1. Dark Current Measurements and Simulations Chris Adolphsen 2/4/15

2. 10 cm copper to stop and measure dark current 2.5 m Ion chamber 1.2 m below beam line Dose meters DESY CM Measurements in 2004

3. DESY 2004 CM measurements with and without a ‘hot’ cavity detuned Measure ~ 1 Rad/hr CW equivalent at 25 MV/m, 2.5 m from CM with hot cavity off – roughly what we would expect for the level of dark current they observe. Cryomodule Dark Current Measurements 3

4. FNAL ILC Prototype CM (CM2) Measurements No response below 20 MV/m – however cavities not phased for acceleration

5. Thomas Jefferson National Accelerator Facility Page 5 TTC 14 JLAB C100 Field Emission Onset Potential for beamline contamination during assembly process Creation of new field emitters during testing

6. Thomas Jefferson National Accelerator Facility Page 6 TTC 14 Estimated captured current based on our simulations An FNAL HTS test last August concluded "we do not see radiation coming from the cavity at the noise level (~2mR/hr) [up to about 20 MV/m]". 100 nA 10 nA 1 nA 0.1 nA Single Cavity Tunnel Radiation vs Gradient

7. He Processing at JLAB During the assembly process, the cavity vacuum of the C100-4 cavity string was compromised. Acceptance testing in the Cryomodule Test Facility revealed unusually poor performance for C100 cavities. High field emission with low onset gradients Several cavities with unusually low quench gradients Not enough power do high gradient conditioning so tried He processing: 1.Cryomodule at 2K 2.Inject high purity He into the beamline vacuum raising partial pressure ~1e-4 Torr 3.Operate cavities at maximum sustainable gradient 60 – 90 minutes. Establishing a frequency lock and dealing with multipacting barrier was tricky Once a good lock was established, stable operations was easy to maintain Integrated voltage increased from 90 MV to 113 MV Average FE onset increased from 9.3 MV/m to 13.1 MV/m 7 M. Drury LCLS-II FAC Review, February 5-6, 2015

8. Before and After Q Versus Gradient Plots LCLS-II FAC Review, February 5-6, 2015 8 M. Drury

9. 9 Simulation of dark current generation, propagation and losses in the LCLS-II SC Linac M. Santana, Xu. Chen, Lixin Ge, Zenghai Li, M. Carrasco

10. Particle Tracking Snapshot Lixin Ge

11. 11 Dose at the cryomodules – elevation view 3 cryostats 5 cryostats In these simulations, electrons that stay in the beam pipe after passing through the quad in one CM are not accelerated in the next CM M Santana Current accelerated equally upsteam and downstream

12. 12 Dose at the cryomodules – elevation view Dark Blue ~ 0.1 Rad / hour / nA, Red ~ 1 kRrad / hour / nA M Santana 7.2 kG Quad at 4 GeV Normalized to captured current

13. 13 Epoxy (~ Silicon) Dose Equivalent Yellow ~ 1e8 Rad / 20 years / nA, Purple ~ 1e4 Rad / 20 years / nA M Santana

14. Radiation Hardness of Wire Insulation

15. 15 End View – Lower RAD Levels above CM 10 kRad / 20 y / nA M Santana

16. Energy deposited in the cavities [mW/nA] 16 CM 2 K Heat Load < 50 mW / nA to be compared to RF load of 100 W / CM M Santana

17. 17 Energy Deposition [mW/nA] in CM Components 1.73 0.22 0.06 0.17 0.01 0.13 3.01 0.37 0.12 0.79 M Santana

18. Multi-CM Captured Current Propagation Particles Enter from Right Red: Distribution of all initial particles; Green: Distribution of initial particles which hit upstream; Blue: Distribution of particles hit upstream L Ge ~ 10% of captured current that makes it through a quad in one CM (red) stays in the beam pipe by the time they reach the next quad (blue) Green particles correspond to the blue ones, but at the upstream end

19. Impact Energy vs Cavity Separation Impact electron distribution 8-cavity module DC vs cavity length Z. Li & L. Ge 5.75  Cavity Spacing 6.25  Cavity Spacing 6.0  Cavity Spacing

20. Dark Current Summary Proper cleaning/installation techniques should yield < 1 nA/CM captured current at 16 MV/m At 1 nA/CM 1e8 Rad dose in 20 years near/in the quad – this is about 50 times below the Kapton survivability level 1e4 Rad dose in 20 years above the CM – most electronics would survive 2 K heat load < 50 mW/CM and not an issue Currently stimulating multi-cavity dark current propagation at lower energy He rf processing produces significant improvement – not clear it can be applied in the SLAC tunnel LCLS-II FAC Review, February 5-6, 2015 20