铯原子喷泉钟物理系统的研制
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摘要
铯原子喷泉钟是当今时间频率的基准钟，具有最高的准确度，标校着其它原子钟，在守时、计量和基础物理研究等领域有着广泛的应用价值。
中国科学院国家授时中心（NTSC）是我国专门从事时间频率基础和技术研究的科研机构，担负着国家标准时间的建立、保持和发播任务，是国际原子时TAI建立的重要参加单位。国家授时中心拥有全国最大的守时钟组，但是没有可以对其进行实时校准的铯原子喷泉钟。研制和运行铯原子喷泉钟，可以提高我国标准时间频率产生的自主性，提高我国时间频率服务的质量。
围绕铯原子喷泉钟物理系统的研制，本文主要研究喷泉钟磁屏蔽、微波腔和二维磁光阱部分。磁屏蔽方面，通过在磁屏蔽内部加补偿线圈和运用最小二乘法，实现了从微波激励腔到原子自由飞行区大范围的均匀磁场；激励腔方面，通过分析设计、耦合馈入、铟丝压封等，微波输入功率降至法国喷泉原子钟的十分之一，减小了原子钟微波泄漏频移；选态腔方面，通过真空脂和密封胶注入，成功解决了微波传输线真空漏气问题；二维磁光阱研制方面，设计并实现了二维磁光阱装置，获得了大流量的慢速原子束。主要研究内容和成果归纳为以下三个部分：
（1）完成了喷泉钟磁屏蔽的设计和研制，达到了较高的性能指标。本文首先给出本喷泉钟在磁屏蔽方面的独特设计，即对喷泉钟整体进行了磁屏蔽，并在真空筒内对应小、中屏蔽筒的端盖位置处设计和放置四个小屏蔽盖，延长了轴向磁场的均匀性；其次，在磁屏蔽研制中，形成了一整套磁屏蔽设计、选材、测试和分析的理论和实验方法；针对磁屏蔽系统由于碰撞、变形等原因导致的磁性能下降，以及漏磁和剩磁等严重情况，通过在磁屏蔽内部轴向方向不同位置放置小的补偿线圈，运用最小二乘法和研制的高精度电流源，实现了对漏磁的较好补偿，得到在长达48cm范围内磁场波动小于1.7nT的均匀磁场。达到国内领先水平。
（2）设计并调试了微波激励腔和选态腔，较好满足了喷泉钟性能的要求。激励腔是喷泉钟的核心，激励腔质量关系着喷泉钟的整体性能指标。选用含铜量高达99.99%的零号无氧铜，达到了无磁和高导电率要求；合理设计结构并采用双端微波对称输入，减少了腔相位频移；采用铟丝压封及60mm截止波导方法，减少了微波泄漏；通过理论计算、软件模拟与实验相结合方法，优化各级耦合效率和阻抗匹配；通过在激励腔上方放置导热性能良好的无氧铜管，增加了原子飞行路径上温度的均匀性，有利于喷泉钟黑体辐射频移的测试评估。通过综合调试，在-75dB范围内未发现有微波泄漏，微波腔馈入微波功率只需要法国FO1喷泉钟的十分之一。为了进一步降低腔相位频移，提高喷泉钟性能，分析提出了四端馈入微波的新型微波腔设计思想。选态腔方面，考虑到无磁、低热膨胀系数、低Q值等要求，选用硬铝材料；采用单端侧面微波耦合，简化了装置；微波线与铜支座之间采用特殊焊接方式，铜支座与选态腔侧壁之间采用压铟丝方式，保证了部件连接处的真空和防止微波泄漏；采用微波线灌真空脂与密封胶的方法，实现了3.710-7Pa的长期真空保证，成功解决了微波线真空泄漏问题。
（3）设计并实现了二维磁光阱装置，获得了大流量的慢速原子束，对提高铯原子喷泉钟的稳定度和准确度性能具有重要的意义。二维磁光阱装置由两个独立的阱区构成，并且铯饱和蒸汽压可以调节，以获得最优的性能指标，其实现的慢速原子束流量为2.1×109atoms/s，满足了铯原子喷泉钟的设计要求。
本文的创新点为：
（1）提出并实现了在磁屏蔽筒内部放置补偿线圈弥补漏磁的方法，在48cm的较长范围内磁场波动小于1.7nT，具有国内领先水平；
（2）设计和调试了微波激励腔，微波泄漏小于-75dB，阻抗匹配；分析提出了进一步减小腔相位频移的四端馈入的新型微波腔设计思想。
（3）以上装置通过与控制系统联调，获得了铯原子喷泉钟的标志性信号—Ramsey谐振信号，实现了铯原子喷泉钟整体闭环运行。为铯原子喷泉钟性能的提高和实际应用奠定了基础。
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.
