Design of Standing-Wave Accelerator Structure

Jeff Neilson, Sami Tantawi, and Valery DolgashevSLAC National Accelerator Laboratory

US High Gradient Research Collaboration Workshop February 9-10, 2011

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Outline

• Motivation• Conceptual Approach• Feed System Design• Cavity Design• Fabrication• Conclusions

Motivation

• Provide robust high-gradient (>100 MV/m) accelerator structure

• Potential advantages of parallel fed, πmode standing-wave (SW) structures over travelling-wave structures– minimizes energy available during breakdown– maximizes power distribution efficiency– enhanced vacuum pumping conductance– empirical evidence πmode have lower breakdown rate at

given gradient vs. travelling wave structures

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Approach*

• Individually fed π mode cavities

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RFsource

Directional Coupler Sc = (1 – i + N)-1/2

Accelerator Cavity

Nth Accelerator Cavity

Load

*S. Tantawi,” RF distribution system for a set of standing-wave accelerator structures”, Phys. Rev., ST Accel. Beams,vol. 9, issue 11

Approach - Cont

• Four RF feed ports per cavity– 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 -

12.5 to -3dB

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Coupler Design

RF Arm Feed to Cavity Coupling

• 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

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RFsource

Accelerator Cavity

Load

Cross-guide Coupler

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3.0 dB coupling12.5 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)

RF Feed Using Cross-Guide Couplers

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Biplanar Directional Coupler*

• Can be designed for coupling over desired range• Compact, minimal field enhancement• Planar shape – easy to machine

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*MIT Radiation Laboratory Series, Vol. 8, “Principles of Microwave Circuits”

Electric field for 3dB Coupler

Coupling Sensitivity to Parameter Variation

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• Variation in coupling will reduce average gradient over structure from optimal value

• Monte Carlo calculation performed varying u, v, d by +/- .0025 cm

• 12.5 dB design has significantly more sensitivity than 3dB design

Coupling Histogram for 12.5 dB DesignTolerance = +/- .0025 cm

vu

d

Coupling Histogram for 3 dB DesignTolerance = +/- .0025 cm

Freq

uenc

y of

Occ

urre

nce

Freq

uenc

y of

Occ

urre

nce

Difference from Design Value (%) Difference from Design Value (%)

12.5 dB Coupler Measurement

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Design coupling factor 0.236 (-12.5 dB)

Measured (3 couplers) 0.20 (-14.0 +/- 0.1dB)

Calculated with 0.198 (-14.1 dB)measured offsets of u, v, d

• Three 12.5 dB couplers built with +/- .0025 cm tolerance• Measured coupling values off by 18%

a a

w

• 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

XWR-90

Modal Amplitude a vs w

Biplanar Coupler

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Rc 10mm

Directivity

Coupling

2d d

dPage 14

P 15 MWEmax 17 MV/mHmax 50 kA/m

Rc 10.6mm

Coupling Histogram for 12.5 dB DesignTolerance = +/- .0025 cmVariation u, v, d, and rc

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Freq

uenc

y of

Occ

urre

nce

Difference from Design Value (%)

Improved 12.5 dB Coupler

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Cavity Design

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)

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Cavity Design

• Four port coupling designed to provide– 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

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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

Page 19 Shaped iris

Width and length of coupler arm

Iris radius of curvature

Cavity radius of curvature

Cavity radius

Beam tunnel radius and thickness

Circumference radiusing (Rc)

Simple Cavity Configuration

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Magnetic Field

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] 0.153

Emax/Accel gradient 2.53

Hmax Zo/Accel gradient 1.29

Design Cavity Results for 100 MV/m

Fabrication

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RF Feed Using Biplanar Coupler

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~ 7 cm

~ 3 cm ~ 24 cm

15 MW Input PowerEmax 23MV/mHmax 73kA/m

Return Loss

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Planar Geometry 180 Degree Elbow

Frequency (GHz)

Ret

urn

Loss

Electric Field

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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

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