How does a waveguide work?
If a microwave oscillation is set up at one end of a waveguide, its electric fields cause electric currents to flow in the copper walls. These currents in turn induce new electric and magnetic fields in the waveguide, oscillating with the same frequency as the original microwave. The net effect is that the microwave travels along the pipe. There is some small loss of energy due to the electrical resistance of the copper, but the microwave intensity that arrives at the far end of the pipe is almost as large as the intensity fed in at the beginning. For the very high microwave intensities used at SLAC, the waveguide must be evacuated (placed under vacuum) because the intense electric fields would breakdown through lightning-like spark formation if air were present in the pipe. The waveguides at SLAC are kept at about 10-11 times atmospheric pressure. Such low air pressure is also called “high vacuum”.
