How did the Lord strike Saul down outside Damascus?

Outside Damascus, the Lord struck Saul down with a beam from heaven, described as probably a laser beam as understood now. He fell to the earth and heard a voice saying, "Saul, Saul, why persecutest thou me?" This occurred while he was breathing out threatenings and slaughter against the disciples.

Les explains that Saul was still breathing out threatenings and slaughter against the disciples of the Lord, a term meaning followers or believers, not the twelve. Saul went to the high priest and sought letters to the synagogues so that if he found any who had followed Jesus of Nazareth, men or women, he might bring them bound to Jerusalem. Outside Damascus, the Lord struck him down with a beam from heaven, probably a laser beam as we understand them now. Saul fell to the earth and heard a voice saying, "Saul, Saul, why persecutest thou me?"

Acts 9:3

And as he journeyed, he came near Damascus: and suddenly there shined round about him a light from heaven:

Who did Saul think was speaking to him on the Damascus road?

Saul assumed the voice and light from heaven belonged to Jehovah, the God of Abraham, the covenant God he thought he was serving. As a religious Jew, he recognized only one Lord, Jehovah, so he naturally concluded that Jehovah was speaking when he cried, "Who art thou, Lord?"