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The amount of power dissipated by the fully integrated microfabricated atomic clock is critical to some of the most important applications. For widespread use in portable RF devices such as global positioning system (GPS) receivers and wireless communications devices, the clocks must be able to operate on power supplied by batteries. A typical AA alkaline battery can store enough energy to run a device dissipating 50 mW for about one day. A goal of 30 mW has therefore been set for the maximum power that can be used by the macrofabricated atomic clock, including physics package, local oscillator and control electronics. The NIST design therefore aims to reduce the power dissipated by the physics package to below 10 mW. This is possibly the most challenging aspect of the program, since the atomic vapor cell must be run quite hot (above 80 °C) in order to generate enough atoms in the cell to optimize the performance of the standard. At present, the NIST cesium physics package runs at a cell temperature of 80 °C. In order to raise the cell temperature this far above the baseplate temperature of 46 °C, 69 mW of electrical power needs to be added to the cell. Thermal modelling has indicated that this power is dissipated in three channels of comparable importance. About 30 mW of heat power is dissipated through the solid structure that attaches the cell to the baseplate. Another 24 mW is dissipated through the gold wire bonds that connect the cell heaters to the electrical leads on the baseplate. The remaining 15 mW is presumably lost through convection, conduction and radiation to the environment.
Advanced structural designs are being developed to address these heat-loss channels. Vacuum packaging should reduce the radiated, convected and conducted power from the device surface considerably, and an advanced spacer, designed to thermally insulate the cell from the baseplate, should reduce the remaining dissipation channels to roughly 10 mW.
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