1. Driver Accelerator Design D. Douglas, G. Krafft, R. Li, L. Merminga, B. Yunn

2. 31 May, 20002 System Parameters

3. 31 May, 20003 Design Issues/Requirements Source/Injector Performance Delivery of appropriate beam to FEL Energy Recovery Peculiarities of SRF environment Geometric, schedule constraints

4. 31 May, 20004 Source/Injector Performance Must run source & injector at original IR Demo design parameters (135 pC) with doubled repetition rate (37.5  75 MHz) Traditional concerns: –transverse/longitudinal emittances at high charge –cathode lifetime –injector characterization –injector operability

5. 31 May, 20005 Source/Injector Performance Status: –have had successful initial run at 135 pC emittance, momentum spread “scoped out” emittance marginal (20-30 mm-mrad), momentum spread okay (~0.1%?), bunch length unknown –cathode lifetimes > 0.6 kC; present wafer at > 2 kC –injector increasingly well characterized still need some calibrations, polarity corrections, modeling –injector operability constantly improving (scripting, procedures)

6. 31 May, 20006 Delivery of Appropriate Beam to FEL Transport & transverse/longitudinal matching Beam quality preservation: –space charge –other wakefield/collective effects –CSR

7. 31 May, 20007 Energy Recovery Transport, transverse/longitudinal matching and acceptance Space charge BBU FEL/RF interaction

8. 31 May, 20008 Peculiarities of SRF Environment Cavity focussing HOM coupler-driven skew coupling RF drive system characteristics

9. 31 May, 20009 Geometric, Schedule Constraints Machine must fit in existing vault Machine must be installed around IR Demo (physically and temporally)

10. 31 May, 200010 Design Concept Direct evolution of IR Demo to higher energies: energy recovering CW SRF linac

11. 31 May, 200011 Design Concept Evolutionary path: –10 MeV/5 mA injector  10 MeV/10 mA injector –35-48 MeV linac (1 cryomodule)  80-210 MeV linac (3 cryomodules) –FEL insertion following linac (CSR)  FEL insertion in backleg Have existence proof –narrower, longer –significant fraction can be installed while IR Demo still operable

12. 31 May, 200012 Aside: Need for Design Code Driver in an unusual regime - large acceptance SRF linac with recirculation –acceleration, RF focussing, large momentum offsets - bordering on non-perturbative “Most” design codes are perturbative/do not model acceleration, or are particle pushers with limited beamline modeling capability (no optimization or analysis) At present, using DIMAD but cross-referencing with direct numerical integration

13. 31 May, 200013 Runge-Kutta Integrator Integrate test ray response to ambient magnetic fields Use lab frame coordinates - nonperturbative

14. 31 May, 200014 “Exact”/DIMAD comparison

15. 31 May, 200015 Design Features 3 module linac –high gradient 7-cell module in center slot (reduce over- focussing induced mismatch) Quad telescopes match into/out of recirculator Bates end-loops FODO backleg transport to FEL insertion FEL insertion –quad telescopes matching to/from wiggler –two embedded optical cavities Longitudinal phase space management identical to that in Demo

16. 31 May, 200016 Longitudinal Matching Requirements: –short bunch (high peak current) at wiggler –small momentum spread at dump E  E  E  E  E  longitudinal phase space through acceleration cycle

17. 31 May, 200017 Longitudinal Matching - cont. E  E  longitudinal phase space during energy recovery

18. 31 May, 200018 Layout

19. 31 May, 200019 Beam Envelopes linac FELarc

20. 31 May, 200020 arc FEL

21. 31 May, 200021 Aberrations - Linac to FEL

22. 31 May, 200022 Aberrations - FEL to Linac

23. 31 May, 200023 Simulation of Energy Recovery Tracking with large momentum spread –longitudinal matching/energy compression should work, but requires octupoles larger linac final to to energy ratio (20 to 1) makes energy recovery/compression more difficult –30 mm-mrad emittances “dicey” –skew quad - last module imposes much of effect; might not be far greater than in demo

24. 31 May, 200024 Reinjection

25. 31 May, 200025 Mid-1st Module

26. 31 May, 200026 Between 1st & 2nd Modules

27. 31 May, 200027 Mid-2nd Module

28. 31 May, 200028 Between 2nd & 3rd Modules

29. 31 May, 200029 Mid-3nd Module

30. 31 May, 200030 End

31. 31 May, 200031 End - octupoles on

32. 31 May, 200032 End - skew quad effect included

33. 31 May, 200033 Space Charge Initial application of PARMELA modeling to injector provided successful operation at 135 pC per bunch Ongoing refinement must occur –improved RF calibration factors –benchmarks against machine behavior to verify model and determine parametric sensitivities Certified model will be applied to design from gun to dump

34. 31 May, 200034 SRF Issues Upgrade module RF drive control –new control module needed to take full advantage of upgrade module design capabilities –existing RF control module adequate (with appropriate parametric choices for, e.g., cavity coupling) for energy gains up to 67 MeV 7-cell cavity model HOM effects –coupler driven skew coupling –BBU –longitudinal HOM power-induced heating

35. 31 May, 200035 BBU Certification TTBBU benchmark against IR Demo in progress Analysis will employ measured 5-cell HOM Qs and frequencies and give BBU threshold as function of 7-cell Qs – provide a specification for damping of 7-cell HOMs

36. 31 May, 200036 Wakefields/Impedance Stewardship Initial estimates suggest momentum spread induced by IR Demo-like impedance budget (2” chamber, multiple viewers, BPMs) may be marginally acceptable Characterization ongoing as vacuum system design evolves Plan use of shielded components upstream of wigglers, considering 3” chamber throughout system

37. 31 May, 200037 CSR Certification Detailed CSR model in advanced stage of development – being benchmarked against CERN measurements –cross-checks planned with IR Demo –design concept will be simulated to certify performance Rudimentary model suggests Bates endloop design may be acceptable, but Results depend on details of phase space distribution, so careful analysis with detailed model is necessary

38. 31 May, 200038 Rudimentary Simulation

39. 31 May, 200039 FEL/RF Interaction Initial studies on IR Demo under way Ongoing work will characterize Upgrade system behavior and performance Need a more fully developed model of the FEL

40. 31 May, 200040 What to do? Develop definitive injector model Analyze space charge effects Complete conceptual design beam optics model –incorporate “best knowledge” magnetic fields (fringe, roll-off, etc) –certify skew quad effects Develop engineering design beam optics model –diagnostic, correction systems –error tolerances/component specifications alignment, powering, field homogeneity requirements

41. 31 May, 200041 What to do? CSR –benchmark model using Demo –certify upgrade design BBU –benchmark TDBBU using Demo –certify upgrade design FEL/RF interaction –benchmark model using Demo –certify upgrade design Continue impedance stewardship

42. 31 May, 200042 What to do? Machine modeling –cavity model HOM driven skew coupling 7-cell cavities –“non-perturbative” model for large acceptance linac –operational modeling RF drive system controls - continue work on control module analysis, design Commissioning planning

43. 31 May, 200043 Schedule