Conceptual Design and Development History of the MIRI Cryocooler System on JWST
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The Mid Infrared Instrument (MIRI) of the James Webb Space Telescope (JWST) is a demanding application for the use of space cryocoolers. Used to cool JWST’s ~100 kg MIRI instrument down to 6 K, the MIRI Cryocooler is critical to enabling the mission’s long-wave infrared science associated with studying the post-Big Bang early formation of the Universe. The selection and design of MIRI and its cryocooler system started with the successes and limitations of JWST’s precursor, the Hubble Space telescope. Planned as the follow-on to Hubble, JWST abandoned Hubble’s astronaut-accessible low-Earth orbit for a colder and better-science environment a million miles from Earth. Targeting for this L2 location meant no servicing missions would be possible, and reliability and life would be critically important. JWST’s increased size, with a 6.5 meter diameter mirror, meant significant deployments would be required post launch to allow the observatory to unfurl from its launch-vehicle shroud and separate the cold telescope from its hot and noisy spacecraft bus. The MIRI cryocooler system met these mission constraints by positioning its dual compressors inside the spacecraft bus, while the MIRI instrument and the cryocooler’s cold-end is positioned 10 meters away in the Science Instrument Module. Accommodating the 1.3-m deployment of the telescope away from the spacecraft and minimizing any vibration transmitted up the connecting refrigerant lines was a driving requirement on the cryocooler system. Providing MIRI’s large refrigeration load at 6 K, while simultaneously cooling MIRI’s 18 K radiation shield required an allnew hybrid Pulse Tube/Joule-Thomson cryocooler. This paper provides an overview of the overall MIRI cryocooler system as it evolved to meet these demanding JWST requirements over its nearly 20-year development history. During this period it drew heavily upon ongoing Pulse Tube cooler developments as well as a history of earlier hybrid J–T cryocooler technologies.
詹姆斯·韦布空间望远镜（James Webb Space Telescope, JWST）的中红外仪器（Mid Infrared Instrument, MIRI）是空间低温制冷机的严苛应用场景。该仪器的制冷机用于将韦布望远镜重达约100千克的MIRI冷却至6开尔文，对于开展与大爆炸后宇宙早期形成相关的长波红外科学研究至关重要。MIRI及其制冷系统的选型与设计，始于韦布望远镜的前身——哈勃空间望远镜（Hubble Space Telescope）的成功经验与局限之处。作为哈勃望远镜的继任项目，韦布望远镜摒弃了哈勃可由宇航员抵达的近地轨道，转而选择距地球100万英里的更低温、更适配科学研究的轨道环境。将观测目标锁定在这一L2拉格朗日点，意味着无法执行在轨维修任务，因此系统的可靠性与使用寿命变得至关重要。韦布望远镜的尺寸大幅增加，主镜直径达6.5米，这要求其在发射后完成复杂的展开操作：让望远镜从运载火箭整流罩中展开，并将低温望远镜与高温且存在噪声干扰的航天器总线平台分离。MIRI制冷系统通过将双压缩机置于航天器总线平台内，满足了上述任务约束要求；而MIRI仪器与制冷机冷端则被安置在10米外的科学仪器舱（Science Instrument Module）中。适配望远镜距航天器1.3米的展开距离，并最大限度降低沿连接制冷剂管路传递的振动，是该制冷系统设计的核心约束要求。既要为6开尔文温区的MIRI提供大制冷量，同时还要将MIRI的18开尔文辐射屏蔽罩冷却至目标温度，这要求采用全新的脉冲管（Pulse Tube）/焦耳-汤姆逊（Joule-Thomson）混合式低温制冷机。本文概述了MIRI制冷系统的整体架构，及其在近20年的研发历程中为满足韦布望远镜的严苛要求而不断演进的过程。在此期间，该系统的研发大量借鉴了当时正在推进的脉冲管制冷机技术，以及早期混合式焦耳-汤姆逊（J–T）制冷机的技术积累。
