Download presentation
1
Design of Standing-Wave Accelerator Structure
Jeff Neilson, Sami Tantawi, and Valery Dolgashev SLAC National Accelerator Laboratory Add date US High Gradient Research Collaboration Workshop February 9-10, 2011
2
Outline Motivation Conceptual Approach Feed System Design
Cavity Design Fabrication Conclusions Page 2
3
Motivation Provide robust high-gradient (>100 MV/m) accelerator structure Potential advantages of parallel fed, π mode standing-wave (SW) structures over travelling-wave structures minimizes energy available during breakdown maximizes power distribution efficiency enhanced vacuum pumping conductance empirical evidence π mode have lower breakdown rate at given gradient vs. travelling wave structures What is typical fraction of RF drive energy dumped at end of structure Page 3
4
Approach* Individually fed π mode cavities RF source
Directional Coupler Sc = (1 – i + N)-1/2 Accelerator Cavity Nth Accelerator Load What is initial response (large reflection?) Individually fed π mode cavities *S. Tantawi,” RF distribution system for a set of standing-wave accelerator structures”, Phys. Rev., ST Accel. Beams,vol. 9, issue 11 Page 4
5
Approach - Cont Four RF feed ports per cavity Module of 18 cells
eliminate RF driven dipole modes damp long range wakefields maximizes pump conductance Module of 18 cells 60 MW power (100MV/m) 15 MW each arm directional coupling factors would range from to -3dB -loaded , unloaded power Page 5
6
Coupler Design Page 6
7
RF Arm Feed to Cavity Coupling
source Load Accelerator Cavity Short cavity spacing (1.3 cm) precludes use of inline coupler along axis of accelerator structure Optimal configuration has coupler in same plane as cavity Page 7
8
Cross-guide Coupler 3.0 dB coupling 12.5 dB coupling Provides required range of coupling required but not ideal solution large field enhancement on slot edges high construction complexity space limitation would require half-height waveguide (increased loss) Page 8
9
RF Feed Using Cross-Guide Couplers
Page 9
10
Biplanar Directional Coupler*
Electric field for 3dB Coupler Can be designed for coupling over desired range Compact, minimal field enhancement Planar shape – easy to machine *MIT Radiation Laboratory Series, Vol. 8, “Principles of Microwave Circuits” Page 10
11
Coupling Sensitivity to Parameter Variation
d Variation in coupling will reduce average gradient over structure from optimal value Monte Carlo calculation performed varying u, v, d by +/ cm 12.5 dB design has significantly more sensitivity than 3dB design Coupling Histogram for 3 dB Design Tolerance = +/ cm Coupling Histogram for 12.5 dB Design Tolerance = +/ cm Frequency of Occurrence Frequency of Occurrence Difference from Design Value (%) Difference from Design Value (%) Page 11
12
12.5 dB Coupler Measurement
Three 12.5 dB couplers built with +/ cm tolerance Measured coupling values off by 18% Design coupling factor (-12.5 dB) Measured (3 couplers) ( /- 0.1dB) Calculated with (-14.1 dB) measured offsets of u, v, d Page 12
13
Biplanar Coupler Modal Amplitude a vs w a w X WR-90 Natural coupling value for WR-90 (w=2.3cm) waveguide is very close to 3dB Potential coupling of 0.24 (-12.5 dB) for width ~3.1cm Page 13
14
Directivity Rc 10mm 2d d Coupling Page 14 d
15
Improved 12.5 dB Coupler Rc 10.6mm P 15 MW Emax 17 MV/m Hmax 50 kA/m
Coupling Histogram for 12.5 dB Design Tolerance = +/ cm Variation u, v, d, and rc Frequency of Occurrence Rc 10.6mm P 15 MW Emax 17 MV/m Hmax 50 kA/m Difference from Design Value (%) Page 15
16
Cavity Design Page 16
17
Cavity Design Goals Proof of concept
Achieved results will determine relevant applications of SW approach Nominal goal is CLIC G acceleration gradient 100 MV/m iris a/λ 0.11 (average CLIC G) Page 17
18
Cavity Design Four port coupling designed to provide With
rf drive to beam long range wakefield damping high pump conductance With Minimal pulse heating and electric field enhancement maintain high shunt impedance minimizing construction complexity Page 18
19
Cavity Shape Many design options explored
rf choke coupling optimized iris shaping multiple slots (>4) complex cavity shape All designs had excessive surface heating or minimal improvement over simple cavity shape Shaped iris Page 19
20
Simple Cavity Configuration
Width and length of coupler arm Iris radius of curvature Cavity radius of curvature Cavity radius Beam tunnel radius and thickness Circumference radiusing (Rc) Page 20
21
Design Cavity Results for 100 MV/m
Parameter Beam Tunnel radius (mm) 2.75 Iris thickness (mm) 2 Stored Energy [J] 0.153 Q-value 8580 Shunt Impedance [MOhm/m] 103.5 Max. Mag. Field [KA/m] 342 Max. Electric Field [MV/m] 253 Normalized Max. Mag. Field [290 KA/m] Emax/Accel gradient 2.53 Hmax Zo/Accel gradient 1.29 Magnetic Field Page 21
22
Fabrication Page 22
23
RF Feed Using Biplanar Coupler
~ 7 cm ~ 3 cm ~ 24 cm Page 23
24
Planar Geometry 180 Degree Elbow
Electric Field Return Loss Return Loss 15 MW Input Power Emax 23MV/m Hmax 73kA/m Frequency (GHz) Page 24
25
Page 25
26
Page 26
27
Summary & Plans Conceptual design for parallel fed SW structure completed Primary issues for achieving a structure with superior performance to existing TW designs are: uniformity of rf feed system power coupling pulse heating from waveguide coupling to cavities achieving sufficient HOM suppression Construction of 18 cell structure by October 2011 Page 27
Similar presentations
© 2025 SlidePlayer.com. Inc.
All rights reserved.