Objective Adjusting the temperature of 2nd temperature zone of cryogenic optical system by applying the electrical heating control method will increase heat load on cryocooler, increase needs for electric power and heat dissipation resources of cryocooler, even decrease dependability of cryocooler in space application. When the cryogenic optical system in space payload is applicated on-orbit, a method without extra heat load on cryogenic optical system, which controlling the temperature of 2nd temperature zone accurately, is urgently needed under the condition of limited electric power and heat dissipation resource on the satellites.

Methods In view of the above problems, thermal resistance control method was proposed to adjust the temperature of 2nd temperature zone of cryogenic optical system in this study. A mechanical structure used for thermal resistance control, consisted of screw, screw sleeve and heat insulated pad, was designed (Fig.2), which based on the model of dual-temperature zone of cryogenic optical system (Fig.1). A one dimensional (1D) mathematical model was developed based on this mechanical structure. The mathematical model was used to investigate heat balance temperature level and heat leakage of 2nd temperature zone. The effects of material parameters such as TC4 and 4J36 and geometric parameters such as length of screw, height of heat insulated pad, height of screw sleeve and thickness of heat insulated pad were investigated. Meanwhile, different temperature conditions of vacuum chamber were considered.

Results and Discussions The experiment results show that the temperature fluctuation of 2nd temperature zone is smaller than ±0.5 ℃ within 4 hours in a row, which can be determined that the condition of thermal equilibrium is acquired. Different environmental temperature and different thermal resistance lead to the difference of heat balance temperature of 2nd temperature zone, which verifies the feasibility of thermal resistance method (Fig.5). The computed results of 1D mathematical model are in good agreement with the experimental data. The maximum deviation between the computed results and the experiment results is only 1.6% (Fig.6-7), which proved the reliability of the calculation method in this paper. Users can obtain more parameters of thermal resistance based on different temperatures of 2nd temperature zone by using above mathematical model. When the thermal resistance is taken constant, the temperature of 2nd temperature zone increases with increasing the temperature of vacuum chamber, which is result of increasing heat leakage caused by increasing environmental temperature. When the temperature of vacuum chamber is taken constant, the temperature of 2nd temperature zone increases with increasing length of screw, decreasing thickness of heat insulated pad and decreasing thermal conductivity, but heat leakage decreases. When the temperature of vacuum chamber is 253.15 K, the adjustment range of the temperature of 2nd temperature zone is 111.1-185.2 K, and the corresponding heat leakage decreases from 1.75 W to 1.1 W. When the temperature of vacuum chamber is 293.15 K, the adjustment range of the temperature of 2nd temperature zone is 120.9-219.4 K, and the corresponding heat leakage decreases from 2.58 W to 1.57 W (Fig.8-Fig.11). According to the above results, there is no extra heat load on cryogenic optical system with thermal resistance control method. The effect of heat load on 2nd temperature zone with the electrical heating control method and thermal resistance control method is calculated and compared, while the temperature of vacuum chamber is 293.15 K. With the raising of target temperature of 2nd temperature zone, heat load on 2nd temperature zone decreases with thermal resistance control method, but increases with electrical heating control method. When the temperature of vacuum chamber is around 220 K, the heat load on 2nd temperature zone is 16.8 W with electrical heating control method, which is nearly 11 times higher than that with thermal resistance control method (Fig.12).

Conclusions Users can select the parameters of thermal resistance they need according to the appointed temperature of vacuum chamber and temperature of 2nd temperature zone. The cryocooler’s needs for electric power resources and heat dissipation resources can be reduced by using thermal resistance control method compared with electrical heating control method. This study may provide a reference for the thermal design of similar cryogenic optical system in space application.