1. SCRF for cw operating XFEL
Topical Workshop on High Repetition-Rate XFEL Physics and Technology Shanghai
SCRF for cw operating XFEL
Cavities
Standard TESLA cavity with HOM couplers as for the E-XFEL.
Surface preparation high temperature doping at 800+C.
Goal average Qo(2K) MV/m (operational Eacc).
Comment: Qo depends on material batch, preparation process, residual flux at the cavity location and flux expulsion.
Statistics for 800 cavities of E-XFEL shows that un-doped cavities demonstrated
That experience suggests to allow for a significant margin on the challenging Qo value aimed in the present report.

2. SCRF for cw operating XFEL, cont.
Topical Workshop on High Repetition-Rate XFEL Physics and Technology Shanghai
SCRF for cw operating XFEL, cont.
LHe vessel
Proper diameter of the chimney and 2-phase pipe should allow for higher 2K heat load due to the cw operation.
Magnetic shielding has to be carefully designed. Installation on helium vessel and on end-groups should eliminate flux penetration through the doped Nb.
All materials used in the vicinity of Nb should have low permeability below 1.05.
Comment: This was not discussed during the Workshop.

3. SCRF for cw operating XFEL, cont.
Topical Workshop on High Repetition-Rate XFEL Physics and Technology Shanghai
SCRF for cw operating XFEL, cont.
Tuner
The cold tuner, for example similar to the E-XFEL CEA tuner with stepping motor, should have high resolution <1Hz.
A piezo-actuator should be implemented for both microphonics and Lorentz force detuning compensation.
Comment: This was not discussed during the Workshop.

4. SCRF for cw operating XFEL, cont.
Topical Workshop on High Repetition-Rate XFEL Physics and Technology Shanghai
SCRF for cw operating XFEL, cont.
Input coupler
It should be investigate if the E-XFEL input coupler, with a thicker copper coating of the inner conductor in the warm part, can be implemented for RF-power of ca. 7kW in matched and full reflection condition.
Thermal analysis and test of the coupler seems to be necessary before implementation of an active cooling.
The coupler should be adjustable remotely to match coupling during operation for various beam loading and to compensate for the change of the coupling due to heating of the inner conductor.

5. SCRF for cw operating XFEL, cont.
Topical Workshop on High Repetition-Rate XFEL Physics and Technology Shanghai
SCRF for cw operating XFEL, cont.
Cryomodule
The cool-down procedure for the whole linac should be worked out because this may influence the cryomodule design, for example need for the J-T valve and cool-down valve, confinement of the 2-phase tube in each cryomodule….
Should be an active demagnetization of the vacuum vessel with Helmholtz coils applied in the tunnel?

6. SCRF for cw operating XFEL, cont.
Topical Workshop on High Repetition-Rate XFEL Physics and Technology Shanghai
SCRF for cw operating XFEL, cont.
Microphonics
Measurements of frequencies and amplitudes vibrations in a similar tunnel were presented. How representative is that data for the tunnel of SCLF?
LLRF electronics, including piezo-driver and active noise cancellation, is crucial for stability of the beam energy. It needs to be developed in collaboration with laboratories having experience in that field.
 Comment: This was not discussed during the Workshop.

7. Cryoplant and cryo distribution system
Topical Workshop on High Repetition-Rate XFEL Physics and Technology Shanghai
Cryoplant and cryo distribution system
Is based on well-established European technology by Linde and Air-Liquide
Because Qo is challenging, we propose to increase capacity of the cryogenic system.

8. Experimental Capacity of the Facility
Topical Workshop on High Repetition-Rate XFEL Physics and Technology Shanghai
Experimental Capacity of the Facility
Consider possibility to expand the experimental capability to accommodate future demands.

9. R&D, Test and Assembly Infrastructure
Topical Workshop on High Repetition-Rate XFEL Physics and Technology Shanghai
R&D, Test and Assembly Infrastructure
Proposed infrastructure seems to be sufficient for the R&D and prototyping phase.
For the production phase, to enable the production rate and test of CMs, it should be coupled with a second infrastructure of similar size as the one in construction in the Beijing area.

10. Discussed Injector Topics 1/3
High Repetition-Rate XFEL Physics and Technology Workshop Shanghai, Aug. 10, 2017
Photocathodes. Relevant parameters: quantum efficiency (QE), QE lifetime, intrinsic emittance, work function.
Due to the present (commercial) availability in terms of laser power, and the demonstrated laser shaping capabilities in both visible and UV frequency ranges, cathode QE and work function are less critical parameters when mA-class average currents are required. Semiconductor cathodes operating in the UV (Cs2Te) or in the visible (CsK2Sb, NaK2Sb,…) can both satisfy QE requirements of greater than several percent.
QE lifetime and intrinsic emittance are parameters of primary importance because they can respectively affect the operating efficiency of the user facility, and limit the smallest emittance achievable. Experimental results indicates significant longer lifetimes for Cs2Te cathodes, but a (~20%) lower intrinsic emittance for multi alkali (CsK2Sb, NaK2Sb,…) cathodes.
All semiconductor cathodes require extremely low operational vacuum pressures (~10-10 Torr). The total pressure must be hydrogen dominated with partial pressure for cathode poisoning molecules (H2O, O2, …) at least two order of magnitude smaller than the total pressure.

11. Discussed Injector Topics 2/3
High Repetition-Rate XFEL Physics and Technology Workshop Shanghai, Aug. 10, 2017
Critical electron gun requirements in terms of beam dynamics: electric field at the cathode during photoemission, and output beam energy.
The FEL shortest wavelength goal defines the combined requirement in terms of emittance and peak current at the undulator. Different tradeoffs between the two quantities are possible.
Select the proper tradeoff between emittance and peak current (using start to end simulations) to allow for the desired FEL performance while setting reasonably achievable emittance and peak current conditions. In particular, for ~20 MV/m field at the cathode and 0.75 keV output energy at the gun, the proposed combination of 0.4 um emittance and 1.5 kA peak current at 100 pC, could be somehow conservative in terms of emittance and challenging in terms of peak current. A combination of lower emittances and peak currents could represent a better matching to the gun characteristics.
A careful study should be performed to evaluate if higher fields and energies are required at the gun to obtain acceptable lasing at 20 keV photon energies. A new upgraded gun design could be necessary.
Define the compression required at the injector to obtain the required longitudinal phase space quality. High-order (3rd and above) longitudinal phase space correlation can seriously limit the compression factor along the linac.

12. Discussed Injector Topics 3/3
High Repetition-Rate XFEL Physics and Technology Workshop Shanghai, Aug. 10, 2017
Time jitter. The injector can be a powerful source of temporal jitter. LLRF, RF source, beam parameters stability requirements must be carefully defined
Diagnostics. Electron and laser beam diagnostics at the injector are extremely important. The proposed injector configuration with a warm section between the single-cavity and the 8-cavity cryomodules is highly supported. Such a warm section can locate a complete suite of diagnostics for the 6D phase space characterization of the beam.