Requirement: A yield of 1.5 positrons accepted into the damping ring per electron through the undulator has been chosen for the design as an operational safety factor. Operating the undulator with an energy of at least 150 GeV matches this requirement, see undulator yield. The chosen BCD undulator parameters are listed here.

The undulator-based scheme provides an easy and cheap upgrade path to the ILC option of using polarized positron beams: adjustment of the undulator length plus implementation of photon collimation and additional polarimeters and spin rotators as appropriate.

The auxiliary source provides a conventional positron source with these parameters.

Such an auxiliary source does not only provide flexibility during commissioning and reduces the loss of machine time due to (scheduled or accidental) machine downtime, but provides also a source of polarized electrons for the e-e- and the gamma gamma ILC options, see BCD description and ILC parameters scope document. Concerning the demands on the auxiliary source no further R&D for the conventional source is required.

Simulations indicate that the expected radiation level and obtained radiation dose for the target is comparatively low compared to the thermal stress expected for the target of a conventional ILC positron source, see the simulation results. Also the number of produced neutrons is well below the maximum neutron dose. Therefore a lifetime of 5 to 10 years of operation is expected for the ILC target of the undulator-based positron source. More details, see section Target and capture issues.

The undulator-based positron source does offer some margin on the damping rings aceptance; with the conventional source (dark line marks the effciency needed to fulfill the yield requirement of 1.5), it will be much more difficult to ensure sufficient bunch charge within the damping rings acceptance, see EUROTeV-report.

Furthermore, in the event that there are unexpected limitations on the damping rings acceptance, the undulator-based source offers the flexibility to improve the capture efficiency simply by lengthening the undulator. For further details, see the sections on Target and capture issues and Undulator features.

To provide high flexibility (for instance in the choice of bunch charge and bunch spacing), it is desirable that the fill of the positron damping ring is `self-reproducing': each newly created positron bunch replaces the positron bunch that collides with the electron bunch that created the new positron bunch).

Timing constraints under general conditions have been studied in Basic Timing Requirements for TESLA and Bunch Timing Aspects for the ILC. A detailed analysis for the current ILC baseline design (including two IR's with different crossing angles) can be read in Satisfying Timing Constraints. Serving two IR's with different longitudinal separation may require the insertion of additional delay lines. In the baseline design a scheme with 1.2 km extra tunnel, including a 150 m trombone system, is foreseen.

Fills that are not `self-reproducing' could also be used, but lead to a loss of flexibility since not all fill patterns might then be possible. They would require only a few 100 m additional tunnel.

Providing that the timing constraints that come with an undulator-based source are kept in mind, no substantial problems with timing issues are expected.

Although the Compton scheme is still in an initial stage, it is very attractive due to the independency of both beams. The scheme provides also polarized positrons.

More details of the design and the R&D status concerning the required high power multi-bunch laser and the positron stacking in the damping ring are described in the section laser-compton design.