Abstract
The free-electron laser facilities driven by a superconducting radio-frequency (SRF) linac provide high-repetition-rate electron beam, which makes it feasible to feed multiple undulator lines at the same time. In this paper, we study a method of controlling the energy of multiple electron bunches by off-frequency detuning in the SRF linac. Based on the theoretical analysis, we present the available discrete frequency detunes and the optimal linac energy allocation solutions with given energy overhead and periodic energy pattern. The required off-frequency detune is not larger than half of the beam repetition rate, which can be realized with the frequency tuner in SRF cavities. We adopt the configuration of the high energy upgrade of the Linac Coherent Light Source II (LCLS-II-HE) as an example to discuss the possible schemes in practice to support two undulator lines simultaneously. We also discuss the energy jitter increase due to the off-crest acceleration, the tunable energy range with fixed total SRF linac energy capacity, the beam energy chirp and lattice design for multienergy beam transport.
Highlights
The successful operation of several hard x-ray freeelectron laser (XFEL) facilities [1,2,3,4] over the world represents a revolution in the development of light source, which leads to a substantial improvement in the peak brightness by a remarkable 8-10 orders of magnitude [5,6,7] and opens up vast opportunities for atom and molecule dynamics research at femtosecond scale [8,9]
We study a new scheme for multiple-energy operation in a superconducting radio-frequency (SRF) linac based on an off-frequency detune method
In the off-frequency detune method, electron beams are accelerated at off-crest phases, where the beam energy is more sensitive to the phase jitter compared with the oncrest acceleration
Summary
The successful operation of several hard x-ray freeelectron laser (XFEL) facilities [1,2,3,4] over the world represents a revolution in the development of light source, which leads to a substantial improvement in the peak brightness by a remarkable 8-10 orders of magnitude [5,6,7] and opens up vast opportunities for atom and molecule dynamics research at femtosecond scale [8,9]. To support multiple beamlines and deliver xrays with larger energy range, a few schemes have been adopted such as using a kicker and bypass line [3,4,11] and using subharmonic trigger in radio-frequency units [12] These operation modes further reduces the average brightness due to low repetition rate. The beam extraction system for MHz GeV electron beams requires MHz-triggered submicrosecond highvoltage kickers and strong septum magnets, which make it lengthy and costly to add the extraction bypass beamline to the SRF linac There is another scheme for burst-mode superconducting linac based on energy change of bunch trains inside one macropulse, which has been utilized in FLASH2 to support two undulator lines simultaneously [13].
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