Working Group 3 Summary

Padamsee, Ohmi and Calaga, S. Peggs

Design crab cavity for these beam parameters

Layouts Discussed

Free space = 30 - 60 m

Lateral Space ~ 45 cm ??Small crossing angle (1 mr) does not work, need more space

Gupta:Quad Pairs for (not so) Large Crossing Angle, 4mrad, why not 8 mrad?

Minimum X-ing angle is determined by how close the other beamline can come

Consider the two counter-rotating beams with the first going through a quad.How close the second beam can be?

Displaced quads with the first beam in the quad and counter rotating beam just outside the coil in a field free region.

It is 200 mm for the geometry on the right

Might work for 800 MHz? Later50 m free space !, 45 cm lateral

Most Advanced KEK - Three Damping Strategies

• Coaxial damper is very effective is damping TM010 mode (f = 413 Mhz, MHz, Q < 70), TE111 mode (f = 650, 677 Mhz, Q < 20

• Squashed cavity very effective in raising frequency of unwanted TM110 from 500 MHz to 700 MHz I.e. above the cut-off frequency of large beam pipe.

• Large beam pipe removes all modes f > 1 GHz• Filling factor is low

– < 0.1 m

• We need to come up with different concept with larger filling factor• Multi-cells??

– Each HOM becomes n HOMs– Trapped modes

TM010 Q < 70TE111 Q < 20

RF Absorber

Beam

Stub Support

Crab ModeRejection Filter

RF Absorber

Coaxial InputCoupler

f > 1.3 GHz for Monopole Modef > 1 GHz for Dipole Mode

E

Coaxial Coupler

B

B

B

E

Why squashed cell shape cavity?

TM110 TM010

TM110

TE111

500MHz

500MHz

324MHz

720MHz

UnwantedMode

TM110 - like Mode

500MHz

TM010 - like Mode

413.3MHz

700MHz

650.5 MHz / 677.6MHz

UnwantedMode

Crab ModeCrab Mode

E

B

The squashed cell shape cavityscheme was studied extensively atCornell in 1991 and 1992 forCESR-B under KEK-Cornellcollaboration.

Cavity Radial Size (43 cm)!!

I.R. 20

I.R. 90

I.D. 188

I.D. 120

I.D. 30

I.D. 240

Input Coupler

Monitor Port

I.R.241.5

483

866Coaxial Coupler

scale (cm)

0 50 100 150

1.5 m

KEK-B Cryomodule Size

Argonne ConceptSingle Cells, More Cells in

Cryomodule. Improve Filling Factor

Input coupler

Rejection filter

Power extraction from coax

Use 2-cellsQext - 1100

LBNL Waveguide Damping Concepts

Q ≈ 2000

LBNL ConceptsMulti-cells, Waveguide Dampers

Waveguides to dampLOM, HOM and unwanteddipole mode

•Q ≈ 1500, not < 100•One Monopole mode (0-mode) is trapped due to cavity symmetry• Difficult to be damped either by coaxial insert or waveguides in 3-cell • Consider asymmetry and larger beam pipes

2-Cell super-structure with damping

Waveguide near beam iris to damp unwanted dipole mode (TM) directly- Strong damping on unwanted dipole mode- Modest damping to LOM, 0 mode

Two 2-cell cavity with waveguide in between beam pipe to damp unwanted dipole mode

• Damping TE11 mode in beam pipe • Effective in damping unwanted dipole mode• The waveguide does not couple strongly with the LOMs Best Q’s are still ≈ 1000

Open beam pipe to increase damping to Q ≈ 100

Reduced iris to maximizeR/Q of mode

Tolerances and Other Issues

Sychro-Betatron Coupling Consequences

• Crab cavity causes an increase synchroton tune (for Qx/y < 0.5). Preset tunes 0.31 is OK.

• Instabilities predicted above 1/2 integer betatron tunes. Dispersion makes it worse, may still be OK (Boaz).

• With pair of crab cavities effect is reduced.

Tolerances at 1 mr and 400 MHzLeft-Right crab phase tolerance = Zimmerman

< 0.012o (t<0.08 ps)•at c=1 mrad & 400 MHz

Crab -acc cavity phase tolerance - Zimmerman

< 0.012o (t<0.08 ps)•at c=1 mrad & 400 MHz

To keep emittance growth (due to random offsets) < 10%/hr

< 0.008o •at c=1 mrad & 400 MHz

Ohmi - strong-strong simulation, lumi- lifetime ≈ 1 day (white noise)

< 0.0015o

•at c=1 mrad & 400 MHz

Kick voltage Jitter tolerance - Zimmerman

0.1%

Compare to ILC

Field and Phase Stability Requirements

For Near-Future Projects

Different accelerators have different requirements for field stability!

• approximate RMS requirements:– 1% for amplitude and 1 deg for phase (storage rings, SNS, JPARK)– 0.1% for amplitude and 0.1 deg for phase (linear collider, LCLS)– down to 0.01% for amplitude and 0.01 deg for phase (XFEL, ERL

light sources)

From Matthias Liepe, Cornell, PAC 05

Example: Digital I/Q Control

X

X

90°

0°

X

Vector ModulatorKlystron

Cavity

Master Oscillator

Imeas, Qmeas

Icontrol

Qcontrol

Down Converter

DSP/ FPGA

DAC

DACADC

RFLO

IF

LLRF for J-PARC (KEK)

0 5000

2000

4000

6000

Time [ μ ]s

200 400 6005990

5995

6000

6005

6010

[Time μ ]s

200 400 600-15

-10

-5

0

5

[Time μ ]s

DSP/FPGA Mixer&I/Q

•LLRF system for pulsed n.c. proton linac of J-PARC •FPGA based I / Q control•Field stability exceeds specs (±1% in amplitude and ±1 degree in phase )•Base for STF LLRF system

± 0.04 deg

± 0.08%

S. Michizono et al.

Shown for absolute phase

Cornell LLRF System for ERL operation at QL = 1.2108

Very good field stability demonstrated with 5 mA beam:

0 0.2 0.4 0.6 0.8 112.1

12.2

12.3

12.4

time [sec]acce

lera

tin

g f

ield

[M

V/m

]

0 0.2 0.4 0.6 0.8 19

9.5

10

10.5

11

ph

ase

[deg

]

time [sec]

A/A 1·10 - 4

0.02 deg

How much better can we do for relative cav-cav relative phase ??

Upcoming Test

Final Recommendations

• Size calibration: Crab mode at 400 MHz means fundamental TM010 mode is at 259 MHz…ouch !

• Take a very hard look at 800 MHz, is emittance growth due to non-linearity of RF acceptable?

• Use R12 = 45 m => V = 37 MV for 8 mrad• Use advanced gradient = 10 MV/m• Active length = 3.7 m x 4 (regions)• Filling factor = 0.3 => 12 m• Phase tolerance is x2 more relaxed.

Forwardbeam

ReturnBeam

RF CavityTM010

Can This Work (Maybe 2 Cavities if necessary)

Dampers Dampers

Advantages

• TM010 is the Lowest Order Mode in the Cavity

•No Degenerate Mode to Worry about

•Damping HOMs easier

•Easier manufacturing

•Will the longitudinal E field cause a problem ??