Keeping Cool: SRON Develops Thermal Calibration System for Deep-Space Telescope

• 分析与建模 - 数字孪生/模拟
• 分析与建模 - 预测分析
• 功能应用 - 远程监控系统

• 航天
• 国家安全与国防

• 产品研发
• 质量保证

• 预测性维护
• 数字孪生
• 远程资产管理

• 软件设计与工程服务
• 系统集成

Observing and analyzing regions in outer space where new stars and planets are born requires extremely sensitive detectors. Radiation and overheating can cause these detectors to fail. Using multiphysics simulation, a team at SRON is developing a calibration source for an imaging spectrometer that can operate with such vulnerable equipment. Heat management takes on a unique role in outer space, especially for cryogenic systems that demand extremely low temperatures in order to detect thermal radiation. This was a challenge faced by the engineering team at SRON Netherlands Institute for Space Research when designing the SpicA Far-InfraRed Instrument (SAFARI), an infrared camera that measures the complete far-infrared spectrum for each image pixel. SAFARI will fly aboard the Japanese Space Infrared Telescope for Cosmology and Astrophysics (SPICA). SPICA will look deeper into space than any space telescope has before. Because SAFARI has ultrasensitive detectors, cooled to slightly above absolute zero, it can pick up weaker far-infrared radiation than previous space cameras. Precise on-ground and in-space calibration is crucial to the accuracy of the sensors and the success of the mission. To design and optimize these calibration systems, the team at SRON turned to a COMSOL Multiphysics® simulation as their guide.

SRON Netherlands Institute for Space Research is a leading institute in the field of space research, focusing on the development and application of innovative technologies for space missions. The institute is known for its expertise in designing and optimizing sensitive detectors and calibration systems for space telescopes. SRON's engineering team is dedicated to overcoming the challenges of heat management in cryogenic systems, which are essential for detecting thermal radiation in outer space. The institute collaborates with international partners, such as the Japanese Space Infrared Telescope for Cosmology and Astrophysics (SPICA), to push the boundaries of space exploration and uncover new mysteries of the universe. With a strong emphasis on research and development, SRON aims to contribute to the advancement of space science and technology.

The calibration source for SAFARI contains a blackbody cavity or radiation source that provides radiation with a spectrum depending only on the source temperature, making it a very reliable and accurate calibrator. However, SAFARI’s detectors are so sensitive that the power produced by the source is approximately a million times too high and must be optically diluted using apertures and an integrating sphere. After passing through the integrating sphere, radiation with the correct power and spectral distribution is then reimaged onto SAFARI’s detector arrays for calibration. Between the radiation source and integrating sphere are a mechanical shutter and iris mechanism. The shutter opens and closes the aperture to the radiation source, while the iris fine-tunes and modulates the output power. Thermal management is vital: the system is held in a “super-dark” environment at 4.5 kelvins (K) to decrease the background radiation from the equipment itself. Variation in the base temperature of the detectors, background radiation (affected by the orientation of the spacecraft), and power dissipated by the iris and shutter mechanisms can all disrupt calibration. The radiation source temperature can be set between 95 and 300 K to generate radiation — this creates a large temperature differential between the source and the 4.5 K environment, while available cooling power at these temperatures is limited to just tens of milliwatts. To account for this, the team needed to design a thermally insulating suspension system. The SRON team needed a stiff suspension with a high resonance frequency that would prevent heat transfer from the source to the rest of the device while also protecting it from unwanted vibrations.

• Using COMSOL simulations, de Jonge evaluated the heat load through the suspension and performed modal analyses on suspension concepts with different geometries and materials, seeking a tradeoff between mechanical stiffness and thermal load. COMSOL allowed the team to quickly study different geometries that would otherwise be difficult to analyze. Because of the large temperature gradient over the brackets and thermal properties that change very quickly as a function of temperature, temperature-dependent material properties had to be implemented. Ultimately, the team chose the solution that had the best combination of mechanical stiffness and thermal insulation. Based on the results, the team designed and optimized a configuration of thin (100 μm) stainless steel wires to hold the radiation source to a triangular frame. Because stainless steel has low thermal conductivity at cryogenic temperatures and the cross-section of the wires is very small, heat conduction through the wires was limited, which the simulation confirmed. For a source temperature of 150 K, the experimental analysis showed 10.17 mW of conducted heat. The simulation results were in close agreement, accurate to within 0.01 mW. The design also had a resonant frequency of 720 Hz, high enough to ensure proper functioning of the radiation source.
• Next, de Jonge optimized the coil-driven iris and shutter mechanisms. The iris is driven by a voice-coil actuator and contains four stainless steel blades that rotate around frictionless bearings. The shutter is a magnetic latching device. De Jonge used COMSOL to optimize the iris coil and housing geometry, aiming to minimize the current and dissipated heat during actuation. By performing a parametric sweep over the main design parameters on the air gap and number of coil windings, the team developed an optimal coil design that has a low driving current of 38 mA and a dissipation of just 1.6 mW.

• The experimental analysis showed 10.17 mW of conducted heat for a source temperature of 150 K.
• The simulation results were accurate to within 0.01 mW.
• The design had a resonant frequency of 720 Hz.