File:Cyclotron with glowing beam.jpg

Summary
https://commons.wikimedia.org/wiki/File:Cyclotron_with_glowing_beam.jpg 60-inch cyclotron, circa 1939, showing beam of accelerated ions (perhaps protons or deuterons) escaping the accelerator and ionizing the surrounding air causing a blue glow. (contrast enhanced) This image is a work of the Lawrence Berkeley National Laboratory and as such is a work of the US government and therefore in the public domain.