Multipaction is a process in which a charge particle inside a gap in a vacuum environment (or very low atmospheric pressure), oscillates due to the influence of an externally applied oscillating electric field of large strength, and every time it hits the gap walls it causes other charged particles to be released. The secondary charged particles that have been released then oscillate due to the externally applied electric field and release more charged particles upon impacting the gap wall. Eventually, enough charged particles participate in the process to create a spark, which is undesirable and usually causes damage.