Cavity Performance Testing: Vertical Dewar and Cryomodule

This material is based upon work supported by the U.S. Department of Energy Office of Science under Cooperative Agreement DE-SC0000661, the State of Michigan and Michigan State University. Michigan State University designs and establishes FRIB as a DOE Office of Science National User Facility in support of the mission of the Office of Nuclear Physics.

John PopielarskiRF Measurement and Design Group Leader

Cavity Performance Testing: Vertical Dewar and Cryomodule

SRF Tutorial: Cavity Performance Testing Vertical Dewar and Cryomodule

IntroductionSRF Cavities Acceptance Testing

• Acceptance criteria from specification• Key criteria for vertical test; an example workflow for cavity processing• Acceptance tests prior to vertical test• SRF subcomponents (coupler, tuner, feedthrough, cables)

Vertical Test Area• Vertical testing vs horizontal testing & VTA area• Test area design considerations & maintenance (magnetic fields)• Personnel safety and machine protection• RF circuits used for vertical testing• Cryogenic circuits used for vertical testing• Test Dewars & Inserts• Other sensors: pressure, temperature, magnetic field probes• Data Acquisition Methods

Outline (1/2)

Slide 2J. Popielarski,


Methods in Vertical Test• Typical VTA Results for cavity certification• Calibration & RF Measurement Technique• Measurement Errors & Mitigation • Limitations: Quench, RF, admin, liquid level• Multipacting, Q-Disease, Magnetic Flux Trap

Acceptance Testing of Cryomodules• Acceptance criteria for cryomodules• Continuous testing during cryomodule assembly• RF and cryogenic circuits; bunker layout

Cryomodule Testing Methods• Typical Cryomodule Certification Results• Development in Bunker Tests

SRF Component Validation• Integrated testing of prototype cavities, couplers, tuners with LLRF control• First article testing

Outline (2/2)



Large Scale Dewar Testing of cavities and cryomodules is a necessary step to ensure quality of the SRF systems before installation into the Linac.Cavity testing, done in a timely manner, is the best method to ensure

quality of the processing steps are adequate to deliver the specifications for the SRF cavity.Cryomodule testing is the final step to certify a cryomodule for

installation in an SRF linac, which ensures smooth and timely commissioning efforts.

Introduction



For vertical test, the acceptance criteria is derive from the specifications, but with more margin to cover possible spread in performance.

SRF Cavities Acceptance TestingAcceptance Criteria is Derived From Specifications


From Saito, K., FRIB Project Moving to Production Phase (SRF2015)

Example Specifications for FRIB Linac

Parameter VTA Acceptance Specificationf0 (MHz) Varies, based on tuner

preload322

Eacc [MV/m] 8.9 7.51

Q0 9.2E9 @ 8.9 MV/m 9.2E9

Qext2 2.8E11 to 2.8E12 ~10 dBm to LLRF

Pcav [torr] <1e-8 No Leak

df/dP [Hz/torr] None. < 4

Ka [Hz/(MV/m)2] None. < 4

X-rays [mR/HR] None.

Example Acceptance for FRIB .53 HWR


SRF Cavities Acceptance TestingKey Acceptance Criteria for Vertical Test

Parameter FRIB Symbol andDefault Units

Description

Resonant Frequency f0 (MHz) Resonant frequency at operating temperature and pressure

Accelerating Gradient Eacc [MV/m] Calculated RF model and stored energyIntrinsic Quality Factor Q0 Calculated from stored energy, frequency and measured

powers

Pick-up Coupling Factor Qext2 Calculated from CW measurement4K/ 2K Cavity Pressure Pcav [torr] Measured with ion gauge on insertFrequency sensitivity to pressure fluctuations

df/dP [Hz/torr] Measurement of frequency at different pressures, linearrelationship

Lorentz force detuning Ka [Hz/(MV/m)2] Measurement of frequency at different fields, changes linearlywith the square of the field

X-rays X-rays [mR/HR] Measured with local ionization chamber (outside Dewar walls)



With tight tolerances to fit the SRF cavities into a cryomodule, measurements are done before SRF cavity processing work starts. A reference to the beam line

elements are made to assist in alignments

SRF Cavities Acceptance TestingCavity Dimensions, Fiducials and Frequency


From J. Popielarski, FRIB Tuner Performance and Improvement (SRF2017)

There is a large spread in frequencies during the manufacturing steps. Frequency checking and tuning is

done through out manufacturing steps.

