Hi Philipp
The quick suggestions...We generally have been using CeB6 on the SX-Ultrachron, but have used LaB6 also. We think we get better lifetime out of the CeB6 (three years or so) compared to LaB6, but our statistics are not so good having had only a few of them in there. Generally we bring up a new one at 85% of the optimum heat value, and bring it up to maximum over about six months. Of course, there is no need to ever bring it up to the optimum heat value unless you need that resolution. Always leave the heat on at your operating value unless you have to vent the gun for some reason, so just HV off for sample exchange. For emission current, there are some variables (hardware, firmware, Wehnelt-tip distance). Start at biasing to give 100uA, then align and adjust manually to see if you can find a maximum output, then leave it there for awhile. Good center brightness in gun scan high and gun scan low images is a good target, but at "undersaturated" conditions, you may get some off-axis emission. There is probably a lot of latitude here if you are not so interested in maximum brightness and spatial resolution. If it is stable and image quality is good = success.
Now, here is maybe more information than you want...Our situation may be somewhat different than yours as this machine has a power supply developed to specifically optimize brightness for REE hexaborides, and controls the condensers using the so-called geochron firmware that operates the three condensers independently, which also results in greatly improved current density at the sample. We order cathodes from Applied Physics Technologies, and specify a 30 micron flat tip area (rather than the typical 16um flat). We tune the bias setting mostly based on the gun scan images, getting deep red center figures with both high and low gun scan images. The behavior of the bias is certainly affected by the exact setting of tip height relative to the Wehnelt, as well as the emission area of the tip. Note of course that the Wehnelt assembly itself is different for CeB6 (or LaB6), compared to W.
I am guessing that you have the old type HV power supply and old-style ion pump (four quadrant elements with magnetic shield). We have the new style, off axis ion pump that eliminates the magnetic shield. This difference can be a factor in performance of the gun (vacuum dependent), and we would recommend you switch off the ion pump after any gun maintenance until the chamber vacuum is pretty good, then turn it back on manually. The firmware tends to turn on the ion pump a bit early in the sequence which may actually cause some plasma formation and may not be so good for the cathode so it is good to be patient with the vacuum system overall. We do find that after awhile we need to go back in and bring the tip back very slightly nearer to the Wehnelt as there is some recession with time (there is an effect noted in the bias behavior which you may not see with the old style power supply). We use CeB6 on our old SX50 also, and have been very happy with how it works, with much improved brightness relative to W, and very long lifetimes. We find it requires much less regular alignment, and users need not worry about figuring out saturation heat values.
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