ILC Technology, Inc.

A program was undertaken to develop technol. for producing a K vapor arc lamp with a 3000 h lifetime.The lamp is to be used as optical pump source in a Nd:YAG laser transmitter as part of a space communications system.The lamp lifetime goal was successfully achieved, a characteristic lifetime of approx. 6000 h being demonstrated for lamps built late in the program.In addition, the final lamp satisfied virtually all other design and performance requirements.Most of the effort on the program was directed at developing supporting technol. for the lamp.This work included solving problems of basal plane cracking and K vapor attack of the sapphire envelope, developing a long-lived cathode, determining the most efficient lamp fill, developing an oxidation protection coating for the Kovar endcaps, optimizing the Kovar-to-sapphire brazed seal design, conducting thermal design proof tests (including simulation of zero-g effects), improving the K filling procedure, developing an optimized lamp operating procedure, and conducting vibration tests.

From development work on K-Rb vapor arc lamps with sapphire envelopes, it is proposed that the basal plane cleavage cracking of the sapphire is caused by epitaxial growth of microplatelets of K β-alumina K₂O.12Al₂O₃ into the sapphire along basal planes.The vol expansion associated with formation of these platelets generates wedging stresses that promote parting of the planes.The β-alumina forms during lamp operation by a reaction between sapphire and K of the type: 2K + 1/2O₂ + 12Al₂O₃ → K₂O.12Al₂O₃.A similar reaction to form Ca β-alumina is proposed to explain basal cracking encountered during envelope-end cap brazing operations.Substituting Mo for high alumina parts in envelope-brazing fixtures resulted in complete prevention of basal cracking during the brazing operation.

Research is discussed on developing prototype 6.4 kW (input power) alkali vapor lamps that would emit 2 kW of output radiation in the major pumping bands of Nd:Y Al garnet laser material.The lamps were to be 8 in. long and 18 mm or less in outside diameter, and were to provide an operational lifetime of 100 hr or more.