Continuous cooling at 300mk
Sensing at THz frequencies (wavelengths from 30µm to 3mm) is a rapidly developing field, with applications as diverse as astronomy, atmospheric science, fusion diagnostics, material characterisation, security scanning and even art conservation. The demanding combination of high sensitivity and fast speed of response for THz detectors inevitably requires a technology that is cooled to cryogenic temperatures.
Such temperatures are normally limited to research environments, though exceptions in the civil theatre such as liquid-helium-cooled hospital MRI machines show that the benefits are recognised to outweigh the inconvenience and cost even of handling and consuming liquid cryogens. However, in the last decade a new mechanical cooler, the Pulse Tube Cooler (PTC) has been developed which can reliably reach and hold temperatures of ~3 K. However, the THz detector technologies that offer best performance characteristics (sensitivity and speed of response) for most applications require temperature < 0.5 K. Such temperatures are currently achieved by operating small self-contained 3He adsorption fridges inside a wet (4 K) liquid helium cryostat.
This research, funded by an STFC IPS award, proposes to design a tandem 3He adsorption fridge to cool a third stage continuously to 350 mK from the 4 K stage of a PT cooler. The focus of our work is on the cooling of THz array detectors as it aligns intimately with the product portfolio and development programme of the Astronomy Instrumentation Group at Cardiff and QMC Instruments.
Schematic of the split condenser 3He refrigerator.
Sub-kelvin cooler concept
The schematic (right) shows the tandem fridge design. The cold stage is cooled by contact with the 3He gas circulated via one of two pre-cooler 3He fridges. The fridges can be controlled by the operation of strategically placed heaters to cycle first one and then the other such as to maintain constant reservoir of 3He in the third stage at low pressure and hence temperature.
The final stage is designed to offer a cooling power of approximately 300 µW at 400 mK continuously. This is sufficient to operate a superconducting detector array or to cool the condenser of a mini-dilution system which can achieve temperatures < 100mK.
Full automation has already been achieved for conventional (limited run-time) 3He fridges, and has also been demonstrated in the laboratory for mini-dilution systems. We therefore anticipate little difficulty in producing such an automated system suitable for commercial applications. A nice feature is that the MI LabView-based control system can be accessed remotely to view and control the state of the system.
This programme is supported by an STFC IPS award.