Recent cryocooler research at NIST for increasing the accessibility of temperatures 4 K and below

Pulse tube refrigerators (PTRs) are the preferred tool for cooling to 4 K in laboratory environments, and also serve as precoolers for millikelvin refrigerators. Since becoming commercially available in the early 2000s, PTRs have made temperatures of a few kelvin and below far more accessible to the scientific community than when liquid helium was required. Unfortunately, despite a quarter century of active use, PTRs are still incompletely optimized and understood. In this talk, we summarize the last six years of research at the National Institute of Standards and Technology aimed at improving commercial 4 K PTRs. First, we introduce dynamic acoustic optimization, a novel method for increasing the cooldown speed of these refrigerators by factors up to 3.5. Next, we explain how the poor efficiency of these PTRs is in large part determined by their method of compression, and how this efficiency may be increased by a factor of 3. The real-fluid physics of helium near 4 K gives the regenerators of PTRs an amazing ability to absorb large amounts of heat without affecting the cooling available at the cold end; we explain this phenomenon using power flow analysis and show how to fully utilize it. Finally, we introduce our newest effort, aimed at understanding losses in the thermal buffer tubes of PTRs. Taken together, these topics suggest that low temperature science may soon be performed more quickly, with less energy input, and with smaller cryocoolers.