Overview of cryo-modules for proton accelerators Paolo Pierini (INFN Milano)

1. overview
of cryomodules
for proton
accelerators
Paolo Pierini
INFN Sezione di Milano
Laboratorio Acceleratori e Superconduttività Applicata
Paolo.Pierini@mi.infn.it
19 March 2009 Bilbao

2. outline
• discuss cryogenics & cryomodules design rationales
• intent limited to modules for elliptical cavities and few
considerations for spoke cavities
– not covering other structures, especially QWR case
• often not completely relevant (common vacuum, 4 K operation, small
scale, ...)
• trying not to concentrate on design details, rather explore
interplay with the design choices/requirements of the
machine / supporting systems
March 19 2009 essbilbao initiative workshop - Paolo Pierini 2

3. SRF cavities and ancillaries - 1
cavities and ancillaries design are chosen on the basis of
a complex optimization that depends on:
• accelerated particles
– velocity profile
• beam energy
– variety of resonator shapes
• beam current
– high current asks for consistent HOM damping
– low current CW implies high external Q and tight resonance
• beam quality requirements
– alignment tolerances
– High Order Mode damping requirements
–…
March 19 2009 essbilbao initiative workshop - Paolo Pierini 3

4. SRF cavities and ancillaries - 2
• pulsed operation
– high field is dominant with respect to minimum losses
– Lorentz Force Detuning impact the cavity/tuner design
– active fast tuner required for high field
– high peak power coupler for high current
• CW operation
– high Q, low losses, dominant with respect to maximum field
– microphonics can be crucial
– active fast tuner considered for low current
– high average power coupler for high current
• other machine dependent features
– high filling factor: interconnections, tuner, magnets, etc
– minimization of static losses : long cryo-strings
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5. general considerations
• cryomodules are now more and more integrated in the
concept/optimization of the accelerator
– no longer viewed as the combination of a cavity system and an
independently designed cryostat to contain it with minimum losses
– modules are especially one (important) part of the overall
cryogenic system
• the cryostat is one of the cryomodule components and its
optimization can affect the cavity package design
– in a large size SRF machine the overall cryomodule cost and
performances dominate that of individual components
• components and systems reliability, and the accelerator
availability, are concepts that are now included in the
large accelerator design from the beginning
– redundancy or MTTR (mean time to repair)?
– improve QC for MTBF
March 19 2009 essbilbao initiative workshop - Paolo Pierini 5

6. cryogenic plant: duties
• primary
– maintain cavities at normal operation temperature
• below 2K for elliptical
• below 4.5 K for spokes
– provide fluid flow for thermal intercepts and shields at multiple
temperature levels
– supply liquefaction flow for power leads
– cool-down and fill (and empty and warm-up) the accelerator
– efficiently supports transient operating modes and off-nominal
operation
• including RF on/RF off and beam commissioning
• secondary
– allow cool-down and warm-up of limited-length strings for repair or
exchange of superconducting accelerating components
• to which extent is an important design choice (unit module, strings...)
March 19 2009 essbilbao initiative workshop - Paolo Pierini 6

7. cryogenic distribution system functions
• supports operation of the linac
– within cooldown and warm-up rate limits and other constraints
imposed by accelerating SRF components
• time duration of cooldowns, transient thermal gradients, ...
• guarantees safety
– All cryo component and circuits should be guaranteed not to ever
exceed their MAWP (Maximum Allowable Working Pressure)
during fault conditions
• guarantees machine protection
– RF cavities from over pressurization under faulty conditions that
can hinder performance
• substantial difference with respect to SC magnets!
March 19 2009 essbilbao initiative workshop - Paolo Pierini 7

