Special JDR-500 with Light-Tight Design for Cryogenic Sensors R&D
Special dilution refrigerator system, equipped with customized infrared radiation shields for maximum light-tightness and magnetic vacuum.
A special JDR-500 system has been designed for National Institute of Standards and Technology (NIST), Boulder, Colorado, to be used in cryogenic sensors research and development effort. This study required a special DR stage, equipped with customized infrared radiation shields for maximum light-tightness and magnetic vacuum. The former was achieved by blocking all the light from entering the sample space (note tongue-groove flanges and “pig-tailed” evacuation ports on photos below). Most of the holes were made blank, and clear holes were covered with sealed covers. The Earth's magnetic field could be suppressed by a factor of 100 or more by means of two µ-metal shields, attached to IVC and Still shield, the latter being demagnetized in-situ at low temperature (not shown). Special “non-magnetic” materials, screws, helicoils, and washers were widely used. As with all JanisULT DR’s, this system has only arc-welded or silver-brazed joints and was thoroughly tested by first sharp thermal-cycling to 77 K and then “cold” leak-checking at 77 K.
The system was equipped with an Alcatel Roots pumping station and a very simple-to-use JanisULT manual gas handling system. Two liquid nitrogen charcoal traps and one 4 K trap were part of the standard package. An electronic MKS flow-meter, two digital pressure sensors and LSCI 370S AC resistance bridge insured computer-controlled operation in a Labview environment, using JanisULT software. JanisULT miniature external CMN thermometers and superconducting fixed point devices were used for mixing chamber (MC) thermometry, while Scientific Instruments ruthenium oxide resistors were installed for auxiliary thermometry (1 K Pot, Still and Intermediate Cold Plate [ICP]). JanisULT resistive thermometers, based on Matsushita 47 Ohm resistors, were installed on the MC for calibration and later use as sample temperature sensors.
Extra space has been provided for cooled electronics on the IVC and 1 K Pot flanges, and three large clear-shot ports added for coaxial cable installation all the way down to the MC. Overall structural design has been re-enforced for heavy (100 kg) and big DUT (device under test) installation, including auxiliary equipment for sample manipulation and cold optics.
The very first cool-down surpassed quoted specifications by far – the base temperature, as measured on the gold-plated copper plate attached to the MC bottom, was 7.5 mK at optimal circulation rate, while maximum cooling power achieved at 100 mK was 443 µW. The ICP and its large copper shield surrounding the DUT were below 40 mK at base temperature. Liquid helium consumption with operational DR inside of vapor shielded dewar was about 0.8 liters/hour at optimal circulation rates, and helium transfer was done every 3 – 4 days.
From the data analysis we conclude that even higher cooling power could be reached with higher circulation rate using the standard JDR-500 stage. Modular design of our heat-exchangers and mixing chamber allow rather smooth evolution of the JanisULT DR product line to JDR-1000 platform, with larger pumps and Programmable Logic Controller (PLC) based automated gas handling system.