# Any photon colliders in the past?

People have been thinking about a photon-photon collider (see this and this) as an add-on or to supplement the ILC, the next generation linear collider.

My question is, have there been any photon-photon colliders running in the past*, including those running below the $e^+ e^-$ threshold to study non-linear effects in photon scattering?

*Virtual/Weissäcker-Williams/Equivalent photon-photon colliders don't count.

• It might just be me who is not a lets say a proffessor, but aren't photons massless, and therefore do not interact with each other? What would be the point of such a colider? – Jakob May 12 '14 at 22:08
• Photons interact with each other through higher order effects, where the photons will pair produce $e^+e^-$ pairs, and then those annihilate into new photons. – webb May 12 '14 at 22:49
• Related but not a duplicate: Inverse pair production w/hohlaraum as photon target - is this experiment going to be carried out? – uhoh Apr 5 '18 at 4:50