Development Of Cesium Fountain Clock Physical Systems

Cesium atomic fountain clock is the primary standard clock of the current timeand frequency measurement, with the best accuracy performance and proofreadingother atomic clocks. It is applied widely in fields such as time-keeping system, thefield of metrology, fundamental physics etc.National Time Service Center (NTSC) is the academic institution specializing intime-frequency foundation and technical research; it is responsible for theestablishment, keeping and dissemination tasks of the national standard time. It is animportant participant in establishing the International Atomic Time (TAI). AlthoughNTSC has the domestic maximum time-keeping atomic clock set, it has no cesiumfountain atomic clock, which is able to calibrate the time-keeping atomic clock setwith high performance. Developed and operated cesium atomic fountain clock, canimprove the autonomy of Chinese standard time and frequency, and improve thequality of time-frequency service.Around this theme, the key technology studies of cesium atomic fountain clockare carried out. This paper mainly focuses on the physical parts consisting of magneticshield, microwave cavity and two-dimensional magneto-optical trap device. By layingcompensation coils and using the method of least squares, a uniform magnetic field isobtained in a large area from the microwave cavity to the atomic free flight range; byadvantage of optimized design, double-ended input, two-coupled connected andpressuring indium wire between connecting parts, an interrogation microwave cavityis realized, whose operating microwave power is only one tenth of the FranceSYRTE-FO1atomic clock, and the microwave leakage frequency shift reducedlargely. By injecting vacuum grease and sealant in coaxial line of state selected cavity,the problem of vacuum seal is solved. By designing and implementing atwo-dimensional magneto-optical trap device, a large flow of slow atomic beam isrealized. Specific research contents and results can be summarized as the followingthree parts:(1) The design and construction of the fountain clock magnetic shielding iscompleted with high performance. Firstly, the unique designs of the magneticshielding in fountain clock are given, including the magnetic shield for overallfountain clock and additional four small shield covers inside the vacuum cylinder, which enhance the strength of the magnetic shielding and extend the axial magneticuniformity area. Secondly, in the development of magnetic shielding, forms acomplete set of magnetic shielding design, material selection, testing and analysis ofthe theoretical and experimental methods. Thirdly, in order to conquer the worseningof the magnetic shielding performance induced by collision, the magnetic flux leakage,remanence, and other serious conditions in our case, placing different smallcompensation coils in different positions along the internal axial direction of themagnetic shield, adopting the least squares method and using homemadehigh-precision current sources, the magnetic leakage is well compensated and a largerange uniform magnetic field up to48cm with less than1.7nT fluctuations is obtained.(2) Design and commissioning of the interrogation microwave cavity andstate-selection microwave cavity is completed and a fine performance is achieved inthe fountain clock. As the core of the fountain clock, the performance of interrogationmicrowave cavity will affect the overall performance of the fountain clock.A copper content of up to99.99%oxygen-free copper is selected to avoidmagnetism. A design of the rational structures and double-ended microwavesymmetrical input are applied to minimize the cavity phase shift.To reduce microwave leakage, a60mm cutoff waveguide method and indiumwire between various parts joints are adopted.Theoretical calculations, software simulation and experimental methods are usedto optimize the coupling efficiency at all levels and impedance matching.A oxygen-free copper (OFC) tube with good thermal conductivity is placed onthe microwave cavity to increase the temperature uniformity of the atom flight path,which is conducive to the fountain clock blackbody radiation frequency shift of thetesting and evaluation.By Integrated debugging, little microwave leakage less than-75dB and highcoupling efficiency are achieved, and compared with the France SYRTE-FO1atomicfountain clocks, only one tenth of the interrogation microwave power is needed.In order to further reduce the cavity phase shift and improve cavity performance,design thinking about a novel microwave cavity with quadruple fed microwave isproposed.Taking into account the non-magnetic, low coefficient of thermal expansion,simplifying apparatus and other requirements, a state-selection microwave cavitymade of hard aluminum and with single-ended coupling is realized. At the same time, special weld is introduced between the coaxial line and the copper bearing and indiumwire is adopted between the selection cavity sidewall and the copper bearing, whichensure the vacuum and microwave leakage prevention between the connected parts.By filling vacuum grease and sealant in coaxial line of the selection cavity, a3.710-7Pa vacuum environment is achieved in the long-term work.(3) By designing and implementing a two-dimensional magneto-optical trapdevice, an intense and slow atomic beam is of great significance to improve thestability and accuracy of the cesium atomic fountain clock. Two-dimensionalmagneto-optical trap device consists of two separate traps, and the saturated cesiumvapor pressure can be adjusted to obtain optimal performance. A high flux up to2.1109atoms/s is achieved, which can meet the requirements of the cesium atomicfountain clock.The innovations of this paper are as follows:(1) A method of compensating for the magnetic flux leakage by placingcompensation coils in magnetic shielding cylinder is proposed and realized. A largerange uniform magnetic length up to48cm less than1.7nT fluctuations is obtained,which is in the leading domestic level;(2) Little microwave leakage less than-75dB and high coupling efficiency areachieved by designing and debugging microwave cavity; to further reduce the cavityphase shift, a new type of microwave cavity with four terminal design is proposed.(3) By debugging the fountain clock, the key signal-Ramsey resonance signal ofthe cesium atomic fountain clock is obtained, and the cesium fountain clock overallclosed-loop operation is realized, which are the foundation for the cesium atomicfountain clock performance improvement and practical application.