1. PBG Structure Experiments, AAC 2008 Photonic Bandgap Accelerator Experiments Roark A. Marsh, Michael A. Shapiro, Richard J. Temkin Massachusetts Institute of Technology, Plasma Science and Fusion Center Work supported by DOE HEP

2. PBG Structure Experiments, AAC 2008 Collaborators  Continuing collaboration with Jake Haimson and HRC  6 Cell structure was designed, built, and tested by Evgenya Smirnova, now at LANL  Wakefield simulations in collaboration with Kwok Ko at SLAC, and John DeFord at STAAR, Inc.  Breakdown experiments were designed to be tested, and in collaboration with Sami Tantawi and Valery Dolgashev at SLAC

3. PBG Structure Experiments, AAC 2008 Outline  17.14 GHz Experimental Results Lab 6 Cell Traveling Wave Structure Wakefield Simulations Wakefield Measurements  11.424 GHz Planned Experiments Single Cell Breakdown Structures Design of PBG Breakdown Structure  Future PBG Improvements and Experiments

4. PBG Structure Experiments, AAC 2008 Outline  17.14 GHz Experimental Results Lab 6 Cell Traveling Wave Structure Wakefield Simulations Wakefield Measurements  11.424 GHz Planned Experiments Single Cell Breakdown Structures Design of PBG Breakdown Structure  Future PBG Improvements and Experiments

5. PBG Structure Experiments, AAC 2008 HRC Relativistic beam Klystron: Microwave Power Source 25 MW @ 17.14 GHz 25 MeV Linac: 0.5 m long 94 cells Structure Test Stand MIT 17 GHz Accelerator 700 kV 500 MW Modulator Photonic Bandgap Accelerator

6. PBG Structure Experiments, AAC 2008 Motivation  Acceleration demonstrated but what about HOMs?  2D Theory predicts all HOMs in propagation band  PBG HOM Damping in practice is more complicated 3D Structure with disk loading (irises/plates) Propagation band means damping, but how much?  HOM Simulations need to be backed by experiments  Beam excitation of wakefields using 6 Cell structure

7. PBG Structure Experiments, AAC 2008 Experimental Setup  Structure is unpowered  DC injector produces a train of bunches  Matched load on input port  Diode detector observations made through output port and vacuum chamber windows 1/17GHz = 60ps 100ns Diode Horn & Diode Load

8. PBG Structure Experiments, AAC 2008 Experimental Setup Pictures Chamber Window Matched Load Output Port Window View from Below

9. PBG Structure Experiments, AAC 2008  Bunch train with 1 mm rms bunch length and 17.5 mm spacing driven through structure PBG Multi-Bunch Simulation Matched Load Output Port Chamber window

10. PBG Structure Experiments, AAC 2008  Bunch train with 1 mm rms bunch length and 17.5 mm spacing driven through structure PBG Multi-Bunch Simulation Matched Load Output Port Chamber window

11. PBG Structure Experiments, AAC 2008  Bunch train with 1 mm rms bunch length and 17.5 mm spacing driven through structure PBG Multi-Bunch Simulation Matched Load Output Port Chamber window

12. PBG Structure Experiments, AAC 2008  Bunch train with 1 mm rms bunch length and 17.5 mm spacing driven through structure PBG Multi-Bunch Simulation Matched Load Output Port Chamber window

13. PBG Structure Experiments, AAC 2008  Bunch train with 1 mm rms bunch length and 17.5 mm spacing driven through structure PBG Multi-Bunch Simulation Matched Load Output Port Chamber window

14. PBG Structure Experiments, AAC 2008 Cold Test of PBG HOMs  17.14 GHz Q = 4000 group velocity = 0.0109 c  Lattice HOMs Q < 250 Low Q Lattice HOMs

15. PBG Structure Experiments, AAC 2008 Simulation of PBG Lattice HOMs  Electric field from HFSS simulations of PBG  Train of bunches means harmonics of 17.14 GHz  Dipole mode not observed Fundamental: 17 GHz, Q = 4000 Lattice HOM: 34 GHz, Q = 100

16. PBG Structure Experiments, AAC 2008 Measured 17 GHz Beam Loading  Output Port diode measurement  No fitting parameters, excellent agreement vgvg 0.0109c Q4000 I1.04 dB/m r98 MΩ/m L29.15 mm P b (Theory)

