In operation, tube 12 is evacuated to a pressure lower than {HEAD ~ about 0.1 torr}, preferably lower than {TAIL ~ about 0.001 torr}, and then sealed 
In operation , tube 12 is evacuated to a pressure lower than { HEAD ~ about 0.1 torr } , preferably lower than { TAIL ~ about 0.001 torr } , and then sealed
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH