update on design of standing- wave accelerator structure

Update on Design of Standing-Wave Accelerator Structure

Jeff Neilson, Sami Tantawi, and Valery Dolgashev SLAC National Accelerator Laboratory

5th Collaboration Meeting on X-band Accelerator Structure Design and Test Program 16-18 May, 2011

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, 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

Approach*

•  Individually fed π mode cavities

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RF source

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

Coupler Design

RF Feed Using Cross-Guide Couplers

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)

Biplanar Directional 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”

Electric field for 3dB Coupler

Coupling Sensitivity to Parameter Variation

•  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 Design Tolerance = +/- .0025 cm

v u

d

Coupling Histogram for 3 dB Design Tolerance = +/- .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

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

X WR-90

Modal Amplitude a vs w

Biplanar Coupler

Rc 10mm

Directivity

Coupling

2d d

d

P 15 MW Emax 17 MV/m Hmax 50 kA/m

Rc 10.6mm

Coupling Histogram for 12.5 dB Design Tolerance = +/- .0025 cm Variation u, v, d, and rc

Freq

uenc

y of

Occ

urre

nce

Difference from Design Value (%)

Improved 12.5 dB Coupler

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)

Width and length of coupler arm

Iris radius of curvature

Cavity radius of curvature

Cavity radius Beam tunnel radius

and thickness

Circumference radiusing (Rc)

Cavity Design Parameters

Cavity parameters optimized to maximize shunt impedance with minimum enhancement of magnetic surface field relative to closed cavity

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

45 Degree Wedge of Cavity

F 11.424 GHz Ql 5340 β 0.7 P 6.5 MW for 100MV/m acceleration gradient (center cavity)

On Axis Field

Initial Test Configuration

1

2 3

4

Frequency Response (dB)

Cavity Driven Through RF Feed (F = 11.424GHz)

On Axis Field Phase Error (degrees relative to π shift)

-5

0

+5

-20

0

-40

Phase Arm Error on Last Cavity Feed (30 Deg)

Frequency Response (dB)

--- no phase error

On Axis Field Phase Error (degrees relative to π shift)

-5

0

+5

-20

0

-40

Fabrication

RF Feed Using Biplanar Coupler

~ 10 cm

~ 3 cm ~ 24 cm

15 MW Input Power Emax 23MV/m Hmax 73kA/m

Return Loss

Planar Geometry 180 Degree Elbow

Frequency (GHz)

Ret

urn

Loss

Electric Field

~25 cm 2.7 kg

Current Mechanical Design

Summary & Plans

•  Conceptual design for parallel fed SW structure completed, mechanical design underway

•  Primary issues for achieving a structure with superior performance to existing TW designs are: –  pulse heating from waveguide coupling to cavities –  achieving sufficient HOM suppression

•  Evidence of strong coupling through beam holes suggests alternate configurations (3 cells per feed?) may be preferable

•  Construction and test of 5 cell structure late summer

update on design of standing- wave accelerator structure

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