17. PBG Structure Experiments, AAC 2008 Measured 34 GHz Wakefields  Output Port diode measurement  Awaiting theory, please help… Quadratic fit

18. PBG Structure Experiments, AAC 2008 Experimental Results Summary  Summary of measurements for 100 mA average current  Observations made on Chamber window as well as Output Port  Multiples of 17.14 GHz observed up to 85.7 GHz with heterodyne receiver

19. PBG Structure Experiments, AAC 2008 Outline  17.14 GHz Experimental Results Lab 6 Cell Traveling Wave Structure Wakefield Simulations Wakefield Measurements  11.424 GHz Planned Experiments Single Cell Breakdown Structures Design of PBG Breakdown Structure  Future PBG Improvements and Experiments

20. PBG Structure Experiments, AAC 2008 SLAC Setup  TM 01 Mode Launcher  Single Cell SW Cavity Input and end cells for matching Central test cell New design uses PBG as central test cell

21. PBG Structure Experiments, AAC 2008 X Band PBG Structure Test  SLAC test stand with reusable TM 01 mode launchers  MIT designed PBG structure for high power testing  Under construction Tuning Parameters Input Cell Radius11.627 mm PBG Cell Radius38.87 mm End Cell Radius11.471 mm Coupling Iris Radius5.132 mm PBG Rod Radii2.176 mm PBG Rod Spacing12.087 mm

22. PBG Structure Experiments, AAC 2008 Design Results  Designed to have ½ field in each pillbox coupling cell, only full field region is in PBG “test” cell  Coupling optimized by minimizing S 11 reflection from TM 01 Mode launcher Field on axisS 11 Coupling reflection

23. PBG Structure Experiments, AAC 2008 X Band PBG Single Cell Structure  Central PBG test cell  Pillbox matching cells  First iris radius varied to optimize coupling PBG Structure Experiments, AAC 2008 ½ Field Full Field

24. PBG Structure Experiments, AAC 2008 Electric Field Plots  Electric field plots: top and side views  6.6 MW in = 100 MV/m gradient = 180 MV/m surface field on iris

25. PBG Structure Experiments, AAC 2008 Magnetic Field Plots  Magnetic field plots: top and side views  6.6 MW in = 100 MV/m gradient = 0.8 MA/m surface field on inner rod

26. PBG Structure Experiments, AAC 2008 Outline  17.14 GHz Experimental Results Lab 6 Cell Traveling Wave Structure Wakefield Simulations Wakefield Measurements  11.424 GHz Planned Experiments Single Cell Breakdown Structures Design of PBG Breakdown Structure  Future PBG Improvements and Experiments

27. PBG Structure Experiments, AAC 2008 PBG Structures, The Next Generation  1 st PBG structure test made using: a/b = 0.18 Triangular lattice of cylindrical rods 3 rows of rods  Relatively high pulsed heating on inner row of rods  Next generation: PBG with low pulsed heating, high gradient, low lattice HOMs  Planned additional tests of improved PBG structures at 11.424 GHz, at SLAC and at 17.14 GHz, at MIT

28. PBG Structure Experiments, AAC 2008 Summary and Conclusions  Measured beam loading in PBG structure  Excellent agreement with theory  Measured HOMs at 34 GHz (waiting for theory…)  X-band standing wave PBG structure designed for SLAC, under fabrication  First high gradient, breakdown tested PBG structure  Future Plans Better PBG structures Testing at SLAC, and MIT

29. PBG Structure Experiments, AAC 2008  Any Questions? Thank You

30. PBG Structure Experiments, AAC 2008 Abstract Damping wakefields is a critical issue in the next generation of high gradient accelerators. Photonic bandgap (PBG) structures have unique properties that offer significant wakefield damping. Experimental measurements of wakefields excited by an 18 MeV electron beam in a 6 cell, 17.14 GHz metallic PBG traveling wave accelerator structure are reported. Theory calculations including traveling wave beam coupling, and wakefield simulations using T3P and Analyst are discussed. Good agreement is obtained between theory and experiment. Design and status of an 11.424 GHz standing waves PBG breakdown experiment to be performed at SLAC are discussed. Current status and future plans for design work including future X-band PBG breakdown structures, and improved pulsed heating performance PBGs will be discussed. Work supported by DOE HEP, under contract DE-FG02- 91ER40648