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Subashini De Silva

Center for Accelerator ScienceDepartment of Physics, Old Dominion University

andThomas Jefferson National Accelerator Facility

ODU/SLACRF-DIPOLE PROTOTYPE

LHC Crab Cavity Engineering Meeting – FNAL 13-14 December, 2012

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Introduction• An overview of the ODU-SLAC cavity development and

construction at 400 MHz toward a crab cavity for the LHC

• Cavity design and present status including– Deflecting mode characteristics– HOM spectra– Damping schemes– Coupler configurations and associated choices should be

addressed

• Cavity fabrication, treatment and recent test results

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Current LHC Crabbing Cavity Requirements

• Local crabbing scheme frequency – 400 MHz• Requires a crabbing system at two interaction

points (IP1 and IP5) Vertical crossing at IP1 Horizontal crossing at IP5

• Beam aperture diameter – 84 mm• Transverse dimensions ~ 145 mm • Transverse voltage – 10 MV per beam per

side• Transverse voltage per cavity – 3.4 MV• Awaiting on beam tolerances based on field

non-uniformity across the beam aperture 42 mm

<150 mm

194 mm

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RF Dipole Cavity Geometry• Operates in a TE-like mode

E Field

H Field

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Characteristics of the RF-Dipole Cavity• Properties depend on a few parameters

– Frequency determined by diameter of the cavity design– Bar Length ~λ/2– Bar height and aperture determine EP and BP

– Angle determines BP/EP

Cavity Length

Bar Length

θ

84 mm

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400 MHz Crabbing Cavity DesignsODU Design SLAC Design

Peak electric field (EP*) 3.86 3.75 MV/m

Peak magnetic field (BP*) 6.9 6.85 mT

BP* / EP* 1.79 1.83 mT / (MV/m)

Stored Energy (U*) 0.18 0.17 J

Geometrical factor (G = QRS)

115.0 152.9 Ω

[R/Q]T 315.7 331.1 Ω

RTRS 3.6×104 5.1×104 Ω2

At ET* = 1 MV/m

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Square RF-Dipole Cavity• Square-type rf-dipole cavity to further reduce the transverse dimensions• Frequency is adjusted by curving radius of the edges• Similar to cylindrical rf-dipole design

– Bar Length ~λ/2– Bar height and aperture determine EP and BP

– Angle determines BP/EP

Height and Width

< 290 mm

x

y

E Field H Field

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HOMs and Wakefields

1.0E-02

1.0E-01

1.0E+00

1.0E+01

1.0E+02

1.0E+03

0 500 1000 1500 2000

R/Q

(Ω)

Frequency (MHz)

Ex, Hy Ez Ey, Hx

• No lower order modes and widely separated HOMs

• Separation from the fundamental crabbing mode is ~200 MHz

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HOM Damping Options

Waveguide Damping• Strong damping can be achieved with

waveguide couplers

Coaxial Coupling• Coaxial couplers requires a high pass

filter to exclude the operating mode

• SLAC ACE3P Suite – Zenghai Li

Two-stage high-pass filter

Input Coupler

Three-stage high-pass filter

HOM Couplers & Damping, Zenghai Li

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Current Status on HOM DampingWaveguide Damping Coaxial Coupling

Two-stage high-pass filter

Input Coupler

Coupler configurations and associated choiceswill be presented in: HOM Couplers & Damping – Zenghai Li

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Multipacting Analysis• Particle tracking code in the SLAC ACE3P Suite – Zenghai Li and

Lixin Ge

- 0.5MV to 2.6 MV - 1.8 MV to 2.8MV - 3.0 MV to 6.0 MV

Modified end plates to suppress multipacting at lower fields

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Field Non-Uniformity and Multipoles(A) (B) At a transverse voltage of 1 MV

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0 2 4 6 8 10 12 14 16 18 20

δVT

/ VT

Offset (mm)

