Spill is a term for the beam which is extracted from the accelerator and transported to external experimental areas during the course of an accelerator cycle.
Slow spill is beam which is extracted over a relatively long time period.
Fast spill refers to one or more pulses which are extracted in a very short time. During a previous run one typical mode of operation was to have three 2 ms fast pulses interspersed in the twenty seconds of slow spill which occurred at Tevatron flat top. One of the fast pulses happened after the slow spill had ended. However, the slow spill actually had to stop briefly when the other two pulses were extracted. Even though fast spill comes out in a short time the intensity in the pulses is large.