Procedures for ESS
Superconducting Cavities Turn on
2015-04-08
Version 2.2(not complete)
Rihua Zeng, Anders Sunesson, Morten Jensen,
Stephen Molloy, Christine Darve, David McGinnis
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Visiting address: ESS, Tunavägen 24
P.O. Box 176
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www.esss.se
1. Motivation and Background ............................................................................................................... 4
2. Cavity Turn On Procedures ................................................................................................................ 7
2.1 Procedure for without Beam[9,10,11] .................................................................................................... 7
1. RF Cables Calibration ................................................................................................................................................ 7
2. Technical Interlock / Sensors. ............................................................................................................................... 7
3. RF source / Waveguides / LLRF. .......................................................................................................................... 7
4. Cooldown to 2K ........................................................................................................................................................... 7
5. Cavity Spectra measurements ............................................................................................................................... 7
6. Cavity Tuners Test...................................................................................................................................................... 7
7. Cavities On Resonance. ............................................................................................................................................. 7
8. Input RF Couplers and Cavities Conditioning. ................................................................................................ 8
9. Cavity Gradient Ramp up. ........................................................................................................................................ 9
10. Cavity Quench Threshold Identification. ..................................................................................................... 10
2.2 Procedures for with Beam ......................................................................................................................... 11
11. Scheme A: Phase Scan ----- ΔT method[15,17].......................................................................................... 11
12. Scheme B: Phase Scan -----signature matching [16] ............................................................................... 12
13. Scheme C: Beam Loading ------Drift beam [12] ......................................................................................... 13
14. Scheme D: Beam Loading ------Zero Crossing [13] .................................................................................. 13
15. Scheme E: Single bunch transient [14]......................................................................................................... 13
3 Cavity Test Procedures ...................................................................................................................... 18
3.1 Single point Ql and Detuning measurement in decay curve ......................................................... 18
3.2 Dynamic Ql and Detuning measurement in open loop ................................................................... 18
3.3 Dynamic Ql and Detuning measurement in closed loop ................................................................. 18
3.4 Dynamic Ql and Detuning measurement in noisy environment ................................................. 19
3.5 Cavity pass band modes ............................................................................................................................. 19
3.6 Klystron input-output characteristics (power) at different modulator voltage ................... 19
3.7 Klystron input-output characteristics (phase) at different modulator voltage..................... 19
3.8 Klystron ripple frequency and amplitude ........................................................................................... 19
3.9 Lorentz force detuning at different cavity field levels .................................................................... 19
3.10 Lorentz force to cavity tuning transfer function............................................................................. 19
3.11 Piezo tuner to cavity tuning transfer function (frequency domain) ....................................... 19
3.12 Piezo tuner to cavity tuning transfer function (time domain) .................................................. 19
3.13 Moto tuner to cavity tuning transfer function ................................................................................. 19
3.14 Microphonics spectrum ........................................................................................................................... 19
3.15 System open loop matrix ......................................................................................................................... 19
3.16 System closed loop matrix ...................................................................................................................... 19
3.17 Phase and amplitude setting .................................................................................................................. 20
3.18 Cavity field behaviour close to and at quench ................................................................................. 20
3.19 Fast fault recovery ..................................................................................................................................... 20
4 Other Procedures ................................................................................................................................. 20
Reference:................................................................................................................................................... 20
Appendix 1: Beam modes limitation identification studies in phase & amplitude setting
........................................................................................................................................................................ 22
Appendix 2: Proposal of cavity tests for ESS cavities (Julien Branlard, DESY) .................. 24
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VERSION HISTORY
Verion
Reason for version
Date
1.0
New Document
20-Sep-2013
2.0
Corrections from S. Molley.
16-Oct-2013
Add procedures for with beam.
2.1
Corrections from M. Eshraqi.
14-Mar-2014
Add “drifting beam method” procedures.
Add appendix from Julien Branlard(DESY), on test proposal for ESS cavities,.
Add Appendix of identifying beam modes limitation under beam loading with
different beam peak current and different beam pulse length.
Add references.
2.2
Add procedures recommend by Mircea Stirbet (JLAB), on power coupler and
cavity conditioning
15-Mar-2015
Add content in motivation and background
Modify title order for sub-sections.
3
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Visiting address: ESS, Tunavägen 24
P.O. Box 176
SE-221 00 Lund
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1. Motivation and Background
The start point of this document is trying to solve the problem “When the cavity is installed in
tunnel at ESS, how to turn on RF power to reach required gradient, and what to be done
when the beam comes?”
For superconducting cavities, a complex set of procedures are required in different situation to
check, test and calibrate every components, determine configuration parameters, and identify
operational limitations. When it is better to automate most of procedures in software, which ensures
procedures reuse and consistent result interpretation, it is still acceptable to get sophisticate
procedures describe in text in documentation when automation is not applicable.
