## 2 Linac  
The PAL-XFEL LINAC is divided into four acceleration sections (L1, L2, L3, and L4), three bunch compressors (BC1, BC2, BC3), and a dogleg transport line to the undulators, as shown in Figure 2. The L1 section consists of two RF stations, where both are comprised of one klystron and two S-band structures, while L2 has 10, L3 has four, and L4 has 27 RF stations where each station has one klystron, four accelerating structures, and one energy doubler. A laser heater to mitigate micro-bunching instability is placed right after the injector, and an X-band cavity for linearization is placed right before the BC1. The major parameters of PAL-XFEL are summarized in Table 2.  
\begin{table}
\begin{tabular}{l l} \hline \hline
**LINAC** & \\ \hline FEL radiation wavelength & 0.1 nm (Hard X-ray)/1 nm (Soft X-ray) \\ Electron energy & 10 GeV \\ Slice emittance & 0.5 mm-mrad \\ Beam charge & 0.2 nC \\ Peak current at undulator & 3.0 kA \\ Pulse repetition rate & 60 Hz \\ Electron source & Photo-cathode RF-gun \\ LINAC structure & 5-band normal conducting \\ \hline
**Undulator** & \\ \hline Type & out-vacuum, variable gap \\ Length & 5 m \\ Undulator period & 26 mm (HX)/35 mm (SX) \\ Undulator min. Gap & 8.3 mm (HX)/9.0 mm (SX) \\ K value & 1.9727 (HX)/3.3209 (SX) \\ Peak B (in Tesla) & 0.8124 (HX)/1.0159 (SX) \\ Vacuum chamber dimension & 13.4 \(\times\) 6.7 mm\({}^{2}\) \\ \hline \hline \end{tabular}
\end{table}
Table 2: Major parameters of PAL-XFEL.  
Figure 2: Schematic layout of PAL-XFEL.  
The total length of the LINAC tunnel is about 710 m. There are 176 S-band accelerating structures and 42 energy doublers. The major high power devices of the 10-GeV linear accelerator are the modulators, the klystrons, the energy doublers (ED), and the accelerating structures (AS). An energy doubler increases the peak power of the RF pulse by reducing the RF pulse length to increase the energy gain at the accelerating structure. The energy doubler was designed by PAL and fabricated by a domestic company. There are 50 modulators for the S-band klystrons, and there is one modulator for the X-band klystrons that is used for linearizing the electron beam. The LINAC requires 46 S-band klystrons to obtain an electron beam energy of 10 GeV. One S-band klystron is dedicated for the RF gun, and three RF stations are designated for deflectors to measure the electron bunch length.  
The klystron requires an RF drive signal at the level of a few hundred watts. A low-level radio frequency (LLRF) and a solid-state amplifier (SSA) are necessary to supply the drive signal to a klystron. To achieve beam energy stability of below 0.02% and an arrival time jitter of 20-fs for PAL-XFEL, the LINAC RF parameters should be as stable as 0.03 degrees for the RF phase and 0.02% for the RF amplitude for S-band RF systems, and 0.1 degree/0.04% for the X-band linearizer RF system. The pulse-to-pulse klystron RF stability is determined by the klystron beam voltage driven by a modulator. Therefore, the klystron modulator beam voltage should be as stable as 50 ppm for the 0.03-degree S-band RF and 0.1-degree X-band RF [3].  
An LLRF system consists of an SSA, a phase and amplitude detector (PAD), and a phase and amplitude control (PAC) unit. The function of the PAC is to control the phase and amplitude of the RF drive signal to a klystron, to provide a pulsed RF signal, and to reverse the RF phase of the klystron drive signal in the middle of the pulse by turning on the Phase Shift Key (PSK) 180-degree phase shifter). The RF pulse length of the drive signal is 4 \(\upmu\)s and the PSK is on after 3.17 \(\upmu\)s from the starting time of the pulse. The PAL-XFEL LINAC tunnel and the klystron gallery are shown in Figures 3 and 4, respectively.  
Figure 3: LINAC tunnel of PAL-XFEL.