BCD -- Design of the ILC Positron Source

The undulator-based positron source was recommended by the ILC sources working group because it was regarded as the most matured design that offers the lowest risk in meeting the baseline ILC specifications. More details are given in this talk.

Baseline design: undulator-based source

BCD Baseline Source In the ILC Baseline Configuration Document (BCD) the undulator-based positron source has been chosen as baseline design. A short description and the complete description of the BCD positron source is linked; the layout can be seen on the right picture and is explained in more detail in this talk.

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 concept of generating polarized positrons via radiation in an helical undulator has been tested in the experiment E166 at SLAC, more details can be found in the section Undulator prototypes: E166 at SLAC.
Prototypes of helical undulators for ILC beam requirements are also currently under construction at the Rutherford Appleton and Daresbury laboratories, U.K., see more details also in the sections Undulator prototypes: ILC undulator/UK and Undulator prototypes: Undulator at Cornell

Auxiliary positron source -- conventional source

Parameters of the auxiliary source In the Positron Source White Paper, it has been agreed that a keep-alive source of 10% nominal intensity should be sufficient to meet all needed requirements. However, the absolute need for such a source, in addition to a second ``hot spare'' positron conversion target, see plot above, is still debatable, see availability studies.

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.

Target considerations

Parameters of the auxiliary source A critical topic for all three kinds of sources are target issues: the lifetime and the radiation level and activation. This has been studied in detail for two undulator source layouts ( electron beam with 150 and 250 GeV) and compared with expectations for a conventional target. For the undulator-based source a target of 0.4 radiation length titanium alloy, rotating with a speed of 100 m/s has been chosen for the baseline design, more details see in this talk.

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.

Capture issues

Capture efficiency of undulator, EUROTeV-report To reach high luminosity, it is necessary not only to generate sufficient numbers of positrons from the source, but also to ensure efficient capture of these positrons by the damping rings. Presently, the transverse acceptance of the damping rings is specified as A_x+A_y<0.09 m-rad (definition of parameters), and (just as important) the energy acceptance is specified as 1% full width. Both specifications are important.

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.

Timing constraints

Due to the link between the electron beam and the generation of the positrons, both beams can not be treated fully independently from each other: since the positrons are produced by undulator photons which are produced by electrons within previous machine pulses, special issues of timing controls have to be taken into account when discussing, for instance, the damping ring circumference, location of the two IR's (if some longitudinal separation) and beam line path lengths.

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.

Alternative design: Compton-based scheme

Laser-compton sources at a LC In the ILC Baseline Configuration Document the laser-compton scheme has been chosen as alternative design. A description of the proposed design in the BCD is linked. More details are given in this talk.

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.

The concept of generating polarized positrons via laser-compton backscattering has been tested at the KEK-ATF, more details can be found in the section Prototype facilities.



Gudi Moortgat-Pick
Last modified: 6-September-2006