Abstract:
Background The high-repetition-rate hard X-ray free electron laser (HXFEL) facility offers a dynamic diagnostic capability that is crucial for research fields such as national defense, energy industry, aerospace science, and beyond. To achieve the desired output photon energies of up to tens of keV, further reduction of the beam slice emittance is needed. At repetition rates on the order of hundreds of MHz, bunch-by-bunch high-precision measurement of beam emittance poses a key challenge.
Purpose To address this challenge, this paper proposes a non-integer-harmonic transverse deflecting cavity (TDC) as a solution. It can impart different deflection angles to individual bunches based on automatic phase slip, realizing bunch-by-bunch slice emittance measurement.
Methods The operating frequency of TDC are determined through constraint analysis and comparison of various frequency selecting method. A physical design of the deflecting cavity is obtained through CST simulation. Using the simulated electromagnetic field distribution, a self-developed program is developed to verify the bunch separation and modulation effects. Then a typical beamline is constructed to study the feasibility of slice emittance measurement using the non-integer-harmonic TDC.
Results Simulations revealed that the most balanced separation effect is achieved under operating frequency of 210 MHz. A typical resolution of better than 2 ps is achieved using the beam transport settings, enabling effective measurement of beam slice emittance.
Conclusions The non-integer-harmonic deflecting cavity system presented in this paper provides a feasible diagnostic tool for high-precision bunch-by-bunch slice emittance measurement at hundred-MHz-level repetition rates. It is expected to support China's research platforms for high-repetition-rate low-emittance photocathode injectors.
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