Photonic Band Gap Accelerator Experiments Roark Marsh Massachusetts Institute of Technology, Plasma Science and Fusion Center Accelerator Seminar 1/27/2009

Talk Outline  Introduction  Photonic Band Gaps  Photonic Band Gap Higher Order Modes  MIT Accelerator  Photonic Band Gap Wakefield Measurements  Photonic Band Gap Breakdown Experiments

Introduction  Standard Model  Large Hadron Collider: LHC  International Linear Collider: ILC  Compact Linear Collider: CLIC  High Gradient Acceleration

Higgs Boson  Remaining/Open issues for Standard Model Unitarity of Z,W interactions All Field Theory particles massless  Higgs Mechanism is the Standard Model solution to these problems

Large Hadron Collider  LHC is a 14 TeV proton collider  Construction complete, being commissioned  Will discover Higgs Boson

xkcd LHC

International Linear Collider  ILC is a superconducting electron-positron linear collider  500 GeV in 30 km  Precision Higgs physics after LHC discovery  31 MV/m gradient

CLIC  Compact Linear Collider  Multi-TeV 2 Beam accelerator concept  Feasibility study being done at CERN  ~100 MV/m normal conducting high gradient structures

High Gradient Acceleration  Gradient Limits Trapping Breakdown Pulsed Heating  High frequency  Wakefields scale with frequency cubed:

Wakefields and HOMs  Wakefields: beam excitation of unwanted modes A bunch of highly relativistic charges transits a cavity Electric field “wake” can be written as a sum over cavity eigenmodes These modes can be excited by a bunch Modes are now resonating in cavity, can affect subsequent bunches

Summary  Standard model predicts Higgs Boson  LHC will discover the Higgs Boson  ILC required for precision Higgs physics  Normal conducting high gradient structures required for next generation of linear colliders  High frequency structures require wakefield damping

Photonic Band Gaps  One dimensional example  Two dimensional formalism  Parameters  Experimental work

Photonic Band Gaps  Frequency range in which there is total reflection  1D Example: Bragg reflector Band Gaps

Dispersion Relations TMTE  Bloch wave vector, k Γ→X→J → Γ  Plot ω versus k Lines on curve are modes  For a given frequency, what if there is no solution? No propagation

a/b Ratio  Only one free parameter in design: rod radius to rod spacing ratio  Frequency used to fix one of a or b  Ratio determines gap properties a b b b TM

No HOMs Higher Order Modes?  2D Theory says complete band gap  No higher order eigenmodes: no HOM wakefields  Frequency tunable material Looks like a wall for operating mode Looks like vacuum for higher frequency modes  Solves Wakefield issue Operating mode confined Wakes leak out

PBG Accelerator Structures  First PBG structure designed, built, tuned and tested with beam  Structure achieved 35 MeV/m * limited by available power and structure design for first results * Smirnova et al. PRL, 2005

No HOMs 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?

Summary  Bragg filters are a 1D example of a PBG  2D is more complicated  Only one free parameter: ratio a/b  No HOMs expected in PBG accelerators  PBG accelerator demonstrated at MIT

PBG Simulations  High Frequency Structure Simulator: HFSS  Operating Modes  Higher Order Modes: HOMs  Structure Cold Test

High Frequency Structure Simulator  Full-wave 3D EM field solver: HFSS by Ansoft  Used for both eigenmode and driven solutions

Operating Modes PillboxPBG a/b=0.15 PBG a/b=0.3  TM 01 on-axis electric field for acceleration  Pillbox walls confine fields  Rods confine mode because it is in the Band Gap

Dipole Modes? Pillbox PBG a/b=0.15 No HOMs  Dipole modes observed in simulation  Artifact of metallic boundary?  Perfectly Matched Layer

Lattice HOMs PillboxPBG a/b=0.15 PBG a/b=0.3 No HOMs HOMs Q=9000 Q=100Q=1000  Quality factor gives quantitative gauge of damping  HOMs present, but strongly damped in 3D

Cold Test of PBG HOMs  GHz Q = 4000 group velocity = c  Lattice HOMs Q < 250 Low Q Lattice HOMs

Summary  HFSS used for field simulations  Operating mode in PBG like pillbox TM 01  HOMs in fact observed in simulations  Lattice HOMs: very low Q from high diffractive loss  Low Q Lattice HOMs seen in PBG structure cold test

PBG Wakefields  MIT HRC 17 GHz Accelerator  Experimental Setup  Simulations  Theory  Measurements

HRC Relativistic beam Klystron: Microwave Power Source 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

Accelerator Schematic Klystron RF Auxiliary Output DC Gun Steering Lens Chopper Prebuncher Bias Beam Monitor Linac Toroidal Lens Haimson Deflector

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

Experimental Setup Pictures Chamber Window Matched Load Output Port Window View from Below

PBG Multi-Bunch Simulation Matched Load Output Port Chamber window 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 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 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 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 Bunch train with 1 mm rms bunch length and 17.5 mm spacing driven through structure

