  The full exploitation of the physics potential of a future Linear Collider
requires the development of polarised positron beams. A very promising scheme
for the technical realisation is the use of helical undulators, generating
circular polarised photons of several MeV which are then converted in a thin
target to longitudinally polarised positrons. The experiment E-166 tests this
scheme. It uses the low-emittance 50-GeV electron beam at the Final Focus Test
Beam (FFTB) at SLAC, passing through a 1 meter-long helical undulator. The flux
and polarisation of the undulator photons as well as the properties of the
positrons will be measured and will be compared with simulations.
