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Photonic Band Gap Accelerator Experiments
Roark MarshMassachusetts Institute of Technology,
Plasma Science and Fusion Center
Accelerator Seminar1/27/2009
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:
1.0 10.0 100.0Frequency (GHz)
0.01
0.10
1.00
TrappingBreakdownPulsed Heating
Gra
die
nt
(GeV
/m)
SLC
NLC
HRC MIT
CLIC
2 MV
3 MV
40 K
120 K
3W
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
TM TE
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
Pillbox PBG a/b=0.15 PBG a/b=0.3
TM01 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
Pillbox PBG a/b=0.15 PBG a/b=0.3
No HOMs HOMs
Q=9000Q=100 Q=1000
Quality factor gives quantitative gauge of damping
HOMs present, but strongly damped in 3D
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
Summary
HFSS used for field simulations
Operating mode in PBG like pillbox TM01
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 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
Accelerator Schematic
Klystron
RF Auxiliary Output
DCGun
SteeringLens
ChopperPrebuncher
Bias BeamMonitor
LinacToroidalLens
HaimsonDeflector
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
PBG Multi-Bunch SimulationMatched Load Output Port
Chamberwindow
Bunch train with 1 mm rms bunch length and 17.5
mm spacing driven through
structure
PBG Multi-Bunch SimulationMatched Load Output Port
Chamberwindow
Bunch train with 1 mm rms bunch length and 17.5
mm spacing driven through
structure
PBG Multi-Bunch SimulationMatched Load Output Port
Chamberwindow
Bunch train with 1 mm rms bunch length and 17.5
mm spacing driven through
structure
PBG Multi-Bunch SimulationMatched Load Output Port
Chamberwindow
Bunch train with 1 mm rms bunch length and 17.5
mm spacing driven through
structure
PBG Multi-Bunch SimulationMatched Load Output Port
Chamberwindow
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 17.14 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
vg 0.0109c
Q 4000
I 1.04 dB/m
r 98 MΩ/m
L 29.15 mm
Measured 17 GHz Wakefields
Output Port diode measurement No fitting parameters, excellent agreement
Pb (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 17.14 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 TM01 Mode Launcher
Standard rectangular waveguide to cylindrical TM01 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 #1Pillbox #2Pillbox #3
*Dolgashev,AAC 2008
X Band PBG Structure Test
SLAC test stand with reusable TM01 mode launchers MIT designed PBG structure for high power testing Under high power testing
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
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
Pillbox Choke PBGStored Energy [J] 0.298 0.333 0.3157
Q-value 8.38E+03 7.53E+03 6.28E+03
Shunt Impedance [MOhm/m] 51.359 41.34 35.937
Max. Mag. Field [A/m] 4.18E+05 4.20E+05 8.86E+05
Max. Electric Field [MV/m] 211.4 212 208
Losses in one cell [MW] 2.554 3.173 3.65
Single cell breakdown experiment structures All for 100 MV/m accelerating gradient
PBG Structures, The Next Generation
1st PBG structure test made using: a/b = 0.18 3 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
Simulation f 11.424 GHz
Q0 7663
Measured f 11.4322 GHz
Q0 7401
Structure Installed
Structure Bunker
Scope Traces 5 MW in, 92 MV/m gradient, 150 ns pulse length
Pow
er [
wat
ts]
Time [seconds]
4 . 1 0 7 6 . 1 0 7 8 . 1 0 7 1 . 1 0 6
5 .0 1 0 6
1 .0 1 0 7
1 .5 1 0 7
2 .0 1 0 7
2 .5 1 0 7
3 .0 1 0 7
K lys tr o n P o w e r
R e v e r s e P o w e r
F o r w a r d P o w e r
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 Rate for 150ns pulse
0.1
1
10
100
1000
0 20 40 60 80 100
Gradient [MV/m]
Bre
akd
own
Rat
e [#
/hou
r at
60
Hz]
PBG Breakdown Data Preliminary data for PBG structure 300 ns Pulselength
PBG Breakdown Rate for 300ns pulse
0.1
1
10
100
1000
0 20 40 60 80 100
Gradient [MV/m]
Bre
akd
own
Rat
e [#
/hou
r at
60
Hz]
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 theUS 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
Bead Pull Comparison
0
0.2
0.4
0.6
0.8
1
0 1 2 3 4 5 6
Axial Distance [cm]
|E|
fie
ld [
No
rma
lize
d]
Beadpullbefore test
Beadpullafter test
DesignSimulation
