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RF Commissioning
Andy Butterworth BE/RF
Thanks to: J. Tuckmantel, T. Linnecar, E. Montesinos, J.
Molendijk, O. Brunner, P. Baudrenghien, L. Arnaudon,
P. Maesen, F. Dubouchet, D. Valuch, D. Van Winkle, C.
Rivetta and many others
Outline
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Introduction
Equipment checkout
Warm commissioning
Cavity conditioning
LLRF commissioning
Data acquisition and diagnostic tools
Controls
Summary
Introduction
• Based on experience with LHC and LEP SC RF
commissioning
• Assumptions:
– Power plant derived from 800MHz IOT amplifier system
currently being specified for the SPS Landau cavities
– Low Level RF similar to the LHC 400MHz system
– Controls derived from LHC and SPS RF and integrated into
the LHC control system
Introduction: LHC and SPS systems
• LHC acceleration system: 16 SC cavities @ 400MHz
– each driven by 300kW klystron
• Power system (slow controls) using PLCs
– high voltage power supplies, klystrons, RF power distribution,
cryostats and ancillary equipment
• Low Level system (Cavity servo controller and Beam
Control)
– digital, FPGA at 80 MS/s, DSP for turn-by-turn (11 kS/s)
– in parallel with fast analog for transient beamloading
– 2 VME crates per cavity
• SPS Landau cavity system: 2 travelling wave cavities
@800 MHz
– each driven by 4 x 60kW IOT amplifiers (currently in
procurement)
Commissioning: Equipment check-out
• RF and control cables: individual testing
– reflection (short circuit) for cable identification
and initial validation
– transmission for phase sensitive signals
– reflection tests with termination to assess
damage, faulty connectors etc.
– > 100 cables per cavity in LHC...
• Control racks equipped and tested in lab
– after installation, signals test up to PLC control
level
• Data exchange with vacuum, power
converters, cryogenics
• Interlock system: individual test and
validation, access, radioprotection
D. Valuch
Warm commissioning: amplifier
• Initial commissioning of power amplifiers done during
reception tests
Technical Specification for
the Power Amplifier for the SPS 801 MHz RF system
12.8
Site acceptance tests
• 48 hour full power operational test, during which the frequency, phase, linearity, output power,
stability and efficiency will be monitored.
• Measurement of gain.
• Output voltage and current.
• Output voltage ripple at specified frequency ranges.
• Power factor at full output.
• Harmonic injection into the mains supply at selected levels including full output.
• Time taken to switch to zero output on receipt of the ‘fast switch-off’ command.
• Energy deposited into a short-circuited output.
• Effectiveness of internal protection circuits.
• Temperature rise during ‘soak’ operation at full power of transformers, inductors and semiconductors.
• Compliance with electromagnetic noise requirements (includes RF, x-rays).
• Compliance with acoustic noise requirements.
• Purity of IOT RF spectrum.
• Thermal run.
• Reproducibility check.
• Harmonic analysis.
• Inrush current measurement.
• Power factor correction test.
• Mains regulation tests.
• Line regulation tests.
• High voltage DC test to be carried out at an over-voltage to be agreed.
• Power Amplifier RF tests.
• etc...
Warm commissioning
• ... of complete power system with RF
– waveguides short-circuited to isolate cavities
• HV and RF interlocks: individual tests
– water flows, WG arc detectors etc
• Bring amplifier slowly to full RF power
– calibration of power measurements (directional couplers)
and attenuators for signal distribution
– test and adjust circulator and load
• Long-term power test (100 hours)
If SPS type 800MHz power plant is used, most of the
procedures will already be well defined
Low power measurements
• ... on cold cavities
• to confirm measurements
from test stand
– loaded Q
– tuning range
• using resonant frequency and
bandwidth measurements
with network analyser
– drive signal injected via coaxial
transition on waveguide
– return signal from cavity field
measurement antenna
We can now remove the short circuit and
proceed with high power testing...