When preparing these procedures, it is important to keep in mind ESS challenges and tough facts
expected to face during the phases of commissioning, normally operating and maintaining for all
the normal conducting cavities and superconducting cavities at ESS. During the stage of procedures
investigation and formalization, it is equally important, if not more, to identify and find suitable
solutions to address these challenges, and to understand better cavity system and get to know its
limitations, thereby testing, controlling and operating the cavity system efficiently and effectively.
The challenges we have to address at ESS are:
1. Cavity Control Challenges. How to deal with the new challenges: longer pulse, higher
beam intensity, higher beam power, higher gradient, spoke cavities and high demands for
energy efficiency and availability.
2. Cavity Operation Challenges. How to deal with wide spread of cavity parameters:
maximum operable gradient, Q load value, operating gradient and synchronous phase,
Lorentz force detuning coefficients, major mechanical modes, power overhead variations,
etc. The same type of cavities (especially superconducting cavities) behaviour not
identically as in theory, due to the fact of complexities and uncertainties in cavity system
manufacturing, surface processing, and assembling.
3. Cavity Test Challenges. How to learn as much as possible from a variety of RF tests
carried out at different test stands and final accelerator tunnel, in order to better understand
the cavity system and know its limitations, thereby operating the cavity system efficiently
and effectively.
When facing up to new challenges, the traditional control, operation and test method and algorithms
fall short in many areas. For example, for the long pulse (~3.5 ms RF pulse, almost 3 times longer
than SNS and DESY), Lorentz force detuning compensation with driving the piezo tuner by a
simple half-cycle sinusoid pulse might no longer suit for ESS superconducting cavities; For the
higher beam intensity, the beam loading in cavities become heavier. The resulting field stability
issue in normal conducting cavities and serious power overshoot in superconducting cavities can
not be well addressed in feedback plus simple adaptive-feedforward system, especially during beam
commission phase; For the higher beam power, the same situation of RF setting errors at SNS (up to
2° in phase and 2% in amplitude) might not be acceptable at ESS due to probably higher beam loss
at high power linac of 5MW; For the spoke cavities, which will be the first time to be operated in
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the linac, uncertainties require a flexible scheme to quickly change operation points to avoid any
unexpected consequences; For the high energy efficient requirement, much effort should be paid to
minimize the power overhead required for cavity field control, aiming to reduce 30% power
overhead to 10%; For the high availability requirement, it is essential to establish the schemes of
fast recovery from quench and fast recovery from single/multiple LLRF, klystron, modulator,
cavity, cryomodule failures.
An extensive investigation has been carried out regarding to these new challenges at ESS, and the
design of cavity control strategies has been made with keeping these new challenges firmly in mind,
highlighting the advantages and disadvantages of different methods and novel ideas newlyproposed in other accelerator facilities, and exploring possible applications at ESS for these
methods and novel techniques through discussion with experts and detailed review in the literature.
The overall control strategies to deal with new challenges are listed but not limited as follows:
The number before each items are the priorities(1, highest priority. 4, lowest). Reference of
each item are listed as well.
 2. Adaptive compensation for Lorentz force detuning at long pulse via piezo tuner Ref:
LORENTZ FORCE COMPENSATION FOR LONG PULSES IN SRF CAVITIES.

1. Beam loading compensation at commissioning phase with different beam peak current
and different beam pulse length.
Ref: Power Overhead Reduction Considerations in Beam Commissioning.pdf

1. Beam loading comensation at normal operating phase
Ref:RECENT PROGRESSES IN THE LLRF FPGA CONTROL SYSTEM OF THE J-PARC LINAC

1. Klystron ripple and droop compensation
Ref: LATEST RESULTS AND TEST PLANS FROM THE 100 mA CORNELLERL INJECTOR SCRF
CRYOMODULE


1. Residual cavity detuning compensation from pulse to pulse
Ref: Some Considerations on Predetuning for Superconducting Cavity
2. Klystron linearization
Ref: Characterization and compensation for nonlinearities of high-power amplifiers used on the FLASH and
XFEL accelerators



2. Phase drift calibration under big temperature variations
3. Strategies and method to deal with slow variations of system environment and operation
conditions
1. Iterative learning (adaptive feed forward) and optimal control to reduce tracking error in
an optimal way, while keeping the deviation of each step from power amplifier output small.
Ref: An Iterative Learning Algorithm for Control of an Accelerator Based Free Electron Laser

3. Improve cavity phase and amplitude setting accuracy
Ref: Determination of field amplitude and synchronous phase using the beam-induced signal in an unpowered
superconducting cavity
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

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3. High precision measurement of basic cavity parameters: Q load value, R/Q, and dynamic
detuning.
