2021-06-16 Seminar about SwissFEL Cs2Te cathode and low emittance measurements Sara's comments before the talks: I'm working on undulators at XFEL, and we are interested in higher photon energy up to 70 keV, and this needs low emittance. That's why I triggered the discussion with Houjun and Frank Brinker. My notes for Romain's cathode talk 1) cern design, exchange of plugs with DESY? 2) CuBe2, 315 C 3 hours, 11 pC dark current@100 MV/m, dark current from RF spring 3) 5 nm rms, copper roughness after all steps? 1e-4 QE 4) Cs2te, 10 mm, 40 nm, 15 nm Te +24 nm Cs, started Co-deposition 5) CU_25? 33%?-->20%, QE island 6) 6E-11 mbar 7) CU_32 codeposition, QE map 8) CU_31 QE hole Question for Romain's talk (incomplete): MK:thickness of the Cs2Te cathode comment from Laura: 25 nm Cs is from film thickness monitor, but the cathode itself is not 40 nm. FS:Cu machining of the plug, any polishing, the sign of machining is visible on the copper plug? Romain: just turning, no polishing, the roughness is ~5 nm Comment from Laura: the Mo plug at DESY after polishing is ~7 nm roughness Sandeep: why the color looks red for the Cs2Te Romain: the Cs might not cover the Te part fully, there is a black or blue part in between Cs2Te and Cu Comment from Laura: the distance between mask and the plug can change the deposition... Laura:25 nm Cs is not enough for 15 nm Te, maybe is the reason for low QE of SwissFEL plugs Romain: the QE starts to drop after more Cs beyond 25 nm Houjun: I checked the depositon curve from Laura, with 25 nm Cs, LASA cathode with 15 nm Te also has ~1% QE, but with more Cs, QE keeps increasing until >10%. Maybe we should try to reduce our Cs amount to stop at 1% QE, if this is the reason for low thermal emittance of SwissFEL CsTe cathode. Houjun: is the 33% QE nonuniformity peak to peak or rms value? is the 200 pC working point in the linear or saturation regime. Romain: 33% is peak to peak. SwissFEL gun emission field is 80 MV/m, the 200 pC emission is still in linear regime. Houjun: is it possible to exchange cathodes with DESY? Romain: due to vacuum compatibility and different plugs, it's difficult. Question for Eduard's talk (incomplete): FB: with the 1 pC beam and large beta function of 70 m, how is the signal intensity in slice emittance measurement for thermal emittance measurement Eduard: The camera setup can has enough signal FS: Are the beam measurments consistent with FEL performance Eduard: The emittance fits FEL gain curve, but we have large energy spread Mikhail: The large energy spread might be correlated with the small emittance, I can send you a study for that. Eduard: Recently, we reduced the energy spread from 15 keV to 8 keV, and mainly due to intra beam scattering, and more importantly due to microbunching before the bunch compressor, from the laser heater chicane, this is very interesting. We don't need laser heater because of the high energy spread, ~8 keV is what we need from laser heater. Houjun: we measured 1.6 keV at PITZ injector, and the beam is roughly 5 meter away from cathode, result very close to simulations. Due to the low energy, large emittance and short distance, we suffer less from intra beam scattering. FS: Can you turn off the laser heater chicane. Houjun:When QE of Cs2Te drop from 1% (CsTe-8) to 0.1% (CsTe-17) level, the measured thermal emittance did not change. Eduard: The higher thermal emittance from slide 12 is from a single laser diameter measurement, not complete emittance scan, not reliable. The thermal emittance for the 1% QE on slide 14 has a bigger error bar. Houjun: what's the typical beam size during emittance scan? Eduard: For measurements after BC2, emittance dominated by screen resolution (slide 18), for emittance after BC1, screen resolution is fine. Houjun: Emittance is sensitive for SwissFEL, do you routinely reproduce low emittance, and do you observe FEL performance on injector emittance? Eduard: we usually optimize injector for low projected emittance, and it's reproducible. FEL performance hard to say. Houjun: we nerver reached such low thermal emittance as SwissFEL at PITZ measurements. Eduard: do you measure projected emittance for thermal emittance, the head and tail may have impacts. Houjun: we lower the charge to 0.05-0.1 pC, and use a different method to measure thermal emittance. Laura: may also check flash results Romain: are the emittance consistent between PITZ and XFEL? Houjun: projected emittance not consistent in the past due to different measurement methods, recently we use different analysis methods for PITZ, and I belive we are now more consistent for 250 pC. XFEL does not measure thermal emittance. Forgot to ask: YAG 10-24 um rsolution? optical or material?