Would a small electron microscope in a cubesat work well in space at 400 km?

Question

There is now an electron microscope in space!

Suppose that once I win a lottery I can build a small, modest SEM column and put it in a 6U cubesat that opens one side to allow the electron optics to be open to the vacuum of space rather than providing any cumbersome vacuum pumping.

The cubesat is released from the ISS so it's exposed to the environment at 400 km. It's equipped with a big 100m x 1 cm loop of kapton or thin metal foil on a motor so that it collects micrometeorite (or Andromeda strain) impacts and then studies them, at least that's my excuse for launching it, hopefully they'll let me navigate it around the ISS to study damage to the various materials on the outside of the ISS.

Question: Just how good is the vacuum at 400 km for a SEM. Can it be expressed in Torr? In other words, 1E-03? 1E-06? 1E-10? How high can it rise during a period of high solar activity?

My little SEM has mostly encapsulated solenoid electromagnets (100 gauss to say 1 kG) along with some small scan coils for imaging and electrostatic lenses in the gun. Will that be a problem in terms of charged particles up there or the Earth's field?

What about the hot tungsten filament in the electron gun? Will that be a problem?

Will my secondary electron detector be swamped by electrons in space?

Even if the vacuum turns out to be pretty good, are there species that will attack materials in my SEM?

Answer 1

It looks like the atmospheric pressure at the ISS's 400km altitude is in the high vacuum range of 1E-6 torr. (760 torr = 1 bar). This may be somewhat complicated by the fact that the thermosphere wildly varies in temperature.

This level, by itself, is good enough for most of the high-vacuum technologies such as electron guns, mass spectrometers, etc, but it is not at the ultra-high-vacuum level (1E-9 torr) needed for some of the most modern and most advanced vacuum technologies. As a general rule, miniaturization helps vacuum and charged particle technology work at mediocre vacuums.

It's common for the different parts of an electron microscope to be pumped to wildly different qualities of vacuum, with the column sometimes being pumped down to insane levels of UHV while the sample chamber simply cannot be pumped to that level because it must contain the sample, which has a nonzero vapor pressure. If you want to use a UHV column (needed for some of the longest-lasting and most technologically advanced electron emitters, less so for the traditional tungsten filament) you can probably just mostly close it off and run ion pumps (no moving parts!) to pump it down even further -- there is no need to seal it.

You may have some concerns about outgassing from materials in your cubesat -- the interior of the column should still probably be made only of materials that a normal SEM would be made of (stainless steel, copper, aluminum, gold, silver, nickel, glass, ceramic, tungsten, and molybdenum, plus the absolute minimum quantity of Viton elastomer or PEEK plastic when you absolutely need it).

Unfortunately I cannot give much of an answer about the magnetic fields or electrons in space except that the latter may be able to be dealt with by appropriate electrostatic screening.