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7 Technology Requirements PROVED DISTURBANCE COMPENSATION SYSTEMS (DISCOS) Major unprovements are needed in the DISCOS in order to achieve enhanced performance in the laser gravitational radiation observatory. These would include both reduction in the distur- bance level below the 10~~/T2g/~-spectral amplitude expected for the initial laser gravitational radiation antenna in space and extension of this performance to periods longer than 104 s. MODERATE-POWER FREQUENCY-STABIIIZED LASERS Frequency-stabilized single radial and longitudinal mode la- sers will be useful in many areas of fundamental physics and in as- tronomy carried out in space. The gravitational wave observations proposed by this study as well as the optical interferometry being considered by the Task Group on Astronomy and Astrophysics will require moderate-power 100- to 1000-mW lasers that will function trouble-free for periods of years. For space applications, it is im- portant that the lasers be efficient; the ratio of the optical output power to the pump power should be larger than in conventional laboratory systems. ss

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56 A good candidate laser system is the Nd:YAG solid-state laser pumped by laser diodes. Solid-state laser systems using neodymium (Nd) in other host lattices may prove to be even bet- ter candidates. The development of high-performance moderate- power lasers is not a major military concern (to the task group's knowledge) and would benefit from funds provided by the space program. CRYOGENIC CAPABIIITY TO TRANSFER HELIUM IN ORBIT Low-temperature experiments such as the Infrared Astronomy Satellite (IRAS), Gravity Probe B (GPB), the Cosmic Background Explorer (COBE), and the Shuttle Infrared Telescope Facility (STRTF) are limited by the lifetime of the liquid helium in the spacecrafts' dewar. A very important capability to develop for the future is the ability to transfer liquid helium in space, especially to eject a transfer for low-temperature free flyers. Ultimately, this should be a routine service function of the Space Station, but it could be extremely useful if developed earlier for the Shuttle. DEVELOPMENT OF CLOCKS The hydrogen maser now has a long-term stability of better than 10-~5. An biller version of this clock was flown several years ago on Gravity Probe A to measure the gravitational red shift to an uncertainty of 10-4 in a suborbital highs. The hydrogen maser is the logical candidate to use in the second-order red shift experiment proposed for the Starprobe mission. Building a space- worthy improved hydrogen maser is an important technological development program for NASA. The present development of trapped ion clocks is not as far along as the hydrogen maser but holds the possibility of stabili- ties of 10-~7 to 10-~. The continued development of these clocks opens the possibility of a sensitive test of the strong principle of equivalence by intercomparison of clocks with different mixtures of electronic and nuclear energy terms. The intercomparison could first be carried out on Earth, where the solar gravitational poten- tial changes over the course of the year due to the Earth's eccentric motion about the Sun, and later on a spacecraft to sample a larger change in the solar field.