4. Energy efficiency increasing in cavity control and cavity operation
4. Ability to work at nonlinearities
4. Ability to work close to limitation
4. Ability to change operation point quickly and correctly
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The methods and novel ideas listed above become possible as consequences of advances in modern
technologies (flexible FPGA, faster CPU, bigger memory, and faster communication speed), novel
control concept, as well as innovative measuring techniques. High quality data with high resolution,
high accuracy and completeness are the essential basics to succeed in using these methods and
novel ideas. For example, adaptive compensation for Lorentz force detuning requires accurate
measurement of transfer function from piezo tuner to cavity detuning, and accurate measurement of
cavity detuning; Feed forward compensation for beam loading requires information of beam current
and beam pulse length, as well as high resolution data of beam arrive time at cavities; Feedforward
compensation for klystron ripple and droop requires accurate measurement of modulator voltage
variations; Feed forward compensation for residual requires accurate and real-time measurement of
cavity detuning and QL; Klystron linearization requires good knowledge of klystron input-output
power characteristic curves at different modulator voltages; Other advanced control and demanding
operation methods require even more strict data quality and special data sets, which will be seen in
the later sections.
Further more, wide spread of cavity parameters make it impossible to employ an single “uniform’
control algorithm and control configuration data even for the same type of the cavities. Instead,
adjustments and modifications are needed for each cavity and customized approaches are preferred
to reflect cavity’s individual performance and characteristics. Under this context, a large number of
data are expected and required for all the cavity systems in ESS linac. A controlled, centralized and
searchable data storage is therefore necessary to save the data effectively and ensure to obtain the
data immediately when using them.
When preparing to collect data, it is important to keep in mind which kinds of data are required, and
to what extent the data quality should be. Although it is the best way to verify the data and refine
the requirements in the normal operating cavities, it is always not practical and too late to generate
requirements at final stage. Instead, the data and general data quality required could be estimated
earlier from results of test stands or similar cavity tests in other accelerator facilities, as well as the
prediction of theory models. As an example, the time-domain transfer function data of piezo tuner
to cavity detuning would be required to compensate the longer pulse Lorentz force detuning,
according to the experiments carried out at Fermilab. Another example is that, the data resolution of
better than 100ns can be concluded from theory model prediction for feedforward table to
compensate beam-loading effects in normal conducting cavities.
In a word, data is the central core to understand the cavity system and to develop advanced methods
and algorithms to address the challenges. Moreover, as it will be seen in the following sections, the
combination and interaction of data, model and tests/experiments will make great contribution to
better system development.
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2. Cavity Turn On Procedures
2.1 Procedure for without Beam[9,10,11]
This procedure includes the steps for cavity test and normal cavity operation. In practical operation,
turning on the cavity after a short-term trip can start from step 10, when all preparations such as RF
cables calibration, interlock check, system check, cavity conditioning and cavity on resonance have
already done.
1. RF Cables Calibration
 Time Domain Reflectometer (TDR) cables check
 Directional Couplers / Circulators: get calibration data.
 Calibrate RF power measurement cables with attenuators at 352.21MHz/704.42MHz
 Make RF calibration summary table
2. Technical Interlock / Sensors.
 Check the sensors (e-, arc detector, water flow, temperature, etc)
 Set the hardware interlock thresholds
 Check the interlock
3. RF source / Waveguides / LLRF.
 Klystron / LLRF check on the load
 Waveguides visual check (how to check those in tunnel?)
 System check / RF leak check at low power
4. Cooldown to 2K
5. Cavity Spectra measurements
(how to measure it for cavity in cryomodule?)
 Measure the fundamental mode spectra
 Measure the cavities HOMs spectra and Qload
 Calibrate the cold RF cables at 2K
6. Cavity Tuners Test
 Test the cavities step-motor frequency tuners and record the motor tuner to cavity
detuning transfer function.
 Tune the cavities to the 352.21MHz/704.42MHz using the Network Analyzer
7. Cavities On Resonance.
 Cavities fine-tuning to the 352.21MHz/704.42MHz by using piezo tuner
 Qload, Kt calibration ( Eacc=Kt(Ptrans)1/2 )
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8. Input RF Couplers and Cavities Conditioning.
 Find the cavities/couplers limits at low repetition rate
 Run the standard cavity/coupler conditioning program (the following is an example at
Desy):
20, 50, 100, 200 s pulse lengths up to 1MW (minimum 700kW, different for different
sections of ESS linac),
300, 400 s up to 330 kW,
500 s + 100, 200, 400, 800 s flat top pulse up to 250 kW,
Cavities high peak power (HPP) test is part of the conditioning (automatic),
if the cavities limits are lower, run up to the minimum cavity limit.
