Sentence:  Amplification has taken place because in the <WSD>buncher</WSD> grids the electrons are affected very little, but in the drift space this effect is given time to grow and the bunching becomes much more pronounced....After leaving the <WSD>buncher</WSD> field the electrons travel towards the plate, are slowed down, stopped and then reattracted by the <WSD>buncher</WSD>. 
Meaning of the word marked with <WSD> tags:  A circuit that causes electrons or other charged particles in a particle beam to group together.