An overview about the ILC parameters and the demands on the positron source is given in this talk: The ILC requires a large amount of positrons, about three orders of magnitude higher per pulse than at the positron source of SLC (see picture).

Positrons have to be produced from photons of some MeV energy via pair production in a target: either the photons are produced via bremsstrahlung from electrons in electromagnetic cascade processes in a rather thick target or via radiation processes of an electron beam and the direct conversion in a rather thin target.

Three kinds of positron sources for the ILC are under discussion:
a) conventional source (unpolarized positrons only)
b) gamma-based source via undulator radiation
c) gamma-based source via laser backscattering

More technical details, BCD design, current R&D status (in particular of prototypes ) and still critical issues like target, availability, stacking are linked.

The use of polarized electrons with a polarization of about 78% led to the best measurement of the electroweak mixing angle in a single experiment (see picture and talk).

The quantitative gain factors for some physics examples are listed in the summary table. The case of having both beams polarized is compared with the case of using only polarized electrons, in most cases the factor refers to (P(e-),P(e+))=(80%,60%) versus (P(e-),P(e+))=(80%,0%) at sqrt{s}=500 GeV.

Some introductory remarks about the use of polarized beams in physics studies, a choice of selected plots and recent talks about the physics case of polarized positrons at the linear collider are linked here.