## 2 Flash Free-Electron Laser Facility
### Flash Linac  
The available photon parameters are based on the extremely high quality electron bunches produced by the FLASH linac whose main parameters are described in Table 2. The FLASH linac is based on superconducting L-band (1.3 GHz) 9-cell cavities originally developed for the TESLA linear collider project [58] and nowadays also used as the backbone of the European XFEL [59]. It is thanks to the high (RF-) duty factor achievable with superconducting technology, that FLASH provides up to 800 \(\upmu\)s long RF flat tops and thus is capable of accelerating thousands of electron bunches per second in 10 Hz burst mode. For user experiments, FLASH provides up to 5000 photon pulses per second. The electron bunches are generated at a normal-conducting 1.6 cell L-band photo-cathode RF-gun. The cathodes are Cs\({}_{2}\)Te films on a Molybdenum carrier plug and are excited by a laser system capable of providing UV pulses with a duration selectable from 1 ps to 6.5 ps (rms). The nominal bunch charge is in the range 300-400 pC depending on experimental demands, but it can be varied from 20 pC to 1.2 nC to accommodate special requirements. The electron beam energy at the exit of the gun cavity is 5.6 MeV with an uncorrelated slice energy spread of about 3 keV. The use of a solenoid close to the gun cavity allows compensating for the linear emittance blow up and to generate bunches with normalized transverse emittances of typically 1 mm mrad at 1 nC and 0.4-0.6 mm mrad at 400 pC [60]. In standard operation, the first accelerating module brings the beam to 165 MeV with a moderately negative energy chirp required for bunch compression. Immediately upstream, a third-harmonic (3.9 GHz) module decelerates the beam by 19 MeV. The relative phases of both modules are adjusted to minimize non-linearities in the longitudinal phase space and optimize the compression. Electron beam compression is done with two bunch compressors. First compression occurs at 146 MeV on a magnetic D-chicane with varying momentum compaction (R\({}_{56}\) = 122-255 mm) set to 142 mm and compressing the beam by a factor 4 to 5. Second compression occurs after the next two accelerating modules at 450 MeV, with further optimization of the energy chirp on the two accelerating modules upstream a \begin{table}
\begin{tabular}{c c c c} \hline \hline
**FEL Parameter** & **Units** & **FLASH1** & **FLASH2** \\ \hline Wavelength & nm & 4.2–51 & 4.0–90 \\ Average single pulse energy & \(\upmu\)J & 1–500 & 1–1000 \\ Pulse duration (fwhm) & fs & \(<\)30–200 & \(<\)30–200 \\ Spectral width (fwhm) & \% & 0.7–2 & 0.5–2 \\ Peak power & GW & 1–5 & 1–5 \\ Photons per pulse & photons & 10\({}^{11}\)–10\({}^{14}\) & 10\({}^{11}\)–10\({}^{14}\) \\ Peak brilliance & photons/(mm rad\({}^{2}\) mm\({}^{2}\) 0.1\% bw) & 10\({}^{28}\)–10\({}^{31}\) & 10\({}^{28}\)–10\({}^{31}\) \\ Average brilliance & photons/(mm rad\({}^{2}\) mm\({}^{2}\) 0.1\% bw) & 10\({}^{17}\)–10\({}^{21}\) & 10\({}^{17}\)–10\({}^{21}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: FLASH FEL radiation parameters available as of 2021. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Electron Beam Parameter** & **Units** & **Current Range** \\ \hline Beam energy & MeV & 380–1250 \\ Bunch charge & nC & 0.02–1.2 \\ Peak current & kA & 0.6–2.5 \\ Slice energy spread & keV & 100 \\ Slice norm. emittance & mm rad & 0.4–1.2 \\ Bunches per train & & 1–500 \\ Bunch spacing & \(\upmu\)s & 1–25 \\ Repetition rate & Hz & 10 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: FLASH electron beam nominal parameters at the undulator in 2021. 6-dipole S-chicane with large momentum compaction flexibility (R\({}_{56}\) = 7-122 mm) set to 75 mm for reaching the final compression of a factor 8 to 10 required for reaching the kA peak current level. Electron beam matching is critical for the final FEL operation and is done downstream of the first bunch compressor using a FODO channel with screens to measure optical mismatch and the injector emittance.