_Article_  
The FLASH Facility: Advanced Options for FLASH2  
and Future Perspectives  
[PERSON] 1, [PERSON] 1, [PERSON] 1, [PERSON] 1, [PERSON] 1,  
[PERSON] 1, [PERSON] 1, [PERSON] 1, [PERSON] 1,  
[PERSON] 1\({}^{\text{\textregistered}}\) \({}^{\text{\textregistered}}\) \({}^{\text{\textregistered}}\), [PERSON] 1, [PERSON] 1, [PERSON] 1, [PERSON] 1,  
[PERSON] 1,2,*, [PERSON] 1 and [PERSON] 1  
Deutsches Elektronen-Synchrotron (DESY), 22607 Hamburg, Germany; [PERSON]RESS] ([PERSON]); [PERSON]S] ([PERSON]); [PERSON]SS] ([PERSON]); [PERSON] ([PERSON]); [PERSON][PERSON]); [PERSON] (M.K.); [PERSON]S] ([PERSON]); [PERSON] ([PERSON]-[PERSON]); [PERSON]RSON]); [PERSON]S.); [PERSON]ESS] (K.T.); [EMAIL_ADDRESS] (M.T.); [PERSON]SS] (R.T.); [PERSON]SS] (M.V.); [PERSON]] (M.Y.); [PERSON]] (J.Z.) Physics Department and Center for Free-Electron Laser Science, University of Hamburg, 22761 Hamburg, Germany  
Correspondence: [EMAIL_ADDRESS]  
Footnote 14:  
taking into account that 20 to 50 \(\upmu\)s are needed to switch bunches between FLASH1 and FLASH2 [4]. The independent photocathode lasers also ensure that the bunch charge can be adjusted individually for the two FEL lines. In combination with fast radio frequency (RF) changes in the time window needed for switching, this enables different compression schemes and hence different pulse durations for user experiments in FLASH1 and FLASH2 [4]. The most important parameters for FLASH2 are shown in the Table 1. While the original FLASH1 FEL line is equipped with fixed gap undulators, which requires a change in the electron beam energy to change the photon energy, the new FLASH2 FEL line has variable gap undulators, which allow for scanning of the photon energy. Furthermore, the possibility for tuning the undulators in FLASH2 has opened up the opportunity to implement and test a variety of novel lasing schemes. First results and future plans will be discussed below.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Electron Beam** & **Value** \\ \hline Energy range & 0.45–1.2 GeV \\ Peak current & \(\leq\)2.5 kA \\ Bunch charge & 0.02–1 nC \\ Normalized emittance & 1.4 mm mrad \\ Energy spread & 0.5 MeV \\ Average \(\beta\)-function & 6 m \\ Rep. rate & 10 Hz \\ Bunch separation & 1–25 us \\ \hline
**Undulator** & **Value** \\ \hline Period & 31.4 mm \\ \(K_{rms}\) & 0.5–2 \\ Segment length & 2.5 m \\ Number of segments & 12 \\ \hline
**Photon Beam SASE** & **Value** \\ \hline Wavelength range (fundamental) & 4–90 nm \\ Average single pulse energy & up to 1 mJ \\ Pulse duration (FWHM) & 10–200 fs \\ Spectral width (FWHM) & \(\approx\)0.5–2\% \\ Peak brilliance & 10\({}^{28}\)–10\({}^{31}\) B \\ \hline \hline \end{tabular}
\end{table}
Table 1: Parameters for FLASH2. SASE: self-amplified stimulated emission.  
Figure 1: Schematic layout of the FLASH facility. The electron gun is on the left, and the experimental halls are on the right. Behind the last accelerating module the beam is switched between FLASH1, which is the original undulator line, and FLASH2, which has been in operation since 2015. Also shown is sFLASH, the seeding R&D setup in FLASH1 [6] and FLASHForward [7], which is a plasma wakefield experiment in the FLASH3 beamline under construction planned to start operation at the end of 2017. FEL: free-electron laser.