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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Space flyable Hg/sup +/ frequency standards

John D. Prestage, Lute Maleki

Published Dec 17, 2002• 4 citations
Publisher

Abstract

We discuss a design for a space based atomic frequency standard (AFS) based on Hg/sup +/ ions confined in a linear ion trap. This newly developed AFS should be well suited for space borne applications because it can supply the ultra-high stability of a H-maser but its total mass is comparable to that of a NAVSTAR/GPS Cs clock, i.e., about 11 kg. This paper compares the proposed Hg/sup +/ AFS to the present day GPS Cs standards to arrive at the 11 kg mass estimate. The proposed space borne Hg/sup +/ standard is based upon the recently developed extended linear ion trap architecture which has reduced the size of existing trapped Hg/sup +/ standards to a physics package which is comparable in size to a Cesium beam tube. The demonstrated frequency stability to below 10/sup -15/ of existing Hg/sup +/ standards should be maintained or even improved upon in this new architecture. This clock would deliver far more frequency stability per kilogram than any current day space qualified standard.>

DOI
10.1109/freq.1994.398252

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We discuss a design for a space based atomic frequency standard (AFS) based on Hg/sup +/ ions confined in a linear ion trap. This newly developed AFS should be well suited for space borne applications because it can supply the ultra-high stability of a H-maser but its total mass is comparable to that of a NAVSTAR/GPS Cs clock, i.e., about 11 kg. This paper compares the proposed Hg/sup +/ AFS to the present day GPS Cs standards to arrive at the 11 kg mass estimate. The proposed space borne Hg/sup +/ standard is based upon the recently developed extended linear ion trap architecture which has reduced the size of existing trapped Hg/sup +/ standards to a physics package which is comparable in size to a Cesium beam tube. The demonstrated frequency stability to below 10/sup -15/ of existing Hg/sup +/ standards should be maintained or even improved upon in this new architecture. This clock would deliver far more frequency stability per kilogram than any current day space qualified standard.>
The authors report the development of a fieldable frequency standard based on /sup 199/Hg/sup +/ ions confined in a hybrid RF/DC linear ion trap. This trap permits storage of large numbers of ions with reduced susceptibility to the second-order Doppler effect caused by the RF confining fields. A 160-mHz-wide atomic resonance line for the 40.5-GHz clock transition is used to steer the output of a 5-MHz crystal oscillator to obtain a stability of 2*10/sup -15/ for 24000 second averaging times. For long averaging intervals measurements are limited by instabilities in available hydrogen maser frequency standards.>
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 ~1-2x10-13 at 1 second, averaging to 10-15 at 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 1-2x10-13 at 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 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 ~ 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. 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.
We report the development of a fieldable frequency standard based on 199Hg+ ions confined in a hybrid r.f./dc linear ion trap. This trap permits storage of large numbers of ions with reduced susceptibility to the second-order Doppler effect caused by the r.f. confining fields. A 160 mHz wide atomic resonance line for the 40·5 GHz clock transition is used to steer the output of a 5 MHz crystal oscillator to obtain a stability of 2 × 10−15 for 24 000 s averaging times. For longer averaging intervals, measurements are limited by instabilities in available hydrogen maser frequency standards. Measurements with 37 mHz linewidth for the Hg+ clock transition demonstrate that the inherent stability for this frequency standard is at least as good as 1 × 10−15.
The frequency stability obtained with an ion-trap-based frequency standard where /sup 199/Hg/sup +/ ions are confined in a hybrid RF/DC linear trap is described. The 40.5-GHz clock transition is measured to be as narrow as 17 mHz representing a quality factor of 2.4*10/sup 12/. A hydrogen maser interrogation of a 37-mHz Hg/sup +/ resonance is used to measure 1*10/sup -15/ stability of the ion resonance at 10000-s averaging time. In another measurement the frequency of the superconducting cavity maser oscillator is steered to follow the Hg/sup +/ clock transition, again providing frequency stability of 1*10/sup -15/ at 10000-s.>
The frequency stability of an atomic standard based on /sup 199/Hg/sup +/ ions confined in a hybrid RF/DC linear trap is described. The 40.5-GHz clock transition has been measured to be 17 mHz wide, representing a quality factor greater than 2*10/sup 12/. A 160-mHz line is used to steer the output of a 5-MHz crystal oscillator to obtain a stability of 2*10/sup -15/ for 24000-s averaging times. In a separate measurement, a 37-mHz line is used to steer the output of the superconducting cavity maser oscillator to reach 1*10/sup -15/ stability at 10000 s.>