From S. Miller, FRIB Cavity and Cryomodule Performance (SRF2019)


RF Input Couplers: The fundamental power couplers (FPC) sometimes follow extensive RF testing and multipacting conditioning process. Tuners: External tuner mechanisms are assembled and work is done

to preloaded / pretension the mechanism to the cavity along with frequency measurements.Magnetic Shielding: First articles are extensively checked with

measurements during cryomodule testing.RF cables (which go inside the cryomodule): Each cable is checked

before and after assembly in the cryomodule.RF/ Vacuum Feedthroughs: As with the cables, these are checked

for acceptance from the vendor and checked again during the cryomodule assemblyMagnetic Material: Stainless steel parts can become magnetized,

these are checked and degaussing is done to the parts if needed

SRF Component Acceptance TestingSome Other Critical SRF Components



The vertical test is designed to test a specific set of parameters.• Test systems are designed specifically for that test, e.g.

» Input coupling is made to match the cavity so that lower power may be used for the test and RF circuits are better matched.

» Precision control of elements that can influence the SRF cavity performance, such as temperature, pressure, and magnetic fields.

» Diagnostics are set-up to monitor specific variables, such as X-rays, beamline pressure, temperature maps, quench detectors

Horizontal testing is designed to test system integration• The emphasis of horizontal test is showing multiple components work

together, such as a fully assembled cryomodule• Some key areas for SRF integration tests are:

» FPC / Cavity integration (multipacting, heat load of coupler)» Tuner + Cavity testing (tuning range, RF control)» Low Level RF integration with tuner

Vertical TestVertical Test // Horizontal Test



Dedicated Cryoplant & Test Dewars Test inserts Test insert preparationShielding considerationMagnetic fieldsRack areasMaintenanceExpansion / upgrading

Vertical Test AreaTest Area Design Considerations



During vertical test, potential hazards exist that should be considered in design phase.• Pressure/Vacuum Vessel (Stored Energy)• RF Systems• Radiation• Cryogenics Systems• Area Hazards: Falls, Overhead Crane Operation, Oxygen Deficiency,

Use Daily Checklists to make sure systems have not changed.• Dewar guard vacuum pressure, reliefs, valve configuration • Process Variables (data logging/ archive via EPICS)• Dewar Pit is ready for cool down (e.g. if test was prepared in the previous

shift)

Use configuration control to keep processes and practices known• If necessary, label parts that are part of the known configuration

Personal Safety and Machine ProtectionKnow the hazards and the risks.



Design RF circuits in VTA for precision RF measurements. The main result relies on the RF power measurement.• Eliminate errors from standing wave patterns due to mismatch by adding

padding when possible.• Avoid wear and tear on components by keeping circuits assembled.• Use dedicated calibration cables.

Instruments used in vertical testing include:RF Amplifier, Circulators, Directional Couplers (High Level RF Part)

Signal Generator with DC FM modulationPower MetersMixer, Amplifier, Phase Shifters

RF Circuits Used for Vertical Testing


(Low Level RF Part)


The Dunk Test• We know the SRF cavity is immersed completely in the helium bath when

the liquid level is higher than the cavity.• The Dewar used for the dunk test is filled with liquid helium

The Not-a-Dunk Test• Liquid helium flows through the cavity helium vessel and a storage tank

above it. All FRIB cavity tests are done this way.• The Dewar used for this type of test is used as a vacuum vessel

Piping and Interface Diagrams• The P&ID is the “schematic” of the system. From looking at the P&ID, we

can see where valves, temperature sensors, pressure gauges, overpressure reliefs are located.

• The P&ID is a way to communicate the configuration of all the pieces of the system to all the stake holders, and it is used for configuration control, e.g. the configuration cannot be changed without updating the P&ID and conducting the appropriate reviews if needed.

Cryogenic Circuits Used for Vertical Testing



The SRF cavity is mounted to the Dewar insert (a Dewar top lid with an SRF cavity hanging from it. Coincidentally, the cavity hangs from the

lid vertically. (Vertical Test) The inserts are prepared in the clean room

and the insert prep mezzanine before being installed into the Dewar. For FRIB SRF cavity vertical testing, the

Dewars act as a cryostat. Alternately, the Dewar can be used to

“dunk test” an SRF cavity. The SRF cavity is installed into the Dewar and the Dewar is filled with liquid helium from the cryoplant

Test Inserts and Dewars


From C. Zhang, Large Scale Dewar Testing… (NAPAC2019)

From W. Hartung, Performance of FRIB Production… (NAPAC2019)

From J. Popielarski, FRIB SRF Cryomodule Performance Testing … (HB2018)


X-ray Sensor // Gamma Detector: Used to measure the X-rays that are generated from the SRF cavity when multipacting or field emission occurs.(Helium) Bath Pressure Sensor: Use to measure bath pressure. In 2K liquid, the bath pressure is sub atmospheric (in vacuum), and an accurate pressure gauge can be used to infer the bath temperatureTemperature Sensor: Temperature sensors are placed in different parts of the test insert to monitor a verity of process, e.g. sensors on the helium supply and return monitors cool down rate, sensors are RF cables could give the tester an indication of undesirable RF heatingMagnetometers: Magnetometers are used to check the presence of magnetic field inside the magnetic shield. This can be checked before cooling downHelium Leak Checkers: Are used to leak check the cryogenic circuits prior to installation into the Dewar

Instrumentation Used in Vertical Test



Level Sensor: The level sensor is installed in the Dewar or in the liquid helium tank about the cavity and reports the liquid level.RGA: A residual Gas Analyzer can be used on the beam line

vacuum to confirm there is a helium leak to the beamline vacuum. We don’t typically use one in production certification tests – we assume any leak to the space is helium anyway.Vacuum Gauges: Vacuum gauges are monitor the vacuum spaces

in the vertical test. • Beamline Space (the vacuum inside the cavity)• Insulating space (the vacuum in the space between the liquid helium and

the outside walls)

Instrumentation Used in Vertical Test



Serial / Network Communication to measurement instruments• Measurement instruments which are serial devices use adaptors to

communicate over the network (e.g. Moxa)• A PC using Labview or similar software can communicate with the devices

and record the signals of the process variables (PVs) for the testing.• Alternately, like in the FRIB case, the test instruments are monitored by an

input-output controller (IOC) which puts the PVs on EPICs• Data is archived• Control done in UI

Data Acquisition Systems for Vertical Test


FRIB VTA1 RF Test AreaFRIB VTA Cryogenic Instruments


Typical metrics used to “PASS” Vertical Test:• Max achieved accelerating gradient

(Ea), Quality Factor (at a given field), Beamline pressure, External Coupling of the pick-up probe, frequency

Alternate metrics:• X-rays. Large X-rays is a sign of a

“dirty” cavity. Typically, Xray “star” greater than 100 mR/hr is enough to trigger a rework, or very early FE onset

• Frequency Sensitivity to Pressure, or df/dP, is not typically a pass/fail, but a large difference from the expected result could be a concern

• Lorentz force detune

Methods in Vertical TestTypical VTA Results for Cavity Certification



In R&D settings, more attention can be paid to the effect of certain processes changes the cavity performance.Emphasis should be made for repeatable test

setups, since so many variables can effect the performancePrototyping testing qualifies the manufacture and

processing steps.

Methods in Vertical TestProcess Development & Prototyping


From W. Hartung, Status report on multicell development for medium velocity beams (PAC2003)

Tests done in development work and prototyping are intended to:• Prove designs• Develop processing

methods• Determine design goals &

specifications


The RF measurement is done with the cavity resonance “locked” in a phase locked loop (PLL). The phase lock loop is done with the signal generator and an error signal.• The error signal is the phase difference between the forward signal (Pf) and

the cavity transmitted signal (Pt). The mixer, a passive RF component, provides the error signal to the signal generator.

• Adjustments to the forward phase puts the cavity at the peak of the resonance: Minimize the reverse power signal (Pr) and maximize the cavity transmitted signal (Pt).

CW Measurement: Measure Pf, Pr, Pt with the power meters.Decay Measurement: Turn off the amplifier and measure the decay

rate of the Pt signal.

