Flux Concentrator Optimization for Future Positron Sources

Future lepton colliders, such as CLIC, FCC-ee or ILC require high quality positron sources. Positrons are created after the collision of electrons on a tungsten target and then focused to match the aperture of the capture structures with a flux concentrator (FC) often called an Adiabatic Matching Device (AMD), a strong pulsed magnet. The FC is a tapered solenoid powered with fast pulses (microsecond) of high current (kA). The current pulse produces a strong magnetic field (3-8 T) at the magnet entrance that rapidly decays (over few cm) to zero. This paper describes the finite element model of the FC and the transient electromagnetic simulation capable of describing the experimental data in terms of current, field and voltage. The computed field map is transferred into particle tracking codes to compute the figure of merit, the positron yield. The coil configuration is optimized to minimize the voltage and Lorentz forces and to maximize the yield. This paper presents the numerical model and the result of the optimization process.