DESIGN OF NEW BUNCHER CAVITY FOR RELATIVISTIC ELECTRON GUN FOR ATOMIC EXPLORATION – REGAE M. Fakhari ∗ , H. Delsim-Hashemi, K. Floettmann, M. Huening, S. Pfeiffer, H. Schlarb, DESY, Hamburg, Germany J. Rossbach, Uni HH, Hamburg, Germany Abstract The Relativistic Electron Gun for Atomic Exploration, REGAE, is a small electron accelerator build and operated at DESY. Its main application is to provide high quality electron bunches for time resolved diffraction experiments. The RF system of REGAE contains different parts such as low level RF, preamplifier, modulator, phase shifter, and cavities. A photocathode gun cavity to produce the electrons and a buncher cavity to compress the electron bunches in the following drift tube. Since the difference between the operating mode of the existing buncher and its adjacent mode is too small, the input power excites the other modes in addition to the operating mode which affects the beam parameters. A new buncher cavity is designed in order to improve the mode separation. Furthermore the whole cavity is modeled by a circuit which can be useful especially during the tuning process. Beam dynamics simulations have been performed in order to compare the new designed cavity with the old one which declare that the effects of the adjacent modes on the beam parameters are decreased. INTRODUCTION The REGAE buncher is formed from four coupled pill- box cavities and so it has four fundamental TM010 normal modes which are named as 0-Mode, π 3 -mode, 2π 3 -mode, and π-mode [1]. The measurement and simulation results for the frequencies of these four TM010 normal modes with Microwave Studio (MWS) and Superfish are shown in Table 1 [2]. According to this table the difference between the π- mode which is the operating mode and its adjacent 2π 3 -mode is only 2 MHz. This very close mode might affect on the REGAE operation and its stability. In fact the input power excites the other normal modes of the buncher in addition to the π-mode that leads to some problems in low level RF operation of the system which might cause unwanted effects on the beam that is going to be bunched. Fig. 1 for exam- ple shows the amplitude of the RF power in the buncher in which some fluctuations are added to the main signal. The frequency of these fluctuations is exactly equal to the differ- ence between the π-mode and the 2π 3 -mode and the origin of this parasitic signal is the poor mode separation in the buncher [2]. The main goal in designing the new buncher is to improve the mode separation in order to remove or at least reduce the effects of other modes on the buncher operation as much as possible. ∗ moein.fakhari@desy.de Table 1: Measured and Simulated Frequencies of the Buncher 0-Mode π 3 -mode 2π 3 -mode π-mode MWS 2983 2988 2993 2995 Superfish 2984.2 2989.4 2994.6 2996.8 Measured 2985 2991 2996 2998 Figure 1: The RF power amplitude in the REGAE buncher. DESIGN PROCESS AND SIMULATION RESULTS In order to improve the mode separation in the buncher one should increase the coupling between adjacent cells which is strongly dependent on the geometry of the cavities such as radius and thickness of the disks between them. It is possible to achieve a better mode separation by changing the physical parameters of the buncher. Parameters that have been investigated are depicted in Fig. 2. In this figure rA, rB, and rC are representing half of the thicknesses of the disks between the cells while yA, yB, and yC represent the radii of these disks openings. Furthermore yD, rD and yE, rE determine the corresponding parameters related to the input and output cells respectively. Figure 2: The physical parameters of the buncher. To find the effects of each parameter it is varied between reasonable end values while the other parameters are kept constant. After simulation and finding the frequencies of the buncher normal modes, the dependency curves of these frequencies on the buncher parameters are drawn. As an example Fig. 3 shows the dependency of δ f 3 on the men- tioned physical parameters, where δ f 3 is difference between the π-mode and the 2π 3 -mode. Similar curves are extracted Modes in MHz 5th International Particle Accelerator Conference IPAC2014, Dresden, Germany JACoW Publishing ISBN: 978-3-95450-132-8 doi:10.18429/JACoW-IPAC2014-THPRI033 THPRI033 3840 Content from this work may be used under the terms of the CC BY 3.0 licence (© 2014). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI. 07 Accelerator Technology Main Systems T06 Room Temperature RF