A Hot New Topic
Stuart A. Wolf
Naval Research Laboratory
In the absence of the President, the President-Elect Mr. Lettieri called the 2059th meeting to order at 8:19 p.m. on April 19, 1996. The Recording Secretary read the minutes of the 2058th meeting and they were approved.
Mr. Lettieri introduced Stuart A. Wolf, Head, Material Physics Branch, Naval Research Laboratory to discuss “Cryo-Electronics — A Hot New Topic”.
Developments in super- and semi-conducting cryoelectronics in the last ten years (that is, since Ed Edelsach retired from the NRL in 1986) have already found application in wireless communication, networking and computing. Cryogenic superconductivity has achieved wide-spread, consumer-oriented utility in very high fidelity superconducting radio frequency filters and low noise amplifiers used in cellular phone base stations to increase their range and number of simultaneous users. The development of practical high temperature superconductivity offers even greater prospects. It may yet be that, as V. I. Ginsberg wrote in 1971, “Next to controlled thermonuclear fusion, development of high temperature superconductivity will have the greatest impact on technology.”
Breakthroughs in high temperature superconductivity depend upon progressing from currently available materials that are hard to cool but relatively easy to make, to materials that are easier to cool and not too hard to make. In order to be practical a material must exhibit superconductivity at least the temperature of liquid nitrogen, not liquid helium. Superconductivity is characterized by a zero resistance to direct electrical current. One research group has observed electric current flowing in a superconducting ring for seven years. In addition to perfect conduction, superconducting materials also exhibit perfect diamagnetism, called the Meisner effect, in which the material expels magnetic fields while it is superconducting. Superconductors do not have zero resistance for alternating current, but exhibit a resistance that is proportional to the frequency over several orders of magnitude.
Some space experiments in high temperature superconductivity (HTS) have been proposed. HTSSE-I was supposed to demonstrate the survivability and viability of HTS devices and cryocooling in space. Unfortunately, that experiment failed to orbit. HTSSE-II, presently scheduled for May 1997, is designed to test more advanced cryogenic systems. Other cryogenic electronics have been tested by Rockwell in Air Force experimental communication satellites, and in space ranging devices.
Other potential uses of HTS are microwave frequency channelizer filters, cueing receivers that tell what channels are being broadcast, low noise microwave receiver that are 2–4 times better than conventional receivers, digital multiplexer using Josephson junctions, mobile phase interferometers, NMR spectrometers that can detect signals four times weaker or 16 times shorter than current NMR receivers, cruise missiles, ship radar, and Doppler radar receivers 100 times as sensitive as current models.
Computing should benefit from major advances in transistor technology which has the potential for achieving ten times more speed at an order of magnitude lower power using cryogenic systems. Josephson junction technology has the greatest potential in analog-to-digital high-thruput switches. These super- and semi-conducting Josephson junctions typically have switching speeds speed on the order of 10-13 sec. Complementary metal oxide superconductors (CMOS) also have great potential. Cooling regular PC chips to 220K (-55°C) increases their speed by 30–50%. Using CMOS circuits should provide even greater chip speeds, but such chips will require very reliable, low cost coolers.
Cryogenic refrigeration in the coming years will thus be required for PC's as well as cars and space vehicles. Peltier coolers will probably prove to be the most efficient means for achieving the cryogenic cooling that will be necessary for these many applications.
Mr. Wolf kindly answered questions from the audience. Mr. Lettieri thanked the speaker on behalf of the Society, announced the speaker for the next meeting, restated the parking policy, and adjourned the 2059th meeting at 9:27 p.m.
John S. Garavelli