Design Considerations for the Stability Improvement of Klystron-Modulator for PAL XFEL

The PAL (Pohang Accelerator Laboratory) 2.5-GeV linac is planed to be converted to a SASE-XFEL facility (PAL XFEL) that supplies coherent X-rays down to 0.3-nm wavelength. The PAL XEL requires a new 1.2-GeV linac that will be combined to the existing linac to increase a beam energy upto 3.7-GeV. This XFEL linac should supply highly bright beams with emittance of 1.2 mm-mrad, a peak current of 3.5 kA, and a low energy spread of 0.03%. The RF stability of 0.02% is required for both RF phase and amplitude to get stable SASE output. This stability is mainly determined by a low level RF drive system and klystron-modulators. The stability level of the modulator has to be improved 10 times better to meet the pulse stability of 0.02%. The regulation methods such as traditional de-Q'ing and precision inverter charging technology are reviewed to find out suitable upgrade shceme of the modulators. Design considerations for the stability improvement of klystron-modulators for PAL XFEL are presented.