Abstract
X-ray free electron laser (XFEL) facility based on electron linear accelerator (LINAC) is regarded as one kind of the fourth-generation light source with the characteristics of high intensity, exceptional brightness, ultrashort pulse duration, and spatial coherence. In electron linear accelerator, energy of beam bunches is provided by high-power electromagnetic microwaves which are generated by a microwave tube called klystron. The stability of beam voltage of klystron occupies a key position in both the stability of output RF (Radio Frequency) power and the jitter of output RF phase, furthermore, it plays an extremely important role in beam energy stability of electron linear accelerator. In this paper, high power RF fluctuation and phase jitter of klystron output caused by beam voltage instability of klystron are analyzed and calculated. Influence of klystron beam voltage instability on beam energy gain in linear accelerator have also been further analyzed and calculated. The calculating procedure is particularly valuable for us to understand the relationship between pulse modulator stability and beam energy gain fluctuations. Finally, relevant experimental results measured by Shanghai Soft X-ray Free Electron Laser Test Facility (SXFEL-TF) is presented.
Highlights
X-ray free electron laser (XFEL) [1, 2] is considered to be the generation light source, which involves potentially higher brilliance, a higher degree of coherence, and ultrashort pulses [3]
Shanghai Soft X-ray Free Electron Laser Test Facility is the first coherent X-ray light source in China based on high-gain harmonic generation (HGHG) and echo-enabled harmonic generation (EEHG) scheme
According to the klystron principle [27], Electron beams generated by heated cathode get kinetic energy by highpower voltage of pulse modulator and modulated by a low-level Radio Frequency (RF) signal coupled into input cavity
Summary
X-ray free electron laser (XFEL) [1, 2] is considered to be the generation light source, which involves potentially higher brilliance, a higher degree of coherence, and ultrashort pulses [3]. Shanghai Soft X-ray Free Electron Laser Test Facility is the first coherent X-ray light source in China based on high-gain harmonic generation (HGHG) and echo-enabled harmonic generation (EEHG) scheme. The microwave system of C-band accelerator structure used a 110 MW pulse modulator and a 50 MW klystron. The first one is the stability of pulse modulator’s high voltage, which will lead to fluctuation of microwave amplitude of klystron RF output [13]. The second one is the imperceptible change of pulse modulator’s voltage, which will bring about tolerance of microwave phase of klystron RF output [14] It is the pulse-to-pulse instability of modulator that will cause the high-power microwave amplitude fluctuation and phase tolerance of klystron, and cause the change of beam energy gain [15]. At the end we found a special phase point, where the beam energy gain is insensitive to the modulator voltage fluctuation
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