Nuclear Inst. and Methods in Physics Research, A 888 (2018) 250–256 Contents lists available at ScienceDirect Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima RF emittance in a low energy electron linear accelerator Sh. Sanaye Hajari *, S. Haghtalab, H. Shaker, M. Dayyani Kelisani Institute for Research in Fundamental Sciences (IPM), School of Particles and Accelerators, Tehran 193955531, Iran article info Keywords: Linear accelerator Electron beam Transverse beam dynamics RF emittance abstract Transverse beam dynamics of an 8 MeV low current (10 mA) S-band traveling wave electron linear accelerator has been studied and optimized. The main issue is to limit the beam emittance, mainly induced by the transverse RF forces. The linac is being constructed at Institute for Research in Fundamental Science (IPM), Tehran Iran Labeled as Iran’s First Linac, nearly all components of this accelerator are designed and constructed within the country. This paper discusses the RF coupler induced field asymmetry and the corresponding emittance at different focusing levels, introduces a detailed beam dynamics design of a solenoid focusing channel aiming to reduce the emittance growth and studies the solenoid misalignment tolerances. In addition it has been demonstrated that a prebuncher cavity with appropriate parameters can help improving the beam quality in the transverse plane. 1. Introduction The IPM Electron Linac is an 8 MeV (upgradable to 11 MeV) S- band traveling wave electron linear accelerator under development at the Institute for Research in Fundamental Science (IPM), Tehran, Iran. As the first practice in design and construction of particle accelerators at IPM, the linac is mainly regarded as a research project providing hands-on experience in accelerator science and technology. In an effort to establish the domestic accelerator knowledge, it has been decided to build the accelerator based on the available technologies in Iran. Therefore, nearly all components of this accelerator are designed and constructed within the country including the RF power amplifier system (klystron and modulator), RF cavities, magnets, and beam diagnostics systems. The linac could serve as an X-ray source or play an injector role for a larger facility. The project is meeting its final stages and the linac commissioning is due in a few months. The layout of the IPM Electron Linac is shown schematically in Fig. 1. A thermionic electron gun provides a beam with an energy and current up to 50 keV and 10 mA, respectively. The electrons are bunched through a traveling wave (TW) buncher and then accelerated in some constant impedance TW accelerating tubes. Beam dynamics and RF design of this buncher is described in [1,2] and the construction process in [3–6]. The buncher and the accelerating structures are connected together and fed with a 2 MW klystron at 2997.9 MHz frequency. The beam energy at the end of the 30 cm length buncher will be around 1.4 MeV. The accelerating structure is composed of two accelerating tubes of 60 cm length each. First and second tubes provide an energy * Corresponding author. E-mail address: sanayehajari@ipm.ir (Sh. Sanaye Hajari). gain of 3.2 MeV and 3.1 MeV, respectively. A third tube can be added if a higher energy is required. The buncher and the accelerating structures are embedded in a solenoidal magnetic field for the focusing. In this paper, this solenoidal field is referred as the main focusing channel. Two small solenoids between the gun and the main focusing channel serve as a matching cell. The matching cell ensures the appropriate beam parameters at the entrance of the main channel. A characteristic feature of this linac is its low gradient long TW buncher. Such a structure offers a notable bunching performance. A detailed discussion on the bunching system and the corresponding longitudinal beam dynamics can be found in [7]. The initial beam parameters are obtained by cst simulation of the electron gun. The cathode with a diameter of 8 mm is warmed up to around 1100 ◦ C. The beam parameters after the electron gun are listed in Table 1, for different gun voltages. It can be shown that such a beam with a current at mA level is emittance dominated. In other words, the behavior of the beam envelope is determined by its emittance rather than the space charge forces [8]. Even more, the beam dynamics simulations show that the space charge effect can be neglected in this problem and one obtains rather the same results turning on or off the space charge forces. In the transverse plane the main issue is to limit the emittance growth. The dominant emittance growth mechanism here is the one induced by the RF forces. Time dependent forces violating Liouville’s theorem allow for the emittance growth. The transverse RF force in an axisymmetric TW structure in which the principle wave is dominant can https://doi.org/10.1016/j.nima.2018.01.085 Received 28 October 2017; Received in revised form 20 January 2018; Accepted 25 January 2018 Available online 31 January 2018 0168-9002/© 2018 Elsevier B.V. All rights reserved.