High power tests: cavity conditioning
• Run cavity up to full power and voltage while observing
vacuum and cavity field emission
• Vacuum
– in practice, we are looking at the vacuum in the main input
coupler
– difficult to see outgassing in cavity (pumped by the cold
surfaces)
• Field emission in the cavity
– shows as He pressure excursions  RF trips
– in LEP, feedback on X-ray emission was possible (total
cryomodule voltage of 40MV)
– radiation not detectable in LHC (single cavity, 2MV)
• The input couplers are equipped with a DC bias voltage
to suppress multipacting during normal operation: this is
switched OFF during conditioning
Cavity conditioning method
• Pulsed FM modulated RF power is applied to the cavity in a
controlled way with vacuum feedback
• Two loops
– Fast vacuum feedback
– Slow computer controlled loop to generate AM envelope and increase
field and power as conditioning progresses (pulse to pulse at 50Hz)
Programmable Conditioning Envelope (Not to scale)
flat-top
rate of fall
rate of rise
RF Power
0 thru 300kW
Rep-rate 20ms
pulse width: 10,20,50us thru 1,2,5,10ms,CW
Total Envelope <= 90s
J. Molendijk
Cavity conditioning method
A
Increase Power using a fixed pulsewidth until Pmax is reached.
B
Increase Pulse width and proceed with the Power as above.
C
Finish with CW and proceed with the Power as above.
J. Molendijk
Conditioning: Implementation
• Conditioning system is fully integrated in LHC Low Level RF
Cavity Controller
Tuner Loop
VME CPU
Switch &
Limit
Conditioning
DDS
Cavity Controller Tuner/Klystron crate
J. Molendijk
The Conditioning DDS Module
Vacuum Loop
CPLD
RF Generator 2
500MHz Synthesizer
(DDS clock)
RF Summing &
Gain Control
4 Channel DDS
RF Generator 1
2008-03-27
J.C. Molendijk - CWRF2008
14
Conditioning DDS – I/Q plot of Dual FM sweeps
IcFwd
1500
1000
500
0
-3000
Q
-3500
-2500
-2000
-1500
-1000
-500
0
500
1000
1500
2000
2500
3000
3500
-500
-1000
-1500
-2000
-2500
I
Actual Data Obtained from Forward Current, I/Q memory in the Tuner loop module
J.C. Molendijk - CWRF2008
15
2008-03-27
Cavity P Fwd
Controls
LHC SC RF Conditioning GUI
Cavity V gap
Pulse
Envelope &
Power
Vacuum
Loop
Dual FM
Generator &
Status
Global
J. Molendijk/F. Dubouchet
Conditioning: Summary
• Typical conditioning time to full power and voltage
for an LHC cavity was a few days to 1 week
• Highly automated, but still requires regular human
supervision to adjust parameters
• Integrated conditioning system in LLRF hardware has
proved very efficient, and allows conditioning of
multiple cavities in parallel
• Main power coupler DC bias switched on only after
conditioning
LHC cavity Low Level
SWITCH/
PROTECTION
CONDITIONING DDS
SWITCH
TUNER LOOP
Phase
Shift
Ic fwd
Digital
IQ
Demod
Fwd
Digital
IQ
Demod
X
Ic rev
Dir.
Coupler
Tuner
Processor
Rev
Digital
IQ
Demod
Tuner Control
Digital
IQ
Demod
Digital RF feedback (FPGA)
60 dB
DAC
1 kHz
SUM
SET
POINT
SUM
40 dB
20 dB
SUM
Set Point
Generation
dV
I0
Vcav
Q0
Voltage
function
dp
From long.
Damper
RF FEEDBACK
SUM
1 kHz
1-Turn Feedback
Cavity Servo Controller.
Simplified Block Diagram
DAC
Phase
Equalizer
Analog IQ
Demodulator
I Q
Analog RF feedback
Single-Cell
Superconducting
Cavity
X
Klystron
Polar Loop
(1 kHz BW)
Ic fwd
Digital
IQ
Demod
X
ADC
SUM
Master F RF
Gain Cntrl
X
noise
Ig fwd
X
RF
Phase Shifter
Digital
IQ
Demod
DAC
DAC
Circ
X
Var Gain RF
Ampifier
Analog IQ
Modulator
Baseband
Network
Analyzer
300 kW Klystron
RF MODULATOR
ANALOG
DEMOD
ADC
1-Turn Feedforward
Analog:
Technology:
Digital I/Q pair:
Analog I/Q pair:
DSP
CPLD or FPGA (40 or 80 MHz)
Analog RF
DAC
ADC
Analog IQ
Demodulator
Signals: Digital:
Wideband
PU
Low Level RF commissioning
• Tuner loop
– tuner phase adjustment to set the cavity on tune when the loop is closed
• RF feedback
– phase alignment of digital and analog feedback branches
– adjustment of feedback gain and phase before closing loop
– measure closed loop response: important for
• beam loading response
• cavity impedance seen by beam
• bandwidth of cavity voltage control
– measure phase noise
• Amplifier phase/amplitude loop
– adjust gain and phase setpoints
– adjust dynamic loop responses
Lots of work with a network analyser, but new tools
are at hand...
