Investigation of the Effect of Space Charge in the compact-Energy Recovery Linac Ji-Gwang Hwang and Eun-San Kim * , Kyungpook National University. 1370 Sankyok-dong, Buk-ku, Daegu, 702-701, Korea Tsukasa Miyajima, KEK, Tsukuba, Ibaraki 305-0801, Japan Abstract Compact energy recover linear(ERL) accelerator is a prototype of the 5 GeV ERL accelerator at KEK. The injec- tor system has two SRF cavities which have the frequency of 1.3 GHz. It accelerates the bunches to the energy of 5 MeV. This beam was injected to the main ring and then it was accelerated to energy of 35 MeV at the main super- conducting RF linac. Due to the low beam energy on the main ring, the investigation of the effect of space charge (SC) which causes the growth of the energy spread is im- portant to produce the low emittance beam. For the pro- duction of the low emittance beam, the optimization of the merger was performed. To obtain smaller emittance at the exit of merger, the effect of the energy spread was also in- vestigated by changing of the k d which is defined by the ratio of energy spread to length of the bunch. In this calcu- lation, we got the noralized transverse emittance of 0.735 mm·mrad at the exit of merger section. INTRODUCTION The Energy Recovery Linear accelerator (ERL) is one of the candidates for the fourth generation light sources that can meet these requirements. The main feature of the ERL is production of low-emittance( pm) beam with energy re- covery in the main linac. The ERL requires sophisticated technology of superconducting accelerator. The generation of ultra-low emittance beams is need to demonstrate before constructing Multi-GeV ERL. The compact-ERL at KEK, in the final stage, will provide a beam energy of around 125 MeV and a bunch charge of 77 pC, which is a prototype for the future 5 GeV ERL at KEK. The layout of the compact- ERL is shown in Fig 1. The c-ERL consists of an injec- tor system, a merger section, a superconducting RF (SRF) section, two return loops and two straight sections[3]. In the early comissioning phase of the compact-ERL, the en- ergy is 35 MeV with a bunch charge of 7.7 pC. The elec- tron injector system consists of a 500 kV photo cathode DC gun, two solenoid magnets, a buncher cavity, three super- conducting RF cavities, seven quadrupole magnets and a merger section. In the second comissioning phase, the in- jector produces electron beams with a bunch charge of 77 pC, beam energy of 5 MeV and bunch length of 0.6 mm rms. The beam energy is increased by 30 MeV with two 9 cell SRF cavities. Since the beam energy in c-ERL is a * eskim1@knu.ac.kr low with high charge, we need to consider the several ef- fects, e.g., the space charge effect, the coherent synchrotron radiation (CSR) effect, the wake function, ion effects and beam break up[4]. In the case of low energy, the elec- tric force which caused the growth of the energy spread is more stronger than the magnetic force. It called SC effect. The emittance growth due to the space charge (SC) effect is dominated for the case of low-energy, around 5 MeV [5], and causes growth of the energy spread. The energy spread induced in an achromatic cell results in the growth of pro- jection emittance at the exit of the achromatic cell. It is known that this emittance growth can be compensated by setting the cell-to-cell betatron phase advance at an appro- priate value[6]. Figure 1: Layout of a compact-ERL. ENERGY SPREAD GROWTH DUE TO THE SC EFFECT The low energy beam injected from the injector system merges with the circulating high energy beams. For the beam mergence, after passing the merger section, the ra- tio of circulating energy to injected energy should be large because the circulating beam is also kicked and needs to be bumped at the merger section. A merger section with 3-dipole was adopted for the flexible beam transport of the high energy circulating beam. The layout of the 3-dipoles merger is shown in Fig. 2. Figure 2: Layout of a merger section.