James Sweet said:
You can't wire electron guns in series! They need only be grounded, just
charge the target to 50kV.
The problem with high-voltage electron guns is flash-over. The 50kV
between the cathode and the anode that is accelerating the electrons
that you want to produce at the cathode can accelerate other charge
carriers, and if the dynamic are wrong the curent in this unintended
sneak path can rise very rapidly to create an arc.
This process is called a flash-over, and can discharge the capacitance
of the cable carrying the 50kV current in a few nanoseconds.
When I was first working on electron microscopes at Cambridge
Instruments, we used a cheap-skate cable termination at the electron
gun that wasn't properly coaxial, and we'd get hundreds of amps of
current running around the ground connections during a flashover,
which had a tendency to blow up sensitive inputs if you didn't protect
them with catching diodes.
When my boss finally got his way and we went over to proper coaxial
connectors at both ends of the high voltage cable, the ground currents
during a flashover confined themselves to the ground return (screen)
of that cable and the electron microscopes became a lot more reliable.
The sources of the unintended sneak paths were interesting and
various. One of the more interesting problems turned out to be organic
vapour contamination in the "high vacuum" in the region of the
electron gun. Basically, it is very difficult to avoid having a single
molecule layer of lubricating oil on every metal surface inside an
electron microscope, and if this shows up in the electron gun the film
is exposed to 50kV X-rays, which knocks off hydrogen atoms, converting
the film into an electrically conducting graphite-like layer of
carbon, which promply adsorbs own its single molecule layer of
lubricating oil. In areas of high electric field, the intially smooth
surface develops sharp points, which in turn develop into sharp
needles, where the tip is a single hydrocarbon molecule, which
eventually becomes a cold field-emission tip emitting electrons in
competition with the intended cathode.
As the field emission current builds up, the needle stops being cold,
and becomes a heated emitter, emitting a lot more current, before
breaking off and hitting the anode with loads of energy where it
generates a load of positive ions, who travel back to the cathode,
accelerated by the 50kV field ....
Very high voltage electron guns - 100kV and higher, are always
"bakeable" with a private vacuum pump which allows you to generate an
"ultra-high vacuum" in the electron gun, which is to say a vacuum high
enough to prevent the single-molecule thick layer of lubricating oil
from building up. IIRR physicists worked out the technique to allow
them to work with bare metal surfaces back in the 1950's, but it's
adaption to commercial electron microscopes was still problematic in
the 1980's when I was working for Cambridge Instruments.