RF Measurement Technique


mixer


β2 = 𝑄𝑄0 𝑄𝑄𝑒𝑒𝑒𝑒𝑒𝑒2⁄ ,

RF Measurement TechniqueDetermine Q and Ea from CW Measurement



𝛽𝛽1 = �1 + |S11|1 − |S11|

�±1

𝛽𝛽2 =|S21|2

1 − |S11|2 − |S21|2

|S11| = �𝑃𝑃𝑟𝑟 𝑃𝑃𝑓𝑓⁄

|S21| = �𝑃𝑃𝑒𝑒 𝑃𝑃𝑓𝑓⁄

𝑃𝑃𝑑𝑑 = 𝑃𝑃𝑓𝑓 − 𝑃𝑃𝑟𝑟 − 𝑃𝑃𝑒𝑒

𝑄𝑄𝐿𝐿 = 2𝜋𝜋𝑓𝑓0 log10(𝑒𝑒)∆𝑒𝑒∆𝐴𝐴

𝑄𝑄0 = (1 + 𝛽𝛽1 + 𝛽𝛽2)𝑄𝑄𝐿𝐿

𝑈𝑈0 =𝑃𝑃𝑒𝑒𝑄𝑄𝑒𝑒𝑒𝑒𝑒𝑒2

2𝜋𝜋𝑓𝑓0 𝑄𝑄0 =

2𝜋𝜋𝑓𝑓0𝑈𝑈0

𝑃𝑃𝑑𝑑 𝐸𝐸𝑎𝑎𝑎𝑎𝑎𝑎 = 𝑘𝑘𝑎𝑎�𝑈𝑈0

For the Q vs Ea curves

ka From cavity model or parameter table

CW Measurements

DecayMeasurement



RF Measurement TechniqueAdvantage of unity coupling and weak pick-up



𝛽𝛽1 = �1 + |S11|1 − |S11|

�±1

𝛽𝛽2 =|S21|2

1 − |S11|2 − |S21|2

|S11| = �𝑃𝑃𝑟𝑟 𝑃𝑃𝑓𝑓⁄

|S21| = �𝑃𝑃𝑒𝑒 𝑃𝑃𝑓𝑓⁄

𝑃𝑃𝑑𝑑 = 𝑃𝑃𝑓𝑓 − 𝑃𝑃𝑟𝑟 − 𝑃𝑃𝑒𝑒

𝑄𝑄𝐿𝐿 = 2𝜋𝜋𝑓𝑓0 log10(𝑒𝑒)∆𝑒𝑒∆𝐴𝐴

𝑄𝑄0 = (1 + 𝛽𝛽1 + 𝛽𝛽2)𝑄𝑄𝐿𝐿

𝑈𝑈0 =𝑃𝑃𝑒𝑒𝑄𝑄𝑒𝑒𝑒𝑒𝑒𝑒2

2𝜋𝜋𝑓𝑓0 𝑄𝑄0 =

2𝜋𝜋𝑓𝑓0𝑈𝑈0

𝑃𝑃𝑑𝑑 𝐸𝐸𝑎𝑎𝑎𝑎𝑎𝑎 = 𝑘𝑘𝑎𝑎�𝑈𝑈0

For the Q vs Ea curves

CW Measurements

DecayMeasurement

1

10

0 ka From cavity model or parameter table

U=Pd*2*pi*f0


From the previous set of equations, we see that we can generate a Q vs E curve from the CW measurements of the forward power.• Accurate measurements of Pf, Pr, Pt are needed. To do this, you need:1. Cable calibration routine (every test). Try to do the cable calibration such that the

RF circuit is not completely broken down. Note any difference to previous calibration.

2. Avoid mismatches. Mismatches happen at adaptors, feedthroughs, mixers, splitters, directional couplers, phase shifters, instrument ports, etc. A small mismatch receiving the Pt signal from the cavity can generate a large error, since the mismatch at the cavity is very large due to the weak coupling. The same is true for the cryomodule test. If necessary, add attenuators to reduce the standing wave in the line. The error in the mismatch changes with line length.

3. Do “looking back” checks. Measure the input impedance where you are reading the power.

4. Measure the Qext’s on a bench before the cavity prep and vertical test, and do room temperature |S21| checks before cool down to ensure the couplers and cables are properly installed. (Do a clean room |S21| check before VTA prep)

Measurement Errors and Mitigations



In the end, we also record what was the limiting factor in the vertical test. This is useful for later, when we look over the test data for many SRF cavities.• Administrative Limit: Sometimes, we place administrative limits on the cavity

field. The intention is to prevent a deconditioning event where a field emitter can be turned on, lowering the maximum achievable gradient. Other reasons to use administrative limits include: » To prevent wear and damage to the High Level RF equipment, cables and

feedthroughs» Avoid excessive radiation in unshielded areas (if any) causing trips or requiring

surveys• Liquid Level Gone: When liquid level is gone the time for the testing has run

out. If the SRF cavity passes all the test metrics, it is likely the test will not need to continue.

• RF Power Limit: Reaching the RF power limit is often a good sign (unless there is a large Q-slope)

• Self Modulation/ Quench/ Thermal Breakdown: In this case, a higher field cannot be reached.

Limitations in the test



Multipacting: Multipacting in FRIB cavities is very common but can be conditioned. At FRIB, most of the testing time is dedicated to multipacting conditioning; fortunately the conditioning is usually done in a few hours.• SYMPTOMS: When raising the forward power, the field level does not increase; While at

high(er) field, the field level suddenly drops; Xrays. As forward power increases and the cavity field stays constant, a vertical line can be seen on the Q vs Ea trace.

