“Direct” Injection

1. “Direct” Injection D. Douglas, C. Tennant, P. Evtushenko JLab

2. 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

3. linac centerline 0.15 m current sheet or field clamp “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 injected beam 0.075 m accelerated and recovered beams in linac recirculated beam, reinjected for energy recovery

4. Direct Injection/Extraction cross-sectional view of both passes of beam (first = blue, second = pink) looking down linac from injection to dump

5. 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

6. Head-Tail RF-Driven Emittance Dilution • Reviewed head-tail issue • assumed beam was 8 degrees long (6s, 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

8. 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: • sx ~ sqrt(be)=sqrt(10*5e-6/(100/0.51099906)) ~0.5 mm • sx’ ~sqrt(e/b)=(5e-6/10/(100/0.51099906)) ~50 mrad • with stated assumptions about the bunch length get ~ ± ½ sigma motion – over the full (6s) bunch length Conclusion: emittance dilution may not be too bad; look at more carefully…

9. 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

10. Results – Theory/Simulation • 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-07-011 • Power into HOMs depends on injection offset GPT simulation of beam size in single-module linac (C. Tennant)

11. Bunches Traveling Through Linac: Animation Injected on-axis Injected 10 mm off-axis C. Tennant and D. Douglas | July 24, 2008

12. Machine Study • Measured impact of injection offsets on beam quality in JLab IR Upgrade • Aperture limited to ~1 cm offsets • Able to run CW @ 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

13. 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

14. Steering • “off-axis” emittance tests: steer off into 1st 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 2nd pass transmission note path-length/phase/energy effects in arc…

15. 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

16. 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

17. Transverse Emittance (5 MeV injection) • Measured with 3 methods: • “multislit” in injector • quad scan at end of linac • tomography in recirculatorbackleg • Results generally consistent and roughly match values w/ full energy injection

18. Emittance Data @ 5 MeV Injection Quad scan: ~ 12-15 mm-mrad Multislit: ~ 13 mm-mrad Multislit courtesy P. Evtushenko Tomography: ~ 10 mm-mrad reconstructed phase space beam spot tomography courtesy C. Tennant

19. “Direct” Injection @ 5 MeV • Test of “merger-less” merger • Low-loss operation with large (~ cm) injection offsets • Beam behavior ~independent of injection orbit

20. 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