This is a waveguide from a decommissioned radiation therapy system. When electrons are launched in to the waveguide, the smooth copper interior of the waveguide speeds them up so fast, it turns them in to beams of radiation that can be aimed at cancerous tumours.
<a href="//commons.wikimedia.org/w/index.php?title=User:Julia_ImageX&amp;action=edit&amp;redlink=1" class="new" title="User:Julia ImageX (page does not exist)">Julia ImageX</a>
CC BY 4.0
<span class="int-own-work" lang="en">Own work</span>