Patent Number: 048067679
Section: claims

1. An electron lens assembly, comprising: at least two magnetic pole pieces disposed in opposition to each other and each having a bore for allowing an electron beam to pass therethrough, said magnetic pole pieces defining a space therebetween;  an exciting coil for generating a magnetic field between said magnetic pole pieces;  a yoke constituted by at least two divided yoke members so as to be capable of accommodating said exciting coil and coupled to said magnetic pole pieces, at least one of said divided yoke members being coupled detachably to one of said magnetic pole pieces;  a metal O-ring disposed on a surface of said detachable yoke member so as to prevent the air from penetrating into the space defined between said magnetic pole pieces along said surface of said detachable yoke member from a space accommodating said exciting coil therein; and  an electron beam passage defining pipe disposed along an electron beam path except for said space defined by said magnetic pole pieces, said pipe being integrally coupled to said detachable yoke member at an end portion near to the magnetic pole piece located close to said pipe.  first and second magnetic pole pieces disposed in opposition to each other and each having a bore for allowing an electron beam to pass therethrough;  a yoke including a first yoke portion and a second yoke portion separable from said first yoke portion, said first yoke portion having one end magnetically coupled to said first magnetic pole piece and the other end, said second yoke portion having one end magnetically and separably coupled to said second magnetic pole piece and the other end magnetically coupled to said other end of said first yoke portion, said second yoke portion further including an exciting coil disposed around outer periphery of said second yoke portion, said second yoke portion having a through-hole formed therein for allowing the electron beam having passed through said first and second pole pieces to pass through said yoke;  a pipe disposed within the through-hole formed in said second yoke portion so as to enclose the electron beam passing through said through-hole, said pipe being formed integrally with said second yoke portion in the vicinity of said one end of said second yoke portion;  a non-magnetic spacer having first and second ends connected gas-tightly to said first and second yoke portions, respectively, said exciting coil being disposed within a space defined by said spacer and said first and second yoke portions; and  a metal O-ring disposed between said second end of said spacer and said second yoke portion for coupling separably and gas-tightly said spacer and said second yoke portion to each other. 2. An electron lens assembly according to claim 1, wherein said pipe is directly welded to said detachable yoke member at the end portion near to said magnetic pole piece located near to said pipe. 3. An electron lens assembly according to claim 1, wherein said pipe is welded to said detachable yoke member through an interconnecting member at the end portion of the pipe located near to said detachable magnetic pole piece. 4. An electron lens assembly according to claim 1, wherein said metal O-ring includes a metal pipe having a cut-out portion extending longitudinally, and a helical spring disposed within said metal pipe. 5. An electron lens assembly, comprising: 6. An electron lens assembly according to claim 5, wherein said pipe is directly welded to said second yoke portion. 7. An electron lens assembly according to claim 5, wherein said pipe is welded to said second yoke portion through an interconnecting member.