It is very important to clarify the mechanism of discharge induced by a laser-produced plasma channel as a basic study on the laser-induced lightning. We measured the temporal evolution of the electron density and the neutral particle density in one of plasma beads in the channel using Thomson and Rayleigh scattering methods, and also performed discharge induction experiments for a short gap. It was found that the induced discharge is greatly influenced by the temporal evolution of the electron density and the neutral particle density in a plasma. In this paper, we measured the electric field along the channel using an optical sensor at the discharge induction to study the leader propagation process in a long gap. The induced discharge in a long gap changes in the order of electron avalanche, streamer, leader, and final sparkover similarly as in a long gap discharge in the atmosphere. To clarify the leader propagation, we measured the change in the electric field due to the leader propagation between the electrodes, and estimated the velocity of the leader propagation. The electric field was measured using an optical sensor based on the Pockels effect
