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

Abstract

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.

Power balance in highly loaded fluorescent lamps

“…Self-consistent numerical models of these lamps under standard operating conditions (400 mA discharge current or 0.035 A cm −2 , 0.8 Pa (6 mTorr) Hg, 400 Pa (3 Torr) Ar) have reproduced experimental measurements reasonably well [8][9][10][11][12]. However, Langmuir probe measurements of electron densities [3,7] and absorption spectroscopy measurements of mercury excited level densities [4] in highly loaded lamps show considerable disagreement with values predicted by the selfconsistent models. Further, the measured electron densities are inconsistent with the calculated electrical conductivity necessary to describe the electrical characteristics of these lamps [5,6].…”

“…The design of the discharge used in the experiments was described in [4] and is illustrated in figure 1. The lamp forms a closed tubular loop that passes through two toroidal ferrite cores, each wound with an induction coil, so that the voltage in the lamp is induced by a magnetic field contained entirely within the ferrite cores.…”

Fundamentals of cavity-enhanced polarimetry for parity-nonconserving optical rotation measurements: Application to Xe, Hg, and I

“…The production of Xe metastable states 6s 2 [3/2] o 2 and Hg 3 P J has been realized using low-pressure electrical discharge lamps [41,42] or optical pumping [43], yielding steady-state densities of about 10 12 cm −3 , allowing column densities of about 10 14 cm −2 (over a single-pass path-length of 100 cm). Similarly, high iodine atom densities of ∼ 10 16 cm −3 have been achieved in glow discharges (requiring high precursor and carrier gas pressures).…”

Cavity-Enhanced Parity-Nonconserving Optical Rotation in Metastable Xe and Hg

“…To obtain measurable PNC optical rotation signals, column densities of ∼ 10 18 cm −2 thermal atoms are typically required [4][5][6], so as to achieve about 20 absorption lengths, for which optical rotation is ∼ 5 R. Such column densities of excited-state atoms are generally not available due to the short excited-state lifetimes. The metastable states Xe 3 P 2 and Hg 3 P J have been produced at steady-state densities of about 10 12 cm −3 , using electrical discharge lamps [19,20] or optical pumping [21], allowing column densities of about 10 14 cm −2 (over a path length of 100 cm). For these column densities and calculated values of R, an enhancement factor of about 10 4 is necessary to obtain signal levels comparable to previous optical rotation experiments [4][5][6].…”

“…We note that all even isotopes will have similar spectra, whereas odd isotopes will have hyperfine structure; in addition, all eight Xe and seven Hg stable isotopes are commercially available (each low pressure lamp requires ∼1 µmol of isotopically pure gas). We plot the PNC optical rotation, 2Nϕ PNC , multiplied by the transmission, as a function of the laser frequency about the absorption line center, for metastable 202 Hg column densities of 3.8 × 10 18 cm −2 (∼107 absorption lengths) [19] , and 132 Xe column densities of 1 × 10 18 cm −2 (∼20 absorption lengths) [20]. Collisional line broadenings for Hg and Xe are estimated to be 20 MHz and 10 MHz respectively.…”