Transcript crab cavity - Indico

CLIC Crab Cavity Infrastructure
Amos Dexter and Benjamin Woolley
CERN
27th October 2011
October 2011
Crab cavity development for CLIC
Main Collaborators
Lancaster University
SLAC
CERN
ASTeC
Manchester University
Graeme Burt, Praveen Ambattu
Valery Dolgashev
Alexej Grudiev, Walter Wuensch, Rogelio Tomas
Peter McIntosh
Roger Jones, Ian Shinton
October 2011
Crab cavity action
Without a correctly functioning crab cavity CLIC looses 90% of its luminosity
The crab cavity system cannot be compromised.
Detector
quadrupoles
quadrupoles
20 milli-rad crossing
travelling wave
crab cavity
Bunches pass through deflecting cavities phased to give zero kick for bunch centres
A deflecting cavity phased in this way is called a crab cavity
For a bunch with length
• The crab cavity kicks the bunch front to start rotating away from the other beamline
• The crab cavity kicks the bunch rear to start rotating towards the other beamline
Perfect alignment of bunches occurs only at the IP
October 2011
Current work
REMAINING EUCARD TASKS
• Gradient tests of a cell design excited in a dipole mode
• Manufacture a multi-cell un-damped cavity for high power tests at CERN
• Complete phase measurement sampling electronics
NEW CERN COLLABORATION
• Development of a damped structure with elliptical cells
 (including a prototype for bench measurements)
• Engineering design work to enable prototype cavities to be tested at CERN
 (cooling, vacuum, mounting, instrumentation etc.)
• Experiments to understand stability of the RF distribution system
• R&D as necessary to improve stability of RF distribution system
• Design of components for active control of the RF distribution path length
ALSO NEEDED
INTEGRATION
October 2011
Planned CLIC crab high power tests
Travelling wave 11.9942 GHz
phase advance 2p/3
TM110h mode
Input power ~ 14 MW
Test 1:
Middle Cell Testing – Low
field coupler, symmetrical
cells
Test 2:
Coupler and cavity test –
Final coupler design,
polarised cells, no dampers.
Test 3:
Damped Cell Testing – Full
system prototype
Cavity length < 200 mm
Cavity width ~ 200 mm
CLIC TW Crab Cavity and Load Cooling ~ 100 W
October 2011
Crab cavity optics
Crab cavities are located near the final focus in linear colliders to get a
large rotation for a given voltage (high R12). (15.2 metres from IP)
Unfortunately this also means a large R34 the effect of wakes are magnified
October 2011
Crab cavity issues
• Wakefields - cause kicks and emittance growth
• Poor Phase Stability - gives large horizontal kicks
• Beam-loading - has large unpredictable fluctuations
on a time scale of tens of ns
Key required outcomes
• Damp, measure and confirm the predicted wakes.
• Establish feasible/achievable level of phase control
performance. (Requirement looks beyond state of the art)
• Need solution which is insensitive to beam-loading
October 2011
RF solution at 12 GHz
Wakefields
Large irises
Small number of cells
Strong damping
Phase and amplitude control
Passive during 156 ns bunch train
Beamloading compensation
High energy flow through cavity
* small number of cells
* high group velocity
* low efficiency
Phase synchronisation
Same klystron drives both cavities
Active waveguide RF length adjustment
Phase reference
Mixing reflected pulses
Optical interferometer
Phase stability
Thick cavity irises
Strong cavity cooling
No mode conversion in RF distribution
Highly stable waveguide
October 2011
Beam timing
If cavities have perfect synchronism then timing issues on one beamline (here the
positron line) can be understood by considering a long bunch symbolised by a
dashed line through the actual bunch. Beam timing errors give head on collisions
with a displaced IP and slight defocusing.
cDterror
t6
t5
Positron crab
cavity position
t3
Electron crab
cavity position
t2
Dxerror
t3
t2
t1
qc
t1
dip
IP
Position of positron
bunch centre when
electron bunch
centre is at IP
Position where
bunches pass
October 2011
Cavity synchronisation
CLIC bunches ~ 45 nm horizontal by 0.9 nm vertical size at IP.
