Thomas Jefferson National Accelerator Facility 1 of 20 Distribution State A “Direct” Injection D. Douglas, C. Tennant, P. Evtushenko JLab

Thomas Jefferson National Accelerator Facility 2 of 20 Distribution State A Acknowledgements Initial funding provided by ONR Recent work supported by AES under JTO funding Initial simulations (sanity check!), useful feedback provided by John Lewellen, discussions with Steve Benson, operational help from Kevin Jordan

Thomas Jefferson National Accelerator Facility 3 of 20 Distribution State A “Direct” (off-axis) Injection Rather than merge beams using DC magnetic fields, inject beam into linac at large amplitude and use RF focusing & adiabatic damping to bring orbit into line Can use reverse process for extraction of energy-recovered beam 0.15 m current sheet or field clamp linac centerline m injected beam recirculated beam, reinjected for energy recovery accelerated and recovered beams in linac

Thomas Jefferson National Accelerator Facility 4 of 20 Distribution State A Direct Injection/Extraction cross-sectional view of both passes of beam (first = blue, second = pink) looking down linac from injection to dump

Thomas Jefferson National Accelerator Facility 5 of 20 Distribution State A Issues & Solutions Concerns Possible emittance dilution from finite phase extent of bunch in RF fields (thanks to Steve Benson for pointing this out…) Potential for HOM excitation/BBU instability Approach Estimates & analysis (emittance, BBU) Simulation (PARMELA, GPT) Beam studies on JLab Upgrade Driver

Thomas Jefferson National Accelerator Facility 6 of 20 Distribution State A Head-Tail RF-Driven Emittance Dilution Reviewed head-tail issue assumed beam was 8 degrees long (6  head to tail  (~Jlab injected length) Simulated RF steering of injected beam with simple cavity matrix model Results: Propagated beam envelopes vary only slightly Differential steering not dramatic

Thomas Jefferson National Accelerator Facility 7 of 20 Distribution State A

Thomas Jefferson National Accelerator Facility 8 of 20 Distribution State A head/tail (orbit) centroid move ~ ±0.2 mm in position, ±30 microrad in angle. compare to the beam size – for 5 mm-mrad normalized emittance at 100 MeV, with 10 m beta:  x ~ sqrt(  )=sqrt(10*5e-6/(100/ )) ~0.5 mm  x’ ~sqrt(  /  )=(5e-6/10/(100/ )) ~50  rad with stated assumptions about the bunch length get ~ ± ½ sigma motion – over the full (6  ) bunch length Conclusion: emittance dilution may not be too bad; look at more carefully…

Thomas Jefferson National Accelerator Facility 9 of 20 Distribution State A Detailed Study Performed as part of JTO-funded AES merger study Three part investigation More careful analytic estimates Simulations with space charge Beam study on Jlab IR Upgrade Conclusions: emittance growth very modest; tolerable for IR systems BBU thresholds unaffected; additional power goes into HOM loads Several cm pass-to-pass possible

Thomas Jefferson National Accelerator Facility 10 of 20 Distribution State A Results – Theory/Simulation GPT simulation of beam size in single- module linac (C. Tennant) Estimates  emittance growth negligible for IR FELs Emittance growth negligible in simulation Beam quality not degraded Analysis  BBU threshold independent of injection offset C. Tennant, JLAB-TN Power into HOMs depends on injection offset

Thomas Jefferson National Accelerator Facility 11 of 20 Distribution State A Bunches Traveling Through Linac: Animation Injected on-axis Injected 10 mm off-axis C. Tennant and D. Douglas | July 24, 2008

Thomas Jefferson National Accelerator Facility 12 of 20 Distribution State A Machine Study Measured impact of injection offsets on beam quality in JLab IR Upgrade Aperture limited to ~1 cm offsets Able to run 1 cm  BBU tests possible Tested at nominal (9 MeV) and low (5 MeV) injection energy Conclusion: No observable impact on beam quality; BBU-related measurements underway

Thomas Jefferson National Accelerator Facility 13 of 20 Distribution State A Machine Study: Method Measure injected emittance (multislit) Quad scan emittance measurement after linac On axis & several displacements Tomography in recirculator BBU – look at power into HOMs in 7-cell module

Thomas Jefferson National Accelerator Facility 14 of 20 Distribution State A Steering “off-axis” emittance tests: steer off into 1 st module, grab at end of module where RF focusing bring (nearly ) to node (no offset downstream) “BBU” tests: steer off into linac, resteer in recirculator to maintain 2 nd pass transmission note path-length/phase/energy effects in arc…

Thomas Jefferson National Accelerator Facility 15 of 20 Distribution State A Machine Study: Results Transversal beam sizes and profiles largely independent of injected orbit over ±1 cm offsets in H and V Machine drift much higher impact than orbit offset Initial data analysis of emittance data  emittance unaffected by steering (to resolution of measurement) Working through error propagation BBU: set up CW configuration, acquired initial signals, whereupon machine crashed (refrigerator trip); lost rest of run to LCW line break before follow-on shifts will schedule more study time over the summer

Thomas Jefferson National Accelerator Facility 16 of 20 Distribution State A Beam Profile At End of Linac x=-10 mm x=0 mm x=+10 mm y=-10 mm y=0 mm y=+10 mm (some scraping) profile measurement by P. Evtushenko & K. Jordan

Thomas Jefferson National Accelerator Facility 17 of 20 Distribution State A Transverse Emittance (5 MeV injection) Measured with 3 methods: 1. “multislit” in injector 2. quad scan at end of linac 3. tomography in recirculator backleg Results generally consistent and roughly match values w/ full energy injection LocationMethodResult (mm-mrad) InjectorMultislit~ 13 End of linacQuad Scan~10-15 Backleg of recirculatorTomography~10 (horizontal)

Thomas Jefferson National Accelerator Facility 18 of 20 Distribution State A Emittance 5 MeV Injection Multislit: ~ 13 mm-mrad Tomography: ~ 10 mm-mrad beam spot reconstructed phase space Quad scan: ~ mm-mrad tomography courtesy C. Tennant Multislit courtesy P. Evtushenko

Thomas Jefferson National Accelerator Facility 19 of 20 Distribution State A “Direct” 5 MeV Test of “merger-less” merger Low-loss operation with large (~ cm) injection offsets Beam behavior ~independent of injection orbit

Thomas Jefferson National Accelerator Facility 20 of 20 Distribution State A Conclusions Direct injection provides possible alternative to traditional merger Beam quality requirements are key likely appropriate for IR systems, may not be quantitatively appropriate for, e.g. shorter wavelength applications Lower frequency better (i.e. “easier”, more available aperture!) Few-several cm separations possible Still need to evaluate emittance data (error analysis) and measure HOM power deposition