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