Hi Philipp,
On the SX100 here at UTAS (SX846, installed 2003) we originally couldn't get to 30kV when using LaB6.
Above something like 27kV we'd have arcing.
This is one of the main reasons why in 2007 the system was modified to have the new side-mounted ion pump design that the newer microprobes have. Originally we had the ion pump with the 4 elements around the Wehnelt. The modified system was tested successfully up to 30kV for LaB6 (which was the specification) but not higher. We actually never used it at voltages like that. Most of the time we wouldn't go beyond 20kV, maybe 25kV occasionally for specific trace element methods.
Cheers,
Karsten