S-BAND RF SYSTEM FOR 0.1 nm SASE FEL AT PAL*

Pohang Accelerator Laboratory, PAL, has been proposing a 0.1 nm SASE FEL. This machine will be designed with an S-band rf linear accelerator to produce a 10.053 GeV electron beam. The output power of a klystron is 80 MW with a pulse width of 4μs and a repetition rate of 30 Hz. The beam energy spread and rf phase stability are 0.037% (rms) and 0.1 (rms), respectively. The SASE FEL needs a modulator stability of 0.1% (rms). We have developed a modulator DeQing system for the existing modulator systems that are “line type modulator system”. We are also considering an inverter power supply to meet the required specification of the FEL machine. We are developing a phase amplitude detection system (PAD) and a phase amplitude control (PAC) system to obtain the required rf stability. This paper describes the rf system for the PAL XFEL (PxFEL). INTRODUCTION The PxFEL is a 4 generation light source to produce a coherent X-ray free electron laser. This machine is designed with an S-band rf linear accelerator to obtain a 10.053 GeV electron beam. The rf stability is a key issue to get stable beam for the PxFEL. As shown in Table 1, the rf design parameters for the XFEL need to have much better than those of present PLS Linac. In the PxFEL, the specifications of the beam energy spread and rf phase are 0.1 % (rms) and 0.1 (rms) respectively [1]. The rf system and klystron-modulator have to provide stable beam operation. The rf frequency, phase, and power are very important factors in the linac operation. A change in these factors influences the electron beam energy and the energy spread. The long-term drift caused by environmental condition can be corrected by the rf phase feedback system. The phase amplitude detection system (PAD) and phase amplitude control (PAC) system also improve the rf stability. Also, the beam voltage stability of the klystron is directly related to a PFN (pulse forming network) charging voltage of the modulator. Therefore, a good regulation of the PFN charging voltage is essential in the modulator. The short-term variations can be corrected by a stable modulator system. We have developed a deQing system to keep the modulator beam voltage stable by less than 0.01 % (rms). In addition, we have also developed a modulator with a constant current source using inverter power supply. This paper describes the microwave system and new modulator system considering the improvement of the rf stability for the PxFEL. Table 1: Design Parameters for the PxFEL Parameters PLS Linac PxFEL Beam Energy 2.5 GeV 10.053 GeV Energy Spread 0.6% 0.037% (rms) Phase Stability ±3.5 0.1 (rms) Amplitude Stability ±0.5% 0.1% (rms) MICROWAVE SYSTEM FOR PXFEL The S-band microwave system of PxFEL is divided into two parts as shown in Figure 1 and Figure 2. One is a drive system and the other is a waveguide system. The drive system consists of an RF signal source (2856MHz), MDL (main drive line), SSA (solid-state amplifier), PAD (phase and amplitude detector), and PAC (phase and amplitude control) units. The waveguide system in one module consists of a s-band klystron, a SLED, and two constant gradient accelerating sections. A short x-band rf section, operating at 11.424GHz, requires a modest power source to operate at 37 MV/m over a length of 0.6m to generate the needed 22 MV of the x-band rf [2]. Figure 1: Microwave system for the PxFEL Figure 2: High power microwave system for the PxFEL ___________________________________________ *Work supported by MOST and POSCO # hohwang@postech.ac.kr TUPC28 Proceedings of FEL2009, Liverpool, UK FEL Technology I : Accelerator