Transcript LCWS12_B_Woolley_Crab_Cavity_Synchronisation

Synchronization system
for CLIC crab cavities
Amos Dexter, Ben Woolley, Igor Syratchev.
LCWS12, UTA
October 2012
Crab cavity action
Without a correctly functioning crab cavity CLIC looses 90% of its luminosity
The crab cavity system cannot be compromised.
Detector
20 milli-rad crossing
quadrupoles
quadrupoles
40.6 m
travelling wave
crab cavity
@12GHz
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
2
Synchronisation Requirement
Cavity to Cavity Phase
synchronisation requirement
Target max. luminosity
loss fraction S
0.98

720  x f
cc
1
S4rms
 1 degrees
f
(GHz)
x (nm)
c
(rads)
frms
(deg)
12.0
45
0.020
0.0188
Dt (fs)
Pulse
Length (ms)
4.4
Estimate RF to beam synchronisation ~ 100 fs (0.43 degrees)
0.156
RF Distribution
1: A single klystron with
high level RF
distribution to the two
cavities.
2: A klystron for each
cavity synchronised
using LLRF/optical
distribution.
• Klystron phase jitter gets
sent to both cavities for
identical path length. Δφ=0.
• Will require RF path lengths
to be stabilised to within 1
micron over 40m.
• Femtosecond level stabilized
optical distribution systems
have been demonstrated
(XFELs).
• Requires klystron output
with integrated phase jitter
<4.4 fs.
Integration
Use over-moded
waveguide from klystron
to the tee
~35m of waveguide from
the Tee to the cavities,
20.3m from cavities to IP
Waveguide Choice
Timing
error/0.3°C
length
498.9 fs
507.8 fs
No of
modes
45.4%
57.9%
Timing
error/0.3°C
Width
210.5 fs
189.3 fs
TE01
TE01
66.9%
90.4%
804.9 fs
279.6 fs
315.9 fs
471.4 fs
7
17
Mode
Transmission
45.4%
57.9%
Timing
error/0.3°C
length
19.04 fs
19.69 fs
No of
modes
TE10
TE10
Timing
error/0.3°C
Width
8.13 fs
6.57 fs
TE01
TE01
66.9%
90.4%
30.8 fs
10.7 fs
12.1 fs
18.02 fs
7
17
Waveguide type
35 meters COPPER
Expansion = 17 ppm/K
WR90(22.86x10.16mm)
Large Rectangular
(25x14.5mm)
Cylindrical r =18mm
Cylindrical r =25mm
Mode
Transmission
TE10
TE10
Copper coated extra pure
INVAR 35 meters
Expansion = 0.65 ppm/K
WR90(22.86x10.16mm)
Large Rectangular
(25x14.5mm)
Cylindrical r =18mm
Cylindrical r =25mm
1
2
1
2
Rectangular invar is the best choice as it offers much better temperature stability->
Expands 2.3 microns for 35 m of waveguide per 0.1 °C.
6
CLIC LLRF Timing
0.05 km
2.75 km
21 km
Chicane
stretching
PETS
2.75 km
Drive beams
12 GHz
Crab
cavity
21 km
Chicane
stretching
Crab
cavity
PETS
IP
compressor
linac
linac
compressor
Klystron
LLRF
LLRF
LLRF
positrons
Bunch timing pick-ups
2 GHz bunch rep
Crab bunch
timing pickups
could be 2.75 km
away from LLRF
Booster Linac
561 m
2.2 to 9.0 GeV
LLRF
LLRF
electrons
Bunch timing pick-ups
Synchronisation to main beam after booster linac.
For a 21 km linac we have 140 ms between
bunches leaving the booster and arriving at the
crab cavities.
RF path length measurement
RF path length is continuously measured and adjusted
4kW 5ms pulsed
11.8 GHz Klystron
repetition 5kHz
Cavity coupler
0dB or -40dB
Waveguide path length phase and
amplitude measurement and control
Forward
power
main
pulse
12 MW
-30 dB
coupler
-30 dB
coupler
Expansion joint
Single moded
copper plated Invar
waveguide losses
over 35m ~ 3dB
Expansion joint
Magic
Tee
LLRF
Reflected power
main pulse ~ 600 W
LLRF
Reflected power
main pulse ~ 500 W
Phase shifter
trombone
Phase shifter
trombone
(High power joint has
been tested at SLAC)
Main beam
outward
pick up
Cavity coupler
0dB or -40dB
Waveguide from
high power Klystron
to magic tee can be
over moded
Phase
Shifter
Main beam
outward
pick up
From oscillator
48MW 200ns pulsed
11.994 GHz Klystron
repetition 50Hz
Control
Vector
modulation
12 GHz
Oscillator
LLRF Hardware Layout
• Fast phase measurements during the pulse (20-30 ns).
