Peter J. Chantry

A simple formula for diffusion calculations involving wall reflection and low density

Diffusion theory is often employed to calculate the effects of wall destruction on the local concentration of an active species immersed in a scattering gas. In many situations the spatial dependence of the concentration is given to a good approximation by the fundamental diffusion mode, and the local loss frequency can be calculated using the container’s fundamental mode diffusion length Λ. The additional assumption that the density of the active species may be taken to be zero at the container boundaries gives a value of Λ=Λ0 which depends only on the container dimensions, but use of Λ0 can be seriously in error if the diffusion mean free path λm is comparable to the dimensions, or if the particle reflection coefficient R becomes of significance. An improved boundary condition may be written simply in terms of the linear extrapolation length λ, whose inverse is the logarithmic gradient of the particle density at the boundary. The equation λ=2(1+R)λm/3(1−R) allows the representation of the full range of ...

Dielectric properties for SF6 and SF6 mixtures predicted from basic data

We have calculated α and η, the ionization and attachment coefficients, and (E/N) *, the limiting breakdown electric‐field–to–gas‐density ratio, in SF6 and SF6 mixtures by numerically solving the Boltzmann equation for the electron energy distribution. The calculations require a knowledge of several electron collision cross sections. Published momentum transfer and ionization cross sections for SF6 were used. We measured various attachment cross sections for SF6 using electron‐beam techniques with mass spectrometric ion detection. We determined a total cross section for electronic excitation of SF6 by comparing the predicted values of α, η, and (E/N) * with our measured values obtained from spatial current growth experiments in SF6 in uniform fields over an extended range of E/N. With this self‐consistent set of SF6 cross sections, together with published He and N2 cross sections, it was then possible to predict the dielectric properties of SF6‐He and SF6‐N2 mixtures. Published experimental values of α fo...

Dielectric properties for SF/sub 6/ and SF/sub 6/ mixtures predicted from basic data

We have calculated α and η, the ionization and attachment coefficients, and (E/N) *, the limiting breakdown electric‐field–to–gas‐density ratio, in SF6 and SF6 mixtures by numerically solving the Boltzmann equation for the electron energy distribution. The calculations require a knowledge of several electron collision cross sections. Published momentum transfer and ionization cross sections for SF6 were used. We measured various attachment cross sections for SF6 using electron‐beam techniques with mass spectrometric ion detection. We determined a total cross section for electronic excitation of SF6 by comparing the predicted values of α, η, and (E/N) * with our measured values obtained from spatial current growth experiments in SF6 in uniform fields over an extended range of E/N. With this self‐consistent set of SF6 cross sections, together with published He and N2 cross sections, it was then possible to predict the dielectric properties of SF6‐He and SF6‐N2 mixtures. Published experimental values of α fo...

A critique of methods for calculating the dielectric strength of gas mixtures

Three currently available methods for predicting the dielectric strength of gas mixtures are discussed and compared. All are based on the common principle of linear addition of some property of the individual gases in proportion to their concentration in the mixture. The simplest approach uses a weighted sum of the dielectric strengths of the separate gases. The other approaches are based on predicting the limiting reduced field (E/N)*m at which the net ionization coefficient of the mixture (ᾱ/N)m = 0. Wieland’s approximation for ᾱm involves a weighted sum of the ᾱ’s for the separate gases, and requires a knowledge of these over the range (E/N)*w<(E/N)<(E/N)*s, where w and s refer to the weakest and strongest gas involved. It is shown that this approach in general gives a mixture strength which varies monotonically with the fractional concentrations and lies within the above range of E/N. The simplest approach, using weighted linear addition of strengths, and the empirical rule proposed earlier by Takuma ...

Towards a miniature laser-pumped cesium cell frequency standard

With the goal of minimizing the overall size of a gas cell frequency standard the authors have characterized cylindrical Cs cells with inner dimensions R=2 mm and L=18 mm. Under conditions of present interest the lifetime in the absence of a tuned microwave field tends to be dominated by Cs-Cs collisions. Wall collisions have only a marginal effect on the lifetime, provided appropriate conditions on the N/sub 2//Ar buffer gas pressure, are satisfied. Using a Te/sub 101/ mode rectangular cavity the microwave resonance lineshape signal parameters were measured over a range of optical and microwave powers at various cell temperatures and used to predict the achievable short term stability. For the optimum combination of optical and microwave powers the linewidth is dominated by power broadening, and the short term stability, sigma ( tau ), predicted from these measurements decreases from sigma ( tau ) tau /sup 1/2/=6.4*10/sup -12/ to 4.2*10/sup -12/, where tau is in seconds, as the cell cold spot temperature is raised from 50 degrees C to 65 degrees C.<<ETX>>

Acoustooptic techniques for comparing high-speed digital data

The possibility of using acoustooptic techniques for recording and comparing high-speed digital data is explored. Suitable architectures for high-speed acoustooptic (AO) comparators capable of determining the occurrence, nature, and location of errors are presented. The analysis indicates that the AO Bragg cell must have a minimum center frequency of twice the data clock frequency and a minimum bandwidth equal to the clock frequency. Initial experimental results verify the analysis and demonstrate the feasibility of the proposed techniques at clock frequencies of at least 330 MHz. Available AO technology should allow the application of this approach at clock frequencies in excess of 1 GHz.