2. Develop techniques for forming and welding utilizing iinimum power. 3. Conduct Shuttle experiments of planned fabrication techniques using selected resin systems. 5.3.3 Solar Cell Technology Item — GaAlAs Solar Cells Criticality — Critical to program success Most of the solar cells in use today are made from silicon; however, the resulting array are very heavy. Use of GaAlAs cells reduces total SPS vehicle mass to more acceptable levels since the GaAlAs cells are thinner and more efficient. This mass reduction is dependent on the development and demonstration of lightweight, high efficiency cells. Present Capability: Single-crystal GaAlAs cells have demonstrated efficiencies of 16- to 20-percent in laboratory tests. Cells operating at 16-percent efficiency could be put into production now. All of these cells are very thick at about 250 p or more and consequently are too heavy for SPS applications. Polycrystaline cells on the order of 5 p thick have been fabricated and tested in the lab. Cells this thin are sufficiently light for SPS; however, they have demonstrated only about 5-percent efficiency. Required Capability: Use of GaAlAs cells on SPS will require polycrystaline cells 1-3 p thick operating at greater than 14-percent efficiency. Otherwise, total system mass in orbit may be excessive. Development Plan: Additional effort is required to better understand the energy loss mechanisms in polycrystaline cells. Techniques need to be developed that reduce the losses until efficiencies of at least 14 percent are achieved. Item ~ GaAlAs Radiation Damage Annealing Criticality — Enhance probability of program success The radiation environment at GEO causes a long-term degradation of solar cell efficiency. Even greater degradation can occur during slow transport through the Van Allen belts. Elimination of the degradation permits the SPS to be reduced in size for a given final power output on the ground. Present Capability: In laboratory tests GaAlAs cells were exposed to radiation doses comparable to that received from 30 years at GEO. Cell efficiencies decreased from 15 percent to 10.5 percent. After the cells were heated to 125-150°C, tests revealed that cells were operating at nearly 15-percent efficiency again. Hence, heating of the cells appears to correct radiation damage.