Holographic Dark Energy Model and Scalar-Tensor Theories

We study the holographic dark energy model in a generalized scalar tensor theory. In a universe filled with cold dark matter and dark energy, the effect of potential of the scalar field is investigated in the equation of state parameter. We show that for a various types of potentials, the equation of state parameter is negative and transition from deceleration to acceleration expansion of the universe is possible.Comment: 11 pages, no figure. To appear in General Relativity and Gravitatio

Collision cross sections and swarm coefficients of water vapour ion clusters (H

The ion swarm transport coefficients such as reduced mobility, diffusion coefficients and reaction rates of three water vapour ion clusters (H2O)nH+ (with n = 1, 2 and 3) in N2 and O2 have been determined from a Monte Carlo simulation using calculated and measured elastic and inelastic collision cross sections. The elastic momentum transfer cross sections have been determined from a semi-classical JWKB approximation based on a rigid core interaction potential model. The inelastic cross sections have been deduced from the measured ones in the case of similar ion cluster. Then, the cross sections sets are fitted using either the measured reduced mobility at low electric field in the case of (H2O)nH+ in N2 or the zero-field mobility calculated from the Satoh's relation and the measured ones in N2. From the sets of elastic and inelastic collision cross sections thus obtained in pure N2 and O2, the ion transport and reaction coefficients for (H2O)nH+ are then calculated in dry air and also extended over a wide range of reduced electric field in N2 and O2. These ion data are very useful for modelling and simulation of non-equilibrium electrical discharges more particularly in humid gases at atmospheric pressure

Measurement of electron swarm coefficients in C

We have measured the electron drift velocity, the effective ionization coefficient, and the longitudinal diffusion coefficient of electrons in the C2F4-Xe mixtures with 1% and 5% amount of C2F4. The electron drift velocity curves show a well defined region of negative differential drift velocity, and a similar effect is found for the density-normalized longitudinal diffusion coefficients at low E/N values. Previous findings of possible Penning ionization effects on the pulse shapes in the C2F4-Ar mixtures were not found in C2F4-Xe. This effect is further explored in this paper and fully supported by the simulation of avalanche development in the C2F4-Ar mixtures in which, apart from electron transport and effective ionization, the formation of metastable Ar∗ followed by ionization of C2F4 has been included. In most cases the comparison between simulations and measurements is very good, thereby giving further support to the original hypothesis

Ion swarm data of N

The ion swarm data such as reduced mobilities, diffusion coefficients and reaction rates of N4+ in N2, O2 and dry air (80% N2, 20% O2) have been determined from a Monte Carlo simulation using calculated and measured elastic and inelastic cross sections. The elastic cross sections used have been determined from a semi-classical JWKB approximation based on a rigid core potential model. The inelastic cross section of N4+ in N2 has been deduced from the measured experimental rates whereas for N4+ in O2 the measured inelastic cross sections have been extended to low and high energies by appropriate approximations. Then the cross sections sets have been validated from comparison of calculated and measured ion swarm data. From the cross sections sets obtained in pure N2 and O2, the ion swarm data for N4+ in dry air are then calculated for a large E/N range [1–104] Td. Finally, the influence of N4+ ions on the streamer development was analyzed with a 2D fluid model in the case of dry air at atmospheric pressure for a point-to-plane electrode configuration

Ion mobilities and transport cross sections of daughter negative ions in N2O and N2O-N-2 mixtures

A pulsed Townsend apparatus is used to measure the mobility of a single negative ion species in N2O and N2O-N-2 gas mixtures. The range of the density-normalized electric field, E/N, is 6.5-100 Td (1 Townsend = 10(-17) V cm(2)) over the pressure range 10-250 Torr and temperature range 295-300 K. Based on previous work it is shown that the most likely drifting ion is N2O2-. A reaction scheme involving ion conversion and electron detachment is presented, ending with the formation of a stable N2O2- ion. The momentum transfer integral cross section for N2O2- in pure N2O and N-2 gases is derived from the above measurements. The unfolded cross sections are used as an initial guess and then further improved by ensuring good agreement between Monte Carlo calculated mobilities and the experimental results for the N2O-N-2 mixture

Negative ion clusters in oxygen: collision cross sections and transport coefficients

Using a pulsed Townsend experiment, we have observed the formation of two negative ion species in oxygen over the pressure range 100–600 torr, and the density-normalised electric field strength, E/N, from 2 to 14 Td. The peculiar shape of these transients has led us to propose a scheme of three-body ion-molecule reactions leading to the formation of O4- and O6-, which is substantiated by a curve fitting procedure. The resulting mobility data of these two ionic species have been used to calculate their respective momentum transfer collision cross sections, together with the dissociation cross sections that are needed to extend the range of calculation of mobility and diffusion (transverse and longitudinal) to 1000 Td. These calculations were based on an optimised Monte Carlo algorithm, using collision cross sections obtained from a JWKB approximation (Jeffreys-Wentzel-Kramers-Brillouin) or taken from literature