• Full scale linear phase measurements to centre mixers and for calibration.
• High accuracy differential phase measurements of RF path length difference (5 μs, 5 kHz).
• DSP control of phase shifters.
DBM
10.7GHz
Oscillator
DBM
Power
Meter
To
DSP
Linear Phase
Detector
Amp
LPF
DBM
Amp +
LPF
DSP
Wilkinson
splitters
From
DAC2
ADC
ADC
-30 dB
coupler
To Cavity
Piezoelectric Phase Shifter
Calibration Stage
DAC2
DAC1
To Phase
Shifter
-30 dB
coupler
Magic
Tee
To Cavity
Piezoelectric phase
shifter
Analogue Board Development
Front end electronics to enable phase to be measure during
the short pulses to an accuracy of 2 milli-degrees has been
prototyped and dedicated boards are being developed.
MCU
PLL controller
10.7 GHz VCO
Wilkinson splitter
Digital phase
detector
DBMs
400 ns span:
RMS: 1.8 mdeg
Pk-Pk: 8.5 mdeg
90 s span:
Drift rate : 8.7 mdeg/10s
Total drift: 80 mdeg
Power Meters
Inputs
Waveguide Stability Experiments
Long Waveguide
section
12 GHz
source
Waveguide
Splitter
Clamps
Phase Measurement
Electronics
Amplifier
Coax Cable
Loud-speaker
Long Waveguide
section
1 kHz sine wave from 1 W RMS speaker
detected as a 25 milli-degree pk-pk phase
variation.
ADC+DSP scheme
DSP clock
signals
Signal from phase
measurement
system
ADC
ADC
clock
Buffer1
Buffer16
ADC clock
signals
• Use DSP as opposed to FPGA for
increased flexibility, due to low
duty cycle DSP is fast enough.
• Sixteen, 16-bit D flip-flops store
data coming from a single ADC
controlled by the ADC’s clock.
The system then uses pulses
from the DSP to shift the data
once it has been read and
processed.
12
DSP
Sampling Boards
ADC
Buffer board
DSP
System has been manufactured and implemented with 13 noise
free bits. Multiplexer is available for increased I/O.
13
Future
• Comprehensive investigation into phase stability of X-band
klystrons using the 12 GHz stand alone test stand at CERN. (50MW
XL5, Scandinova Modulator)
• Development of feed-forward and/or feedback system to stabilise
the klystron’s output.
• Continued development of electronics to obtain stand alone phase
measurement/correction system.
• Design/procurement of the waveguide components needed.
• Demonstration RF distribution system, with phase stability
measurements. Stand alone or parasitic on CTF-3 dog leg?
• Measure phase noise across the prototype cavity during a high
power test.
14
Summary
• Requirements for synchronisation have been established
• Synchronisation scheme(s) to meet specification has been formulated
• Prototyping of electronics for necessary phase measurements/correction
• Co-ordinated effort on the synchronisation scheme with CERN
• Future high power experiments outlined
Klystron phase and amplitude
control
Phase synchronisation (4.4 fs)
Passive + active feed forward during 156 ns bunch train
1)Same klystron drives both cavities, stabilized waveguide
2)Separate, stabilised klystrons for each cavity.
Phase measurement
Calibration stage and DBM.
Down conversion to ~ 1 GHz, with digital phase detection.
Waveguide phase correction
DSP correction system with waveguide trombone phase
shifters. (Choke mode flange).
Extra Slides
Waveguide Stability Model
Use ANSYS to
find
“dangerous”
modes of
vibration for a
1 m length of
waveguide
fixed at both
ends.
Fundamental
mode 65.4 Hz
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. Develop UK
manufacturing.
Test 2:
Coupler and cavity test –
Final coupler design,
polarised cells, no dampers.
Made with CERN to use
proven techniques.
Test 3:
Damped Cell Testing – Full
system prototype