The tungsten filament on our SX100 blew overnight and so this morning I went in to replace it. After replacing it and pumping down I decided to bring up the last column conditions just to get started with the gun alignment. The filament had failed on a metallurgical sample which we were using 20 keV, so I loaded that column condition since it was the most recent one saved.
I saw about 5 nA reading on the picoammeter, so I then proceeded to use the Align Gun button in the Cameca PeakSight softwtare. It went through its usual cycle, but at the end gave me a gun alignment failed message, which is quite unusual.
So I checked the filament saturation, increased the beam current and tried the gun align again, but each time it ended with a gun alignment failed message (except for one setting which I will get to below).
So then I'm thinking what is different about this and then I thought, OK I'll try 15 keV. And then it aligned fine! And the alignment was actually quite good, about +/-200 to 300 for the high and around +/-30 to 50 for the low.
The only time the gun align worked at 20 keV was when I ran the saturation up to 240 or so, though the actual saturation was around 205. And when it finally worked at this high saturation, the 20 keV gun alignment was essentially the same as it was at 15 keV.
I don't usually align the gun at 20 keV (though I know I have in the past), but does anyone have any suggestions or insight into why the gun align works at 15 keV, but fails at 20 keV?