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Chinese Journal of Quantum Electronics

Cooling effect of different buffer gas of trapped mercury ions in a linear ion trap

Yang Yu-na, Hao Liu, Yue-Hong He, Chen Yi-he, She Lei, LI Jiao-mei

Published Jan 1, 2012• 0 citations
OpenAlex

Abstract

Buffer gas cooling is the most effective and practical method to cool ions in ion trap.The kind and quantity of buffer gas are the key technologies in mercury ion microwave frequency standard experiments.Buffer gas made of helium,neon or argon was studied to cool trapped mercury ions(~(199)Hg~+) in a linear ion trap by introducing a resistance term in Mathieu's equation.It's found that the decay time of motion of ~(199)Hg~+ in argon gas is the shortest,and the frequency shift of the clock transition(40.5 GHz) is minimum when the pressure of helium is 10~(-5) Torr or the pressure of neon gas is 2.4×10~(-5) Torr.Neon is the most suitable buffer gas among helium,neon and argon,considering the decay time-constant of motion of ~(199)Hg~+ in buffer gases and ~(199)Hg~+ clock transition shift's sensitivity to the change of the pressure of buffer gas.

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Buffer gas cooling is the most effective and practical method to cool ions in ion trap.The kind and quantity of buffer gas are the key technologies in mercury ion microwave frequency standard experiments.Buffer gas made of helium,neon or argon was studied to cool trapped mercury ions((199)Hg+) in a linear ion trap by introducing a resistance term in Mathieu's equation.It's found that the decay time of motion of (199)Hg+ in argon gas is the shortest,and the frequency shift of the clock transition(40.5 GHz) is minimum when the pressure of helium is 10~(-5) Torr or the pressure of neon gas is 2.4×10~(-5) Torr.Neon is the most suitable buffer gas among helium,neon and argon,considering the decay time-constant of motion of (199)Hg+ in buffer gases and (199)Hg+ clock transition shift's sensitivity to the change of the pressure of buffer gas.
The influences of different buffer gas, neon and helium, on Hg-199(+) clock transition are compared in trapped Hg-199(+) linear trap. By the technique of time domain's Ramsey separated oscillatory fields, the buffer gas pressure frequency shifts of Hg-199(+) clock transition are measured to be (df/dP(Ne)) (1/f) = 1.8 x 10(-8) Torr(-1) for neon and (df/dP(He)) (1/f) = 9.1 x 10 8 Torr(-1) for helium. Meanwhile, the line-width of Hg-199(+) clock transition spectrum with the buffer gas neon is narrower than that with helium at the same pressure. These experimental results show that neon is a more suitable buffer gas than helium in Hg-199(+) ions microwave frequency standards because of the Hg-199(+) clock transition is less sensitive to neon variations and the better cooling effect of neon. The optimum operating pressure for neon is found to be about 1.0 x 10(-5) Torr in our linear ion trap system.
The influences of different buffer gas, neon and helium, on199Hg+clock transition are compared in trapped199Hg+linear trap. By the technique of time domain’s Ramsey separated oscillatory fields, the buffer gas pressure frequency shifts of199Hg+clock transition are measured to be(d f /dPNe)(1/ f) = 1.8 × 10-8Torr-1for neon and(d f /dPHe)(1/ f) = 9.1 × 10-8Torr-1for helium. Meanwhile, the line-width of199Hg+clock transition spectrum with the buffer gas neon is narrower than that with helium at the same pressure. These experimental results show that neon is a more suitable buffer gas than helium in199Hg+ions microwave frequency standards because of the199Hg+clock transition is less sensitive to neon variations and the better cooling effect of neon. The optimum operating pressure for neon is found to be about 1.0 × 10-5Torr in our linear ion trap system.
We report measured buffer gas collision shifts of the 40.507347996xx GHz mercury ion clock transition using inert helium, neon, and argon gases and getterable molecular hydrogen and nitrogen gases. The frequency shift due to methane was also examined. At low partial pressures methane gas did not impact the trapped ion number but was observed to strongly relax the population difference in the two hyperfine clock states thereby reducing the clock resonance signal. A similar population relaxation was also observed for other molecular buffer gases (N/sub 2/, H/sub 2/) but at much reduced rate.
Individual Ba ions are trapped in a gas-filled linear ion trap and observed with a high signal-to-noise ratio by resonance fluorescence. Single-ion storage times of $\ensuremath{\sim}5\phantom{\rule{0.3em}{0ex}}\mathrm{min}$ $(\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{min})$ are achieved using He (Ar) as a buffer gas at pressures in the range $8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}--4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\phantom{\rule{0.3em}{0ex}}\text{torr}$. Trap dynamics in buffer gases are experimentally studied in the simple case of single ions. In particular, the cooling effects of light gases such as He and Ar and the destabilizing properties of heavier gases such as Xe are studied. A simple model is offered to explain the observed phenomenology.
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.