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 ??
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