The grid electrode is curved along a short side direction thereof and includes portions having different thicknesses, {HEAD ~ the portions} being arranged in {TAIL ~ the short side direction} 
arranged in
The grid electrode is curved along a short side direction thereof and includes portions having different thicknesses , { HEAD ~ the portions } being arranged in { TAIL ~ the short side direction }
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 REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL