1. Odu/slac rf-dipole prototype
LHC Crab Cavity Engineering Meeting – FNAL
13-14 December, 2012
Odu/slac rf-dipole prototype

2. 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

3. 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

4. RF Dipole Cavity Geometry
Operates in a TE-like mode
E Field
H Field

5. 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

6. 400 MHz Crabbing Cavity Designs
ODU 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
RTRS
3.6×104
5.1×104 Ω2
At ET* = 1 MV/m

7. 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
E Field
H Field x y

8. HOMs and Wakefields No lower order modes and widely separated HOMs
Separation from the fundamental crabbing mode is ~200 MHz

9. HOM Couplers & Damping, Zenghai Li
HOM Damping Options
SLAC ACE3P Suite – Zenghai Li
Waveguide Damping
Strong damping can be achieved with waveguide couplers
Coaxial Coupling
Coaxial couplers requires a high pass filter to exclude the operating mode
Two-stage
high-pass filter
Input Coupler
Three-stage
high-pass filter
HOM Couplers & Damping, Zenghai Li

10. Current Status on HOM Damping
Waveguide Damping
Coaxial Coupling
Two-stage
high-pass filter
Input Coupler
Coupler configurations and associated choices
will be presented in: HOM Couplers & Damping – Zenghai Li

11. Multipacting Analysis
Particle tracking code in the SLAC ACE3P Suite – Zenghai Li and Lixin Ge
Modified end plates to suppress multipacting at lower fields
- 0.5MV to 2.6 MV
- 1.8 MV to 2.8MV
- 3.0 MV to 6.0 MV

12. Field Non-Uniformity and Multipoles
(B)
At a transverse voltage of 1 MV
Cylindrical Cavity
Square Cavity
Modified Square Cavity
Units b3
3.0×102
4.1×102
1.0×102
mT/m b4
0.0
mT/m2 b5
-4.6×104
-4.1×104
-2.2×104
mT/m3 b6
mT/m4 b7
-1.03×107
-2.0×107
-6.9×107
mT/m5
Voltage deviation at 20 mm
Horizontal: 5.0%  0.2%
Vertical: 5.5%  2.4%

13. 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
RTRS
4.0×104
3.6×104 Ω2
Transverse voltage per cavity
3.4
Peak magnetic field (BP)
35.4
35.0
Peak electric field (EP)
64.7
62.6
Operating temperature
2.0
4.2 K
Surface Resistance (RS)** (Rres = 10 nΩ)
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
Q0 **
12.2
1.2
10.2
1.6
×109
At ET* = 1 MV/m ** Estimated
Cylindrical Cavity
Square Cavity

14. Cavity Prototype Fabrication
400 MHz Prototype
499 MHz Prototype
110
220
500
Input Power Coupler
Pick Up Probe
Pick Up Probe

15. Prototype Test Plan Variable power coupler Cavity processing
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

16. Final 400 MHz Crab Cavity Design
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)

17. 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

18. 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