Cavity to Cavity Phase
synchronisation requirement
Target max. luminosity
loss fraction S
0.98
f
(GHz)
12.0
x
(nm)
45

720  x f
cqc
1
S4rms
qc
(rads)
frms
(deg)
0.020
0.0188
 1 degrees
Dt (fs)
Pulse
Length (ms)
4.4
0.156
So need RF path lengths identical to better than c Dt = 1.3 microns
October 2011
RF layout and procedure
travelling
wave cavity
Laser interferometer
Control
Waveguide with micronlevel adjustment
Magic
Tee
Waveguide with micronlevel adjustment
LLRF
Main beam
outward
pick up
LLRF
Phase
Shifter
From
oscillator
main beam
outward pick up
Pulsed
Modulator
12 GHz Pulsed
Klystron
( ~ 50 MW )
Control
Vector
modulation
12 GHz
Oscillator
Once the main beam arrives at the crab cavity there is insufficient time to correct beam to cavity errors.
0. 1 Send pre-pulse to cavities and use interferometer to measure difference in RF path length. (option1)
0. 2 Send off frequency pre-pulse and measure phase difference of reflections. (option2)
0. 3 Use measurement from last high power pulse. (option 3)
1. Perform waveguide length adjustment at micron scale
2. Measure phase difference between oscillator and outward going main beam
3. Adjust phase shifter in anticipation of round trip time and add offset for main beam departure time
4. Klystron output is controlled for constant amplitude and phase
5. Record phase difference between returning main beam and cavity
6. Alter correction table for next pulse
October 2011
Distribution stability requirement
• r.m.s. cavity to cavity synchronisation requirement is 4.4 fs
• hence r.m.s klystron to cavity stability requirement is 3.1 fs (two paths)
• Stability requirement for one transmission leg = 13.4 milli-degrees
Control waveguide temperature to say 0.3oC.
Copper expansivity 17  10-6 K-1.
Width expansion example for circular TE11 (12 mm)
A 12 mm radius could expand by 61 nm hence phase velocity changes by 1146 m/s
For a 45 metre length phase change = 1.55 degrees = 115 times the allowance
Width expansion example for circular TE10 (40 mm)
A 40 mm radius could expand by 204 nm hence phase velocity changes by 281 m/s
For a 45 metre length phase change = 0.52 degrees = 39 times the allowance
Length expansion example for circular TE10 (40 mm) without expansion joints
A 45 metre waveguide could vary in length by 230 mm.
Waveguide wavelength ~ 27.0 mm
Phase shift ~ 3.07 degrees which is 229 times the allowance!
It is clear that the differential transmission length must be measured on each
pulse and corrected. One would also seek to minimise disturbing influences.
October 2011
Waveguide consideration
Assume 45 m waveguide run from Klystron to each Crab cavity
For copper s =5.8e7 S/m and at 11.994 GHz
Attenuation
Transmission
Over moded
Rectangular TE10 EIA90 (22.9 x 10.2 mm)
0.098 dB/m
36.2%
no
Rectangular TE10 special (24 x 14 mm)
0.073 dB/m
47.2%
no
Circular TE11 (r = 9.3 mm)
0.119 dB/m
29.3%
no
Circular TE11 (r = 12 mm)
0.055 dB/m
56.7%
TM10
Circular TE01 (r = 40 mm)
0.010 dB/m
90.1%
extremely
Available klystron has nominal output of 50 MW
Divide output for two beam lines = 25 MW
For standard rectangular waveguide we have 9.1 MW available (OK for 12 cells)
For special rectangular waveguide we have 11.8 MW available
For circular 12mm TE11 waveguide we have 14.2 MW available (OK for 10 cells)
(note that mode conversion from circular TE11 to circular TM10 is
vanishingly small for properly designed bends)
October 2011
Waveguide choice
Probably 40 mm diameter over moded TE01
(as RF path length less dependent on diameter variation.)
Waveguide needs isolation from influences
(changing the RF path length by more than ~ 0.5 microns in 20 milli-seconds.)
The dynamic range of the fast waveguide phase shifters is not likely to be more
than 100 times the range adjustment required between pulses.
Have a choice
use slow waveguide phase shifter to compensate for long term temperature
drifts (differential between cavities on timescales > hours)
or
temperature control.
40 mm waveguide
Damping material with
good thermal mass.
October 2011
Civil engineering
Machine By-Pass added
Emergency escape
tunnel
Survey gallery added
October 2011
Klystron position?
Crab cavities
Klystron
October 2011
Waveguide routing?
klystron in detector
cavern
preferred waveguide route
in own bore then cavern
detector
beam tunnel
crab
cavity
15.2 m
beam tunnel
crab
cavity
awkward waveguide route in
beam tunnel, then detector tunnel,
then cavern
October 2011
CLIC detector halls
Crab cavity klystron?
October 2011