X-band Accelerator Structures
R&D at SLAC
Juwen Wang
SLAC/LLNL Discussion
March 5, 2011
1. Introduction• Brief history• Achievements
2. Basics of X-Band Accelerator Structures• Design Principle and Description• Accelerator Structures Performance
3. Structure Assembly Technology• Mechanical QC and Microwave QC• Chemical cleaning• Accelerator parts joining (diffusion bonding, brazing
and welding)• Microwave tuning and characterization• Vacuum baking• Alignment
4. Discussion on Fabrication Technology and Locations
Outline
Contributors
SLAC: C. Adolphsen, N. Baboi, K. Bane, G. Bowden, D.L.
Burke, J. Cornuelle, S. Doebert, V. Dolgashev, A. Hasse, H.
Hoag, E. Jongewarrd, K. Jobe, R.M. Jones, R. Kirby, k. Ko, Z.
Li, G.A. Loew, J. Lewandowski, R.J. Loewen, D. McCormick,
R.H. Miller, C. Nantista, C.K. Ng, E. Paterson, C. Pearson, N.
Phinny, T. Raubenheimer, M. Ross, R.D. Ruth, S. Tantawi, K.
Thompson, J. Van Pelt, F. Wang, J. W. Wang, P.B. Wilson.
KEK: Y. Funahashi, Y. Higashi, T. Higo, N. Hitomi, H. Kudo, T.
Kume, H. Matsumoto, Y. Morozumi, K. Takata, T. Takatomi, N.
Toge, K. Ueno, Y. Watanabe.
FNAL: T. Arkan, H. Carter, D. Finley, I. Gonin, T.
Khabiboulline, S. Mishra, G. Romanov, N. Solyak.
LLNL: J. Klingmann, K. Van Bibber.
1. Introduction• Brief history
• Achievements
Achievements of X-Band
Structures R&D at SLAC
• Motivation:Main Linac for the Future Linear Colliders (NLC, GLC and CLIC)
• Brief History:1. 1988 – 2004 X-Band accelerator structures R&D for the NLC/GLC
in collaboration with KEK, FNAL and LLNL.
Designed, fabricated and tested 50 X-Band accelerator structure sections.
(Among them, 8 were made with KEK collaboration and 12 were fabricated by
FNAL).
2. 2007 – Present X-Band accelerator structures R&D for the CLIC
main linac in collaboration with CERN and KEK. Participated the design, fabrication and testing of more than 12 X-Band
accelerator structure sections.
3. Ongoing Support Work for LLNL Project of the Compton
Scattering Light Source MEGa-Ray.
Evolution of Structures
New typesof couplers
Optimized cell shape
Contribution to the Accelerator Technology
through NLC/GLC X-Band Structures R&D
• Theoretical analysis for full understanding of HOM suppression in RF
accelerator structures.
• Damped and Detuned Structures can be applied to any low emittance,
high beam loading accelerators.
• Simulation methods for beam-structure interaction: structure wakefield,
emittance growth and analysis of structure alignment and dimension
tolerances.
• Optimization of accelerator parameters for highest RF efficiency and
dimension determination with sub-micron precision.
• Manifold damping gives structure position monitor with micron transverse
sensitivity and frequency multiplexed longitudinal resolution of the order
of several cells.
• Fabrication technologies for normal conducting accelerator structures
such as precision machining, diffusion bonding and long structure
alignment.
• Extensive studies for high gradient RF operation to meet the NLC
requirement rate at 65 MV/m: new types of couplers, Procedure for
structure treatments.
2. Basics of X-Band
Accelerator Structures
• Design Principle and Description
• Accelerator Structures Performance
Requirements of Accelerator
Structures for Linear Colliders
• High Accelerating Gradient to
Optimize Length and Cost.
• Control of Short and Long-
Range Wakefields to Ensure the
Preservation of Low Emittance for
Multi-Bunch Beams.
Transverse wakefields - II
Computed transverse δ-function wake potential per cell for S-Band SLAC structure.
Solid line: Total wakeDashed line: 495 modesDot-dashed line: lowest frequency dipole
mode (λ=7 cm)
Multi-bunch Beam Breakup due to the long range transverse wakefields.
Single Bunch Emittance Growth (Head-Tail Instability) due to the short range transverse wakefields
Early Studies on Two Types
of Heavily Damped Structures
Example of structure with radial slots in iris.Example of structure with circumferential-slot coupling: crossed-waveguidestructure, with two half-cells and one full cell.
