This is an old revision of the document!

Changing the ebeam filament

Introduction	We have a Varian e-Gun 980-0003, powered by the e-Gun Control Unit Model 922-0020. Read the manuals FIRST before working on the instrument. They are short and informative. However the information about changing the filament (in the e-Gun Manual) is a little confusing, and we ended up successfully completing this task using our own method. This guide will explain the steps for how we changed the filament. You'll need several small Allen keys, some flathead screwdrivers, and 0.01“ and 0.04” feeler gauges.
	Here's what you see when looking at the e-gun and crucible holder from the front. The filament is around the back (you'll need to remove both sheet metal walls from the chamber to reach it).
	Here's behind the crucible holders. The filament is the curly piece of metal. To change the filament, we'll need to remove the filament, the two blocks holding the filament legs, and the cathode shield together as a unit. Before we do this, we'll need to disconnect the voltage leads from the filament. Before unscrewing anything, UNPLUG the e-Gun control unit!
	Here's the diagram from the manual showing the various parts. The filament is the cathode, while the crucible and crucible holder are the anode.
	The voltage leads are shown here. There are two set screws, one on each side of the blocks, holding them to the filament. These can be unscrewed to loosen them from the filament and blocks. Then go to the other end of the voltage leads to loosen them, where they go out of the chamber. (next picture)
	Here the labeled screws can be loosened (but don't remove them) to allow the voltage leads to move slightly so they can be maneuvered out of the blocks.
	Once the leads are removed, we can unscrew the two large screws shown here. Partially unscrew the left screw first, and remove the “wing”. Then proceed to unscrew it the rest of the way, followed by the right screw. The blocks, filament, and cathode shield will come off together as a set (careful not to drop them!)
	This is what it looks like. Place on a clean beta wipe. The filament is held in place by the upper two set screws, which can be loosened to allow the filament to be removed.
	This is what the setup looks like with the filament and blocks removed. The giant metal U underneath is the magnet. The magnetic field extends up through the vertical pieces of metal separating the crucible cavities. The blocks holding the filament screw into the ceramic tubes in the center of the magnet.
	The cathode shield was clamped between the blocks and the ceramic tubes, so it just slides out. The right screw has two ceramic washers (one on the outside surface, one underneath). Disassemble by removing the screws and washers, then unscrewing the hinge (aka ceramic spacer) screws. Clean all the parts (blocks, washers, spacer) with a magenta Scotch-brite pad and IPA in the fume hood. (perhaps bake before putting back into instrument? We forgot to.)
	The replacement filaments are in this green box (hopefully still in the cupboard with the deposition materials and spare crystals).
	Here's what the old filament looked like compared to the new one. It looks like the old filament is coated in some shiny metal (maybe Pt?). Slide the filament back into the contraption but don't tighten the set screws. We need to put everything back into the chamber in order to get the correct positioning of the filament.
	With the filament replaced, this whole unit needs to go back into the chamber. This part can be a little tricky but it is doable. Carefully carry this to the chamber and gently screw the main screws into the ceramic tubes from whence they came. Don't forget the cathode shield! Also don't tighten the screws too much as the distances between the cathode and anode, and between the filament and cathode shield, will need to be adjusted.
	Using the 0.01“ and 0.04” feeler gauges, adjust the distances as shown in the diagram from the e-Gun manual. Take your time and make sure everything is as even and accurate as possible. Misalignment here can cause misalignment of the electron beam. We adjusted the anode to cathode shield distance first, and tightened the main screws to hold in place. Then we adjusted the cathode shield to filament position, and tightened the filament set screws to hold it in place.
	The filament should stick out from beneath the cathode shield by about 1/3 of its width. You can see this by looking down on the filament from above (sorry, didn't get a photo of this). Reattach the wing last, by tightening everything then slightly loosening the left screw to place the wing as it was originally. Once that's all secured reattach the voltage leads and tighten on both ends.
	That's it! There may be an increase in the pressure during the first rough pump-down due to off-gassing; if so, vent the chamber and restart. You may have to change the beam position slightly using the beam position control knob. The full range of motion is 1/8“ (according to the manual). I found the beam position was easy to see when shooting a crucible full of Cr chunks, as the chunk that glowed first/brightest was where the beam was hitting.