## 6 Conclusions  
Perspectives towards the sub-angstrom working regime of the European XFEL are given based on the present state of the facility. Through start-to-end electron bunch optimization for a peak current close to 4 kA by the end of the accelerator beamline, it is demonstrated that a central slice emittance of about 0.24 \(\upmu\)m in front of the undulators can be achieved for a 100-pC electron bunch created via photoemission by applying detailed transverse laser pulse shaping and a cathode peak accelerating gradient of about 56 MV/m. The temporal characteristics of the cathode drive laser are kept the same as in current user operation. When reducing the cathode gradient down to 40 MV/m, a central slice emittance of about 0.33 \(\upmu\)m is obtained before the undulators. Based on obtained electron bunch qualities, the FEL performance in the sub-angstrom working regime, in terms of SASE pulse intensity and FEL gain length, is discussed at the varied electron beam energy levels available at the facility. Specifically, at 14 GeV electron beam energy, millijoule-level SASE intensity is obtained for a photon energy of 25 keV (i.e. 0.496 A in wavelength) using a gain length of about 7.4 m. Reference studies were performed, which show that 2.3 mJ and 4.4 mJ lasing intensities at 20 keV and 9.3 keV using gain lengths of 3.9 m and 6.3 m, respectively. Millijoule-level SASE performance can be reached at about 0.5 A photon wavelength at 14 GeV electron beam energy. Towards a photon wavelength less than 0.5 A  while boosting the energy level to 17.5 GeV, the obtained electron bunch qualities allow lasing at 30 keV with above 1.5 mJ intensity at a gain length of about 7.5 m. At 40 keV, close to a half-millijoule SASE performance can still be reached. Accessing an even higher photon energy of 45 keV, a fairly good lasing intensity of 0.16 mJ is shown, although the corresponding gain curve just steps into the saturation regime within the available total undulator length of the facility. Note that the existing undulators are used for this study, and that a possible upgrade to superconducting undulators with a controllable undulator period down to 2 cm and below is under consideration, which can further boost the accessible photon energies [13; 14; 59]. In addition, the obtained electron bunches, produced with lower cathode accelerating gradients of 40 MV/m and 50 MV/m, are used for SASE simulations to demonstrate the bunch qualities at a reduced final beam energy of about 7 GeV. This is relevant to a potential continuous-wave mode operation of the facility. At accessible photon energies using the existing undulators, above 1.5 mJ intensity is obtained at 7.8 keV while more than 2.3 mJ is achieved at 5.3 keV. A higher cathode accelerating gradient of 50 MV/m over 40 MV/m results in a smaller emittance, leading to superior FEL performance. The advantage becomes more pronounced towards higher photon energies. As an outlook, further developments of the existing cathode laser shaping system are in progress. The measurement programs based on the obtained results as presented in this article are currently under active discussion and dedicated facility developments at the European XFEL in the sub-angstrom working regime are planned. \begin{table}
\begin{tabular}{l l l l l l l} \hline \hline \(\mathbf{E_{e}}\) & \(\mathbf{E_{p}}\) & \(\mathbf{\lambda_{p}}\) & \(\mathbf{P}\) & \(\mathbf{I}\) & \(\mathbf{L_{g}}\) & \(\mathbf{E_{cath}}\) \\ \hline
**[GeV]** & **[keV]** & **[nm]** & **[W]** & **[m]** & **[m]** & **[MV/m]** \\ \hline
7.3 & 5.3 & 0.2339 & \(1.30\times 10^{11}\) & 2.38 & 3.42 & 40 \\ \hline
7.3 & 5.3 & 0.2339 & \(1.36\times 10^{11}\) & 2.39 & 3.28 & 50 \\ \hline
7.3 & 7.8 & 0.1590 & \(9.64\times 10^{10}\) & 1.51 & 4.32 & 40 \\ \hline
7.3 & 7.8 & 0.1590 & \(1.22\times 10^{11}\) & 1.86 & 4.10 & 50 \\ \hline \hline \end{tabular}
\end{table}