Beam loading compensation for the low frequency linacs and compressors in the NLC

The low frequency rf linacs and bunch compressors in the NLC are heavily loaded by beam currents up to 0.9 A. Beam loading generates bunch-to-bunch energy deviations, which need to be compensated to operate with long bunch trains in the linear collider. There are two basic compensation techniques, namely ΔT and ΔF schemes. The ΔF scheme is operationally straightforward but requires additional rf systems which are powered at slightly different frequencies. In contrast, the ΔT scheme does not require any additional rf and is thus expected to be less expensive. Here, the loading compensation is realized by early injection combined with rf phase and amplitude modulation. The phase/amplitude modulation is performed with fast phase shifters at low rf power before amplification by the klystrons. With combined phase and amplitude modulation, the ΔT scheme can be applied to both the accelerator and compressor linacs. In this paper, we present simulations of the ΔT compensation scheme for the NLC low rf accelerator and compressor linacs. We discuss different rf configurations using the ΔT scheme as well as some issues related to phase/amplitude modulations in such layouts