L. D'Addario
Last changed 1998-November-13

See also:

• MMA Memo No. 184, "Cryogenics options for the MMA, "   1997-09-20. General background information.
• MMA Memo No. 185, "Review of Gifford-McMahon refrigerators for 4K, "   1997-09-25.

SUMMARY

The cryogenics plan for the MMA includes spending the first half of the D&D phase (through about March 2000) on technology assessment and development, followed by detailed design and construction of a prototype refrigeration system. The latter will be delivered in time for integration with the prototype receiver. The cryogenic system includes not only a refrigerator but also the thermal design of the receiver package, and hence it interacts closely with the receiver design. After thorough testing and documentation, it is expected that volume production of refrigeration systems for the array will be handled by a build-to-print contract to industry.

The technology assessment and development is expected to yield improved performance and reliability compared with earlier designs. Very high reliability and low maintenance are important because of the large number of systems and the remote site of this array. The following investigations are being carried out: detailed performance evaluation of the NRAO J-T refrigerator and of commercial 4K Gifford-McMahon refrigerators; evaluation of new componets for J-T cryocoolers, including expanders, reactive gas cleaners, and oil-free pumps with non-contacting seals; evaluation of pulse tube refrigerators, possibly including purchase of a prototype; and experimental studies of infrared filtering materials and structures. In addition, the thermal design will be proved by construction of a thermal mockup of the MMA receiver. This will produce the same conduction, radiation, and dissipation loads as the final receiver (supporting all bands)and will include a cryocooler suitable for testing; it will contain no mm-wavelength electronics, but will be available long before the actual receiver.

Construction of the antenna evaluation receivers will include use of cryocooler hardware previously developed at the NRAO or available commercially, and is therefore not covered in this chapter. Nevertheless, some novel design and construction techniques worked out during cryogenics development may be incorporated into the evaluation receivers if time permits.

6.1 OVERVIEW

The MMA cryogenics design is presently only roughly determined because, first, the cooling requirements of the receivers are not yet accurately known; and, second, because we intend to use the MMA development phase to achieve substantial improvements relative to the cryocoolers that have heretofore been conventional in radio astronomy. The large number of antennas and the remote location of the MMA imply that two kinds of improvement are highly desirable: power consumption and reliability.

Existing NRAO systems capable of cooling 1W to 4K use about 9.7kW from the power line; this is an efficiency of about .01%, compared to the ideal theoretical (Carnot) efficiency of about 1.2% and practical efficiencies achieved in other (much larger) systems of up to 0.2% (20x better). An improvement by a factor of 2 to 4 (to .02% to .04%) should be feasible in our size range, as described below. In addition, careful design to minimize the cooling requirements of the receivers should keep the 4K load below 0.5W and the loads at other stages below 10W.

Most cryocoolers in radio astronomy now use Gifford-McMahon refrigerators. These are highly developed commercially and are fairly reliable, but they nevertheless have parts that wear out and require overhauls at intervals of 12 to 18 months. In addition, random failures occur from leaks in joints and flexible hoses, from helium contamination, and in compressors. As a result, use of the traditional technology can be expected to result in about 2 shutdowns per year per cryocooler for maintenance or repair. Even with one system per antenna, this is more than one per week. Technologies now exist (for compressors, expanders, and hoses) that should, in principle, result in a large improvement in reliability. But in most cases the performance we need has not yet been demonstrated, and some components are not yet commercially available. Detailed investigations are planned during the development phase before making a final choice.

6.2 REQUIREMENTS

The receiver plan (see chapter 5 of this book) calls for dual-polarization coverage of 30 GHz to 950 GHz in 10 bands, with the lowest three bands (30-116 GHz) using HFET amplifiers as the initial stages, and the remaining 7 bands using SIS mixers. It is desired to have the HFET amplifiers as cold as 15K; colder would not lead to significant improvement, and temperatures as warm as 50K might be acceptable. The SIS mixers (at least for Nb devices) must be kept below 4.5K in the worst case, and the nominal temperature should be 4.0K or less. Each receiver will have a cold feed horn at the same temperature as its first stage, and optical components outside the dewar are to be minimized. (As we shall see, eliminating all external optics after the subreflector implies large windows at the lower frequency bands, creating substantial heat load.)

(The receiver plan might change, but the cryogenics plan given here is based on the receiver configuration just described.)

With the goal of keeping the cryogenic system as simple and hence as reliable as possible, configurations with heat sinks at only two cryogenic temperatures are being considered (as opposed to the three stages that are traditionally used to reach 4K). The coldest stage would then have to be at 4K, and the other stage might be at about 40K. Some commercially available Gifford-McMahon refrigerators achieve these temperatures. Table 6.1 shows the likely distribution of cooled components for the 2-stage and 3-stage cases.

             Table 6.1:  Temperatures of Cooled Components

          Three-Stage Case			  Two-Stage Case

     Stage 1, 70K                            Stage 1, 40K
   Radiation shields			   Radiation shields
   IR filters at windows		   IR filters at windows
   Cable and WG heat sinks		   Cable and WG heat sinks
   LO multipliers or photodiodes	   HFET amplifiers, RF and IF,
					     stages 3--5 (34 ea.)
					   LO multipliers or photodiodes

     Stage 2, 20K                            Stage 2, 4K
   Cable and WG heat sinks		   All feedhorns (10 ea.)
   HFET receiver feed horns (3 ea.)	   SIS mixers (14 ea.)
   HFET amplifiers, RF, bands 1-3	   HFET amplifiers, RF and IF,
     5 stages (x6)			     stages 1 and 2 only (34 ea.)

