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