Chapline attended a meeting where Sobel'man's work on X-ray lasers was presented. He had learned of the unique underground nuclear tests made on behalf of the Defense Nuclear Agency (DNA), where the burst of X-rays produced by the nuclear reactions were allowed to travel down a long tunnel while the blast itself was cut off by large doors that slammed shut as the explosion approached. These tests were used to investigate the effects of X-rays from exoatmospheric nuclear explosions on reentry vehicles. He realized this was a perfect way to pump an X-ray laser.