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

Coffer, J. G.; Encalada, Nicole L.; Huang, M.C.Y.; Camparo, J. C.

doi:10.1063/1.4899199

Cited by 9 publications

(3 citation statements)

References 30 publications

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“…However, the alkali rf-discharge lamp is a nonlinear device, 28 and its typical operating regime in a Rb atomic clock pushes it close to instability. 29…”

Section: B the Ac Stark Shift Or Light Shiftmentioning

confidence: 99%

The ac stark shift and space-borne rubidium atomic clocks

Formichella

Camparo

Sesia

et al. 2016

Journal of Applied Physics

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Due to its small size, low weight, and low power consumption, the Rb atomic frequency standard (RAFS) is routinely the first choice for atomic timekeeping in space. Consequently, though the device has very good frequency stability (rivaling passive hydrogen masers), there is interest in uncovering the fundamental processes limiting its long-term performance, with the goal of improving the device for future space systems and missions. The ac Stark shift (i. e., light shift) is one of the more likely processes limiting the RAFS' long-term timekeeping ability, yet its manifestation in the RAFS remains poorly understood. In part, this comes from the fact that light-shift induced frequency fluctuations must be quantified in terms of the RAFS' light-shift coefficient and the output variations in the RAFS' rf-discharge lamp, which is a nonlinear inductively-couple plasma (ICP). Here, we analyze the light-shift effect for a family of 10 on-orbit Block-IIR GPS RAFS, examining decade-long records of their on-orbit frequency and rf-discharge lamp fluctuations. We find that the ICP's light intensity variations can take several forms: deterministic aging, jumps, ramps, and non-stationary noise, each of which affects the RAFS' frequency via the light shift. Correlating these light intensity changes with RAFS frequency changes, we estimate the light-shift coefficient, K-LS, for the family of RAFS: K-LS = -(1.9 +/- 0.3) x 10(-12) /%. The 16% family-wide variation in K-LS indicates that while each RAFS may have its own individual K-LS, the variance of K-LS among similarly designed RAFS can be relatively small. Combining K-LS with our estimate of the ICP light intensity's non-stationary noise, we find evidence that random-walk frequency noise in high-quality space-borne RAFS is strongly influenced by the RAFS' rf-discharge lamp via the light shift effect. Published by AIP Publishing

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“…However, the alkali rf-discharge lamp is a nonlinear device, 28 and its typical operating regime in a Rb atomic clock pushes it close to instability. 29…”

Section: B the Ac Stark Shift Or Light Shiftmentioning

confidence: 99%

The ac stark shift and space-borne rubidium atomic clocks

Formichella

Camparo

Sesia

et al. 2016

Journal of Applied Physics

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“…1, optimum atomic signal generation is often achieved with the lamp operating near the "ring-mode" to "red-mode" transition [6], a region of operation where radiation trapping first begins to play a dominant role in the plasma physics [7]. Not only does the visual appearance of the discharge from the lamp change in this transition region, but the lamp can exhibit instability in the form of low-frequency pulsing [8]. Moreover, depending on the interaction of the discharge's complex permeability and the electronic circuit powering the discharge [9], ion-acoustic waves can develop in the discharge giving rise to ~10 kHz light intensity oscillations [10,11].…”

Section: Introductionmentioning

confidence: 99%

Measuring buffer-gas pressure in sealed glass cells

Driskell

Huang

Camparo

2015

2015 Joint Conference of the IEEE International Frequency Control Symposium &Amp; The European Frequency and Time Forum

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In alkali rf-discharge lamps used for optical pumping in atomic clocks and magnetometers, a buffer-gas (Kr or Xe) allows electrons to extract energy from an rf-field, and these energized electrons eventually produce alkali resonant light. Contrary to naïve intuition, rf-discharge lamps can lose their noble-gas buffer over time. Recently, we began a long-term experimental program to better understand the mechanism of noble-gas loss in rf-discharge lamps, and needed a nondestructive means of measuring buffer-gas pressure in sealed glass cells. For this purpose, we employ the Kazantsev, Smirnova, and Khutorshchikov (KSK) technique, which is based on inferring buffer-gas pressure from the collision shift of an alkali ground-state hyperfine transition frequency ν hfs . Here, we discuss the basic the KSK technique and two modifications that we have implemented for its improvement: use of a diode laser for optical pumping, and extrapolation of ν hfs to zero magnetic field. Testing our system's long-term performance with a very low pressure reference cell (i.e., 3.3 torr Xe), we find a reproducibility of 0.2% and an absolute accuracy of 5%. Further, our systematic drift is less than one mtorr/month. I.Recently, it has become clear that slowly, over the course of years, the Kr and Xe buffer gases in the lamp can be lost from the lamp volume [15,16]. This buffer-gas loss not only alters the lamp's operating characteristics, it can eventually lead to the lamp's failure, since without elastic collision partners the electrons cannot extract energy from the rf-field. At present, the most viable theory for buffer-gas loss in discharge lamps involves the Noble-gas Ion Capture (NIC) mechanism of Jaduszliwer and co-workers [17,18]. Briefly, the NIC mechanism posits that the few very high energy

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“…Only recently have researchers come to understand that the ring-mode to red-mode transition in the ICP (where rf-discharge lamps routinely operate) derives from radiation trapping, 15 and that this transition-region is a regime of lamp instability. 16 Researchers still do not have theoretical justification for the proportionality between Rb light emission and Rb vapor pressure; they do not understand the threshold temperature for rf-discharge lamp operation, and they have no clear sense of how that threshold temperature scales with the rf-power driving the discharge.…”

Section: Introductionmentioning

confidence: 99%

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

2018

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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.

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

Cited by 9 publications

References 30 publications

The ac stark shift and space-borne rubidium atomic clocks

The ac stark shift and space-borne rubidium atomic clocks

Measuring buffer-gas pressure in sealed glass cells

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

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