The James Webb Space Telescope (JWST) program have identified the need for cryogenic cooling transport devices that (i) provide robust/reliable thermal management for Infrared (IR) sensors/detectors in the temperature range of 20-30K, (ii) minimize vibration effects of mechanical cryocoolers on the instruments, (iii) reduce spatial temperature gradients in cryogenic components, and (iv) afford long continuous service life of the telescope. Passive two-phase capillary cooling technologies such as heat pipes, Loop Heat Pipes (LHPs), and Capillary pumped Loops (CPLs) have proven themselves capable of performing necessary thermal control functions for room temperature applications. They have no mechanical moving part to wear out or to introduce unwanted vibration to the instruments and, hence, are reliable and maintenancefree. However, utilizing these capillary devices for cryogenic cooling still remains a challenge because of difficulties involving the system start-up and operation in a warm environment. An advanced concept of LHP using Hydrogen as the working fluid was recently developed to demonstrate the cryocooling transport capabilities in the temperature range
of 20-30K. A full-size demonstration test loop − appropriately called H2-ALHP_2 − was constructed and performance tested extensively in a thermal vacuum chamber. It was designed specifically to manage "heat parasitics" from a warm surrounding, enabling it to start up from an initially supercritical state and operate without requiring a rigid heat shield. Like room temperature LHPs, the H2-ALHP transport lines were made of small-diameter stainless steel tubing that are flexible enough to isolate the cryocooler-induced vibration from the IR instruments. In addition, focus of the H2-ALHP research and development effort was also placed on the system weight saving for space-based applications.
Requirements for cryocooling of large-area heat sources begin to appear in studies of future space missions. Examples are the cooling of (i) the entire structure/mirror of large Far Infrared space telescopes to 4-40K and (ii) cryogenic thermal bus to maintain High Temperature Superconductor electronics to below 75K. The cryocooling system must provide robust/reliable operation and not cause significant vibration to the optical components. But perhaps the most challenging aspect of the system design is the removal of waste heat over a very large area. A cryogenic Loop Heat Pipe (C-LHP)/
cryocooler cooling system was developed with the ultimate goal of meeting the aforementioned requirements. In the proposed cooling concept, the C-LHP collected waste heat from a large-area heat source and then transported it to the cryocooler coldfinger for rejection. A proof-of-concept C-LHP test loop was constructed and performance tested in a vacuum chamber to demonstrate the feasibility of the proposed C-LHP to distribute the cryocooler cooling power over a
large area. The test loop was designed to operate with any cryogenic working fluid such as Oxygen/Nitrogen (60-120K), Neon (28-40K), Hydrogen (18-30K), and Helium (2.5-4.5K). Preliminary test results indicated that the test loop had a cooling capacity of 4.2W in the 30-40K temperature range with Neon as the working fluid.
To provide cryocooling across a gimbaled joint still remains a difficult challenge for spacecraft engineers. Gimbaled cryogenic infrared payloads have very difficult-to-meet requirements for 2-axis motion and low torque. Thus the difficulty of making a cryogenic thermal connection across a gimbaled joint cannot be overstated. Because of the cryogenic nature of the connection, thermal joint flexibility, durability, reliability, material compatibility, differential expansion/contraction, and parasitic heat loss, are all complex technical concerns. Thus two primary issues of the proposed across-gimbal passive cryocooling system - management of heat parasitics and flexible/durable thermal connection between heat sources (infrared sensors/detectors) and heat sinks (cryocooler coldfingers) - are the main focus of the current development effort. A cryogenic Advanced Loop Heat Pipe proof-of-concept test loop that was developed in Phase I demonstrated a heat transport capability of 50W-m (20W over a distance of 2.5m) in 3/32"O.D. stainless steel
lines. But more importantly, the test loop started up and operated reliably even in a 300K environment. In the follow-on Phase II, the research focus shifted to the development and demonstration of a low-torque durable flexure mechanism for a 2-axis gimbal.
Next generation space infrared sensing instruments and spacecraft will require drastic improvements in cryocooling technology in terms of performance and ease of integration. Projected requirements for cryogenic thermal control systems are: high duty cycle heat loads, low parasitic heat penalty, long transport distances, highly flexible transport lines, and lower cooling temperatures. In the current state of cryocooling transport technology, cryogenic Loop Heat Pipes (CLHPs) are at the forefront of intensive research and development. CLHPs are capable of dispersing heat quickly from an IR heat source and transporting it to remotely located cryocoolers via small and flexible transport lines. Circulation of working fluid in a CLHP is accomplished entirely by capillary action developed in fine pore wicks of the system capillary pumps. Thus they contain no mechanical moving parts to wear out or to introduce unwanted vibrations to the spacecraft. A recently developed CLHP using Hydrogen as the working fluid performed extremely well in the temperature range of 20-30K under the most severe operating conditions. However, it was not optimized for spacecraft applications due to cost and schedule constraints of the initial research phase. Design optimization of the Hydrogen Advanced Loop Heat Pipe is the main objective of the follow-on research. Chief among the system improvements is the weight and volume reduction of the loop components.