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Experimental measurements of Hg excited state densities and the positive column electric field have been made in a highly-loaded low-pressure Hg–Ar discharge for current densities ranging from 0.05 to 0.6 A cm−2 and Hg vapour pressures from 1.2 to 13.2 mTorr. Resonance level (Hg 6s6p 3P1, 1P1) and metastable level (Hg 6s6p 3P0, 3P2) column densities across the positive column have been obtained using white light absorption spectroscopy. The experimental results are compared with numerical predictions from two existing and independent codes. Several physical mechanisms, which are not generally included in numerical models of the Hg–Ar positive column, are suggested as possible causes of the observed discrepancies.

Low-pressure gas discharge modelling is reviewed, both from a historical perspective and for current industrial applications. An ovetview of the basic mathematical and physical models used to describe low-pressure discharges is given, together with a summary of the most common numerical techniques which have been adopted. Modelling of the oc glow discharge and discharges maintained by high-frequency (RF and microwave) electromagnetic fields is reviewed, with illustrations of the validity of these models in predicting discharge properties and explaining and interpreting experimental results.

We have performed close-coupling calculations of electron scattering from the ground state of mercury atoms in the 4–500 eV energy range. Total scattering cross section and differential and integrated cross sections for elastic scattering and excitations of the 6s6p 1P1, 6s6p 3P0,1,2, 6s7p 1P1, 6s7s 1S0 and 6s7s 3S1 states are presented. Comparison with available experimental and theoretical results is presented. Good agreement with experimental differential cross sections in both shape and absolute values was found for most of the transitions.

A dense, lowtemperature, plasma target driven by an immersed, inductive antenna AIP Conf.The electrical conductivity is an important parameter in understanding the mechanism by which power is coupled to a radio-frequency ͑rf͒ discharge plasma, as well as in determining the external electrical characteristics of the discharge. We present the results of computations of the resistive and reactive components of the collisional impedance of an argon plasma at 13.56 MHz. The plasma conductivity is computed from the two-term solution to the Boltzmann equation, and includes the velocity dependence of the electron collision frequency, as well as non-Maxwellian electron energy distribution functions. We compare these results with those obtained from the widely used classical expression for plasma impedance, in which the electron collision frequency is computed either in the dc or high frequency limit. Our results show that neither of the classical limiting expressions are adequate for discharge pressures in the range of few mTorr to a few Torr, which includes the region of operation for many rf discharges used in many applications of plasma technology. Further, the classical formula assumes that in the high-frequency limit the plasma reactance is due entirely to electron inertia. We demonstrate that the plasma reactance may be strongly influenced, and in some cases dominated, by electron collisions. Results are presented in graphical form, which are useful in evaluating the importance of these effects on the interpretation of experimental results and the modeling of rf discharges.

ObjectiveTo estimate the rate of combination antiretroviral treatment change and factors associated with combination antiretroviral treatment change among patients recruited in the Australian HIV Observational Database (AHOD). MethodsAnalyses were based on patients in the AHOD who had commenced combination antiretroviral treatment after 1 January 1997. Combination antiretroviral treatment change was de®ned as the addition or change of at least one antiretroviral drug. A random-effect Poisson regression model was used to assess factors associated with increased rates of combination antiretroviral treatment change. ResultsA total of 596 patients in the AHOD were included in the analysis, with a median follow-up of 2.3 years. The overall rate of antiretroviral treatment change in this group was 0.45 combinations per year. In a multivariate analysis, a low CD4 count (, 200 cells/mL) at baseline was associated with an increased rate of treatment change [rate ratio (RR) 1.43; 95% con®dence interval (CI), 1.13, 1.80; P 0.003)]. Combinations including a nonnucleoside reverse transcriptase inhibitor were also associated with slower rates of change than treatment combinations including a protease inhibitor (RR 0.64, 95% CI, 0.51, 0.80, P , 0.001). ConclusionInitiating combination antiretroviral at a CD4 cell count , 200 cells/mL may be associated with poorer patient outcomes. However, the possibility that clinician or patient concerns about low immunological status led to faster rates of treatment change in this group cannot be discounted.

Using a 1D, two-fluid description, the influence of the magnetic field on the plasma boundary region adjacent to the cathode target in a magnetron has been investigated. Assuming the transport of magnetically confined electrons above the 'race-track' to be dominated by diffusion and mobility, with plasma ions falling directly to the cathode, the spatial variation in the electron and ion densities and velocities in the sheath and pre-sheath have been determined, together with the electric field structure. By assuming an electric field remnant in the plasma, the fluid equations have been integrated through the space-charge sheath up to the cathode, allowing smooth joining of the sheath and pre-sheath regions. Modelling with both constant and spatially varying magnetic fields has shown that realistic secondary electron fluxes are predicted for only a limited range of initial discharge parameters, indicating that Bohm diffusion of electrons is the most appropriate transport mechanism.Sheath widths are found to decrease with increasing magnetic field strength at the cathode and increase with cathode potential. In the constant-B -field case and for deep sheath potentials (>10 electron temperatures equivalent), the sheath behaviour is consistent with the Child-Langmuir law. However, the inclusion of a grad B force in the spatially varying magnetic field case gives rise to electron densities in the sheath, a significant fraction of the ion density (>10%).