This limits the power allowed to the cathode because as the power increases, a level is soon reached where {HEAD ~ the edge} of the ring touching {TAIL ~ the target} is damaged 
touching
This limits the power allowed to the cathode because as the power increases , a level is soon reached where { HEAD ~ the edge } of the ring touching { TAIL ~ the target } is damaged
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 OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH