Abstract
The weakly-ionized, alkali inductively-coupled plasma (ICP) has a long history as the light source for optical pumping. Today, its most significant application is perhaps in the rubidium atomic frequency standard (RAFS), arguably the workhorse of atomic timekeeping in space, where it is crucial to the RAFS’ functioning and performance (and routinely referred to as the RAFS’ “rf-discharge lamp”). In particular, the photon flux from the lamp determines the signal-to-noise ratio of the device, and variations in ICP brightness define the long-term frequency stability of the atomic clock as a consequence of the ac-Stark shift (i.e., the light-shift). Given the importance of Rb atomic clocks to diverse satellite navigation systems (e.g., GPS, Galileo, BeiDou) – and thereby the importance of alkali ICPs to these systems – it is somewhat surprising to find that the physical processes occurring within the discharge are not well understood. As a consequence, researchers do not understand how to improve the spectral emission from the lamp except at a trial-and-error level, nor do they fully understand the nonlinear mechanisms that result in ICP light instability. Here, we take a first step in developing an intuitive, semi-quantitative model of the alkali rf-discharge lamp, and we perform a series of experiments to validate the theory’s predictions.
Highlights
Since the early 1960s1–3 alkali rf-discharge lamps have been used as optical-pumping light sources in atomic physics[4] and atomicphysics based devices
Rb rf-discharge lamps are crucial elements for atomic signal generation in the devices flying on GPS, Galileo, and BeiDou global navigation satellite system (GNSS) spacecraft,[8,9] playing a critical role in the achievable timekeeping performances of those atomic clocks
From an applied physics and technology perspective understanding the alkali inductively-coupled plasma (ICP) is critical for understanding the performance limitations of GNSS, and the pathways for GNSS improvement
Summary
Since the early 1960s1–3 alkali rf-discharge lamps (i.e., weakly-ionized inductively-coupled plasmas or ICPs) have been used as optical-pumping light sources in atomic physics[4] and atomicphysics based devices (e.g., atomic clocks[5] and magnetometers[6]). The plasma physics of the discharge is complex, and nonlinear.[17] it is unlikely that a complete first-principles theory of rfdischarge lamp operation will ever be developed. Our purpose here is to work towards a semi-quantitative conceptual model of alkali ICP spectral emission that makes a fair attempt at capturing the important elements of the plasma physics, yet simultaneously offers a vehicle for developing an intuitive picture of the lamp’s behavior.
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