Indium and Vacuum Seals

• Both seal surfaces and the indium should be kept as clean as possible.
• Acetone does an adequte job cleaning surfaces and indium.
• When making a seal out of a wire of indium, cut the wire to length, beveling the edges to make the joint. The freshly cut edges will stick together readily.
• Not all sealing schemes make reliable indium seals. An improperly designed seal geometry can often be sealed initially, and then after a couple of days, will open up. For example, we attempted to make a seal between a knife edge and a flat. It initially worked, but after a couple of days, the indium crept inwards, over the knife edge, and a leak opened up. The problem was solved by machining washers which prevented the indium from flowing inwards.
• A good rule of thumb for determining the number of fasteners needed to secure an indium seal is - as many as you can possibly fit on the surface!
• Indium seals can be reliable down to very cold temperatures. The filltube was reliably sealed to the LHe reservoir with .125" indium.
• Indium can be reformed into useful seals after use. Dan Gezari (GSFC) successfully extruded indium wire from scrap indium. He first cleaned the indium with acetone. His extrusion mechanism consisted of a hollow cylinder with a bolt screwed into one end, a plunger and a stand. The bolt had a hole drilled lengthwise through which the indium was extruded. A 0.070" hole yielded indium with a .079" diameter. He used a hydraulic press to extrude the indium. Smaller diameter indium required larger forces (500lbs for 0.125" wire, 2000lbs for 0.080" wire).