Phase Synchronisation Systems
Dr A.C. Dexter
Overview
Accelerator Synchronisation Examples
Categories of Timing Problems
Oscillators
Clock to Accelerator Cavity
Phase Locked Magnetrons
RF Interferometers
CLIC Crab Cavity Synchronisation
Laser Timing Distribution
Laser to RF
Particle Accelerator Engineering, London, October 2014
Accelerator Examples
Bunch to RF
Off crest acceleration
Voltage gain as
function of relative position
Bunch position when RF field is maximum
Free Electron Laser
Crab Cavity System
Electrons
IP
Positrons
Crab cavity
Crab cavity
quadrupole
quadrupole
25 m
Particle Accelerator Engineering, London, October 2014
Categories of Timing Problems
•
Stability
 Oscillators shift period with temperature, vibration etc.
 Voltage Controlled Oscillator (VCO) shifts period with applied voltage
 Atomic clock Df/f ~ 10-14 ~ 60 fs per minute
•
Synchronisation
 Two clocks with different periods at same place (Phase Locked Loop)
 Identical delivery time/phase at two places (Crab Cavity Problem)
 Same clock at two places
 Resynchronisation requires constant propagation time of signal
 Detector with high resolution and low noise
•
Trigger an event at a later and a different location
 Needs two stable clocks which are synchronised (FEL problem)
 Must be able to generate event from clock pulse with tiny jitter
 Work at DESY and MIT suggest 10fs achievable
Particle Accelerator Engineering, London, October 2014
Oscillators
VCO or Magnetron Oscillator
Oscillator using amplifier
sensitive to
temperature
RF Output
Filter
RF Output
Input and
reflection on
output port
DC Input
(changes frequency)
reflection on
output port
DC Input
(Changes phase)
Phase Locked Loop
(Synchronises oscillators at different frequencies, jitter follows performance of
microwave oscillator and long term stability follows crystal oscillator)
Crystal
Oscillator
Frequency
divider /N
Phase
Detector
Frequency
divider /R
Particle Accelerator Engineering, London 2014
Low Pass Filter /
integrator
Microwave Voltage
Controller Oscillator
Clock to Cavity
LLRF control - feedforward
to next pulse based on last
pulse and environment
measurements
Optical clock signal
Locked microwave oscillator
Solid state amplifier
IQ modulator
Solid state amplifier
TWT amplifier
Waveguide
Absolute timing impossible
as every component and
connector adds phase
uncertainty
Klystron
Waveguide
Pulse compressor
Waveguide
sensitive to
temperature
Cavity
Extremely
sensitive to
modulator
voltage
Magnetron Exciting Superconducting
Cavity
Demonstration of CW 2.45 GHz magnetron driving a specially manufactured
superconducting cavity in a vertical test facility at JLab and the control of
phase in the presence of microphonics was successful.
First demonstration and performance of an injection locked continuous wave magnetron to
phase control a superconducting cavity
A.C. Dexter, G. Burt, R. Carter, I. Tahir, H. Wang, K. Davis, and R. Rimmer,
Physical Review Special Topics: Accelerators and Beams, Vol. 14, No. 3, 17.03.2011, p. 032001.
http://journals.aps.org/prstab/abstract/10.1103/PhysRevSTAB.14.032001
Circuit for Phased Locked Operation
Phase
shifter
Double Balance
Mixer
Spectrum
Analyzer
2.45 GHz
Panasonic 2M137
1.2 kW Magnetron
Oscilloscope
controls power
Load
3
Phase
shifter
Stub
Tuner 2
Loop
Coupler
1W
Amplifier
Circulator
3
Stub
Tuner 1
Circulator
2
Load 2
Loop
Coupler
Cathode
heater
control
Load 1
IQ Modulator
(Amplitude &
phase
shifter)
DAC
ADC
DAC
Digital Signal
Processor
Oscilloscope
Digital Phase
Detector
HMC439
÷2
Agilent E4428 signal generator
providing 2.45 GHz
÷2
Unwanted
300 V DC +5% 120 Hz ripple
LP Filter
8 kHz cut-off
High Voltage
Transformer
42 kHz Chopper
Pulse Width
Modulator SG 2525
1.2 kW Power
Supply
Control Voltage
Sets current
from modulator
and can be in
control loop to
minimise phase
change through
magnetron,
(or to source)
Phase Control Performance
0
0
Injection but
magnetron
off
Power spectral density (dB)
-20
-30
Injection +
magnetron on
+
control
-10
Power spectral density (dB)
-10
-40
-50
-60
-70
-80
-90
-100
-20
-30
-40
-50
-60
-70
-80
-90
-110
-120
-500
-250
0
Frequency offset (Hz)
250
500
Power spectral density (dB)
Cavity phase error (degrees)
0
Injection +
magnetron
on
-10
-20
-30
-40
-50
-60
-70
-80
-250
0
Frequency offset (Hz)
250
500
-250
0
Frequency offset (Hz)
250
500
45
Control on
35
Control off
25
15
5
-5
-15
0.00
-90
-100
-500
-100
-500
0.01
0.02
0.03
Time (seconds)
0.04
0.05
RF Interferometer
Synchronisation when
return pulse arrives at time
when outward pulse is sent
Position along
cable
Far location
adjust effective
position of far
location with a
phase shifter
180o
0o
Near location
time
Interferometer line length adjustment
synchronous
output
Precision reflector
synchronous
output
digital phase
detector
digital phase
detector
loop
loop
filter
filter
coax link
master
oscillator
phase
shifter
directional
coupler
directional
coupler
Phase
shifter
VTF Phase Control Tests
IF
Load
Power meters
Power meters
phase
detector
board A
DBM
divide to
1.3 GHz
synchronous
reference signals
Manual
phase
shifter
Load
cavity
control
16 bit
A/D
DSP
does IQ
conversion
then PI
control
vector
mod.
D/A
Divider
Load
Loop
filter
phase
shifter
Phase
shifter
interferometer line length
adjustment circuits
phase
detector
board B
divide to
1.3 GHz
Manual
phase
shifter
~ 15 metre
low loss
(high
power)
coax link
cavity
control
16 bit
A/D
Load
Rhode & Schwarz SG
used to generate 3.9 GHz
Manual
Phase
Shifter
Load
Phase
shifter
Loop
filter
DSP
does IQ
conversion
then PI
control
D/A
precision reflector circuit
vector
mod.
Daresbury Test 2009
Period
Jitter (degrees)
1
Cavity to cavity control off
10 secs
0.7942
2
Cavity to cavity control on
10 secs
0.0852
3
Cavity to cavity control on
0.05 secs
0.0743
4
Cavity to cavity no interferometer
10 secs
0.0888
5
Cavity to cavity no interferometer
0.05 secs
0.0763
6
Cavity to source 1
0.05 secs
0.0576
7
Cavity to source 1
10 secs
0.0600
CLIC 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
cc
1
S4rm s
c
(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
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
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 40m ~ 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
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
Particle Accelerator Engineering, London, October 2014
12 GHz
Oscillator
Laser Distribution
Diagram from Florian Loehl, Cornell University
Laser to RF
j
t
VLF
Loop filter
The pulses sit on
the zero-crossings
of VCO output when
it is locked.
F(s)
VCO
f = f0 + KVLF
j
Balanced
detector
t
Ti:sapphire
ML-laser
2GHz phase modulator
100MHz
Rep rate
Diagram from J.W.Kim et al. MIT
lRF/2
p/2