Fluorescence quenching and the “ring-mode” to “red-mode” transition in alkali inductively coupled plasmas

Abstract

The ring-mode to red-mode transition in alkali metal inductively coupled plasmas (ICPs) (i.e., rf-discharge lamps) is perhaps the most important physical phenomenon affecting these devices as optical pumping light sources for atomic clocks and magnetometers. It sets the limit on useful ICP operating temperature, thereby setting a limit on ICP light output for atomic-clock/magnetometer signal generation, and it is a temperature region of ICP operation associated with discharge instability. Previous work has suggested that the mechanism driving the ring-mode to red-mode transition is associated with radiation trapping, but definitive experimental evidence validating that hypothesis has been lacking. Based on that hypothesis, one would predict that the introduction of an alkali-fluorescence quenching gas (i.e., N2) into the ICP would increase the ring-mode to red-mode transition temperature. Here, we test that prediction, finding direct evidence supporting the radiation-trapping hypothesis.

Spectral emission from the alkali inductively-coupled plasma: Theory and experiment

“…o C + (4463 ± 65) K for low rf-power (37) T Xe = (7.9 ± 3.5) T L − 80 o C + (4080 ± 59) K for high rf-power (38) As anticipated by the examination of Fig. 12a, there is a strong temperature dependence of T Xe at low rf-power levels.…”

“…The Rb-metal ICP (and its associated electronics) was obtained from a commercial atomic clock, 36 and the glass bulb containing the discharge was thermally bonded into a metal base and placed within the induction coils of a Hartley oscillator; the glass bulb had R = 0.4 cm and L = 1 cm ⇒ λ L = 4.7 cm 3 , and it contained 4.57 ± 0.21 torr Xe as a buffer gas. 37 An optical fiber, held in place by an X-Y-Z stage, was placed in front of the discharge lamp and connected to an Ocean Optics spectrometer (∆λ = 3 nm). A transistor at the metal base was used as the primary heating element, and in addition to the rf-signal that could be developed by the rf-oscillator's "internal" circuit, an external rf-signal was supplied.…”