The way that the electrtons for the beam are produced is actually quite low-tech and is similar to old TV sets. There is a thin piece of wire with current in it that heats up, The electrons get excited and are "boiled off" the surface. Then they are attracted by a large positive voltage from an anode. The Diamond website talks abiut a linac - a linear accelerator. Many of these use a series of alternate anodes - the voltages are made positive in front of approaching electrons and negative behind them. The website talks about RF cavities for accelerating electrons. This will be like a klystron tube in reverse - see recent post. http://www.diamond.ac.uk/Science/Machine/Components/linac.html#
