A dielectric loaded accelerator for accelerating charged particles, such as electrons, ions and/or protons, is described herein. The dielectric loaded accelerator accelerates charged particles along a longitudinal axis and towards an outlet of the accelerator. The dielectric loaded accelerator accelerates the charged particles using oscillating electromagnetic fields that propagate within the accelerator according to an electromagnetic mode. The dielectric loaded accelerator described herein includes an electromagnetic mode with a phase velocity that increases towards the outlet of the accelerator and matches a velocity of the charged particles being accelerated along the longitudinal axis of the accelerator. By matching the phase velocity of the oscillating electromagnetic fields to the velocity of the charged particles, the accelerator reduces phase slippage between the fields and the charged particles and, therefore, efficiently accelerates charged particle towards the outlet.