Grid control X-ray tube versus dual X-ray tube?

A Grid Controlled X-ray Tube is a triode in which a third electrode is introduced between the Anode and Cathode. This electrode is normally neutral so the x-ray production can take place. However, one can decide when to cut the exposure by introducing negative charge on this. This will then stop (repel) the electron from reaching the Anode and x-ray production is stopped. Therefore, the elecrode with a negative charge can as a timer or if chrge is given intermittently, it acts as a device to have flouroscopy in pulses.