Field Enhancement of a Superconducting Helical Undulator with Iron

The productions of positrons in sufficient quantities is one of the necessities for either the TESLA or the S-Band Linear Collider project. One of the promising possibilities is to guide the high energy electron beam through a superconducting helical undulator producing synchrotron radiation which would in turn be directed onto a target for positron production [1]. A helical undulator has two advantages as compared to a planar wiggler: The energy of the primary electron beam can be lower, since high photon energies can be reached with a short period, high field device. While a source with a planar wiggler can operate only with a minimum beam energy of at least 150GeV, the operation of a source with a helical undulator seems to be possible down to an energy of ∼100-120GeV. In addition the photons of a helical undulator close to the radiation axis are circularly polarized. Highly polarized positrons can be generated if the outer part of the photon beam is scraped off. Since this requires an overproduction of photons it can in general only be realized at higher energies of the primary electron beam. Thus the same helical undulator could be used for the production of positrons with a low polarization (∼30 %) at lower beam energy (∼120GeV) and for highly polarized positrons (∼60%) at high beam energies ( ≥ 250GeV). Primary calculations (based on an analytical formula for the field of an iron free helical undulator) yield the following undulator parameters[2]: minimum electron beam energy ∼150 GeV period λ = 1.2 cm magnetic field B = 0.9 T total undulator length L = 150 m