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Discussion on SRF Technology for cw XFELs Topical …

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Topical Workshop on High Repetition-Rate XFELPhysics and TechnologyShanghai 11.08.2017
SCRF for cw operating XFELCavitiesStandardTESLA cavitywith HOM couplersas for the E-XFEL.Surfacepreparationhigh temperature doping at800+C.GoalaverageQo(2K)=2.7E10@16 MV/m(operational Eacc).Comment:Qodepends on materialbatch, preparationprocess, residual fluxatthe cavitylocation and flux expulsion.Statistics for 800cavitiesof E-XFEL showsthatun-dopedcavitiesdemonstrated Qo=1.5E10@16MV/m.Thatexperiencesuggeststo allow for a significant margin on thechallengingQo valueaimed in thepresent report.
Topical Workshop on High Repetition-Rate XFELPhysics and TechnologyShanghai 11.08.2017
SCRF for cw operating XFEL, cont.LHevesselProper 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 ofNbshould have low permeability below 1.05.Comment: This was not discussed during the Workshop.
Topical Workshop on High Repetition-Rate XFELPhysics and TechnologyShanghai 11.08.2017
SCRF for cw operating XFEL, cont.TunerThe 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.
Topical Workshop on High Repetition-Rate XFELPhysics and TechnologyShanghai 11.08.2017
SCRF for cw operating XFEL, cont.Input couplerIt 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.
Topical Workshop on High Repetition-Rate XFELPhysics and TechnologyShanghai 11.08.2017
SCRF for cw operating XFEL, cont.CryomoduleThe 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?
Topical Workshop on High Repetition-Rate XFELPhysics and TechnologyShanghai 11.08.2017
SCRF for cw operating XFEL, cont.MicrophonicsMeasurements 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.
Topical Workshop on High Repetition-Rate XFELPhysics and TechnologyShanghai 11.08.2017
Cryoplant and cryo distribution systemIs based on well-established European technology by Linde and Air-LiquideBecause Qo is challenging, we propose to increase capacity of the cryogenic system.
Topical Workshop on High Repetition-Rate XFELPhysics and TechnologyShanghai 11.08.2017
Experimental Capacity of the FacilityConsider possibility to expand the experimental capability to accommodate future demands.
Topical Workshop on High Repetition-Rate XFELPhysics and TechnologyShanghai 11.08.2017
R&D, Test and Assembly InfrastructureProposed infrastructure seems to be sufficient for the R&D and prototyping phase.For the production phase, to enablethe production rate and test ofCMs, it should be coupled with a second infrastructure of similar size as the one in construction in the Beijing area.
Discussed Injector Topics 1/3
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 whenmA-class averagecurrents 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 (~10-10Torr). 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.
High Repetition-Rate XFEL Physics and Technology Workshop Shanghai, Aug. 10, 2017
Discussed Injector Topics2/3
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 theundulator. Different tradeoffs between the two quantities are possible.Select theproper 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 0.75keVoutput energy at the gun, the proposed combination of 0.4 um emittance and 1.5 kA peak current at 100pC, 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.Acareful study should be performed to evaluate if higher fields and energies are required at the gun to obtainacceptable lasing at 20keVphotonenergies. A new upgraded gun design could be necessary.Definethe compression required at the injector to obtain the required longitudinal phase spacequality. High-order (3rdand above) longitudinal phase space correlation can seriously limit the compression factor along thelinac.
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. Electronand laser beam diagnostics at the injector are extremely important. The proposed injector configuration with a warm section between the single-cavityand the 8-cavitycryomodulesis highly supported. Such a warm section can locate a complete suite of diagnostics for the 6D phase space characterization ofthe beam.

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Discussion on SRF Technology for cw XFELs Topical …