-awareness and to persuade consumers to see a new film. Data from the United States, Britain, and France showing the disproportionate distribution of income and fame among stars confirm their role as persuaders. Ultimately, film producers extended the life of their products by licensing their instant, tradable brands to other consumer goods industries.
Evanescent-wave microscopy: a new tool to gain insight into the control of transmitter release
Authors: Oheim Martin , Loerke Dinah , Chow Robert H. , Stühmer Walter
Published: July 2002

Evanescent–wave excitation was used to visualize individual fluorescently labelled vesicles in an optical slice near the plasma membrane of bovine adrenal chromaffin cells. A standard upright microscope was modified to accommodate the optics used for directing a laser beam under a supracritical angle on to the glass–water interface on top of which the cells are grown. Whereas epi–illumination images appeared blurred and structureless, evanescent–wave excitation highlighted acridine orange–labelled vesicles as individual pinpoints. Three–dimensional (3D) trajectories of individual vesicles were obtained from time–resolved image stacks and used to characterize vesicles in terms of their average fluorescenceFand mobility, expressed here as the 3D diffusion coefficient D(3). Based on the single–vesicle analysis, two groups of vesicles were identified. Transitions between these states were studied before and after stimulation of exocytosis by repetitive or maintained membrane depolarizations by elevated extracellular [K+]. Findings were interpreted as sequential transitions between the previously characterized pools of vesicles preceding the fusion step. The observed approach of vesicles to their docking sites was not explained in terms of free diffusion: most vesicles moved unidirectionally as if directed to their binding sites at the plasma membrane. Vesicle mobility at the membrane was low, such that the sites of docking and fusion were in close vicinity. Both the rim region and confined areas in the centre of the footprint region were the site of intense vesicle trafficking.
Effect of changes in action potential shape on calcium currents and transmitter release in a calyx–type synapse of the rat auditory brainstem
Authors: Borst J. G. G. , Sakmann B.
Published: July 2002

We studied the relation between the size of presynaptic calcium influx and transmitter release by making simultaneous voltage clamp recordings from presynaptic terminals, the calyces of Held and postsynaptic cells, the principal cells of the medial nucleus of the trapezoid body, in slices of the rat brainstem. Calyces were voltage clamped with different action potential waveforms. The amplitude of the excitatory postsynaptic currents depended supralinearly on the size of the calcium influx, in the absence of changes in the time–course of the calcium influx. This result is in agreement with the view thact at this synapse most vesicles are released by the combined action of multiple calcium channels.
Molecular pathology of dentatorubral--pallidoluysian atrophy
Authors: Kanazawa Ichiro
Published: July 2002

Dentatorubral–pallidoluysian atrophy (DRPLA) is an autosomal dominant disorder characterized clinically by myoclonus, epilepsy, cerebellar ataxia, choreoathetosis and dementia. Cardinal pathological features of DRPLA are a combined degeneration of both the dentatorubral and the pallidoluysian systems. Although the early sporadic cases were reported by Western neuropathologists, a strong heritability and an age of onset–dependent variability of the clinical features were carefully deduced by Japanese clinicians. The disease is fairly common in Japan, but extremely rare in Caucasians. Since the gene was identified in 1994, DRPLA is known as one of the CAG repeat expansion diseases, in which the responsible gene is located on chromosome 12p and its product is called atrophin 1. DRPLA shows prominent ‘anticipation’, which is genetically clearly explained by a marked instability of the expanded CAG repeat length during spermatogenesis. Moreover, the instability of the CAG repeat length also seems to occur in the somatic cells, resulting in ‘somatic mosaicism’. Possible mechanism(s) underlying the neuronal cell death in DRPLA are discussed in terms of molecular pathological points of view.
The loss of episodic memories in retrograde amnesia: single–case and group studies
Authors: Kopelman Michael D. , Kapur Narinder
Published: July 2002

Retrograde amnesia in neurological disorders is a perplexing and fascinating research topic. The severity of retrograde amnesia is not well correlated with that of anterograde amnesia, and there can be disproportionate impairments of either. Within retrograde amnesia, there are various dissociations which have been claimed—for example, between the more autobiographical (episodic) and more semantic components of memory. However, the associations of different types of retrograde amnesia are also important, and clarification of these issues is confounded by the fact that retrograde amnesia seems to be particularly vulnerable to psychogenic factors. Large frontal and temporal lobe lesions have been postulated as critical in producing retrograde amnesia. Theories of retrograde amnesia have encompassed storage versus access disruption, physiological processes of ‘consolidation’, the progressive transformation of episodic memories into a more ‘semantic’ form, and multiple–trace theory. Single–case investigations, group studies and various forms of neuroimaging can all contribute to the resolution of these controversies.