Prof. Mircea Stirbet recommend the following procedures for input couplers and
cavities conditioning at ESS:
8.1
a)
b)
c)
d)
Check coupler instrumentation and interlocks:
If vacuum gauges are operational
If electrometer for electron activity are operational
If radiation monitors are functional
If cooling fluid (water on the antenna, He gas on the outer conductor) have the right
specifications (flow, temperature). By the way, cooling FPC antennae with water is a
risky choice in SRF applications.
e) If the temperature sensors are reading supposed temperature ( RT, 50 or 70K intercept,
2K)
f) If arc detector(s) on couplers are operational
g) If ice ball prevention heater is operational (this is a power supply, heating tape and a
temperature sensor which should keep the air side of the coupler at 25 – 29 C. If this
heater is not operational, moisture could condense, eventually will froze and overnight a
nice ice ball could be generated).
h) If the coupler has capacitor for DC bias, this should be OFF (short circuited) during RF
conditioning.
8.2
a)
b)
c)
d)
e)
Proceed with RF conditioning:
Start with low pulse duration
Start with low RF amplitude
Find the cavity and lock on the Pi mode resonance. Presume active cavity tuning.
Set vacuum feedback loop threshold at a convenient initial value (1e-8 – 5e-8 mbar).
Increase RF amplitude (computer controlled operation). If there is vacuum and electron
activity perform RF processing using vacuum feedback loop.
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f)
g)
h)
i)
j)
k)
l)
m)
n)
Once vacuum and electron activity is less intense, continue increasing RF pulse
amplitude up to the cavity specification.
Eventually change vacuum feedback loop threshold to a higher value (1 10-7 mbar). Do
not go higher than 2.5 e-7 mbar.
Once maximum RF power amplitude is reached, perform a “phase “conditioning,
changing the RF phase in steps of 2-5 degrees.
Go back to the initial phase and decrease the RF pulse amplitude.
Change pulse duration and repeat steps 8.2 (steps d - g).
Continue until the coupler is operational at maximum pulse duration and RF amplitude
is achieved. OBS. The cavity could interfere with FPC conditioning. Sometimes, helium
boiling, temperature runaway, field emission onset or cavity quench events could
manifest before reaching the maximum RF power amplitude. On cavity, probably the
administrative limit on Eacc max should be used.
If field emission manifest during cavity processing (higher than background ionizing
radiation signal) perform eventually cavity RF processing. This step could be time
consuming.
Once RF coupler processing was demonstrated with maximum pulse duration, decrease
RF amplitude, turn the RF OFF and set DC bias ON.
Turn RF On and ramp the using the maximum pulse duration, up to the administrative
Eacc. Observe vacuum and electron activity. There should be none.
8.3 If possible, perform operations specified at 8.1 and 8.2 on all couplers, at the same time
(in parallel) on a cryo-module. If conditioning is done sequential, one coupler after the
other, once all couplers and cavities are conditioned, a test should be done with all 2 (or
4) couplers RF powered at the same time. Perform a long term run (at least one hour) at
constant cavity gradients. Keep an eye on Helium level, helium temperature, ionizing
radiation levels, and coupler’s temperatures.
8.4 The cryo-module should be ready for beam operation. And this is another story.
9. Cavity Gradient Ramp up.
 Load feed forward tables for low gradient operation. Simulated beam with synchronous
phase is included in the feed-forward signals, and preliminary injection time is
calculated according to measured QLoad.
 Modulate the phase of RF power during filling time to track the cavity resonance
frequency, in order to minimize RF power required compensating detuning effect during
filling time.
 Adjust motor tuner and adjust injection time to obtain a constant cavity field at low
gradient operation where Lorentz force detuning is small. The adjusted injection time is
recorded and employed in feedforward tables and feedback setpoints.
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



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


Set a proper pre-detuning via motor tuner to compensate synchronous phase operation
and to minimize the Lorentz force detuning effect at nominal gradient. The Lorentz force
detuning is measured in advance or is predicted by models.
Scale up the feed forward tables to reach higher gradient.
Raise gradient slowly.
When rising close to nominal gradient, update feedforward table by measuring dynamic
cavity detuning in previous pulses from forward, reflected, and transmitted powers. In
such a way, Lorentz force detuning and pre-detuning are expected to be compensated.
Optimize the pre-detuning to reduce the RF power.
Apply feedback once the open loop response is close to the desired closed loop response.
Increase gain to nominal.
Optimize feedback parameters for minimum residual amplitude and phase fluctuation
during flat-top.
10. Cavity Quench Threshold Identification.
 Cavity maximum gradient measurement at low repetition rate
 Cavity maximum accelerating gradient measurement at nominal repetition rate with cryo
losses (Qo) and radiation measurements
 Radiation / Dark Current measurements, if needed.
 Quench detection can be made by detecting Qload , since there is sharp drop in Qload when
quench occurring. Fast quench handling can be therefore made in next RF pulse by
reducing or shut off the RF power.
