Abstract
Future light sources such as synchrotron radiation sources driven by an energy recovery linac, free electron lasers, or THz radiation sources have in common that they require injectors, which provide high-brilliance, high-current electron beams in almost continuous operation. Thus, the development of appropriate highly brilliant electron sources is of key importance. With its superconducting radio-frequency photo-injector (SRF gun) the Helmholtz-Zentrum Dresden-Rossendorf provided a promising approach for this key component, which has since been adopted in other laboratories. Nevertheless, some limitations occur caused by electron multipacting, which should be suppressed in order to further improve the gun. In this contribution, we present a detailed analysis of multipacting in the critical area of the SRF gun and different suppression techniques for it. The analytical predictions on the threshold for multipacting are qualitatively comparable with numerical simulation results and experimental data. Finally, we present specific surface structuring as an effective method to mitigate the multipacting phenomenon from the photocathode channel.
Highlights
The evolution of theories on the multipactor effect and several experimental developments are reviewed by Kishek [1] and Chang [2] in detail
The transit time of the electrons for the two-sided multipactor should be an odd integer number (2n − 1) of half rf periods [7,9,1,13,14] whereas the transit time for one-point MP is mostly determined by the electron initial longitudinal velocity and the normal electric field [1,15,16,17] The other criterion is that the secondary emission yield (SEY) of the impacted surface material is larger than 1
Since our studies revealed that a grooved structure is a promising suppression technique for the superconducting radio frequency (SRF) gun, we optimized the model
Summary
The evolution of theories on the multipactor effect and several experimental developments are reviewed by Kishek [1] and Chang [2] in detail. In the 1930s and 1940s, MP was analysed theoretically as well as experimentally for a flat gap This led Gill and von Engel to introduce a parameter k equal to the ratio of the impact velocity of the primaries to the emission velocity of secondaries that they assumed to be constant [5]. Hatch and Williams reformulated this assumption based on other theories and their own experimental and theoretical findings, which allowed the establishment of susceptible zones for the fundamental and higher order MP modes in the 1950s [6,7]. The phase stability for the case of non-zero starting energy is introduced [8,10] The latter theory claims that the negative starting phase substantially widens the MP zone width.
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