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Mercury Trapped-Ion Frequency Standard for the Global Positioning System

Robert L. Tjoelker, Eric A. Burt, Sang‒Ho Chung, R. Gläser, R. Hamell, Lawrence Lim, Lute Maleki, John D. Prestage, Nasrat A. Raouf, Tom Radey, Canales Sepúlveda, G. F. Sprague, Blake Tucker, Brent C. Young

Published Nov 27, 2001• 6 citations
OpenAlex

Abstract

We report on progress towards the development of a small, low-mass and -power, high-stability mercury trapped-ion frequency standard for the Global Positioning System. The design performance goal is a frequency stability reaching into the 10(-16) range using technologies that allow for more than 10 years of continuous operational life. Key features include using a multi-pole ion trap to minimize sensitivity to ion-number-dependent effects and a nitrogen buffer gas for long vacuum pump life. The development program is structured in three phases with the goal of gaining early flight experience and keeping development costs in check.

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Trapped mercury ion is a promising candidate for portable and operational atomic clocks. It operates at the 40.5GHz ground state hyperfine transition of 199Hg ions. Mercury ions are confined in a linear RF trap, buffer-gas cooled, with a long ion storage lifetime in an ultra-high vacuum enclosure without an active pump. The clock transition is optically-pumped and detected with a Hg discharge lamp without need of a laser light source. This approach has proven to provide simplicity in design and implementation and robustness in operation. In this paper, we report our recently effort on developing a micro and low-power mercury trapped ion clock in the DARPA ACES program. In a 5 mm x 10 mm linear trap (0.25 cc), we have achieved the clock fractional frequency stability of .7 × 10-12τ-1/2and reached down to 7 × 10-14after 1000 seconds of averaging. We will discuss the clock performance under different conditions and implications toward a micro mercury trapped ion clock with low power and high stability.

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The authors have recently completed a breadboard ion-clock physics package based on Hg ions shuttled between a quadrupole and a 16-pole rf trap. With this architecture the authors have demonstrated short-term stability ~2-3times10-13at 1 second, averaging to 10-15at 1 day. This development shows that H-maser quality stabilities can be produced in a small clock package, comparable in size to an ultra-stable quartz oscillator required for holding 1-2times10-13at 1 second. This performance was obtained in a sealed vacuum configuration where only a getter pump was used to maintain vacuum. The vacuum tube containing the traps has now been under sealed vacuum conditions for nearly a year with no measurable degradation of ion trapping lifetimes or clock short-term performance. We have fabricated the vacuum tube, ion trap and UV windows from materials that will allow a ~400 C tube bake-out to prepare for tube seal-off. This approach to the vacuum follows the methods used in flight vacuum tube electronics, such as flight TWTA's where tube operation lifetime and shelf life of up to 15 years is achieved. They use neon as a buffer gas with 2-3 times less pressure induced frequency pulling than helium and, being heavier, negligible diffusion losses will occur over the operation lifetime

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Ion traps are exciting candidates as future precision frequency sources. Recent developments demonstrate that mercury ion frequency standards are capable of a stability performance comparable to commercial cesium standards. There is, however, considerable room for improvement with regard to the signal to noise problem. The 40 GHz microwave frequency implies that a careful design should be implemented to ensure the elimination of the unwanted side bands in the microwave pump signal. A long life, high performance light source to be used in a trapped mercury ion microwave standard must be developed and the long term performance of a trapped mercury ion microwave standard must be investigated. While newly proposed two photon pumping schemes in conjuction with mercury ions promise exciting developments for both microwave and optical frequency standards, other ions that may be potential candidates should be evaluated for their usefulness.

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Realization of a compact mercury ion microwave clock with frequency stability of 6.3 × 10− 16

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Abstract The mercury ion microwave clock is considered to be one of the leading candidates for new generation satellite navigation atomic clocks due to its excellent long-term frequency stability, extremely low frequency drift rate, high reliability and good potential for miniaturization. In recent years, the performance and maturity of the space mercury ion microwave clock have improved rapidly, and the reported long-term stability results are mostly in low 10 − 15 level. In this article, we present a compact prototype of mercury ion microwave clock aiming to spaceborne applications with the long-term stability of 10 − 16 . By regulating the physical effects contributing to the clock transition frequency shifts, the clock maintains a white frequency noise Allan deviation of 2.8×10 − 13 / τ 1/2 even with the averaging time τ over 10 5 s and achieves the long-term stability of 6.3 × 10 − 16 for averaging times of 200000 s. The space mercury ion microwave clock with such level performance will benefit the Global Navigation Satellite Systems and a wide range of space science and missions.

