Sentence:  The pulse width can be decreased by means of a <WSD>buncher</WSD>, which consists of a longitudinal re-entrant cavity. The first electrons to reach the <WSD>buncher</WSD> are accelerated by a weaker longitudinal field than those that reach it at later times. 
Meaning of the word marked with <WSD> tags:  A circuit that causes electrons or other charged particles in a particle beam to group together.