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 an amount of {HEAD ~ electron emission} from {TAIL ~ the cathode} 
from
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 an amount of { HEAD ~ electron emission } from { TAIL ~ 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 OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL