SEM back online

A couple of months ago, Milly started having beam issues. At the time it seemed like emitter trouble. New emitter modules run about $3k, so I took the opportunity to look into manufacturing my own.

But that’s a long story for another time.

8mm across, 1mm high, 0.1mm thick. And the tip is just a tungsten atom or two.
8mm across, 1mm high, 0.1mm thick. And the tip is just a tungsten atom or two.

The short version is that I’ve learned a lot in the last couple of months:

  1. Cold cathode tungsten emitter tips are really, really tiny. I knew that of course, but you don’t truly have an appreciation for something until you try to make one.
  2. Spot welding tungsten is harder than you might think. It has the highest melting point of any element (3422 C) and gets quite brittle after heating.
  3. Before jumping right into emitter maintenance, be sure to check all of your fuses.

In the end, it turned out that I had blown two fuses in the electromagnetic lensing power supply. This was the cause of the beam trouble, not the emitter itself.

Why two fuses? This circuit uses two 10A fuses in parallel. Each half is supposed to carry 7A.

Why two fuses in parallel instead of a single 20A fuse? I have no idea. The original manufacturer thought it was a great idea. But with fuses in parallel, whenever one blows the other one does too, often in a spectacular fashion.

After changing the fuses I decided to put the original emitter back in place. One 24 hour bake later, she’s back online.

Some random junk that happened to be under the beam.
Some random junk that happened to be under the beam.

She’s not quite 100% yet… There’s a little trouble with the noise cancelling pre-amp, and I need to take the time to properly realign the column. But thankfully she’s up and making images again.

More on my DIY emitter adventure in a future post.

2 thoughts on “SEM back online”

  1. When you are messing around with the emitter make sure to used ESD procedures. Static charge leaking from the tip is enough to blow it.

    1. Thanks for the tip! (No pun intended…)

      I kept the connector contacts shorted with aluminum foil. I do the same when I bake the system. Not foolproof, but better than nothing.

      When I get closer to making a new emitter tip, I’ll certainly break out the wrist strap.

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