power. At 532 nm (doubled YAG wavelength), 9% of full power would require 14 kW of laser power if a 2.5 meter mirror is used [12], assuming perfect atmospheric compensation. Although many issues need to be resolved, perhaps this could be done with lasers now existing or under development in the laboratory, such as the AVLIS copper-vapor laser or frequency-doubled YAG lasers, with existing beam-directors and adaptive optical systems used for the test. Finally, during the discussion of radiation damage, another application was suggested, that of using a ground-based laser to heat up solar cells to anneal radiation damage. This could even use laser types which operate at wavelengths that are not good for power conversion but are now available in high power. It would, however, require high-temperature design of the arrays, which is possible, but has not been currently implemented on existing satellites. Experiments Needed The next workshop question was, what experiments need to be done now in order to verify key assumptions about laser receivers? All high-powered lasers available now or in the near future at the wavelength range of interest are pulsed. Investigation of the effect of the pulse format on the cell response is a major concern. The AVLIS copper vapor laser, currently the highest continuous average-power laser in operation at wavelengths below 1 micron, has a pulse format with a pulse width of ~50 ns and a repetition rate up to 26 kHz. The wavelength can be varied somewhat by pumping a dye with the copper-vapor light at 511 and 578 nm. Of the free-electron lasers under consideration, the RF FEL will typically have a pulse width of 10 ps, with a repetition rate on the order of a GHz, while the induction FEL would have a pulse width on the order of 20 ns, with a repetition rate of twenty kHz. A frequency-doubled Nd: YAG laser would require a pulsed output in order to achieve high efficiency on the doubling crystal without thermal distortion, since the efficiency of frequency doubling is directly proportional to the intensity. Various pulse formats would be possible for this laser, as long as the peak-to-average ratio is sufficiently high to reach good doubling efficiency. If cell operation at 1.06 microns is possible, it may be possible to use a Nd: YAG laser without frequency doubling in CW operation. Experiments reported at this SPRAT showed the response of cells to pulsed lasers is significantly different than the response to CW laser illumination [8,9], and suggested that this response may be dependent on laser wavelength [10], Thus, it was suggested that pulsed laser experiments in the wavelength range of 750-850 nm GaAs cells should be done, to learn as much as possible about GaAs cell response at the most efficient operating wavelength of GaAs cells. This could potentially be done using a Ti-sapphire laser or a dye laser. Experiments done previously showed difficulties with lasers using the induction FEL or copper-vapor pulse formats, and suggested that novel cell and circuit design techniques, such as monolithically integrated cells, wide flat conductors, and integral capacitors, could ameliorate some of the difficulties. Experiments should be done to test some of these possibilities, as well as to gather further data on cell response at these pulse formats. Few tests so far have been done using the RF laser format, and the tests done to date have not resolved the picosecond micropulse structure of the laser. Since this is an increasingly attractive laser format, further tests on RF lasers should be done. Operating wavelengths were discussed later in the workshop. It was suggested that if cell operation at longer wavelengths is desirable, cell testing at the desired wavelength should begin immediately.
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