A combined ΔT+ΔF beam loading compensation scheme for power saving

The NLC low frequency (S and L band) rf linacs are heavily loaded by a beam of about 130 ns in macropulse length (90 bunches) and a current up to 2.75 Amps. Beam loading generates a large energy spread along the bunch train. For these linacs, the bunch train is much shorter than the filling time of these linac structures, and the energy of the beam drops approximately linearly with time during the pulse as a result of beam loading. For such a case, there are two natural choices for beam loading compensation, ΔT (which achieves compensation by injecting the beam before the accelerator structure is full) and ΔF (which compensates by having some accelerator sections at f₀±ΔF). There are, however, disadvantages with these methods: non-local compensation in the ΔF method (except using many short ΔF sections) and low efficiency in the ΔT method due to short filling time and amplitude modulation required for compensation. In this paper, we discuss a combined ΔT+ΔF method. In this scheme, the filling time of the structure is optimized for ΔT compensation for phase-I operation. In phase-II operation, ΔF sections are used to compensate the extra 50% beam loading due to higher current. Simulations have shown that up to 30% of power can be saved by using this method