Linac Design for Dalian Coherent Light Source*

Dalian Coherent Light Source (DCLS) will use a 300MeV LINAC to produce fully coherent photon pulses in the wavelength range between 150-50nm by high gain harmonic generation free electron laser (FEL) scheme [1]. Beam quality requirements for the linear accelerator (linac) are critical, including not only the beam brightness, but also the stability and the reliability. In this paper, optimization study is performed for the linac. Based on beam stability simulation in the longitudinal direction, the tolerant budget is formed for the short period jitter. For the transverse orbit error, beam orbit correction technique is implemented by beam dynamics simulations and the transverse jitter is also presented accordingly. Measurement method for the beam quality is also described in the paper. INTRODUCTION Dalian Coherent Light Source (DCLS) is a FEL user facility based on the principle of single-pass, high-gain harmonic generation scheme, which is located in northeast of China. According to the FEL physical design and corresponding tolerance requirement [2], as shown in table 1, this paper gives the linac design considerations and numerical investigations on the stability for DCLS. Beam measurement method and further upgrade considerations are also presented briefly in the end of the paper. Table 1: Beam Specification for DCLS Linac Parameter Average (Unit) Charge 0.5 (nC) Energy 300 (MeV) Energy Spread 0.1 (%) Beam length 1 (ps) Emittance 2~4 ( mrad) LINAC DESIGN AND TOLERANCE Photo-cathode injector is adopted for DCLS linac and one bunch compressor system is employed to generate the required peak current. Before the chicane, two S-band accelerating structures generate the required energy chirp for bunch compression and the following four S-band accelerating structures are adopted to get the required beam energy, while cancelling the energy spread. The overall layout for DCLS linac is shown in figure 1. Figure 1: Layout of DCLS linac. Linac Layout and Parameter Optimization Initial bunch length is optimized after the bunch charge is fixed to 0.5nC. For this kind of relatively high intensity current, space charge effect is dominated. The emittance dilution effect and bunch lengthening process are obvious shown in figure 2. The initial bunch length, 7 ps, is chosen as a balance between the conflicting requirements of the peak current and the needs to minimize the emittance. Figure 2: The emittance dilution effect and bunch lengthening process depending on the different initial bunch length. Next, the position and the strength of the magnetic chicane are optimized to the jitter effect by careful choice of the linac acceleration and compression parameters [3]. After this process, the beam energy for the bunch compressor is at 128MeV and the R56 is set to -50mm. Accordingly, the design parameters for DCLS linac are fixed as summarized in table 2. ____________________________________________ *Work supported by the Knowledge Innovation Program of Chinese Academy of Sciences. #[email protected] TUPEA042 Proceedings of IPAC2013, Shanghai, China ISBN 978-3-95450-122-9 1226 C op yr ig ht c ○ 20 13 by JA C oW — cc C re at iv e C om m on sA tt ri bu tio n 3. 0 (C C -B Y3. 0) 02 Synchrotron Light Sources and FELs A06 Free Electron Lasers Table 2: Beam Parameters along the Linac Beamline Beamline Eout (MeV) zout(m m) out(%)