Abstract
Energy recovery linacs provide high-energy beams, but decelerate those beams before dumping them, so that their energy is available for the acceleration of new particles. During this deceleration, any relative energy spread that is created at high energy is amplified by the ratio between high energy and dump energy. Therefore, energy recovery linacs are sensitive to energy spread acquired at high energy, e.g., from wakefields. One can compensate the time-correlated energy spread due to wakes via energy-dependent time-of-flight terms in appropriate sections of an energy recovery linac, and via high-frequency cavities. We show that nonlinear time-of-flight terms can only eliminate odd orders in the correlation between time and energy, if these terms are created by a beam transport within the linac that is common for accelerating and decelerating beams. If these two beams are separated, so that different beam transport sections can be used to produce time-of-flight terms suitable for each, also even-order terms in the energy spread can be eliminated. As an example, we investigate the potential of using this method for the Cornell x-ray energy recovery linac. Via quadratic time-of-flight terms, the energy spread can be reduced by 66%. Alternatively, since the energy spread from the dominantly resistive wakefields of the analyzed accelerator is approximately harmonic in time, a high-frequency cavity could diminish the energy spread by 81%. Because both methods of reducing energy spread require separate sections for the two beams within the linac, such sections have been included in Cornell’s x-ray energy recovery linac design.
Highlights
Energy recovery linacs (ERLs) accelerate high-current particle beams to high energy in a linac
We have investigated the potential of using time-offlight terms in ERL loops to reduce wake-driven correlated energy spread
One could use the time of flight in the ERL return pass that contains the x-ray sources, but this method requires undesirable correlated energy spread in this pass
Summary
Energy recovery linacs (ERLs) accelerate high-current particle beams to high energy in a linac. These are used in x-ray [1,2], FEL [3], or nuclear physics [4,5]. The energy spread at high velocity is relatively small, but relevant because it limits the bandwidth of the x-ray radiation It is multiplied during deceleration by the ratio between high energy and dump energy, which is approximately 500 in the case of the Cornell ERL. While energy recovery of EA in linac A demands that the bunch center return to linac A after an odd multiple of half the rf period, a possible deviation tCE is included above. The second half of the accumulated effect of wakefields is added:
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