Page 1

Suba De SilvaJean Delayen, Rocio Olave, Todd Satogata, Geoff Krafft

Center for Accelerator ScienceDepartment of Physics, Old Dominion University

DEFLECTING CAVITY OPTIONS FOR RF BEAM SPREADER IN LCLS II

December 4th, 2013

Page 2

RF Spreader System Requirements

• Three rf cavity design options– Superconducting rf-dipole cavity– Normal conducting rf-dipole cavity– Normal conducting 4-rod cavity

Parameter Value Unit

Electron energy 4.0 GeVAngle of deflection (θdef) 0.75 / 1.0 mradTransverse voltage (VT) 3.0 / 4.0 MV

RF frequency (f) 325 MHz

CDR CHAPTER 7: ELECTRON COMPRESSION AND TRANSPORT

3-Way Beam Spreader

Septum

Vertical RF Transverse Deflector

To Dump

x

zV

t

y

xB

Septum

y

xB

Vertical Beam Separation

Page 3

Superconducting RF-Dipole Cavity

SC RFD Cavity Units

Frequency 325 MHz

Nearest HOM 508 MHz

VT* 0.46 MV

Ep* 2.6 MV/m

Bp* 3.6 mT

Bp*/Ep

* 1.4 mT/(MV/m)

U* 0.049 J

[R/Q]T 2133 Ω

Geometrical Factor 91.5 Ω

RTRS 1.95×105 Ω2

At ET* = 1 MV/m

• RF-Dipole Design

• RF Fields and Surface Fields

• Beam aperture of 40 mm– Considering cavity processing– Low wakefield impedance budget

• Any dimensional constraints ?

Electric field

Magnetic field

CavityLength =

70 cm

Bar length = 41 cm

Bar height = 4.4 cm

θAngle = 50 deg

Cavity diameter =

34 cm

Page 4

Superconducting RF-Dipole Cavity• Required deflection can be achieved by

one cavity

• Compensation for beam loading– Only fundamental deflecting mode is considered– At a beam offset of 5 mm with a transverse voltage

variation of δVT = 0.002VT – Average beam current of 0.02 mA

• Multipacting is expected to be processed easily

VT 4.0 MV

Ep 23 MV/m

Bp 32 mT

Operating Temperature 2.0 / 4.2 K

Surface Resistance (RS) [Rres = 10 nΩ] 10.9 / 58.7 nΩ

Q0 8.4 / 1.6 ×109

Power Dissipation (Pdiss) 0.9 / 4.8 W

QL 5.5×106

Loaded Bandwidth 59 Hz

Compensation for beam loading 1.4 kW

• No lower order modes• Widely separated HOMs

• Reduced field non-uniformity with increased bar height

-0.01

-0.005

0

0.005

0.01

-10 -5 0 5 10

δVT

/ VT

Offset across the beam aperture (mm)

Vertical Horizontal

1.0E-01

1.0E+00

1.0E+01

1.0E+02

1.0E+03

1.0E+04

0 500 1000 1500

R/Q

[Ω]

Frequency [MHz]

Deflecting VerticalAcceleratingDeflecting Horizontal

183 MHz

Page 5

Field Non-Uniformity• Voltage deviation at 20 mm

– Horizontal: 5.0% 0.2%– Vertical: 5.5% 2.4%

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

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

(A)

(B)

• Shaped loading elements– To reduce filed non-uniformity across

the beam aperture– Suppress higher order multipole

components

Page 6

Current RF-Dipole Cavities499 MHz Deflecting Cavity for Jefferson Lab 12 GeV Upgrade

750 MHz Crabbing Cavity for MEIC*

*A. Castilla et.al., in Proceedings of the 3rd IPAC, New Orleans, Louisiana (2012), p. 2447.

• Total crabbing voltage – 13.4 MV per beam per side

• Per cavity – 3.4 MV

400 MHz Crabbing Cavity for LHC High Luminosity Upgrade

• Deflecting voltage – 3.8 MV

• Crabbing voltage– Electron beam – 1.5 MV– Proton beam – 8.0 MV

Page 7

Properties of RF-Dipole Cavity Designs

Frequency 499.0 400.0 750.0 MHz

Aperture Diameter (d) 40.0 84.0 60.0 mm

d/(λ/2) 0.133 0.224 0.3

LOM None MHz

Nearest HOM 777.0 589.5 1062.5 MHz

Ep* 2.86 3.9 4.29 MV/m

Bp* 4.38 7.13 9.3 mT

Bp*/Ep

* 1.53 1.83 2.16 mT/(MV/m)

[R/Q]T 982.5 287.0 125.0 Ω

Geometrical Factor (G) 105.9 140.9 136.0 Ω

RTRS 1.0×105 4.0×104 1.7×104 Ω2

At ET* = 1 MV/m

750 MHz Crabbing Cavity for MEIC at Jefferson

Lab*

499 MHz Deflecting Cavity forJefferson Lab 12 GeV Upgrade

400 MHz Crabbing Cavity for LHC High Luminosity Upgrade

24 cm

44 cm

34 cm

53 cm

19 cm

35 cm

*A. Castilla et.al., in Proceedings of the 3rd IPAC, New Orleans, Louisiana (2012), p. 2447.

Page 8

499 MHz RF-Dipole Cavity• Multipacting was easily

processed during the 4.2 K rf test

• Design requirement of 3.78 MV can be achieved with 1 cavities

• Achieved fields at 2.0 K– ET = 14 MV/m– VT = 4.2 MV– EP = 40 MV/m– BP = 61.3 mT

1.0î 108

1.0î 109

1.0î 1010

1.0î 1011

0.0 5.0 10.0 15.0

Q0

ET (MV/m)

1.0î 109

1.0î 1010

1.0î 1011

0.0 1.5 3.0 4.5

Q0

ET (MV/m)

1.0î 109

1.0î 1010

1.0î 1011

0.0 14.3 28.6 42.9

Q0

ET (MV/m)

1.0î 109

1.0î 1010

1.0î 1011

0.0 21.9 43.8 65.7

Q0

ET (MV/m)

1.0E+09

1.0E+10

0.0 1.5 3.0 4.5 6.0 7.5

Q0

VT (MV)1.0E+09

1.0E+10

0.0 1.5 3.0 4.5 6.0 7.5

Q0

EP (MV/m)1.0E+09

1.0E+10

0.0 1.5 3.0 4.5 6.0 7.5

Q0

BP (mT)

1.0E+09

1.0E+10

0 5 10 15 20

Q0

ET (MV/m)

Quench

3.78

Page 9

400 MHz RF-Dipole Cavity• Multipacting levels were

easily processed

• Achieved fields at 4.2 K– ET = 11.6 MV/m– VT = 4.35 MV– EP = 47 MV/m– BP = 82 mT

• Limited by rf power at 4.2 K• Achieved fields at 2.0 K

– ET = 18.6 MV/m– VT = 7.0 MV– EP = 75 MV/m– BP = 131 mT

1.0E+09

1.0E+10

0 5 10 15 20

Q0

ET (MV/m)1.0E+09

1.0E+10

0.0 1.5 3.0 4.5 6.0 7.5

Q0

VT (MV)1.0E+09

1.0E+10

0.0 1.5 3.0 4.5 6.0 7.5

Q0

EP (MV/m)1.0E+09

1.0E+10

0.0 1.5 3.0 4.5 6.0 7.5

Q0

BP (mT)

Multipacting levels observed below 2.5 MV

1.0E+08

1.0E+09

1.0E+10

0 5 10 15 20

Q0

ET (MV/m)1.0E+09

1.0E+10

0.0 1.5 3.0 4.5 6.0 7.5

Q0

VT (MV)1.0E+09

1.0E+10

0 20 40 60 80

Q0

ET (MV/m)1.0E+09

1.0E+10

0 28 56 84 112 140

Q0

ET (MV/m)

3.4 5.0

QuenchLimited

by rf power

Multipacting levels observed below 2.5 MV

Design goal – 10 MV1.0E+09

1.0E+10

0 5 10 15 20

Q0

ET (MV/m)

Page 10

Normal Conducting RF-Dipole Cavity

NC RFD Cavity Units

Frequency 325 MHz

Nearest HOM 518 MHz

VT* 0.46 MV

Ep* 3.2 MV/m

Bp* 3.8 mT

[R/Q]T 8367 Ω

Geometrical Factor 48.3 Ω

RTRS 4.0×105 Ω2

At ET* = 1 MV/m

• RF-Dipole Design *

• RF Fields and Surface Fields

• Beam aperture of 25 mm– Due to the dependence on transverse

shunt impedance (RT)– Considering cavity processing

Electric field

Magnetic field

Bar length = 31 cm

Cavity length = 37 cm

Cavity height = 26 cm

Bar height = 1.5 cm

Bar width = 6 cm

Cavity width = 15 cm

* T. Luo, D. Summers, D. Li, “Design of a Normal Conducting RF-dipole Deflecting Cavity”, in Proceedings of the 2013 International Particle Accelerator Conference, Shanghai, China, WEPFI091

