Fabricating Niobium Cavities

Niobium cavities can be constructed using different methods. Here we illustrate some procedures used at Cornell to fabricate sheet niobium cavities.

Step 1: Shearing

The sheet material from the niobium supplier has rectangular shape. The starting blank to form the cavity half cell is usually a disk.

It is made by shearing the sheet repetitively while slowly pivoting the sheet around a center pin.

The plate after being sheared to be round.

Step 2: Deep drawing

Deep drawing is a forming process whereby a blank is pressed into shape using a set of dies. These dies are machined out of 7075-T6 aluminium alloy. The female die is lubed with clean motor oil.

The plate to be pressed is also lubed on the side where the female die will contact it.

The bolts are tightened down to just the right amount using a torque wrench.

The male die.

The sandwich composed of the male die, the niobium plate, and the female die is placed into the 100 ton press.

The deep drawn plate is removed from the die.

Step 3: Trimming

A 1/16 inch cavity would be trimmed on a lathe instead.

After deep drawing, the half cells are trimmed to the final size for electron-beam welding.

For this 1/8 inch wall thickness cavity, a CNC (computer numerical control) milling machine is used to trim the inner and outer diameter. The cavity is held in place by a vacuum chuck.

Step 4:Electron-beam welding

The pieces of the cavity are electron beam welded together inside a vacuum chamber.

Assembling

A 500 MHz cavity is shown here in a clean room. A stainless steel waveguide hat with an input power coupler is being attached to the niobium cavity. Indium wire is used at the seal at the joint.