     Stage 3, 4K
   SIS mixers (7 ea.)
   SIS receiver feed horns (7 ea.)
   HFET amplifiers, IF,
     stages 1 and 2 only (28 ea.)

For the three-stage case, an attempt has been made to estimate all contributions to the heat load for the 10-receiver system. The result is given in Appendix A. The calculation assumes that all receivers are in the same dewar and can use a common refrigerator, but splitting them into separate dewars would affect only the radiation shields, which contribute a relatively small amount. The radiation through each window is assumed to be mostly sunk to the first stage by a fairly thick (several mm) PTFE infrared filter. The filter performance assumed is based on the measurements reported in [1]. A separate window is assumed for each band, with the sizes taken from [2]. The thermal properties of coax cables and waveguides are taken from [3]. The photonic and multiplier LO options (see chapter 7) are treated separately, but each is assumed to include 100 mW of RF dissipation; the multiplier option has slightly more load on the first stage due to its input waveguides.

It is apparent from Appendix A that the dominant load on every cooling stage is from the 300K radiation incident on the windows. This is true for each receiver separately as well as for the grand total. (This depends on using a large number of layers of multi-layer insulation ["superinsulation"] to keep the radiation from opaque surfaces low.) Therefore, minimizing the window area is one of the best things we can do to minimize the load.

Without any special effort, the calculated loads are about 36W, 1.4W, and 0.7W on the three stages. Appendix A also shows some ways to reduce this. A great deal of the first stage load (22W) and second stage load (1.2W) can be eliminated if the large windows of the three HFET receivers could be reduced to 2.2in diameter (from 9.0, 4.5, and 3.2in, respectively) by using external optics to reduce the beam waist diameter. Slightly more could be saved by eliminating the 33-50 GHz band entirely. The calculation assumes that all SIS receiver windows are 2.2 in diameter, which is what's needed for the lowest frequency band; actually, smaller windows can be used at the higher frequencies. After taking these steps, the largest remaining loads are from HFET amplifier dissipation and from conduction through coax cables. If only one SIS receiver is in use at one time, and if the bias is turned off for the other IF stages, significant saving results. (Note that the HFET receivers remain on, and could be used simultaneously.) If the IF coax from 4K to 20K is lengthened to 20cm (at some sacrifice in loss), further savings occur at 4K. With all of these measures, the loads become 6.5W, 0.27W, and 0.37W, respectively.

It should be emphasized that these are theoretical results, and are very preliminary. Whether they represent reality remains to be seen.

6.3 OFF-THE-SHELF TECHNOLOGIES

6.3.1 Two-stage Gifford-McMahon. (See MMA Memo #185.)

       Table 6.2:  Commercial Two-Stage GM Refrigerators for 4K

 Mfgr       Model   Load@4.2K  MinTemp  System Price  Input Power
Daikin[1]   CSW210    0.8 W     3.0 K      $35k	        6.7 kW
Sumitomo[2] SRDK415   1.5	3.2	    41k		7.5
Sumitomo[2] SRDK408   1.0       3.1         39k	        7.5
Sumitomo[2] SRDK205   0.5       3.1         24k		3.4
Leybold	    4.2GM     0.5	3.4	    37.5k	6.5
Boreas[3]   B100      0.9	3.1         38.5k	3.0   

 Model   1stStg Q@T  P1,atm  P2,atm  He flow    Eff/Carnot[4]
CSW210    35W @41K    21.8    7.1    85 Nm^3/h  .041  .0084
SRDK415   30W @40K    ?       ?      ?		.040  .0141
SRDK408   37W @40K    23.4    6.8    80		.041  .0094
SRDK205    4W @40K    ?       ?      30		.018  .0104
4.2GM     50W @50K    22.1    7.5    88		.044  .0054
B100	  2.5W@80K    21      1.0    20		.023  .0211

Notes:
 [1] Represented in U.S. by APD Cryogenics, Inc.
 [2] Represented in U.S. by Janis.  SRDK405 is new, promised in 4Q97.
 [3] Not GM, but a proprietary hybrid cycle, partially recuperative.
 [4] Efficiencies relative to Carnot, first with specified loads on 
both stages, then with zero 1st stage load but same power consumption.

6.3.2 Hybrid three-stage: 2GM + JT. This is the configuration that has been traditionally used in radio astronomy when refrigeration near 4K is required. Specific designs were developed for cooling the superconducting magnets of microwave maser amplifiers in the 1960s, both for radio astronomy and for spacecraft tracking, and essentially the same designs are in use today.

6.4 ADVANCED TECHNOLOGIES

6.4.1 Pulse Tube Refrigerators. [to be written; some notes:]

• Generally not commercially available (but 1-stage 10W@77K now avail from Iwatani)
• Orientation dependence may require tilted installation
• Cyclic, producing temperature fluctuations
• See refs [5--7] and other works cited therein.