## 4 Seeding Lasers
### Seed Laser Beamlines  
Optimal performance of the seeded FEL poses strict requirements to the seed laser properties at the interaction points with the electron bunches (see Table 7). This imposes the design of a beamline capable of preserving the high-quality laser pulses produced in \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Seed Laser 1** & **Seed Laser 2** & **Units** \\ \hline Wavelength & 343 & 297–317 & nm \\ Max. Pulse energy & 50 & 16 & uJ \\ Pulse length (FWHM) & \(\sim\)500 & 50 & fs \\ Spectral width & & & \\ Beam radius (1/e\({}^{2}\)) & 750 & 600 & uJ \\ at the modulator & & & \\ Rayleigh length & 3.4 & 4.0–3.7 & m \\ Beam quality & \(<\)1.5 & 1.5 & M\({}^{2}\) TEM00 \\ Pointing stability & 50 & 50 & uJ \\ at the modulator (rms) & & & \\ Polarization & Hor. & Hor. & \\ Focus position & Modulator 1 center & Modulator 2 center & \\ Temporal structure & Up to 600 pulses & Up to 600 pulses & \\  & at 1 MHz at 10 Hz & at 1 MHz at 10 Hz & \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Seed laser properties at the modulators. the laboratory till the interaction point and methods for constant monitoring and actively control of critical parameters is essential. After the generation of the seeding lasers in the laser laboratory, the two seed lasers, seed laser 1 and seed laser 2, will be transported to the interaction regions via dedicated laser beamlines. Seed laser 1 travels a distance of \(\sim\)22 m from the laser lab to the center of the first modulator, seed laser 2 a distance of \(\sim\)30 m to the center of the second modulator. In-coupling of both lasers to the electron beamline axis in the modulators will be done through injection chicanes (see Figures 3 and 6), allowing the electron beam to safely pass around the in-coupling mirrors. Given the tight constraints of the electron and laser beamline design the injection mirror needs to be placed in both cases into the accelerator ultra-high vacuum. To mitigate the effect of air turbulences on the seed laser spatial pointing and arrival time, but also to protect the sensitive beam transport optics against chemical substances responsible for reduction of reflectivity over time, the seed laser beams will be transported in vacuum. Based on experiences with transporting high-power UV pulses at the present Xseed experiment at FLASH [56], ultra-high vacuum is chosen instead of low vacuum pressure. Each of the two seed lasers is generated on one of the arms of the laser table in the laboratory (see Figure 6) and is transported via throughputs of the laser table into the raised floor where the vacuum chambers hosting the beam transport optics are placed. Traveling within the vacuum pipes, the two laser pulses pass through 2 m of radiation protection wall that separates the laser laboratory from the accelerator tunnel. The two laser beamlines continue in vacuum with dedicated vacuum chambers installed close to the tunnel floor to minimize vibrations till the two injection boards. A few of the mirrors in the laser beamline will be motorized allowing an active control of the laser pointing at the injection boards. Moreover, the design of the vacuum chamber and mirror holder is aiming at providing also diagnostic capabilities for on-line monitoring of laser pointing and other laser parameters along the beamline. At the two in-coupling boards, more diagnostics are placed to facilitate the optimization of the laser parameters (the pulse duration, phase front tilt, pulse energy, mode quality, burst structure, pointing,...) at the interaction points. A small fraction of the laser pulses or, depending on the type of measurement, the complete pulse is planed to be used for monitoring purposes. To improve the capabilities of controlling the optimal transverse superposition between the laser and the electron beam, the laser beamline design also includes additional diagnostics for the laser beams out-coupled from the electron beamline after the interaction point.