We present the recent progress in the development of a hydrogen maser in the TE111 mode. Hydrogen masers use the transition at 1420.405 MHz between the two hyperfine levels F = 0 and F = 1 of the 1s1/2 ground state of the hydrogen atom . Standard hydrogen masers are heavy devices which are based on the use of a TE011 cylindrical cavity with dimensions of the order of 27 cm . In contrast, the TE111 mode is the lowest frequency mode of a cylindrical cavity in the usual regime D/L<0.985 where D and L are the diameter and the length of the cavity, respectively . In comparison with the standard masers, the TE111 mode makes thus possible to reduce dimensions significantly to obtain resonance at 1420.405 MHz, which is very interesting for space applications and in particular in the context of the global positioning system.
The design of the upper und lower parts of the maser were studied in order to obtain a compact model. The cavity is made of aluminum and is composed of two halves which clamp a thin Teflon FEP sheet (0.125 mm). This sheet is used as a septum in order to create two storage regions in the cavity. This is compulsory because the TE111 mode exhibits two regions with opposite directions of the magnetic field. The measured frequency of the cavity at room temperature is 1420.610 MHz. Therefore in order to obtain the resonant frequency in vacuum, the working temperature of the maser should be around 40 °C. The frequency of the cavity is tuned by using a varactor diode, which allows a tuning range of 60 kHz. The loaded quality factor of the cavity with the teflon sheet is 13600. The cavity is surrounded by a thermal screen, a solenoid, three magnetic shieldings and a vacuum bell. Four ovens are used for the temperature control of the maser and a temperature stability of 10−4 K is expected.
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