Test Cells for Damped Structures
in 1980s at SLAC
CERN CLIC
Waveguide Damped (WDS) Structure
• Minimize
E-field
• Minimize
H-field
• Provide
good HOM
damping
• Provide
good
vacuum
pumping
Long Range Dipole Mode Suppression
- Idea of Detuning of Dipole Modes
1
dnk
df
Cells for a Detuned Structure have profiles with Gaussian dimensional distribution.
Dipole mode distributionfor Detuned Structure
In the time domain, the excited wakefield by the cells with Gaussian distribution dipole frequencies has Gaussian amplitude profile.
•Treat each cell as periodic.
•Calculate several sample
cells to obtain dispersion
curves for studying
synchronous kick factor
and avoided crossing
(coupling).
•Fit dispersion curves
of sample cells to
obtain cell parameters
for equivalent circuits.
• Interpolate to obtain
parameters of all cells
• Solve coupled circuit
system.
• Integrate spectrum for
wake in order to provide all
important design
information to optimize
cell-manifold parameters
12
14
16
18
20
22
0 30 60 90 120 150 180
Phase (deg)
F1 (
GH
z)
avoided
crossing
Long-Range Wakefield Calculation
Precision Fabrication
for Accelerator Discs
Profile tolerance 1 μm and Surface finishing better than 50 nm
Microwave QC for Single Disk
Stack Microwave QC Setup
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
0 50 100 150 200
Single-disk RF-QCdel_sf00del_sf0pidel_sf1pidel_sf20
Fre
qu
en
cy D
evia
tio
n [
MH
z]
Disk number
Single-Crystal Diamond Turning Polycrystalline Diamond Turning
Super Precision Machining with Single
Diamond Cutter – Tuning Not Needed
-3
-2
-1
0
1
2
3
0 50 100 150 200
Accelerating mode frequency
2b
off
set
[mic
ron
]
Fre
qu
en
cy [
MH
z]
Inte
grate
d P
hase
Sli
p [
deg
ree]
Disk number
2b_offset
Integrated Phase SlipMeasured Frequency
Regular Precision Machining with
Polycrystalline Diamond Cutter – Tuning Needed
Microwave QC of Fundamental Modes for H60VG4SL17A/B Regular CupsTemperature and humidity corrected
Microwave QC of Dipole Modes for H60VG4SL17A/B Regular CupsTemperature and humidity corrected
Regular Precision Machining with Polycrystalline
Diamond Cutter – Tuning Needed (Continued)
Port for Terminatingand Extracting Dipole Mode Power
High PowerRF Coupler
Prototype Accelerator Structure
for the NLC/GLC Main Linac
Cutoff view of a structure end
A 60 cm structure with most of final design features
Damped Detuned Structures for
the NLC/GLC
DDS1 (Round Damped Detuned) 2π/3 Mode TW StructureSingle diamond turning discs without tuning; Micron level cell-to-cell alignment.
High Gradient Test Structures
One of four T-type Structures --
T53VG3, 60-Cell 2π/3 Mode TWSW20PIL
15-Cell π Mode SW
One of more than
10 High Phase
Advance 5π/6
Mode TW
Structures,
H60VG3S18 with
HOM Slots and
Manifolds.For the LLNL Campton
Scattering Light Source.
Theoretical and Experimental Proof of
Transverse Wakefield Suppression
Comparison of the measurement for a pair of dipole Interleaved
60 cm Damped Detuned X-Band Structures with error bars (red)
and calculated wakefield (black) – Data from early 2005.
RFRF
Beam’s eye view of input coupler.
SEM picture of input matching iris.
Pulse heating was in excess of 100°C.
Performances of some structures
were found to be limited by pulse
heating of coupler matching irises.
Distribution of Breakdowns
(70 MV/m, 400 ns, 10 hr run)
T53VG3
Input coupler Output coupler58 Cells
Rate in cells
.1/hr
Edge Damage on Cavity Side of
Coupling Iris due to RF Pulse Heating
RF Pulse Heating causes:
• Surface Roughening and Cracks
• Local Surface Melting
Field Distribution in Coupler
Region and RF Pulse Heating
Surface temperature
distribution in the region of
coupler iris for 400 ns pulses,
48 MW.
The maximum temperature
increase was 127º C.
