pbg structure experiments, aac 2008 photonic bandgap accelerator experiments roark a. marsh, michael...

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


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


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


HRC Relativistic beam Klystron:

Microwave PowerSource 25 MW @ 17.14 GHz

25 MeV Linac:0.5 m long

94 cells

Structure Test

Stand

MIT 17 GHz Accelerator

700 kV500 MW

Modulator

Photonic Bandgap

Accelerator


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


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

ChamberWindow

MatchedLoad

OutputPort

Window

View from Below


Bunch train with 1 mm rms bunch length and 17.5 mm spacing driven through structure

PBG Multi-Bunch SimulationMatched Load Output Port

Chamberwindow


16 18 20 22 24 26-100

-80

-60

-40

-20

0S21

[dB

]

Frequency [GHz]

Your text

Cold Test of PBG HOMs

17.14 GHz Q = 4000 group velocity = 0.0109

c

Lattice HOMs Q < 250

Low Q Lattice HOMs


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


Measured 17 GHz Beam Loading Output Port diode measurement No fitting parameters, excellent agreement

vg 0.0109c

Q 4000

I 1.04 dB/m

r 98 MΩ/m

L 29.15 mm

Pb (Theory)


Measured 34 GHz Wakefields

Output Port diode measurement Awaiting theory, please help…

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 17.14 GHz observed up to 85.7 GHz with heterodyne receiver


Outline





SLAC Setup

TM01 Mode Launcher

Single Cell SW Cavity Input and end cells for matching Central test cell New design uses PBG as

central test cell


X Band PBG Structure Test

SLAC test stand with reusable TM01 mode launchers MIT designed PBG structure for high power testing Under construction

Tuning Parameters

Input Cell Radius 11.627 mm

PBG Cell Radius 38.87 mm

End Cell Radius 11.471 mm

Coupling Iris Radius 5.132 mm

PBG Rod Radii 2.176 mm

PBG Rod Spacing 12.087 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 S11 reflection from TM01 Mode launcher

Field on axis S11 Coupling reflection


X Band PBG Single Cell Structure Central PBG test cell Pillbox matching cells

First iris radius varied to optimize coupling


½ Field Full Field


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


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


Outline





PBG Structures, The Next Generation

1st 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


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


Any Questions?

Thank You


AbstractDamping 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

pbg structure experiments, aac 2008 photonic bandgap accelerator experiments roark a. marsh, michael...

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