The Cherenkov effect can occur in vacuum. In a slow-wave structure, like in a traveling-wave tube (TWT), the phase velocity decreases and the velocity of charged particles can exceed the phase velocity while remaining lower than c . In such a system, this effect can be derived from conservation of the energy and momentum where the momentum of a photon should be p = ℏ ℏ β β ( β β is phase constant) rather than the de Broglie relation p = ℏ ℏ k . This type of radiation (VCR) is used to generate high-power microwaves.