AAC 2008

Design of Advanced Photonic Bandgap Accelerator

Structures

M. A. Shapiro, R. A. Marsh, B. J. Munroe, R. J. Temkin

MIT Plasma Science and Fusion Center

(see also R. A. Marsh et al., PBG Wakefields Expt.)

Work supported by DOE HEP

AAC 2008

OUTLINE

• Introduction to PBG Accelerator Research

• HOM and Wakefields Simulation

• Advanced PBG Structures

• Conclusions

AAC 2008

Motivation

Advanced Structures Needed for Wakefield Damping Slots for damping

SLAC Damped Detuned Structure

Shintake Choke-Mode Structure

MIT PBG Structure

AAC 2008

Photonic Bandgap Cavity

Pillbox Cavity, TM01 mode

2a

PBG Cavity, triangular lattice a/b=0.15, TM01 –like mode

bdefect

Fundamental mode TM01 in

bandgap 2D Lattice theory says: no

HOM confined in defect For a/b<0.2, only low

frequency bandgap exists

AAC 2008

Accelerator with PBG cells

6.97 mmLattice vector, b

2.16 mmIris radius

25.2 P[MW] MV/m Gradient

1.08 mmRod radius, a

17.140 GHzFrequency

2/3Phase shift per cell

0.013cGroup velocity

23.4 kΩ /m[rs/Qw]

98 MΩ /mrs

4188Qw

6.97 mmLattice vector, b

2.16 mmIris radius

25.2 P[MW] MV/m Gradient

1.08 mmRod radius, a

17.140 GHzFrequency

2/3Phase shift per cell

0.013cGroup velocity

23.4 kΩ /m[rs/Qw]

98 MΩ /mrs

4188Qw

Disk loaded PBG structure.Open to free space for HOM damping.Irises as in disk loaded waveguide.

Accelerator parameters

AAC 2008

Accomplishments: PBG Accelerator Expt.

E. I. Smirnova et al., Physical Review Letters (2005).

First successful experimental PBG accelerator demonstration.

Tested to gradient 35 MeV/m, limited by available power

AAC 2008

OUTLINE

• Introduction to PBG Accelerator Research

• HOM and Wakefields Simulation

• Advanced PBG Structures

• Conclusions

AAC 2008

Lattice Dipole HOMs

HFSS simulations show HOMs in PBG structure Field not confined in central region (defect), but in lattice Low Q, Q<300

Pillbox Dipole Mode 23 GHz Q = 9500

Lattice Dipole Mode 24.9 GHz Q = 63

AAC 2008

Spectrum of Lattice HOMsLo

g A

mp

(dB

)

16 18 20 22 24 26-100

-80

-60

-40

-20

0S21

[dB

]

Frequency [GHz]

Your text

Cold test

HFSS

Lattice HOMs

Simulations

explain HOM continuum spectrum measured

AAC 2008

Wakefields Theory and Simulations

• Conventional wakefield theory (K. Bane et al., 1987) can

be used for PBG wakefields calculation

– Requires r, Q for each mode

– Calculations underway at MIT

• SLAC codes (T3P) can be used to simulate wakefields in

PBG structure

– We collaborate with SLAC and STAAR, Inc. on wakefields

simulations in PBG structures.

AAC 2008

Animation of Beam Transit

AAC 2008

OUTLINE

• Introduction to PBG Accelerator Research

• HOM and Wakefields Simulation

• Advanced PBG Structures

• Conclusions

AAC 2008

Plans for Advanced PBG Structures

• First PBG structure built at 17 GHz

– Tested at MIT for gradient, wakefields

• Second PBG structure being built for gradient

testing at 11.424 GHz (MIT/SLAC collaboration)

• Future PBG structures at 11, 17 GHz are being

designed

– Reduced pulsed heating

– Gradient 100 MeV/m

– Low breakdown rate

– Free of wakefields

AAC 2008

PBG Structure Detuning

23.0 GHz TM11, Q=72

Structure allows detuning dipole modes TM11

17.13 GHz TM01

Lattice rotated by 30 deg. from cell to cell

AAC 2008

Pulsed Heating

SLAC DDS StructureΔT=55OC for 70 MV/m

(Z. Li et al., SLAC-PUB-8647, 2000)

PBG StructureH-field distribution

ΔT=40OC for 70 MV/m gradient,

Hmax=0.56 MA/m, 100 ns pulse

length

AAC 2008

New Ideas for Optimized PBG Structures

Optimize structure for dipole mode

damping and reduced pulsed heating Shaped rods, not circular Distortion of lattice geometry

Similar to proposal of G. Werner et al.

(Colorado) for dielectric PBG design

Example of PBG structure with elliptical

rods to improve pulsed heating

Complex Mag E

Complex Mag H

ΔT=11OC for 70 MV/m gradient,

Hmax=0.3 MA/m, 100 ns pulse length

AAC 2008

Conclusions

• PBG structure under investigation for linear collider

• Wakefields in form of Dipole HOMs calculated.

– Lattice HOMs with low Q

• Calculation results can be compared to PBG wakefields experiment (next talk).

• Collaboration with SLAC and STAAR Inc. on wakefields in PBG structure

• Advanced PBG accelerator under design for testing 11 and 17 GHz

– 17 GHz structure for test at MIT, 11 GHz at SLAC

– Reduced pulsed heating, comparable to DDS

– Extremely low HOM wakefields, much lower than in DDS