How is Dose Rate Controlled for Varian True Beam?

Doserate is controlled by electron gun grid timing. The filament voltage is always constant and grid is fired either in coincidence with RF pulse or fired after RF pulse (delayed). The delayed bunch is wasted. This technique is used because it provides consistent output with each pulse. If pulses would be skipped then electron gun would accumulate a lot more free electrons emitted from filament during period of time and it would affect output of subsequent pulse. So doserate output is controlled by modulating between coincident and delayed gun grid pulses.

While performing beam tuning you may change grid pulse voltage which affects max output. You tune Truebeam to beam about 15% over desired doserate and then DoseServo slows it down doing the trick described above. You want to have these 15% as a safe margin to prevent under dose as linac output may be affected by many factors such as input voltage, poor tuning, temperature and others.