A custom UHV compatible JDR-500 dilution refrigerator (DR) was tested at Janis Research Company in preparation for installation at NIST’s new Center for Nanoscale Science and Technology (CNST). The system includes a 15 Tesla superconducting magnet with a compensation coil and has central access into the UHV space. Using a top manipulator, the large bore central access allows an STM with in-situ prepared sample and probe tip to be translated into the magnet center without warming up the system above 4 K. An integral pre-cooling circuit is used to cool the system to 4 K without the need for any helium exchange gas.
The DR itself has been modified to meet user specifications (see photos below). First of all there is a 1.5” central access hole passes through every plate and chamber, including the Mixing Chamber (MC). There are two additional 0.95” side access holes above the MC. The DR also includes additional smaller ports for use in support of auxiliary needs such as shutter manipulation.
The gas handling system includes a bellows pump/compressor in addition to Alcatel Roots and mechanical vane pumps. The mixture Condensing Unit (MCU) contains a Joule-Thomson (JT) stage as well as a 1 K pot with computer-controlled and manual needle valves. The two condensation stages can be used interchangeably, with the JT stage utilized for low vibrational measurements. Initial mixture condensing is done in 1 K pot mode, while normal circulation is primarily in JT mode. The Janis manual Gas Handling System includes digital gauges for future computer control of the DR operation. UHV compatible RuO2 thermometers were provided by the NIST group (six in total installed), while Janis supplied UHV versions of a CMN thermometer and a Fixed Point Device. After calibrating the CMN thermometer against superconducting fixed points, a laboratory scale was created for this thermometer and was then used to calibrate RuO2 thermometers that were installed at the MC.
The UHV compatible DR insert employs all metal sealed Conflat and VCR flanges (no indium sealed flanges were used). Non-magnetic austenitic stainless steel and copper alloys are used below the IVC flange. All surfaces are treated for low out-gassing, including electro-polished stainless steel and 24 carat gold plated OFHC copper. Structurally the DR stage was reinforced to have high intrinsic resonance frequencies and high axial and torsional strength. Special UHV compatible shielded wires were developed for this application and are now becoming standard in Janis UHV compatible systems. The Insert can be baked at 100 °C in a special baking chamber while the DR unit is thermally controlled using a dedicated pre-cooling line. The same line is used for initial pre-cooling of the DR unit, eliminating the need for exchange gas. A super-efficient vapor shielded liquid helium dewar with a total evaporation rate (including magnet support and DR) of less than 0.7 L/h guarantees a long time (9 – 10 days) between liquid helium transfers.
The first test of the system confirmed that it achieves a 10 mK base temperature with approximately 100 shielded wires attached to the MC, and provides a cooling power of greater than 300 µW at 100 mK in both the 1 K Pot and JT operating modes. No heating was observed at the CMN thermometer at 10 mK while sweeping magnetic fields up to 10 Tesla at moderate rates.
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