A multi-hundred GeV electron beam radiate photons in either a planar or a helical undulator.
Since the radiation intensity in
a helical undulator is two times higher than in a planar undulator,
the helical undulator was chosen as ILC baseline source. In addition it offers the possibility to generate
polarized positrons via the generation of circularly-polarized photons.
The photons produce then positrons via pair production in a direct conversion process at a rather thin target (0.4-0.5 radiation length). The from the target emerging positrons (and electrons) have to be accelerated up to an energy of about 125 MeV in cavities embedded in a focusing solenoid, the electrons and positrons are separated. A pre-acceleration system, accelerates the positrons up to 400 MeV. More details about the capture optics are described in the section Target and capture issues. In a super-conducting booster RF system the positrons reach their final energy of about 5 GeV before the injection into the positron damping ring. |