Page 11

Normal Conducting RF-Dipole Cavity

• Total deflection can be achieved by 6 cavities• Surface heating at the loading elements are reduced by

curving and requires cooling

• RF properties can be further improved with reduced beam aperture

Total Power Requirement

VT 4.0 MV

Ep 28 MV/m

Bp 33 mT

Surface Resistance (RS) 4.7 mΩ

Shunt Impedance (RT) 86 MΩ

Q0 1.03×104

Power Dissipation (Pdiss) 186 kW

Peak dPdiss/dA 158 W/cm2

Per Cavity Power Requirement

VT per cavity 0.67 MV

Ep 4.7 MV/m

Bp 5.5 mT

Q0 1.03×104

No. of Cavities 6

Power Dissipation (Pdiss) per cavity 5.2 kW

Peak dPdiss/dA per cavity 4.4 W/cm2

Page 12

Normal Conducting 4-Rod Cavity

NC RFD Cavity Units

Frequency 325 MHz

Nearest HOM 518 MHz

LOM 226 MHz

VT* 0.46 MV

Ep* 3.4 MV/m

Bp* 7.2 mT

[R/Q]T 1.9×104 Ω

Geometrical Factor 37.3 Ω

RTRS 7.2×105 Ω2

At ET* = 1 MV/m

• 4-Rod Design *

• RF Fields and Surface Fields

• Beam aperture of 25 mm– Due to strong relation with shunt

impedance (RT)

* C.W. Leemann, C. G. Yao, “A Highly Effective Deflecting Structure” in Proceedings of the 1990 Linear Accelerator Conference, Albuquerque, New Mexico, p. 232

Cavity diameter = 45 cm

Cavity length = 45 cm

Rod diameter = 3.1 cm

Rod length = 21 cm

Rod gap = 2 cm

Magnetic field

Electric field

Page 13

Normal Conducting 4-Rod Cavity

• Total deflection can be achieved by 4 cavities• Localized surface magnetic field has higher cooling

requirements per cavity• Surface heating at the end of the rods requires cooling• RF properties can be substantially improved with reduced

beam aperture, compared to NC RFD cavity

Total Power Requirement

VT 4.0 MV

Ep 29 MV/m

Bp 63 mT

Surface Resistance (RS) 4.7 mΩ

Shunt Impedance (RT) 153 MΩ

Q0 8.0×103

Power Dissipation (Pdiss) 104.4 kW

Peak dPdiss/dA 583 W/cm2

Per Cavity Power Requirement

VT per cavity 1.0 MV

Ep 7.3 MV/m

Bp 15.8 mT

Q0 8.0×103

No. of Cavities 4

Power Dissipation (Pdiss) per cavity 6.5 kW

Peak dPdiss/dA per cavity 36 W/cm2

Page 14

499 MHz Normal Conducting 4-Rod Cavity• 499 MHz 2-cell 4-rod cavity*

– Cu coated stainless steel can– Uses parallel cooling mechanism

• RF power coupled using magnetic coupling at the end of the cavity

• Maximum reached rf power = 5.2 kW– Limited by the cooling of rf power coupler

* C. Hovater, G. Arnold, J. Fugitt, L. Harwood, R. Kazimi, G. Lahti, J. Mammosser, R. Nelson, C. Piller, L. Turlington, “The CEBAF RF Separator System”, in Proceedings of the 1996 Linear Accelerator Conference, Geneva, Switzerland, p. 77.

2-cell 4-rod cavity

Frequency 499 MHz

Shunt Impedance (RT) 210 MΩ

QL 2.5×103

Q0 5.0×103

VT ~ 0.75 MV

Max Power Dissipation (Pdiss) per cavity 5.2 kW

Page 15

Summary• Total deflection of 4.0 MV can be

achieved by one cavity using the SC RF-Dipole Cavity

• Considering the distance between rf spreader system and end of linac needs to look into liquid He supply by– A transfer line– A separate refrigerator

• NC RFD requires 6 cavities and has low rf power requirements

• NC 4-Rod cavity requires 4 cavities– Similar rf cavity is currently being used

successfully at Jefferson Lab rf separator system

SC RFD NC RFD NC 4-Rod Units

Frequency 325 MHz

LOM - - None MHz

Nearest HOM 508 518 349 MHz

VT* 0.46 MV

Ep* 2.6 3.2 3.4 MV/m

Bp* 3.6 3.8 7.2 mT

RTRS 1.95×105 4.0×105 7.2×105 Ω2

No. of cavities 1 6 4

VT per cavity 4.0 0.67 1.0 MV

Pdiss per caivty 4.8(At 4.2 K) 5.2×103 6.5×103 W

At ET* = 1 MV/m