 Detailed and high-resolution quench level identification is essential to work close to
limitation. Firstly, ramp up the cavity gradient with 1 MV/m until a quench is detected
and RF safely turned off. After a brief cryogenic recovery time, a second ramp-up is
performed using smaller gradient increments (0.1 MV/m). The two measurements are
correlated and archived, along with the forward, reflected and probe waveforms leading
to the two quenches. Near-quench and safe-operation thresholds are calculated for every
cavity as a function of their measured quench threshold.
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2.2 Procedures for with Beam
1. RF off. Drift low intensity beam (5mA) through Linac.
2. Ramp up the cavity field as mentioned in step 10 in procedure without beam.
3. Set cavity phase and amplitude.
Several methods are applicable here to set the cavity phase and amplitude. At least 2~3 methods
expect to be implemented, so as to cross check. These methods are expected to investigate in details
to find proper implementation in order to improve the setting accuracy. The accuracies obtained at
SNS for different methods are listed in Table 1. The description and general procedure for these
methods are given in the following sections.
11. Scheme A: Phase Scan ----- ΔT method[15,17]
Phase scan methods are referring here to the way of calibrating setting point for RF cavities by
scanning RF phase and amplitude, measuring beam arrival times at downstream locations,
comparing measured phase to model predicted data, and identifying the best-matched data for
calibration. ΔT method is a classical phase scan method and used widely in normal conducting linac
such as in LAMPF, Fermilab, JPARC and SNS.
Linear system response is assumed in ΔT method and it is only valid in the vicinity of design phase
and amplitude. ΔT method is a cavity-by cavity operation, assuming that the cavities upstream to
the one being adjusted are “on”, and the cavities downstream are “off”. Beam phases (or beam
arrival time) are provided by two downstream BPMs. The two BPMs can be neighbouring each
other, or separated by several cryo-modules, which depends on the specific location of cavity (the
sensitivity of beam velocity to energy gain becomes low as beam energy goes high) being adjusted.
The cavities between two BPMs should be detuned by 20 (?) cavity bandwidth.
The general procedures of ΔT method are listed below:
1. Find approximate phase and amplitude set point, by observing BPM signals and beam
loading effect, and doing RF based calibration.
2. Cavity being adjusted is off. Record two downstream BPMs phases ϕbpm1-0 and ϕbpm2-0.
3. Ramp the cavity being adjusted to nominal field calibrated by RF power based measurement
(amplitude accuracy in RF based calibration is around 10%).
4. Turn on beam with repetition rate 14Hz, beam intensity <10mA(see attached table for DTL.
For superconducting cavity, it could be relaxed due to higher cavity bandwidth), and beam
pulse length < 20μs.
5. Record two downstream BPMs phases ϕbpm1 and ϕbpm2.
6. Calculate relative changes of BPMs phases between cavity “on” and “off” Δϕbpm1 = ϕbpm1-0 ϕbpm1 and Δϕbpm2 = ϕbpm2-0 - ϕbpm2. Plot Δϕbpm1 and Δϕbpm2.
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7. Scan the cavity synchronous phase with step 0.5°(?) over the range ±5°(?) of design phase,
and repeat 1~5 at each step, to generate a constant-amplitude, variable-phase curve in
(Δϕbpm1, Δϕbpm2) plane.
8. Calculate the slope of the curve, which depends on cavity amplitude, and compare it with
the slope values of model predict curves at different amplitude. These predicted curves have
a common point of intersection.
9. Use some fitting algorithm to determine best-fit amplitude.
10. Having determined proper amplitude, it is now possible in model to calculate the transfer
function relating Δϕbpm1 and Δϕbpm2 to phase deviation Δϕ and energy deviation ΔW at the
entrance of cavity with respect to nominal value. Δϕ and ΔW can then be determined.
11. Correct the phase set point, and if necessary, correct as well the input energy at cavity
entrance according to the result in step 10.
12. Scheme B: Phase Scan -----signature matching [16]
Unlike ΔT method having a linear system response and small input energy displacement restriction,
signature matching method can work at large displacement of initial conditions. In high energy part,
signature matching methods can easily scan the phase over 360° at different amplitude, and make
good match with model predict curve. However, at low energy linac, cavity phase scan can only be
several ten degrees where beam stay sufficiently bunched to produce good signals at downstream
BPMs, and the accuracy indicated at SNS for low energy part is not good enough. ΔT method is
probably necessary to get a good setting accuracy for ESS spoke cavities.
The general procedures for signature matching are listed below:
1. Ramp the cavity being adjusted to nominal field calibrated by RF power based measurement
(amplitude accuracy in RF based calibration is around 10%).
2. Detuning the downstream cavities by 20 cavity bandwidth to bypass the beam, which locate
between two downstream BPMs.
3. Turn on beam with repetition rate 14Hz, beam intensity <10mA(see attached table for DTL.
For superconducting cavity, it could be relaxed due to higher cavity bandwidth),, and beam
pulse length <20μs.
