Viewing page 36 of 156

This transcription has been completed. Contact us with corrections.

– 24 –

V. Receivers

The Committee was impressed that there has been progress in the development of receivers, even though the staffing for the effort is still well below what is needed for the project. The receiver development is the most difficult part of the project. There are many unknowns at the present time: what kind of refrigerator to use, what refrigerator temperature is necessary, what kind of SIS chip to use at the lower frequencies, how to build receivers at frequencies above 500 GHz. For these reasons, it is essential to build up the staff to the necessary level more rapidly than has so far been possible. Staffing needs include a machinist capable of working at the necessary machining accuracy. There is also a need for a cryogenics engineer, a microelectronics technician to fabricate junctions, a further experienced electronics technician, and a further receiver engineer.

The Committee is pleased to see that a test 230 GHz receiver is being constructed which will be used as the SEST telescope. If it is possible, it would be most useful if that receiver were installed in a cryostat such as may be used in the array. It could be a prototype. The refrigerator which was proposed, a four state system with two JT loops and 2K capability, may be more ambitious than is necessary. Since 4K may be adequate for SIS junctions using Nb and NbN, a 4K system may suffice. Note that the Nobeyama group has had excellent success with a high power 4K system available from Sumitomo at a very reasonable price. Such a system could be purchased, including the dewar, for early testing. It might turn out to be the final choice. An alternative refrigerator that should be investigated is the Gifford-McMahon system being developed by Mitsubishi. They have achieved moderate power and temperatures well below 4K.

In response to queries from the group, the Committee recommends the following start-up system: one cryostat per antenna with provision for 4 – 8 modular receivers but initially with just two receivers. Then provision should be made for (a) doubling the receivers for dual polarization, and (b) a technique for measuring Stoke's parameters one at a time. One receiver should be at 230 GHz for testing as well as scientific observations. Since this is to be a sub-millimeter array, the second receiver should be at as high a frequency as can be put together in time for the initial completion of the array, perhaps 490 GHz.

The Committee feels that the above system is about the most that should be attempted for the initial array. Development of the remaining complement