This large disk are collecting electrode at high temperature density ion collector. These disks are manufactured by melting iridium metal rolled into sheet and cut into required shape.

The Sputtering targets made out of high purity platinum specially 99.99% and with highly polished surface area, the surface area are mirror polished and you can see the reflections in it. You see the same from the image attached. The targets are also available in very pure Silver metal and gold metal for watch industries.