4. Record two downstream BPMs phases ϕbpm1 and ϕbpm2.
5. Scan the cavity synchronous phase with step 0.5°(?) over the full range 360°, and repeat 4~5
at each step, to generate a constant-amplitude, variable-phase curve in (Δϕbpm1, Δϕbpm2)
plane.
6. Predict the values in model for BPM phases (ϕbpm1_calc and ϕbpm2_calc) as a function of
synchronous phase.
7. Spline fit the measured phase difference (ϕbpm1 - ϕbpm2).
8. Match the model predict values with measured ones, by minimizing the difference between
(ϕbpm1 - ϕbpm2) and (ϕbpm2_calc - ϕbpm1_calc) over the range of scanned phase. Phase deviation
Δϕ, input beam energy deviation at entrance of cavity ΔW, and cavity amplitude deviation
ΔV are adjusted in this matching procedure.
9. Correct the phase and amplitude set points according to the result in step 8.
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13. Scheme C: Beam Loading ------Drift beam [12]
The drifting beam technique is based on very strong beam-cavity interactions in the SC cavity for
high current beams. It was proposed several years ago and recently realized at SNS. It uses
measured beam currents and pulse shapes with a beam current monitor (BCM), and beam induced
signals in the SC cavity with the cavity control circuit. Using the measured beam current in a beamcavity model that simulates the beam-loading in the cavity, by comparing model simulation results
with the actual measurement of the cavity, cavity phase and the field amplitude are determined
precisely.
The general procedures are listed below:
1. Measure accurately beam current and beam pulse shape by BCMs.
2. Tune the cavity to resonance frequency as close as possible (<50Hz?)
3. Turn off RF. Turn on beam with repetition rate 1Hz(?), beam intensity 10mA(?), beam pulse
length > 50 μs.
4. Measure the phase and amplitude of beam-induced signal. Measure the phase and amplitude
of noise signal before next beam pulse coming. Subtract noise signal from beam-induced
signal.
5. Repeat the measurement in step (4) for ~10 beam pulses and average the results.
6. Predict the beam-induced signal in model by measured beam current and beam pulse shape.
7. Determine the phase offset and amplitude calibration coefficient by comparing measured
result with model calculations.
8. Set amplitude and phase.
14. Scheme D: Beam Loading ------Zero Crossing [13]
The goal is to adjust the cavity phase such that the beam passes the cavity at the zero crossing of the
accelerating field. This is accomplished by variation of the cavity phase set point until the average
(microphonics !) transients are nulled. The cavity phase is now at -90 deg. or +90 deg. The sign can
be determined from a change in transient when increasing the phase setpoint. The phase is
calibrated by adding a phase offset such that the measured phase reading is correct at zero crossing.
This method might have higher accuracy since no transient from beam loading cause perturbations
to cavity field.
15. Scheme E: Single bunch transient [14]
The concept of the transient detector for the X-FEL is based on nulling method, where the cavity
probe signal is split into two branches, one delayed by a up to 100 ns and phase shifted by 180
degrees before adding the two signals. The nulled signal is amplified by 60-80 dB with an rf
amplifier and the transient induced by a single bunch is detected by a schottky diode based rf vector
detector to achieve the required low noise performance.
13
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14
European Spallation Source ESS AB
Visiting address: ESS, Tunavägen 24
P.O. Box 176
SE-221 00 Lund
SWEDEN
www.esss.se
Warm Linac
Superconducting Linac
Beam Based Calibration
Energy
degrader
(only for
DTL)
Beam Based Calibration
RF based
Calibration
Delt-T
Phase-scan
signature
Phase-scan
signature
Drift Beam
Amplitude
1.7% (at
most) differ
from Delt-T
results[7]
Varied
0~5% with
Delt-T
results
±2.4%[1]
±4%(better
results after
precise
calibration for
BCM)[1]
Phase
1.5 °(at
most) differ
from Delt-T
results[7]
Varied
0~11° with
Delt-T
results
±1°
±1°
Beam pulse
<50us[5]
Beam
Current
<20mA[5]
Repetition
rates
1Hz
Error Source
<50us
<50us[8]
>5mA
1Hz
Beam
paramete
rs
1Hz
BPM
Calibration
Beam
parameters
Beam
loading in
unpowered
cavities
~20us[2]
5~20us[3]
<20mA[2]
5mA is
threshold*[6]
1Hz
BPM
Calibration
Beam
parameters
Beam loading in
unpowered
cavities
15
±5%[1]
(Calibration
uncertainties
could be
checked in test
stand)
>50us[4]
N/A
<20mA[4]
N/A
1Hz
1Hz
BCM
Calibration
Beam
Parameters
LLRF noise
Cavity
detuning
Passband
modes
Measurement
errors
Cavity
parameters
errors
Tab
le
1:
Ove
rvie
w
of
cavi
ty
pha
se
and
amp
litu
de
setti
ng
met
hod
s
use
d at
SN
S
European Spallation Source ESS AB
Visiting address: ESS, Tunavägen 24
P.O. Box 176
SE-221 00 Lund
SWEDEN
www.esss.se
16
European Spallation Source ESS AB
Visiting address: ESS, Tunavägen 24
P.O. Box 176
SE-221 00 Lund
SWEDEN
www.esss.se
17
European Spallation Source ESS AB
Visiting address: ESS, Tunavägen 24
P.O. Box 176
SE-221 00 Lund
SWEDEN
www.esss.se
3 Cavity Test Procedures
This chapter will discuss tests for individual modules separately, and focus on the following issues:




Why do we need these tests
Theory and method behind test and data processing to obtain required data
Standard test procedure, which not only include how to carry out test, measure signals, but
equally important are the data analysis, data organization and data storage
Automation
3.1 Single point Ql and Detuning measurement in decay curve
DESY example: early than 1991(Ql) and early than 1998(detuning)
3.2 Dynamic Ql and Detuning measurement in open loop
Ql and detuning can be derived from power measurement and careful calibration of forward power.
