Design Studies of Positron Collection for the NLC(LCC-0070)

Abstract

The positron source for the NLC project utilizes a 6.2 GeV electron beam interacting in a high-Z positron production target. The electromagnetic shower in the target results in large energy deposition which can cause damage to the target. Optimization of the collection system is required to insure long-term operation of the target with needed high positron yield into the 6-dimensional acceptance of the subsequent pre-damping ring. Positron tracking through the accelerating system indicates a dilution of the initial positron phase space density. Results of simulations indicate that a positron yield of 0.76 can be obtained at the injection into pre-damping ring if the beam is limited by the normalized emittances of {var_epsilon}{sub x}, {var_epsilon}{sub y} {le} 0.03 {pi} m rad, bunch length {sigma}{sub z} = 6.97 mm and energy spread {Delta}E/E = 2%. The value of positron yield can be increased by 1.15 if longitudinal focusing is utilized in 1.9 GeV booster linac. In the regime of polarized positrons injection, the polarization of 0.65 can be obtained for 6.5% of initial polarized positrons emerged from the target. The number of captured polarized positrons is increased by 9.6% keeping the value of polarization as 0.63 if longitudinal focusing in the booster linac is used.