Scientific Reports•Dec 15, 2025•2 citations
We report on progress towards the development of a small, low-mass and -power, high-stability mercury trapped-ion frequency standard for the Global Positioning System. The design performance goal is a frequency stability reaching into the 10(-16) range using technologies that allow for more than 10 years of continuous operational life. Key features include using a multi-pole ion trap to minimize sensitivity to ion-number-dependent effects and a nitrogen buffer gas for long vacuum pump life. The development program is structured in three phases with the goal of gaining early flight experience and keeping development costs in check.
Trapped mercury ion is a promising candidate for portable and operational atomic clocks. It operates at the 40.5GHz ground state hyperfine transition of 199Hg ions. Mercury ions are confined in a linear RF trap, buffer-gas cooled, with a long ion storage lifetime in an ultra-high vacuum enclosure without an active pump. The clock transition is optically-pumped and detected with a Hg discharge lamp without need of a laser light source. This approach has proven to provide simplicity in design and implementation and robustness in operation. In this paper, we report our recently effort on developing a micro and low-power mercury trapped ion clock in the DARPA ACES program. In a 5 mm x 10 mm linear trap (0.25 cc), we have achieved the clock fractional frequency stability of .7 × 10-12τ-1/2and reached down to 7 × 10-14after 1000 seconds of averaging. We will discuss the clock performance under different conditions and implications toward a micro mercury trapped ion clock with low power and high stability.
Abstract The methods of trapping and cooling of atoms and ions have been transformative for atomic clocks due to the reduction, and in some cases elimination, of major systematic frequency shifts. Continuously operating atomic clocks based on trapped mercury ions have existed for decades but until recently have been restricted to terrestrial applications. The recently completed Deep Space Atomic Clock (DSAC) mission demonstrated the first trapped ion clock operation in space. Here we review DSAC as well as follow-on improvements towards the realization of high stability, long life Hg ion atomic clocks for foreseen ground and flight application.
We have recently completed a breadboard ion-clock physics package based on Hg ions shuttled between a quadrupole and a 16-pole rf trap. With this architecture we have demonstrated short-term stability ~1-2times10-13at 1 second, averaging to 10-15at 1 day. This development shows that H-maser quality stabilities can be produced in a small clock package, comparable in size to an ultra-stable quartz oscillator required for holding 1-2times10-13second. This performance was obtained in a sealed vacuum configuration where only a getter pump was used to maintain vacuum. The vacuum tube containing the traps has now been under sealed vacuum conditions for nearly two years with no measurable degradation of ion trapping lifetimes or clock short-term performance. We have fabricated the vacuum tube, ion trap and UV windows from materials that will allow a ~400degC tube bake-out to prepare for tube seal-off. This approach to the vacuum follows the methods used in flight vacuum tube electronics, such as flight TWTA's where tube operation lifetime and shelf life of up to 15 years is achieved. We use neon as a buffer gas with 2-3 times less pressure induced frequency pulling than helium and, being heavier, negligible diffusion losses will occur over the operation lifetime.
The authors have recently completed a breadboard ion-clock physics package based on Hg ions shuttled between a quadrupole and a 16-pole rf trap. With this architecture the authors have demonstrated short-term stability ~2-3times10-13at 1 second, averaging to 10-15at 1 day. This development shows that H-maser quality stabilities can be produced in a small clock package, comparable in size to an ultra-stable quartz oscillator required for holding 1-2times10-13at 1 second. This performance was obtained in a sealed vacuum configuration where only a getter pump was used to maintain vacuum. The vacuum tube containing the traps has now been under sealed vacuum conditions for nearly a year with no measurable degradation of ion trapping lifetimes or clock short-term performance. We have fabricated the vacuum tube, ion trap and UV windows from materials that will allow a ~400 C tube bake-out to prepare for tube seal-off. This approach to the vacuum follows the methods used in flight vacuum tube electronics, such as flight TWTA's where tube operation lifetime and shelf life of up to 15 years is achieved. They use neon as a buffer gas with 2-3 times less pressure induced frequency pulling than helium and, being heavier, negligible diffusion losses will occur over the operation lifetime
Ion traps are exciting candidates as future precision frequency sources. Recent developments demonstrate that mercury ion frequency standards are capable of a stability performance comparable to commercial cesium standards. There is, however, considerable room for improvement with regard to the signal to noise problem. The 40 GHz microwave frequency implies that a careful design should be implemented to ensure the elimination of the unwanted side bands in the microwave pump signal. A long life, high performance light source to be used in a trapped mercury ion microwave standard must be developed and the long term performance of a trapped mercury ion microwave standard must be investigated. While newly proposed two photon pumping schemes in conjuction with mercury ions promise exciting developments for both microwave and optical frequency standards, other ions that may be potential candidates should be evaluated for their usefulness.
Abstract The mercury ion microwave clock is considered to be one of the leading candidates for new generation satellite navigation atomic clocks due to its excellent long-term frequency stability, extremely low frequency drift rate, high reliability and good potential for miniaturization. In recent years, the performance and maturity of the space mercury ion microwave clock have improved rapidly, and the reported long-term stability results are mostly in low 10 − 15 level. In this article, we present a compact prototype of mercury ion microwave clock aiming to spaceborne applications with the long-term stability of 10 − 16 . By regulating the physical effects contributing to the clock transition frequency shifts, the clock maintains a white frequency noise Allan deviation of 2.8×10 − 13 / τ 1/2 even with the averaging time τ over 10 5 s and achieves the long-term stability of 6.3 × 10 − 16 for averaging times of 200000 s. The space mercury ion microwave clock with such level performance will benefit the Global Navigation Satellite Systems and a wide range of space science and missions.