LLRF embedded data acquisition
• All LHC LLRF boards have onboard signal recording memory
• 2 parallel sets of buffers:
– Post-Mortem capture
– User “Observation”
• 64 turns @ 40Ms/s 
128kB/signal
• Revolution frequency tagging
RF feedback board
“Baseband network analyzer” (BBNWA)
I-Output (Norm.)
Counts
• LLRF boards also have embedded memory buffers for “excitation data”:
can inject signals into the loops
White Noise Kernel
• Loops excited with noise
2000
• Output signals recorded
0
digitally
-2000
0
500
1000 1500 2000
2500 3000 3500 4000
4500
Sample #
• Transfer function
1
estimate program
I-Output Signal
0
calculates frequency
domain transfer function
-1
0
0.5
1
1.5
2
2.5
3
x 10
• “Fit” an idealized linear
1
model to the measured
Q-Output Signal
0
data and calculate
-1
recommended
0
0.5
1
1.5
2
2.5
3
Sample #
x 10
adjustments
Q-Output (Norm.)
5
5
MATLAB tools for remote setting-up of LLRF developed with
US-LARP collaborators (D. Van Winkle, C. Rivetta et al.)
D. Van Winkle
Open Loop Alignment
Un-Aligned Open Loop Response and Model Fit (SM18)
Measured Magnitude and Phase with Fit of Open Loop
Polar Plot of measurement and Fit of Open Loop
Gain (dB)
-50
Fit
Data
-60
90
0.002
120
60
0.0015
-70
-80
-60
0.001
150
30
0.0005
-40
-20
0
Frequency (kHz)
20
40
60
180
0
Phase (degrees)
200
0
210
330
-200
240
-400
-60
-40
-20
0
Frequency (kHz)
20
40
Measured Magnitude and Phase with Fit of Open Loop
Polar Plot of measurement and Fit of Open Loop
Gain (dB)
-50
90
Fit
Data
-60
0.002
120
60
0.0015
-70
-80
-60
300
270
60
0.001
150
30
0.0005
-40
-20
0
Frequency (kHz)
20
40
60
180
0
Phase (degrees)
400
200
210
0
330
-200
240
-400
-60
-40
-20
0
Frequency (kHz)
20
40
300
270
60
RF feedback alignment in the comfort of your home
Page 22
D. Van Winkle
BBNWA contd.
• Comparison of closed loop response measured with
instrument vs. embedded BBNWA measurements
Closed Loop measure in SM18 (BBNA)
10
5
Gain (dB)
0
-5
-10
-15
-20
-1
Frequency (MHz)
Network analyzer (Agilent)
-0.8
-0.6
-0.4
-0.2
0
0.2
Frequency (MHz)
0.4
0.6
0.8
BBNWA
D. Van Winkle
1
Automated tuner setup
• MATLAB script for Automatic setup of tuner loop
Tuner sweep to find
resonance
Sweep across
resonance with
different phase
shifter values
Converge to optimum
phase
LLRF: Summary
• It took almost 1 month to set up the Low-Level RF of
the first cavity
• Once the procedures were well defined, the last few
cavities took about 1 day each
• New automated tools using MATLAB and the BBNWA
feature of the LLRF hardware will save a lot of time
 Many thanks to our US-LARP colleagues from SLAC
Controls and software
• PLCs and Low-Level crates
interfaced via FESA (Front
End Software Architecture)
– a “FESA class” software module
is written for each type of
equipment
– communication via the CERN
Controls Middleware
• Application software:
– LabView expert synoptic panels
– Matlab scripts for more
sophisticated specialist
applications
– LSA (standard machine
operations software) manages
settings and sequencing
– Logging and Post Mortem
– Alarms (LASER)
– ...
Summary
• Power system commissioning procedures will be well
known if using existing power source (SPS type
800MHz)
• Controls and application software should be based
on standard CERN controls infrastructure
• LHC low-level electronics has built-in conditioning
and diagnostics facilities, and is already well
integrated into the control system
• Powerful tools are being developed for LLRF settingup which could equally be applied to the crab cavity
system