The third form is that the cathode has a heater so that the cathode temperature is controlled in a range of the room temperature to about 100 degrees Celsius to regulate {HEAD ~ an amount} of {TAIL ~ electron emission} from the cathode 
The third form is that the cathode has a heater so that the cathode temperature is controlled in a range of the room temperature to about 100 degrees Celsius to regulate { HEAD ~ an amount } of { TAIL ~ electron emission } from the cathode
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH