  Particle accelerators that use electromagnetic fields to increase a charged
particle's energy have greatly advanced the development of science and industry
since invention. However, the enormous cost and size of conventional
radio-frequency accelerators have limited their accessibility. Here we
demonstrate a mini-accelerator powered by terahertz pulses with wavelengths 100
times shorter than radio-frequency pulses. By injecting a short relativistic
electron bunch to a 30-mm-long dielectric-lined waveguide and tuning the
frequency of a 20-period terahertz pulse to the phase-velocity-matched value,
precise and sustained acceleration for nearly 100% of the electrons is achieved
with the beam energy spread essentially unchanged. Furthermore, by accurately
controlling the phase of two terahertz pulses, the beam is stably accelerated
successively in two dielectric waveguides with close to 100% charge coupling
efficiency. Our results demonstrate stable and scalable beam acceleration in a
multistage mini-accelerator and pave the way for functioning terahertz-driven
high-energy accelerators.