Simulation of PBG Lattice HOMs  Electric field from HFSS simulations of PBG  Train of bunches means harmonics of GHz  Dipole mode not going to be observed Fundamental: 17 GHz, Q = 4000 Lattice HOM: 34 GHz, Q = 100

Traveling Wave Theory  Use cold test of structure to establish mode properties Insertion loss Group velocity Mode Q  Traveling wave theory for mode excitation  Power emitted by beam can be expressed analytically vgvg c Q4000 I1.04 dB/m r98 MΩ/m L29.15 mm

Measured 17 GHz Wakefields  Output Port diode measurement  No fitting parameters, excellent agreement P b (Theory)

Measured 34 GHz Wakefields  Output Port diode measurement  Simulations within an order of magnitude Quadratic fit

Experimental Results Summary  Summary of measurements for 100 mA average current  Observations made on Chamber window as well as Output Port  Multiples of GHz observed up to 85.7 GHz with heterodyne receiver

Summary  PBG wakefields observed  17 GHz results agree quite well with traveling wave theory  34 GHz results can be explained by wakefield simulations to within an order of magnitude

PBG Breakdown  SLAC standing wave breakdown experiments  PBG structure design  PBG cold test and status  Preliminary results

SLAC Setup  TM 01 Mode Launcher Standard rectangular waveguide to cylindrical TM 01 mode conversion Peak field kept low  Single Cell SW Cavity Consists of input and end coupling cells for matching Central test cell ½ field in matching cells, full field only in test cell New design uses PBG as central test cell

Accelerating Gradient [MV/m] Breakdown Rate vs Gradient Pillbox #1 Pillbox #2 Pillbox #3 *Dolgashev, 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 high power testing Tuning Parameters Input Cell Radius mm PBG Cell Radius38.87 mm End Cell Radius mm Coupling Iris Radius5.132 mm PBG Rod Radii2.176 mm PBG Rod Spacing mm

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

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

Electric Field Plots  Electric field plots: top and side views  5.9 MW in = 100 MV/m gradient = 208 MV/m surface field on iris

Magnetic Field Plots  Magnetic field plots: top and side views  5.9 MW in = 100 MV/m gradient = 890 kA/m surface field on inner rod

Structure Parameters PillboxChokePBG Stored Energy [J] Q-value8.38E E E+03 Shunt Impedance [MOhm/m] Max. Mag. Field [A/m]4.18E E E+05 Max. Electric Field [MV/m] Losses in one cell [MW]  Single cell breakdown experiment structures  All for 100 MV/m accelerating gradient

PBG Structures, The Next Generation  1 st PBG structure test made using: a/b = rows of rods of a triangular lattice of cylindrical rods  Relatively high pulsed heating on inner row of rods 87 K for 100MV/m gradient and 100ns  Next generation Lots of possible tuning parameters with broken symmetry PBG with low pulsed heating, high gradient, damping

Fabricated

Structure Brazed

Structure Cold Test  Non-resonant beadpull  Coupling and Q measurements  Simulations confirm results Mode Properties Simulationf GHz Q0Q Measuredf GHz Q0Q0 7401

Structure Installed

Structure Bunker

Scope Traces  5 MW in, 92 MV/m gradient, 150 ns pulse length Power [watts] Time [seconds]

Analysis Process  Breakdowns counted on Scope Traces  Time for breakdown data from Scope Traces  Power level from same time span Peak Power Meter  Power converted to Gradient, Surface Electric field, Surface Magnetic field using HFSS simulations  HFSS simulations checked against cold test results

PBG Breakdown Data  Preliminary data for PBG structure  150 ns Pulselength

PBG Breakdown Data  Preliminary data for PBG structure  300 ns Pulselength

Summary  Breakdown in PBG structures under investigation  First “realistic” PBG structure  Highest gradient PBG already observed, >100MV/m  Data analysis begun

Ongoing and Future Work  Structure has finished high power testing  Highest pulsed heating structure tested  Only second damped structure test  Analysis proceeding for comparison with undamped geometry

Talk Summary  High gradient research necessary for future linear collider concepts and High Energy Physics advances  High frequency research requires HOM damping  The nature of HOMs in PBG structure are understood  Wakefields have been measured in PBG structures  Results agree very well with theory for fundamental, Results can be explained with simulations for HOMs

Talk Summary  Breakdown in PBG structures is being investigated  Structure fabricated and cold tested successfully  High power testing complete  Very exciting initial results for damped structure  Structure performance to be compared with undamped geometry

Funding Acknowledgement This research is funded by the US Department of Energy, Office of High Energy Physics

Collaboration Acknowledgement  Colleagues at MIT: Rick Temkin, Michael Shapiro, Jags Sirigiri, Brian Munroe  CTR and SPR work done with Amit Kesar, now at Soreq  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 in collaboration with Sami Tantawi and Valery Dolgashev at SLAC. Cold tests with Jim Lewandowski and High power testing with Dian Yeremian.

 Any Questions? Thank You

HFSS Mesh  700k Elements  Run on 8 processors, 32 GB memory

Cold Test Comparison  Comparison of Beadpull tests  Done with Jim Lewandowski