Modelling of low-pressure Ar+Hg discharge with high electric current densities

Abstract

Deals with experimental and theoretical investigations on the characteristics of low-pressure Ar+Hg discharges with very high electric current densities. The positive column of these discharge systems is studied under the following conditions: noble gas pressure 7.7 Torr; tube radius 1.6 mm; cold-spot temperature 313-53 K (corresponding to a saturation mercury vapour pressure of 6-88 m Torr) and electric current 30-600 mA. It is found that the V-I characteristic of the positive column will be positive if the value of the current density is above a certain critical one. A six-level model which can fairly describe the depleted low-pressure Ar+Hg discharges is developed. The output of the theoretical model gives the whole macroscopic plasma properties such as the electric field strength, the electron temperature, the energy transfers of the various collision processes and the radiation output, etc. The E-I and Te-I characteristics calculated according to the model agree qualitatively with the measured results.

Power balance in the positive column of narrow bore (T2) fluorescent lamps

“…P jk ( x, x ) probability density of photons emitted elsewhere in the discharge at position x being absorbed at position x (13) Pjk (r) probability that photons from all other radii 0 < r < R 1 are absorbed at r (14) q em (ε) total elastic momentum transfer cross section with Hg and Ar atoms (9) q s jk (ε) cross section for the transition j → k (12) q t (ε) total electron transport cross section ( 7)…”

“…A fully 'non-local' model of radiation trapping has been incorporated to solve equation (13). The probability term P jk ( x, x ) is evaluated using the concept of frequency dependent cell-to-cell coupling coefficients, and includes the full isotopic and hyperfine structure of the Hg resonance lines, Doppler and resonance broadening, and broadening of Hg lines by Ar atoms.…”

“…Narrow bore FL are also used in backlighting of LCD displays, such as computer screens and TV panels. Experimental studies have been reported in the literature for T4 (OD =14 mm) [8,9], T3 (OD =10 mm) [10], T2 (OD =7 mm) [11,12] and T1 (OD = 4 mm) [13,14]. Probe measurements of electric field and electron density in T1 FL have also been compared with results from a numerical model [13].…”

“…At these levels of current density, mercury depletion at the centre of the discharge (cataphoresis), caused by the different speeds at which Hg ions and atoms diffuse through the plasma, can play an important role in determining the electrical characteristics and light output of the discharge. Such highly loaded lamps have traditionally provided a challenge to modellers [13], due to both the cataphoresis effects and the inadequate data available for ionization processes at high electron densities obtained in these discharges. These issues have been discussed with reference to a second class of highly loaded discharge [17][18][19], in which the power is coupled into the lamp by means of an external inductive coil (an example of so-called 'electrodeless' lamps).…”

Experimental and numerical study of a low-pressure Hg Ar discharge at high current densities

“…Standard positive column models (see section 4) have been less successful in reproducing experimental measurements for highly loaded lamps [15,16]. However, more complex kinetic models have reproduced many of the experimentally measured parameters such as electric field and radiative output for a wide range of power loading in discharges with buffer gas pressures of 1 Torr and greater [9,10].…”