Progress on PEEM3 - An Aberration Corrected X-Ray PhotoemissionElectron Microscope at the ALS Page: 2 of 4
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similar to the PEEM2 objective lens , whose electron optical properties have been optimized to have small
aberrations . An image with a magnification of 11.23 is formed at the entrance plane of the separator section
(indicated by image arrow in Fig. 1) between the field lens and the first quadrupole. This field lens makes the field
ray parallel upon entering the beam separator. The objective lens, together with the field lens, form a telescopic
round lens system. This is necessary for the mirror to run in the so-called symmetric mode in which first-order
chromatic distortion and third-order coma can be cancelled . To cope with the inevitable stray DC magnetic fields
mechanical misalignments and tolerance errors and we employ dodecapole correctors as indicated in Figure 1. The
choice of dodecapoles was driven by the desire to have not only quadrupole and octapole correctors but also a
hexapole component, as three-fold astigmatism has been a problem in PEEM2 which only has octapole correctors.
All correctors are dodecapoles for design consistency.
Separator Section I Intermediate Projector
Objective Field Lens11 Separator I Lensl Lens2 Lens1 Lens2
Sample -13600v -8530v focusing _ 19540v 19371v
-20000v LIs71 -11841 v-18440v -140 -lll
- Lens - 7541v
D DiFOIrator '1? }. l~ I Sr I' ~CCD
I"-s ,- focusing Aperture
- Lens -7541v Various Electrostatic
" -- - sizes dodecapole
..-_] Field Lens 2
Section 02222 Electrostatic
Mirror ----- -7182v ElcrIai
eo Ov ,Quadrupole
- 2290v Elrrorn Electrostatic!
-2170 M Magnetic
0 23870 -dodecapole
Section T Image planes
FIGURE 1. Schematic layout of the electrostatic lenses, separator magnet and electron mirror of the PEEM3 microscope.
Typical lens voltages are given. As a scale indicator, the distance from the sample to the right hand CCD is 1350mm.
The separator section consists of three Einzel lenses, six quadrupoles and the dipole separator magnet itself .
Simple round Einzel lenses were chosen to be the main focusing lenses as the deviation from rotational symmetry
caused by the bending magnet and the remaining aberrations of the lenses are small. The weak electrostatic
quadrupoles provide the required stigmatic image correction and the 4 degrees of freedom per leg for the electron
beam position and angle. On the first pass, the electron beam leaves the separator section chromatically dispersed as
it enters the mirror section
The mirror itself consists of four electrodes with the actual mirror reflecting electrode shaped as a spherical
segment of radius 5.6mm. The entrance electrode is at ground voltage, while the potentials of other electrodes give
three degrees of freedom to determine the focal length, chromatic aberration and the spherical aberrations of the
mirror . In order to cancel coma generated by the mirror, the magnification of the mirror is chosen to be -1 and a
field lens is placed near the image plane (separator exit on the first pass) to ensure that the linear optics are
telescopic for the fundamental ray trajectories. A pair of magnetic/electrostatic dodecapoles effect adjustments of
beam trajectory within the mirror section. The set up of the separator and mirror is not going to be trivial. A
projector lens and CCD detector are located behind the mirror, which is to be used as a diagnostic PEEM. This
PEEM will allow us to independently test and optimize the first half of the beam separator and the incoming beam at
the mirror. In this operational mode, the lens voltages are rearranged and the electron beam passes though a 500 m
diameter hole in the reflecting electrode before being projected onto the diagnostic CCD.
The mirror section has magnification of -1, so the second pass through the separator cancels the chromatic
dispersion of the separator on the first pass. This is the new development for these low energy PEEM microscopes.
A significant amount of design work was put into developing a separator magnet with the initial work directed along
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MacDowell, Alastair A.; Feng, J.; DeMello, A.; Doran, A.; Duarte,R.; Forest, E. et al. Progress on PEEM3 - An Aberration Corrected X-Ray PhotoemissionElectron Microscope at the ALS, article, May 20, 2006; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc889767/m1/2/: accessed November 18, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.