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