James Thomas Dakin

A model of radial variations in the low‐pressure mercury–argon positive column

The low‐pressure Hg–Ar positive column is studied with a model which accounts for radial transport mechanisms. The model calculates, not assumes, the radial density distribution for each of the species and the radial temperature distributions for the electron and the neutral gases. Solutions to the discharge equations are found for specified values of the wall (Hg reservoir) temperature, electric current, Ar pressure, and tube diameter. These solutions compare favorably with available parametric data for 3.6‐ and 1.4‐cm‐diam. discharge tubes. The model is used to investigate effects such as radial cataphoresis, the effect of radiation trapping on radial distributions, and deviations from a radially uniform electron temperature. The work supports some of the simplifying assumptions often made in modeling the discharge at the standard fluorescent lamp operating condition, such as a radially invariant electron temperature, the use of a constant escape factor to describe radiation trapping, and negligible rad...

Frequency dependence of the pulsed high‐pressure sodium arc spectrum

Spectral energy distributions are calculated and measured for a wall‐stabilized high‐pressure Na‐Hg‐Xe arc in pulsed operation. These and other quantities are studied as a function of the pulsing frequency over the range 500–5000 Hz, and as a function of duty cycle over the range 25%–100% for a fixed power input of 150 W. The arc is contained in a 5.5‐mm‐diam channel with an electrode separation of 39 mm containing a gas mixture of 100‐Torr Na, 470‐Torr Hg, and 190‐Torr Xe. The fully time‐dependent energy balance model assumes local thermodynamic equilibrium and one‐dimensional cylindrical symmetry. The radiation transport calculation includes the effects of 28 Na lines in the visible and infrared, 20 of which are treated as optically thick. The amount of nonresonance Na radiation in the spectrum is found to be maximum at the lowest frequency and duty cycle. Under these conditions the model shows the arc center temperature to be lowest prior to the pulse, but to reach the highest peak values well before t...

Wall‐induced inhomogeneities in the low‐pressure Hg‐Ar positive column

Radial variations of the Hg(63P0,1,2) densities are measured in a low‐pressure Hg‐Ar discharge and the results are compared with a model. The 34‐mm‐diam discharge tube is operated in the vicinity of 400‐mA current, 6.4‐mTorr Hg pressure, and 2.81‐Torr Ar pressure. A tunable dye laser beam passes axially through the discharge tube, and densities are measured using both a saturated absorption technique and an interference technique. The model incorporates continuity equations for Hg excited and ionic states, and a two‐temperature approach to parametrizing the electron energy distribution function. The shapes of the radial density distributions of the various species are predicted rather than assumed. Considerable attention is given to the role of tail electrons above the energy threshold for Hg inelastic scattering. Theoretical considerations and the measured variations of the 63P0 and 63P2 metastable densities suggest that the tail electron temperature is lower near the wall than at the center of the disch...

Energy balance of the high-pressure mercury discharge with sodium- and scandium-iodide additives

A high‐pressure Hg discharge with NaI and ScI3 additives is studied through model calculations and spectroscopic diagnostics. The computer model solves the coupled energy, radiation, and chemistry equations for the discharge on a self‐consistent basis. The wall‐stabilized discharge is assumed to be in local thermodynamic equilibrium and to exhibit radial symmetry and axial homogeneity. From a prescription of the boundary conditions, the model predicts radial variations in discharge quantities, such as the temperature, chemistry, and radiation field. Predictions are checked by directly comparing model results with experimental data for a horizontal discharge tube. The arc tube is rotated about its own axis so as to suppress convective effects. Necessary input to the model includes oscillator strengths and broadening constants for a large number of Hg, Na, and Sc spectral lines, as well as scattering cross sections and thermochemical data. The required data have been gathered from available sources and dete...

Metal halide lamp having improved operation acoustic frequencies

Asymmetric discharge electrode means are provided for a metal halide lamp enabling improved operation at acoustic frequencies. A xenon-metal halide lamp employing such improved discharge electrode means is disclosed together with a reflector lamp unit employing such lamp construction for its light source.

Model and diagnostics of the high‐pressure mercury discharge with sodium iodide additives

Spectroscopic data from a high‐pressure mercury arc with sodium iodide additives are compared with model calculations. The fused silica arc tube was oriented horizontally, and rotated about its own axis at sufficiently high speed to center the arc. This mode of operation suppresses convective effects, creating a greater degree of axial symmetry and longitudinal homogeneity than is normally present in a metal halide arc. High‐resolution line‐of‐sight spectra were recorded for all of the principal spectral features and for a series of offsets from the arc axis. Several independent measurements of the arc temperature are extracted from the spectra using standard techniques. The arc model assumes local thermodynamic equilibrium, axial symmetry, longitudinal homogeneity, absence of convection, and time independence. Equations associated with energy conservation, radiation transfer, chemical equilibrium, and multicomponent diffusion are solved simultaneously to determine a self‐consistent arc temperature profil...

Radial variations of Hg(63PJ) in a narrow‐diameter low‐pressure Hg‐Ar discharge

Two laser techniques are used to measure densities of mercury atoms in the three 63PJ states populated in the positive column of a narrow (13.7 mm diameter) low‐pressure Hg‐Ar dc discharge. Relative densities are obtained as a function of radial position using a saturated laser absorption technique, while on‐axis absolute densities are measured using the hook method (an interferometric technique). Results are shown for two mercury vapor pressures and are compared with computer model predictions.Two laser techniques are used to measure densities of mercury atoms in the three 63PJ states populated in the positive column of a narrow (13.7 mm diameter) low‐pressure Hg‐Ar dc discharge. Relative densities are obtained as a function of radial position using a saturated laser absorption technique, while on‐axis absolute densities are measured using the hook method (an interferometric technique). Results are shown for two mercury vapor pressures and are compared with computer model predictions.