Superconducting Receiver Technologies Supporting ALMA and Future Prospects

Abstract

The Atacama Large Millimeter/submillimeter Array (ALMA) is the most sensitive ground-based radio astronomical telescope (see Figure 1) (ALMA observatory; URL: http://www.almaobservatory.org). Located in the Atacama Desert, 5,000 m above sea level in northern Chile, South America, it is a collaboration between East Asia, Europe, and North America. ALMA consists of fifty-four 12-m-diameter antennas and twelve 7-m-diameter antennas. The observation frequencies range from 35 GHz (millimeter waves) to 950 GHz (submillimeter waves). With such radio frequency (RF) observations, it is possible to capture the amplitudes and phases of signals from astronomical objects and by performing a proper interference of those signals, a single large synthesized radio telescope can be formed. This method is called aperture synthesis and was developed by radio astronomer Martin Ryle at Cambridge University (Ryle, 1975), for which he was awarded the Nobel Prize in Physics in 1974. The spatial angular resolution is determined by the antenna array configuration and observation frequency. The distance between ALMA antennas can be extended up to 16 km, allowing a maximum angular resolution of less than 0.01″, which is why the term "large" is used in the name. The achievable angular resolution of ALMA is 10 times higher than those of the Subaru Telescope and Hubble Space Telescope and up to 100 times higher than those of the existing millimeter/submillimeter telescopes.ALMA also offers unpreceded sensitivity. Owing to its large collecting area of over 6,600 m 2 and highly sensitive receivers using superconductor-insulator-superconductor (SIS) mixers along with high-electron-mobility transistor (HEMT) amplifiers, ALMA has a sensitivity 2 orders of magnitude better than those of the existing radio telescopes. Most of the SIS mixers in ALMA utilize superconducting niobium (Nb)