rf-power and the ring-mode to red-mode transition in an inductively coupled plasma

Abstract

The optical output of an alkali-metal inductively coupled plasma (alkali-ICP) plays an important role in both atomic magnetometers and atomic clocks, producing these devices’ atomic signals through optical pumping. Unfortunately, though the alkali-ICP’s optical pumping efficiency grows exponentially with temperature, at relatively high temperatures (∼140 °C) the discharge transitions from “ring mode” to “red mode,” which is a spectral change in the plasma’s output that corresponds broadly to a transition from “good emission” for optical pumping to “poor emission.” Recently, evidence has accumulated pointing to radiation trapping as the mechanism driving the ring-mode to red-mode transition, suggesting that the phenomenon is primarily linked to the alkali vapor’s temperature. However, observations of the transition made in the 1960 s, demonstrating that the ICP temperature associated with the transition depended on rf-power, would appear to cast doubt on this mechanism. Here, we carefully investigate the influence of rf-power on the ring-mode to red-mode transition, finding that rf-power only affects the transition through discharge heating. Thus, the present work shows that the primary effect of rf-power on the ring-mode to red-mode transition can be understood in terms of the radiation trapping mechanism.

Low-frequency, self-sustained oscillations in inductively coupled plasmas used for optical pumping

“…5 2 S 1/2 transition). 22 As we raised the temperature of the discharge, entering the ring-mode to red-mode transition region, we found that the discharge could exhibit self-sustained oscillations as illustrated in Fig. 5 (Multimedia view).…”

“…Evidence suggests that the ring-mode to red-mode transition is driven by radiation trapping. 22 FIG. 1.…”

“…A thermistor attached to the metal base recorded temperature. Similar to our previous work, 22 we calibrated this metal-base temperature, T mb , to the liquid Rb pool's temperature, T Rb , using the Rb 420 nm line recorded by the spectrometer.…”

“…As in our previous work, 22 we placed an "injection antenna" around the discharge bulb and injected rf-power from an external source into the discharge at the oscillator's resonant frequency. In this way, we were able to suddenly change the rf-heating of the discharge by a small amount, and then measure T mb as a function of time in order to assess the thermal time constant, 1/c th .…”

Noble-Gas Loss in Alkali Rf-Discharge Lamps and Its Possible Dependence on Electron Temperature

“…As discussed in [30], rf-power has a significant influence on the lamp's average temperature. In addition, rf-power affects the lamp's temperature gradient, thereby defining the location of the liquid Rb pool in the lamp, and hence the Rb vapor pressure [31].…”