Imaging Elemenls with Permanent Magnets 



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Strahlnchtung 



1, Permanent magnet. 2a, 2b, Disploceable polepieces of 

 the intermediate lens. 3, Insert, at the same time fixed pole- 

 piece at the final projector lens. 4, Disploceable polepiece 

 of the fmal projector lens. 5, Control nut. 6, Bevel gear. 



Fig. 1. Cross section through a two-stage projective system 

 operating with permanent magnets. 



an electron microscope objective lens a permanent 

 magnetic einzel-lens may be used. On its electron 

 optical properties Lenz (9) has published a paper. 

 Such einzel-lenses and any lens system of two or 

 more gaps operated by a single permanent magnet 

 as described e.g. in a paper by v. Borries and Langner 

 (4) show no image rotation, or, if the specimen im- 

 merges in the field as is the case in strong lenses, 

 the image rotation is at least small and does not 

 vary noticeably when the magnification is changed. 

 The size of such a system depends on the niagne- 

 tomotoric force of the permanent magnet. The opti- 

 mum shape of a permanent magnetic lens system has 

 been calculated by v. Borries and Lenz (4), and it 

 can roughly be said that for the diameter D of such 

 a system, provided the best obtainable Alnico 

 magnets with at least 4.5 10'' Gauss-Oersted are 

 used, the following simple rule holds: 



Z) = 7 10 - mm Amp ' /, 



where / is the number of ampere-turns the magnet 

 supplies when in the system. Based on the design 

 of what we think an optimum system for an electron 

 lens system with rotational symmetry three perma- 

 nent magnetic electron microscopes have been com- 

 pleted with / =i 2500 Amp on which papers have 

 been published by v. Borries (1,2). Recently we 

 have built a two-stage projector system of / = 



BOO^.. 



VergroOerung 



lC% 



P 



loy. 



5% 



10 75 



— » ! mm 



ausgenulzler Bereich 



20 



Fig. 2. Magnification and distortion of a two-stage perma- 

 nent magnetic projector system. 



1720 Amp, PU = 60 Amp- Volt for 50 kV, for the 

 use with an immersion objective. The cross section 

 is shown in fig. I . The magnet is outside the vacuum. 

 The intermediate lens is formed by the polepieces 

 2a and 2/?, the axial displacement of which permits 

 the magnification of this stage to be varied. The 

 properties of such lenses have been described 

 earlier (6, 7). A rather large range of magnifica- 

 tion is achieved at the cost of considerable radial 

 distortion. The final projector has therefore been 

 designed as a lens with a variable gap-w idth which 

 yields less distortion but also less variation of magni- 

 fication. Fig. 2 illustrates magnification and radial 

 distortion versus the displacement of the polepieces. 

 The magnification can be continuously varied in the 

 ratio I :20 while the specimen is under observation. 



Furthermore we have enlarged the above-men- 

 tioned system and used a magnet with 3250 Amp. 

 With this two-stage microscope a magnification of 

 2<S,000 has been obtained. Fig. 3 shows the cross 

 section of this system. The objective lens 1 is normally 

 operated with imic gap, but h> axial displacemcnl of 

 an iron piece 2 the second gap opens. So the back 

 focal plane of the first gap can he imaged to obtain 

 tiiree-stage difiVaction diagrams. In the first gap we 

 use a magnetic stigmator 3 which is schematically 

 shown below. Two pieces of iron slide within 

 slotted pieces, so the a/imuth and the streiigtli of 

 the stigmator can be controlled separatel>. 



The projector consists of a polepiece unit 4 which 

 is also axially displaceable and permits a variation 

 of the magnification of 1:11. New in respect to for- 

 mer instruments of that kind is the separate control 

 of the magniticalion in the projector and in the 

 objective. Owing to this it is e.g. possible to image 