Design (A) in xDesign (B) in xDesign (A) in yDesign (B) in y

Voltage deviation at 20 mm– Horizontal: 5.0% 0.2%– Vertical: 5.5% 2.4%

  Cylindrical Cavity

Square Cavity

Modified Square Cavity

Units

b3 3.0×102 4.1×102 1.0×102 mT/m

b4 0.0 0.0 0.0 mT/m2

b5 -4.6×104 -4.1×104 -2.2×104 mT/m3

b6 0.0 0.0 0.0 mT/m4

b7 -1.03×107 -2.0×107 -6.9×107 mT/m5

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Properties of RF-Dipole Designs

Parameter Cylindrical Cavity Square Cavity Unit

Nearest HOM 589.5 597.2 MHz

Deflecting voltage (VT*) 0.375 MV

Peak electric field (EP*) 3.9 3.86 MV/m

Peak magnetic field (BP*) 7.13 6.9 mT

[R/Q]T 287.3 315.7 Ω

Geometrical Factor (G) 138.7 115.0 Ω

RTRS 4.0×104 3.6×104 Ω2

Transverse voltage per cavity 3.4 MV

Peak magnetic field (BP) 35.4 35.0 mT

Peak electric field (EP) 64.7 62.6 MV

Operating temperature 2.0 4.2 2.0 4.2 K

Surface Resistance (RS)** (Rres = 10 nΩ) 11.3 70.0 11.3 70.0 nΩ

Static heat load per cavity ** From cryomodule design specifications W

Dynamic heat load per cavity ** 3.3 20.1 3.6 22.4 W

Q0 ** 12.2 1.2 10.2 1.6 ×109

At ET* = 1 MV/m ** Estimated

Cylindrical Cavity

Square Cavity

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Cavity Prototype Fabrication

110

500

220

499 MHz Prototype400 MHz Prototype

Input Power

CouplerPick Up

Probe

Pick Up

Probe

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Prototype Test Plan• Variable power coupler

– To process multipacting

• Cavity processing– Bulk BCP for 120 μm removal from the surface– High pressure rinsing– Baking for 10 hours at 6000C – Light BCP of 10 μm– In-situ baking

• Cavity assembly– Fixtures to support cavity in the test cage

• RF Test– Low power test while cooling down the cavity– High power test at 2 K and 4.2 K

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• Goal – To design a cavity for testing at SPS and future test at LHC, meeting the design requirements

• Optimize cavity geometry (ODU&SLAC) to,– Suppress multipacting levels– Revise the design to address mechanical specifications

• Stress• Pressure Sensitivity• Lorentz force detuning• Achieve design rigidity with adequate stiffening

– Power and HOM coupler designing• To achieve required damping requirements• Easy chemical processing of couplers

• Cyomodule design– Cavity tuning and He tank designing – HyeKyoung Park (ODU/JLAB)– Cryomodule designing – Dmitry Gorelov (Niowave)

Final 400 MHz Crab Cavity Design

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Summary• The current 400 MHz rf-dipole crabbing cavity design meets current

requirements on– Dimensional constraints– Electromagnetic peak surface field and transverse voltage specifications

• 400 MHz rf-dipole prototype– Is in preparation for surface treatment and VTA assembly – RF testing will be performed early 2013

• Ready to continue working on designing the final cavity desgin• Currently there are several viable electromagnetic design options• The final selection will be based on the requirements on

– Electromagnetic– Mechanical– Dimensional

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Acknowledgments• Work supported by the ODU-Niowave P1 & P2 STTR

• Work also supported by the US LHC Accelerator Research Program (LARP) through US Department of Energy contracts DE-AC02-07CH11359, DE-AC02-98CH10886, DE-AC02-05CH11231, and DE-AC02-76SF00515.

• ODU– Jean Delayen– Subashini De Silva– HyeKyoung Park– Julius Nfor– Alex Castilla

• SLAC– Zenghai Li– Lixin Ge

• Niowave– Terry Grimm– Dmitry Gorelov