Positron source

Future lepton col­lid­ers such as FCC-ee need intense po­si­tron beams. Po­si­trons are made by sending an elec­tron beam onto a thick target of a heavy metal, usually tung­sten: the elec­trons radiate brems­strah­lung photons, and the photons convert into elec­tron–po­si­tron pairs. More po­si­trons need more beam power, and the target can take only so much heat and thermo-me­chan­i­cal stress.

When the elec­tron beam is aligned with a major axis of a tung­sten crystal, the strings of atoms act to­geth­er and their fields add up into a strong axial field. At high energy this field makes the elec­trons radiate far more than brems­strah­lung in amor­phous tung­sten, and photons convert more easily into pairs: the electro­magnetic shower de­vel­ops faster, as if the radia­tion length were shorter.

Two designs use the effect. In the hybrid source a thin ori­ent­ed crystal ra­di­ates and an amor­phous target down­stream con­verts the photons; in the single-crystal source one thick ori­ent­ed crystal does both. Either way the same po­si­tron yield needs less ma­te­ri­al, so less energy is de­pos­it­ed in the target.

We measure the radia­tion and the showers in ori­ent­ed tung­sten crys­tals in beam tests at DESY and CERN, and follow the po­si­trons in simula­tion from the target to the en­trance of the FCC-ee damping ring. With IJCLab the crystal-based design entered the FCC-ee in­jec­tor base­line, and it will be tested at PSI.