Detuning can also be measured from the phase difference between incident wave and probe wave.
The accuracies of each method have to be compared.
3.3 Dynamic Ql and Detuning measurement in closed loop
Ql and detuning are derived from the power measurement (forward, reflected, probe), it is also
found in other labs that better result can be obtained in second derivation: (proposed earlier than
1998)
18
European Spallation Source ESS AB
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dP
 1/ 2  i P  21/ 2 F
dt
1/ 2  
  dP  
Re  P * 

  dt  
Re P * P  2 F 
  dP

Im P * 
 21/ 2 F  
 dt

  
P* P
3.4 Dynamic Ql and Detuning measurement in noisy environment
3.5 Cavity pass band modes
3.6 Klystron input-output characteristics (power) at different modulator voltage
3.7 Klystron input-output characteristics (phase) at different modulator voltage
3.8 Klystron ripple frequency and amplitude
3.9 Lorentz force detuning at different cavity field levels
3.10 Lorentz force to cavity tuning transfer function
3.11 Piezo tuner to cavity tuning transfer function (frequency domain)
3.12 Piezo tuner to cavity tuning transfer function (time domain)
earlier than 2011, test in Fermilab, FLASH, JPARC…
Test Steps:
 Excite the piezo with a series of impulses and sweep the impulse-to-RF delay
(Equivalent to measuring the piezo-to-detuning transfer function using CW)
 The forward, probe and reflected RF waveform recorded at each delay and used to calculate
detuning.
 The response 2-D matrix is then obtained (detuning vs. time at different peizo pulse dealys)
 The response matrix can be inverted to give waveform required to produce a given detuning
profile.

3.13 Moto tuner to cavity tuning transfer function
3.14 Microphonics spectrum
3.15 System open loop matrix
3.16 System closed loop matrix
Start earlier than 1998 (mature system used in Flash and will be in XFEL)
Test step:
19
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1. find an appropriate excitation signal to excite all system dynamics to be modelled.(time domain,
frequency domain?)
2. remove the trend and means from observed data, by averaging, filtering, shift data sequences or
remove outliers.
3. assign model structure(the level of complexity)
4. choose identification algorithm
5. validate the model(compare with the measured system data not used in identification, or with
other physical known parameters )
3.17 Phase and amplitude setting
3.18 Cavity field behaviour close to and at quench
3.19 Fast fault recovery
4 Other Procedures
Reference:
[1] Y. Zhang, et al., Comparision of SNS Superconducting Cavity Calibration Methods.
[2] D. Jeon, et al., Beam Loading Effects on Phase Scan for Superconducting Cavities.
[3] S. Henderson et al., The Commissioning Plan for the Spallation Neutron Source Ring and
Transport Lines.
[4] Y. Zhang et al., Determination of field amplitude and synchronous phase using the beaminduced signal in an unpowered superconducting cavity
[5] J. Galambos, SNS Commissioning Strategies and Tuning Algorithms.
[6] J. Galambos, Pasta- an RF phase scan and tuning application
[7] Dong-o Jeon, Comparision of phase scan vs acceptance scan for the SNS DTL
[8] A.V. Aleksandrov, SNS Warm Linac Commissioning Results
9. Superconducting Accelerating Cryo-Module Tests at DESY.
10. OPERATIONAL ASPECTS OF THE RF CONTROL SYSTEM FOR THE TESLA TEST
FACILITY.
11. LLRF TESTING OF SUPERCONDUCTING CRYOMODULES FOR THE EUROPEAN
XFEL.
12. Determination of field amplitude and synchronous phase using the beam-induced signal in an
unpowered superconducting cavity.
13. TRANSIENT BEAM LOADING BASED CALIBRATION OF THE VECTOR-SUM FOR
THE TESLA TEST FACILITY.
