Cryostat: The Pole STAR cryostat (Figure 7) was constructed by Precision Cryogenics and is based on the successful Brian Ellison hybrid design. It uses a CTI model 350 coldhead to cool the outer and inner radiation shields to ~77 and 12 K, respectively and a 4 liter liquid helium can to maintain the mixers at their operating temperature. This same basic design is used for all AST/RO and CSO receivers. In the single pixel AST/RO receivers, 1.2 in diameter, Teflon coated, crystalline quartz windows are used at 300 and 77 K. Zytex is used as the IR filter on the 12 K radiation shield. With a good vacuum, hold times of ~6 days can be achieved. With Pole STAR, we have 4 such apertures to contend with in the same cryostat. Furthermore, Teflon coated, crystalline quartz windows are no longer available and the emerging beams from our close-packed lens array makes it impossible to fabricate 4 independent vacuum windows. At 77 K, we have simply substituted an additional layer of Zytex for the 50 mil quartz windows. For the 300K vacuum window, we are using a 0.300 in thick disk of HDPE in which 4 cylinders with the diameter of the emerging beams have been milled out to a depth of 0.262 in (see Figure 8). The membrane thickness at the bottom of the cylinders corresponds to 4 lambda at 370 microns. The machining of these windows leaves behind a thick cross member structure which helps keep the window from collapsing under atmospheric pressure. The signal loss through the window is estimated to be <5 %. With all 4 mixers and amplifiers mounted in the cryostat, the measured hold time is ~3 days. The holdtime is much better than you would expect based upon the optimum performance of the single pixel receivers. This was achieved by doing away with the `vent' tube normally used in these cryostats. In the lab we have found that without the vent tube, the fill line is LESS likely to ice up, most likely due to the increased back pressure of helium gas.

Figure 7: Pole Star cryostat with 4-pixel window Click on thumbnail for full-size image! Figure 8: Precision-milled 4-pixel HDPE window Click on thumbnail for full-size image!

Mixers: Each SIS mixer uses a Potter horn with a circular to 1/2 height waveguide transition to couple radiation to a Nb SIS junction. The junctions achieve low noise (~450K) performance through the use of an on-substrate Al tuning structure and a fixed waveguide backshort. The mixers have embedded magnets to suppress the Josephson effect. A four-wire bias system is used to ensure stable operation of the mixers. (The bias system is discussed in detail elsewhere.)

Figure 9: 810 GHz SIS mixer rocket Click on thumbnail for full-size image!