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

31 May, System Parameters

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

31 May, 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

31 May, 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)

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

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

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

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

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

31 May, Design Concept Evolutionary path: –10 MeV/5 mA injector  10 MeV/10 mA injector –35-48 MeV linac (1 cryomodule)  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

31 May, 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

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

31 May, “Exact”/DIMAD comparison

31 May, 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

31 May, 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

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

31 May, Layout

31 May, Beam Envelopes linac FELarc

31 May, arc FEL

31 May, Aberrations - Linac to FEL

31 May, Aberrations - FEL to Linac

31 May, 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

31 May, Reinjection

31 May, Mid-1st Module

31 May, Between 1st & 2nd Modules

31 May, Mid-2nd Module

31 May, Between 2nd & 3rd Modules

31 May, Mid-3nd Module

31 May, End

31 May, End - octupoles on

31 May, End - skew quad effect included

31 May, 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

31 May, 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

31 May, 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

31 May, 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

31 May, 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

31 May, Rudimentary Simulation

31 May, 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

31 May, 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

31 May, 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

31 May, 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

31 May, Schedule