14. SINGLE BUNCH TRANSIENT DETECTION FOR THE BEAM PHASE MEASUREMENT
IN SUPERCONDUCTING ACCELERATORS
15. DEVELOPMENT AND IMPLEMENTATION OF ∆T PROCEDURE FOR THE SNS* LINAC
16. PASTA an RF phase scan and tuning application
20
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17. The Delta-T Tuneup Procedure for the LAMPF 805 MHz Linac,
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Appendix 1: Beam modes limitation identification studies in phase &
amplitude setting
The table below shows whether the perturbations, caused by different beam pulse length and peak
currents, are within cavity field stability ±1%, 1°. (For DTL tank 2. Upper table is result with
feedback control. Bottom table is result without feedback control). For superconducting cavity,
perturbation expects to be less due to large cavity voltage time constant).
, 084- 89: 46; 21.6< 64140/4/1; =1=60.>1?; .4: 301: 2>1@5: /01=; 81! 1A 1: 2>1! 1>038001088; 81.6< 64/7
B0C71*1D1, : //0/19; 45140/4/E1! 1D1F: 6./19; 45140/4/E1" 1D1F: 6./1=60.>140/4E1#1D1F: 6./1@5: /0140/4
G0: < 1H- 880241621 , - ./01.023451621- /7
<I7
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22
"*
*
*
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"
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
European Spallation Source ESS AB
Visiting address: ESS, Tunavägen 24
P.O. Box 176
SE-221 00 Lund
SWEDEN
www.esss.se
, 084- 89: 46; 21.6< 64140/4/1; =1=60.>1?; .4: 301: 2>1@5: /01=; 81! 1A 1: 2>1! 1>038001088; 81.6< 64/7
B0C71*1D1, : //0/19; 45140/4/E1! 1D1F: 6./19; 45140/4/E1" 1D1F: 6./1=60.>140/4E1#1D1F: 6./1@5: /0140/4
G0: < 1H- 880241621 , - ./01.023451621- /7
<I7
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" +% * * * * * * * * * * * * * * " " " " "
$+% * * * * * " " " " " " " " " " " " " "
&+% * * * " " " " " " " " " " " " " " ! !
( +% * * " " " " " " " " " ! ! ! ! ! ! ! !
! *+% * * " " " " " " ! ! ! ! ! ! ! ! ! ! !
! " +% * " " " " " " ! ! ! ! ! ! ! ! ! ! ! !
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23
"*
"
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!
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!
!
!
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!
!
!
!
!
!
!
!
!
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!
!
!
!
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!
!
!
European Spallation Source ESS AB
Visiting address: ESS, Tunavägen 24
P.O. Box 176
SE-221 00 Lund
SWEDEN
www.esss.se
Appendix 2: Proposal of cavity tests for ESS cavities (Julien Branlard,
DESY)
2013-12-06 (From Julien Branlard, DESY)
1. Cavity fundamental mode measurements
a. 5 minute / cavity
b. Measure spectrum with network analyzer
c. Check the center frequency is according to specs
d. Can also measure Ql from width of resonance peak
e. Measure other sub-pi modes at the same time
2. Calibration of cold cables
a. Quick (5 mins)
b. Required in order to get accurate gradient measurement
c. Network analyzer measurement, s11 off resonance
3. Coupler conditioning
a. Crucial step
b. Necessary (especially for 1.5 MW per coupler)
c. Has to be done warm, then cold
d. Time consuming (several days)
e. Requires automation (exception handling is what makes it hard)
f. NOTE: for cold conditioning, cavity gets gradient  more complicated (Xrays, field
emission, field breakdown, quenches, etc…)
4. Cavity gradient calibration
a. Need to measure loaded Q and compare against specifications
b. From loaded Q, you derive kT parameter
c. Vcav proportional to kT SQRT(Pfwd) (I think Q0 is also needed)
d. Just a computation , not an actual test but it’s required
e. Check gradient against specifications
f. Could perform gradient check and verify quench limit
g. Makes sense to do this before cold coupler conditioning
5. Heat load
a. Time consuming because cryo time constant is big
b. Let cavity sit at one gradient, measure heat load
c. Increase gradient, repeat etc…
d. Check dynamic heat load against specifications
24
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6. Tuner
a.
b.
c.
d.
Check functionality
Check tuning range, tuning constant
Performed with network analyzer,
observe peak frequency change as a function of tuner steps
7. Piezo
a. Check functionality
b. Apply DC bias and verify frequency shift
c. Verify maximum tuning range  has an implication on where the slow tuner center
frequency should be
8. Lorentz force detuning
a. Relative fast test
b. Place cavity on resonance
c. Ramp up gradient
d. Measure detuning (phase slope) as a function of gradient
e. Derive LFD constant
f. Could be done in parallel with cold coupler conditioning
25
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Visiting address: ESS, Tunavägen 24
P.O. Box 176
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