Features of the undulator-based (polarized) positron source
In 1979 V.E. Balakin und A.A. Mikhailichenko proposed the scheme to generate polarized electrons and positrons via radiation in a helical undulator. More details are given in this translation of the PhD thesis.
The front end system, i.e. the characteristics of undulator radiation, target properties and capture optics have been studied in detail in this PhD thesis.
The proof of principles of such a scheme, the production of circularly polarized photons and polarized positrons, has been recently performed with the experiment E166 at SLAC. Several prototypes for short period helical super-conducting undulators have been designed, see the sections ILC undulator/UK and Undulator at Cornell.
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.
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.
Yield versus undulator length
Dependence on drive-beam energy
Characteristica: polarization, etc.
Andriy, Tech meeting
Wei Gai, LCWS05
Wei Gai, posipol
Last modified: 3-September-2006