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Miniature coaxial pulse tube cryocoolers are widely used for cooling instruments in space science missions and military exploration fields due to their low vibration, long life, and high reliability and rapid cooling. In this study, a highly compact 0.91 kg miniature coaxial pulse tube cryocooler is developed. When the charging pressure is 4 MPa and 72 W of electrical power is inputted into the cryocooler, it can achieve a cooling power of 2.26 W at 80 K. The relative Carnot efficiency is 8.45% with a reject temperature of 295 K, and the operating frequency is 114 Hz. The cooling power per unit mass reaches 2.48 W/kg, which is the highest value in the 80 K temperature zone publicly reported in the miniature coaxial pulse tube cryocoolers, which means that while maintaining a compact structure, more cooling power can be provided under the same weight. In addition, the influence of input electrical power and charging pressure on the optimum frequency and the influence of frequency on the cooling rate are also experimentally studied. When the input power increases from 20 W to 60 W, the optimum frequency gradually increases. At low input power, there is an obvious optimum frequency to maximize the relative Carnot efficiency, but with the increase of input power, there is a frequency range to keep the performance basically stable, and only deviations from the range can cause the performance to decline significantly. When the charging pressure is reduced from 4 MPa to 3 MPa, the optimum frequency basically remains constant when the cold end temperature is about 51 K, but as the cold end temperature rises to 80 K, the charging pressure changes make the optimum frequency change. The research also found that in the early stage of the process where the cryocooler cools down from the ambient temperature to 80 K, not operating at the optimum frequency for the target temperature zone doesn't have a significant impact on the cooling rate. However, during the process when the cold end temperature drops from 120 K to 80 K, operating at the optimal frequency for the target temperature zone can obviously reduce the cooling time. That is, the influence of the frequency on the cooling time lies in the difference in the time required for the process in which the cold end temperature gradually approaches the target temperature zone in the later stage of cooling. This research provides valuable references for the further study and practical application of miniature pulse tube cryocoolers.
