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Principle of Operation
The Fine Focus RHEED gun RG15 consists of an electrostatic electron source combined with a magnetic lens for long distance
focussing and a double octopole deflector for beam rocking in any direction. The whole gun is m -metal shielded ensuring high stability against ac and dc magnetic fields. From this no serious beam deflection inside the source by dc magnetic fields, e.g. the earth magnetic field will happen.
Electron Source
The Electron source consists of Cathode/Wehnelt assembly mechanically adjustable during operation and a two stage focusing with
selectable illumination apertures.
The filament consists of thoriated tungsten wire which is easy to operate and has a long lifetime. The filament is mounted in a
Wehnelt assembly that can be adjusted mechanically during operation. This helps to get best alignment of the electronoptics with homogenous beam profiles and maximum beam currents.
The first stage focussing consists of an electrostatic unipotential lens that images the cathode on an illumination aperture.
The illumination aperture is selectable by a mechanical linear motion feedthrough. The selectable sizes of this aperture are
1mm, 300m m, 100m m, 30m m and 10m m. The illumination aperture is then imaged onto the sample / screen via a magnetic lens.
Octopol deflector
The electron beam can be deflected in any desired direction via two octopole deflectors. The two deflectors are assigned as
octopole A and B respectively. The first octopole in the beam direction is octopole A.
The sixteen deflector plates of the double octopole are designed in that manner to give special properties of deflection.
Especially beam rocking in any desired deflection plane is accomplished with its shape.
Each octopole gets separate actively adjusted bipolar x and y deflector voltages from the beam control unit. Theyy are applied
onto separate highvoltage dividers where the 4 incoming voltages are splitted into eight. The voltage divider provides potentials what are calculated to give a deflector potential distribution as
homogeneous as possible inside the octopoles.
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