Q-Disease: Q-Disease can happen in cavities that have not undergone heat treatment process (de-gassing). A Q-Disease check can be done to qualify the de-gassing process,• SYMPTOMS: An uncharacteristic drop in Q as field increases, depending on the severity of

the Q-Disease. To check for Q-Disease, an overnight “soak” at ~100K should make this happen.

Excessive Trapped Flux: If magnetic field is present during cool down (niobium transition to a superconductor), a lower Q be observed in the test. This can be avoided by doing good magnetic hygiene and shielding.• SYMTOMS: Measured Q is lower than expected; Q is lower in different test environments

(different shield, magnetic hygiene, etc.)

Other Tests and Symptoms in Vertical Test



For cryomodule test certification, less margin is given to the pass/ fail metric for the cavity field (Vertical Test was +20% for FRIB)Non conforming frequency needs to

be reworked.• Frequency rework on a cryomodule

after a bunker test is very expensive.• As a lesson learned, the SRF cavities

are tested continuously during the cryomodule assembly

Acceptance Testing of CryomodulesAcceptance criteria for cryomodules and continuous testing during

assembly


From W. Chang, Progress in FRIB Cryomodule Bunker Tests. (SRF2019)


In Bunker Testing, high power RF is needed to bring the cavity to full field. • The power levels need for the linac may not be

needed, since there is no beam load in the bunker test.

• Typically, test bunkers are not equipped with enough amplifiers to energize all the cavities at the same time. In FRIB, we have two amplifiers and two LLRF systems for each test.

The bunker needs to have the ability to do 2K testing (if the intended operation is in 2K).• At 2K:

» LLRF control at high field can be done» Dynamic heat load measurement is straight forward

Bunkers generally have concrete walls to form shielding in case the cavities make X-rays

Acceptance Testing of CryomodulesRF and Cryogenic Circuits; Bunker Layouts


From T. Xu, FRIB Cryomodule Design and Production (NAPAC2019)

From J. Popielarski, Performance Testing of FRIB.. (SRF2017)


Cryomodule Testing MethodsTypical Cryomodule Certification Work Flow


Slide from J. Popielarski, Performance Testing of FRIB.. (SRF2017)


After Cooling Down to 4K, Do an |S21| measurement.• Make sure the port connections are

50 ohm, a mismatch in the FPC side could give the wrong QL (BW)

• If the bath pressure is unstable and the |S21| trace has noise, the decay method can be used for a more accurate bandwidth measurement.

Cryomodule Testing MethodsIn-Situ RF Calibration


Popielarski, John, FRIB Linac SRF Commissioning… (TTC2021-01)

(internal mismatch)


Cryomodule Testing MethodsDynamic Heat Load of an offline cryomodule


Slide copied from: Kim, Sang-hoon, Offline and online heat load measurements of FRIB cryomodules and performance after thermal cycling (TTC2021-01)[1] J. Fuerst, W. Hartung, “Dissipated power measurements in the A0 SRF cavity system,” FERMILAB-Conf-00/010-E (2000)

2xCav RF


Like in vertical test, Q and Ea remain the top parameters. In the linac:• The average heat load needs to be close

to the design value.• The average Ea needs to be close to the

design value.• The maximum Ea is defined as the max

field at which the cavity can run stable (long term operation) in the tunnel

Cryomodule Testing MethodsTypical Cryomodule Certification Results



Before mass production, combining critical SRF components into a single “Horizontal” test is an effective way to validate designs or give feedback to make adjustments. It is difficult, expensive, and time consuming to

develop cryomodules to do this type of work. It is useful to do this type of testing in these

systems together with the cavity:• High power RF couplers (thermal performance,

multipacting)• Cavity Tuners (tuning range and control)• Microphonics, mechanical mode damping.

The real performance is known in the cryomodule test, so if possible push forward first article cryomodule testing.

SRF Component ValidationIntegrated Testing Campaigns to Build Confidence in the Design


From J. Popielarski, Cryogenic Testing of Production… (LINAC16)


First article cryomodules have more expansive testing and more ambitious testing goals. • Lock of all the cavities (1 hours+) without trips. • Thermal cycle to check for Q degradation from magnet operation (validate

deguassing Method)• Dynamic load of each cavity (typically we measure only a few cavities for the

normal bunker certification)

SRF Component ValidationFirst Article Testing Example: FRIB QWR Accelerating Module


From J. Popielarski, Performance Testing of FRIB.. (SRF2017)

Documents

Cavity Performance Testing: Vertical Dewar and Cryomodule