New method for positron production at SLAC

The focusing system for the positron beam at SLAC makes use of an adiabatically tapered solenoid whose transverse acceptance remains high over a broad energy band. From the computed distribution in energy of the accepted positrons, one can estimate that approximately one-half of the total number of positrons lies within the energy band from 2 - 4 MeV. Due to the debunching effect over the drift space following the target, only the high energy part (from 4.5 MeV to 10 MeV or more) of the accepted spectrum contributes to the useful current (within 1% energy bin). By decelerating the beam in a special short section very near the converter it is possible to obtain a bunching of all the accepted positrons with energy above 2 MeV within approximately 50, giving an improvement of a factor of two on the analyzed current