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