Go here, then the Chopper controls are in the top left

The Chopper is a power electromagnet that can deflect beam into a beam dump

We can control it to block beam for some number of microseconds (the rest of the time is called the Pulse Width), and to repeat this multiple times per second (Repetition Rate)

The percent of total beam not blocked, is called the Duty Factor

Duty Factor = Pulse Width * (Rep. Rate / 10000)

Even when sending as much beam as possible, it will still block beam for 50 out of every 10000us