中国科学院国家授时中心（NTSC）是我国专门从事时间频率基础和技术研究的科研机构，担负着国家标准时间的建立、保持和发播任务，是国际原子时TAI建立的重要参加单位。国家授时中心拥有全国最大的守时钟组，但是没有可以对其进行实时校准的铯原子喷泉钟。研制和运行铯原子喷泉钟，可以提高我国标准时间频率产生的自主性，提高我国时间频率服务的质量。
围绕铯原子喷泉钟物理系统的研制，本文主要研究喷泉钟磁屏蔽、微波腔和二维磁光阱部分。磁屏蔽方面，通过在磁屏蔽内部加补偿线圈和运用最小二乘法，实现了从微波激励腔到原子自由飞行区大范围的均匀磁场；激励腔方面，通过分析设计、耦合馈入、铟丝压封等，微波输入功率降至法国喷泉原子钟的十分之一，减小了原子钟微波泄漏频移；选态腔方面，通过真空脂和密封胶注入，成功解决了微波传输线真空漏气问题；二维磁光阱研制方面，设计并实现了二维磁光阱装置，获得了大流量的慢速原子束。主要研究内容和成果归纳为以下三个部分：
（1）完成了喷泉钟磁屏蔽的设计和研制，达到了较高的性能指标。本文首先给出本喷泉钟在磁屏蔽方面的独特设计，即对喷泉钟整体进行了磁屏蔽，并在真空筒内对应小、中屏蔽筒的端盖位置处设计和放置四个小屏蔽盖，延长了轴向磁场的均匀性；其次，在磁屏蔽研制中，形成了一整套磁屏蔽设计、选材、测试和分析的理论和实验方法；针对磁屏蔽系统由于碰撞、变形等原因导致的磁性能下降，以及漏磁和剩磁等严重情况，通过在磁屏蔽内部轴向方向不同位置放置小的补偿线圈，运用最小二乘法和研制的高精度电流源，实现了对漏磁的较好补偿，得到在长达48cm范围内磁场波动小于1.7nT的均匀磁场。达到国内领先水平。
（2）设计并调试了微波激励腔和选态腔，较好满足了喷泉钟性能的要求。激励腔是喷泉钟的核心，激励腔质量关系着喷泉钟的整体性能指标。选用含铜量高达99.99%的零号无氧铜，达到了无磁和高导电率要求；合理设计结构并采用双端微波对称输入，减少了腔相位频移；采用铟丝压封及60mm截止波导方法，减少了微波泄漏；通过理论计算、软件模拟与实验相结合方法，优化各级耦合效率和阻抗匹配；通过在激励腔上方放置导热性能良好的无氧铜管，增加了原子飞行路径上温度的均匀性，有利于喷泉钟黑体辐射频移的测试评估。通过综合调试，在-75dB范围内未发现有微波泄漏，微波腔馈入微波功率只需要法国FO1喷泉钟的十分之一。为了进一步降低腔相位频移，提高喷泉钟性能，分析提出了四端馈入微波的新型微波腔设计思想。选态腔方面，考虑到无磁、低热膨胀系数、低Q值等要求，选用硬铝材料；采用单端侧面微波耦合，简化了装置；微波线与铜支座之间采用特殊焊接方式，铜支座与选态腔侧壁之间采用压铟丝方式，保证了部件连接处的真空和防止微波泄漏；采用微波线灌真空脂与密封胶的方法，实现了3.710-7Pa的长期真空保证，成功解决了微波线真空泄漏问题。
（3）设计并实现了二维磁光阱装置，获得了大流量的慢速原子束，对提高铯原子喷泉钟的稳定度和准确度性能具有重要的意义。二维磁光阱装置由两个独立的阱区构成，并且铯饱和蒸汽压可以调节，以获得最优的性能指标，其实现的慢速原子束流量为2.1×109atoms/s，满足了铯原子喷泉钟的设计要求。
本文的创新点为：
（1）提出并实现了在磁屏蔽筒内部放置补偿线圈弥补漏磁的方法，在48cm的较长范围内磁场波动小于1.7nT，具有国内领先水平；
（2）设计和调试了微波激励腔，微波泄漏小于-75dB，阻抗匹配；分析提出了进一步减小腔相位频移的四端馈入的新型微波腔设计思想。
（3）以上装置通过与控制系统联调，获得了铯原子喷泉钟的标志性信号—Ramsey谐振信号，实现了铯原子喷泉钟整体闭环运行。为铯原子喷泉钟性能的提高和实际应用奠定了基础。
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.
引文
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