Development of a closed-cycle, Low Vibration Remote Cooler for Cryogen-Free Dilution Refrigerators

Many applications require cooling in the millikelvin temperature range with extremely low mechanical vibration. Commercially available cryogen-free dilution refrigerators (DR) offer an ideal cooling platform for these low temperatures because they offer large sample spaces, which makes integrating wiring and instrumentation easy, and low operation costs due to cooldown automation and the elimination of liquid Helium consumption, an expensive, non-renewable resource. Enabling this technology is the pulse tube cryocooler, which is used to cool the first two stages of the DR and pre-cool the He3/He4 mixture that is used as the primary refrigerant. The downfall for some applications is that the pulse tube cryocooler generates unwanted vibrations that propagate to the sample platform and adversely effect the measurement. Applications such as scanning probe microscopy, low temperature calorimetry for dark matter searches, and quantum information, science and technology can be inhibited by vibration from the pulse tube cryocooler. We propose to mechanically separate the pulse tube crycooler from the DR for vibration mitigation and provide cooling through two forced flow helium circulation loops. The system will consist of a “cold finger” that will be a direct replacement for the cold head in a typical DR. This will allow both for the retrofit of existing cryogen free DR, as well as very low development costs for existing DR manufacturers to offer low vibration DRs based on this system. The pulse tube cryocooler will be housed in a separate cryostat. The system will use two pumps to circulate helium, which will be cooled by the first and second stages of the pulse tube. The cold helium will then be circulated to heat exchangers on the cold finger to cool the first two stages of the DR to 50K and 3K, nominally. The only connection between the pulse tube cryocooler and the DR will be through two long, flexible transfer lines thus isolating the DR from the vibration of the cold head. During the phase I effort, we propose to design and manufacture a prototype low-vibration cooling system. A substantial part of the work will involve the experiment design and optimization of critical components in the system such as the heat exchangers and transfer lines. We will also test the cooling performance of the prototype unit as a whole.