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To reduce the size and weight of the hydrogen maser atomic clocks, some useful attempts and related research results about sapphire-loaded cylindrical cavities for hydrogen masers were reported by the Beijing Institute of Radio Metrology and Measurement. The fractional frequency stability of the order of 10-15 over 10 000 seconds can be realized. However, because of a large frequency-temperature coefficient in a single sapphire bulb in the cavity, further improvement of the stability in the compact hydrogen clock was restricted. In this work, we chose several small single-crystal chips of SrTiO3 with a large negative frequency-temperature coefficient to compensate the sapphire microwave cavity. Based on the theoretical calculation, the frequency-temperature coefficient in the TE011 mode of a sapphire cavity associated with several small chips of SrTiO3 can be greatly reduced. A sapphire-loaded cavity and 8 compensated chips of SrTiO3 were prepared, and a combined cavity was simulated by finite element method and measured by experiments. When quality factor was kept above 40 000, the frequency-temperature coefficient can be reduced to about 1/5 of its starting value. The experimental results agree very well with the calculation and simulation. Furthermore, this new method was applied in the compact hydrogen maser. Because of the decrease of temperature frequency shift, the hydrogen maser stability at medium- and long- term averaging time from 1 s to 105 s has an obvious improvement compared with the our previous results. [ABSTRACT FROM AUTHOR]

The previous compact hydrogen maser with sapphire microwave cavity at the Beijing Institute of Radio Metrology and Measurement was not suitable for a space application in navigation systems with limitations on volume and weight. To resolve this problem, we present a new design of the sapphire-loaded cavity with optimal parameters in the minimization of volume and maximization of the quality factor. Methods of theoretical calculations, finite element simulation, and related experiments were performed in designing the sapphire-loaded cavity. Based on the analysis, a miniaturized sapphire microwave cavity with the total volume of 3.04 dm³, the quality factor 67 500, and frequency-temperature coefficient -59.7 kHz/°C is developed. The experimental results are completely consistent with calculation values. In addition, the theoretical calculation result shows that the product of the z-component of the magnetic energy filling factor in the bulb region and the cavity TE011-mode Q-factor is 40 800 at 50°C in the miniaturized sapphire cavity. In addition, 2 sapphire-loaded cavities in the Japanese National Institute of Information and Communications Technology and in the Microwave and Optical Communication Research Institute at the University of Limoges, France, are compared with that of our designs. [ABSTRACT FROM AUTHOR]