Surface resistivity
Thermal conductivity
Specific heatMagnetic field Pulse width
Temperature increase
due to RF pulse heatingc
RTHT s
pt
2
Improved Coupler Design
By proper choice of
matching cell b dimension,
matching cell length can be
made equal to standard cell
length.
|Es|max= ~34 MV/m @ 48 MW
|Hs|max= ~98.4 kA/m @ 48 MW Pulse Heating ~ 3° C
NLC Prototype Structures Can Stably Operate at 65
MV/m to Meet the Required RF Breakdown Rate
Average breakdown rates for a series of NLC test structures as a function of accelerating
gradient after 500 hours (upper line) and 1500 hours (lower line) of RF processing.
Breakdown Rate Dependence on Pulse
Length for Various NLC Structures.
Some of KEK/SLAC Made Accelerator Structures
for Testing CLIC Main Linac Design
T18_VG2.4_DSC with SLAC Flanges TD18_VG2.4_DISC with SLAC Flanges
TD18_VG2.4_DISC with KEK Flanges
T28_VG2.9 (T26) with SLAC Flanges
T18_VG2.4_DISC Structure Test
Cumulated
Phase Change120°
Field
Amplitude
5.1~__ inaccoutacc EE
Microwave Tuning and test High power test set-up
RF BKD Rate Gradient Dependence for 230ns Pulse at Different
Conditioning Time
After 250hrs RF
Condition
After 500hrs RF
Condition
After 900hrs RF
Condition
RF BKD Rate Pulse Width Dependence at Different
Conditioning Time
G=108MV/m
G=108MV/m
G=110MV/mAfter 1200hrs RF
Condition
This performance maybe good enough for 100MV/m structure for a warm collider, however, it does
not yet contain all necessary features such as wake field damping. Future traveling wave structure
designs will also have better efficiencies
CLIC Prototype Structures Can Stably Operate at 100
MV/m to Meet the Required RF Breakdown Rate
3. Structure Fabrication
Technology
• Mechanical QC and Microwave QC
• Chemical cleaning
• Accelerator parts joining (diffusion bonding,
brazing and welding)
• Microwave tuning and characterization
• Vacuum baking
• Alignment
Lathe with Twin Spindles and Twin Turrets
Profile tolerance 5 μm and Surface finishing 300 - 400 nm
ZYGO Surface Flatness Measurement
for Typical Cups of T18_VG2.4_DISC Structures
Both sides show less than 1 micron concaved
17D-A 17D-C
14D-C16D-A
Stacking for Body Diffusion
Bonding of a CLIC Structure
Diffusion Bonding of T18_vg2.4_DISC
Pressure: 40 PSI
Holding for 1 hour at 1020º C
Brazing of QUAD with Water Flange
Au/Cu Alloy: 25/75
Brazing temperature: 1041-1045º C
First Assembly Brazing of T18_vg2.4_DISC
Body / Two Coupler Assemblies / Cooling/One Beam Pipe / Tuning Studs
Au/Cu Alloy: 35/65Brazing temperature: 1021-1025º C
Final Brazing of T18_vg2.4_DISC
Au/Cu Alloy: 50/50Brazing temperature: 979-983º C Adding One Beam Pipe
Flange Welding for a Accelerator Structure
Microwave Tuning and Characterization
Tuning and
Structure Characterization
Example of Phases and Amplitudes
along the Axis of a 77-Cell TW Accelerator
Phases and amplitudes
plotted in a complex
plane (5π/6 mode
structure, 2x150º=300º
per cell for reflection)
Electrical field amplitudes along the structure.
There are small amplitude and phase
modulation due to slightly imperfections of the
couplers, which almost no impact to the power
efficiency and beam acceleration.
Typical modulation due to uncompleted tuning of output coupler
Reflection and Transmission
Reflected S11 from input coupler as a function of frequencies.
Transmission S12 from input port to output port as a function of frequencies.
Wiggles due to the beating of reflections from slightly mismatched input/output couplers.
Transmitted power (left to the load) =
fTf
2
1
2
12S
2
21Sf
Vacuum Baking of Two Structures
650° C
10 days
Alignment Measurement
Using CMM Machine
4. Fabrication Technology and
Locations
Fabrication Locations
1. National Laboratories for the X-Band Structures:• SLAC
• KEK
• LLNL
2. Private Vendors for the X-Band Structures:• US
Robertson Precision, Inc. (California)
LeVezzi Precision, Inc. (Illinois)
• Japan
IHI
Morikawa co.
• Europe
VDL Enabling Technologies Group (Netherland)
Manufacturability
• Case of Small Amount Production (less
than few hundreds)
• Case of Mass Production (10k for future
X-Band compact FEL or even 1.8 millions
precisely machined parts for Linear Collider,
which was studied extensively in late 1990s)
Design of Manufacturability (DFM) Studies
with Huge Cost Reduction