Ladderlike ionization of the mercury atom in Hg-Ar low-pressure discharge and its modeling

Discrepancies reported in the literature between numerical predictions and experimental measurements in low-pressure Hg discharges at high current densities are considered. Elements of a one-dimensional fluid model and recent spectroscopic and Langmuir probe measurements are combined in a semi-empirical way to individually examine components of the positive column power balance and the discharge conductivity. At a Hg vapour pressure of 0.81 Pa (6.1 mTorr) and a current density of 300 mA cm −2 , previous discrepancies in the power balance and discharge conductivity are simultaneously resolved by assuming a higher electron density than that obtained from the Langmuir probe measurements. This conclusion is supported by independent measurements of ion density reported in a companion paper. The importance of radial cataphoresis under these conditions, particularly with regard to radiation transport, is highlighted. This work is of particular interest for the design of fluorescent lamps operating at high current densities.

Section: Discussionmentioning

confidence: 99%

Power balance in highly loaded fluorescent lamps

Lister¹,

Curry²,

Lawler³

2004

“…Discrepancies between measurements and calculations of Hg excited level densities in other highly loaded discharges were reported in [17], indicating important mechanisms for depopulating these levels and enhancing the ionization balance are missing from the models. There is clearly a need to better understand the role of multi-step ionization, identified as an important process by Wani [32]. The development of theoretical methods for calculating accurate electron impact cross sections near threshold, and computer programs to implement them [33,34] have opened the possibility that the necessary data might be accurately calculated.…”

Section: Discussionmentioning

confidence: 99%

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

Han¹,

Jiang²,

Zhu³

et al. 2008

Experimental measurements of the electric field and absolute radiance of 254 nm radiation in the positive column of Hg–Ar discharges, corresponding to T2 (outer diameter 7 mm) fluorescent lamps (FLs) are presented. The discharges were operated with 5 Torr argon filling pressure, for discharge currents 20–200 mA and cold spot temperatures 20–80 °C. These narrow bore discharges are ‘highly loaded’, with current densities up to an order of magnitude higher than in larger radius standard FL, and mercury depletion at the axis due to radial cataphoresis can be severe. The cold spot temperature for a maximum production of 254 nm radiation is found to be around 50 °C, compared with ∼40 °C for standard FL. The measurements are compared with the results of a numerical model. The model results are also used to provide a power balance for the positive column, showing the relative importance of all power dissipation mechanisms (radiation, gas heating and particle diffusion to the wall of the discharge).

“…Theoretical and numerical models of the positive column in fluorescent lamps have been developed for more than 40 years, since the pioneering work of Waymouth and Bitter [5] and Cayless [6]. Since that time, all major lighting companies have developed numerical programs to describe the positive column [7][8][9][10][11]. There are also a number of other models, principally by university researchers (e.g.…”

Section: Introductionmentioning

confidence: 99%

“…the electron energy distribution function (EEDF) (expressed in this paper through the electron energy probability function (EEPF)). The first [7][8][9][10][11][12]15]-'fluid' models-assume an analytic approximation for the EEPF, from which a set of rate and transport coefficients may be calculated. A set of rate equations (including the ambipolar diffusion for electrons and ions) is then solved to compute particle densities, electric fields, etc.…”

Section: Introductionmentioning

confidence: 99%

Approximations to the electron energy distribution and positive column models for low-pressure discharge light sources

Lister

Sheverev

Uhrlandt

2002

The applicability of `fluid' models based on analytic approximations of the electron energy distribution function (EEDF) and of kinetic models for low-pressure discharge light sources is discussed. Traditionally, `fluid' models of fluorescent lamps assume that the EEDF is Maxwellian up to the energy of the first excited state. It is shown that such an approach is sufficiently accurate in most cases of conventional as well as of `highly loaded' fluorescent lamps. However, this assumption is strongly violated for many rare gas glow discharges for mercury free light sources. As an example, a neon dc discharge is studied. The densities of the four lowest excited states and the electric field have been measured. The experimental results can be fairly well reproduced by a kinetic positive column model. This article was scheduled to appear in issue 14 of J. Phys. D: Appl. Phys. To access this special issue please follow this link: http://stacks.iop.org/0022-3727/35/i=14/