8. cryogenic distribution system design
• design should be independent of cooldown rates,
cooldown sequences, or pressurization rates
• includes many components to be designed/engineered
– feed boxes
– cryogenic transfer lines
– bayonet cans
– string/modules feed and end caps
– string connecting and segmentation boxes
– gas headers
– ...
• cryogenic distribution system and cryomodules are not
engineered independently
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9. the cryomodule environment: a“cartoon” view
to He production 2K
supports
and distribution 5-8 K
system
40-80 K
RF
all “spurious” sources of heat
RF penetrations
cavities
losses to the 2 K circuits need to
be properly managed and
intercepted at higher temperatures
(e.g. conduction from penetration
and supports, thermal radiation)
these are the accelerator active
devices with tight alignment
constraints for beam “quality”
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10. the efficiency of the thermal cycle
• thermal cycle efficiency
– efficiency of the thermal cycle, to extract heat Q deposited at Tc
we need a work W at temperature Th always greater than the
Carnot cycle
Th − Tc
⋅ηth
W = Q⋅
Tc
– including the efficiency of the thermal machine (20% for Tc = 2 K)
we need 750 W at room temperature for 1 W at 2 K
– all sources of parasitical heat loads need to be carefully avoided if
we do not want to pay such a high price!
– accurate thermal design in order to minimize the heat losses
• Static: Always present, needed to keep the module cold.
• Dynamic: Only when RF is on. Due to power deposition by RF fields.
• N.B. at different intercept temperatures
• when Tc = 4.2 K we have ~ 250 W/W
• when Tc = 50-80 K we have ~ 20-10 W/W
March 19 2009 essbilbao initiative workshop - Paolo Pierini 10

11. heat removal by He
• heat is removed by increasing the energy content of the
cooling fluid (liquid or vapor)
– heating the vapor
– spending the energy into the phase transition from liquid to vapor
• cooling capacity is then related to the enthalpy difference
between the input and output helium (∝ to mass flow)
• the rest is “piping” design to ensure the proper mass flow,
convective thermal exchange coefficient, pressure drop,
…
@2 K 20 J/g latent heat
Premoved [ W] = m flow[g/s] ∆h[J/g ] 40 K to 80 K 5 K to 8 K 2K
Temperature level Temperature level Temperature level
(module) (module) (module)
Temp in (K) 40,00 5,0 2,4
Press in (bar) 16,0 5,0 1,2
Enthalpy in (J/g) 223,8 14,7 4,383
Entropy in (J/gK) 15,3 3,9 1,862
Temp out (K) 80,00 8,0 2,0
Press out (bar) 14,0 4,0 saturated vapor
Enthalpy out (J/g) 432,5 46,7 25,04
Entropy out (J/gK) 19,2 9,1 12,58
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12. isothermal saturated bath
• to operate the cavities the heat load is ultimately carried
away by evaporation in an isothermal bath
– either saturated bath of LHe at ambient pressure (4.2 K)
– or saturated bath of subatmospheric superfluid LHe (< 2.1 K)
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13. state of the art
• two main different solutions
• the TESLA cryostring concept developed for a
superconducting linear collider
– tested in the TTF (now FLASH)
– used for the European XFEL linac construction (1.7 km)
– assumed for the ILC design (~30 km)
– concept studied also for proton machines
• SPL at CERN, Project X at FNAL
• the SNS linac
– short & independent units
– fast replacement of a single faulty unit
– concept used for ADS linac
March 19 2009 essbilbao initiative workshop - Paolo Pierini 13

14. TESLA cryomodule design rationales
• high filling factor
– maximize ratio between real estate gradient and cavity gradient
– long cryomodules/cryo-units and short interconnections
• moderate cost per unit length
– simple functional design based on reliable technologies
– use the cheapest allowable material that respect requirements
– minimum machining steps per component
– minimum number of different components
– low static heat losses in operation
• effective cold mass alignment strategy
– room temperature alignment preserved at cold
• effective and reproducible assembling procedure
– class 100/10 clean room assembly just for the cavity string
– minimize time consuming operations for cost and reliability
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15. consequences/I
• The combined request for a high filling factor [machine
size] and the necessity to minimize static heat losses
[operation cost] leads to integrate the cryomodule
concept into the design of the whole cryogenic
infrastructure
– Each cold-warm transition or cryogenic feed require space and
introduces additional static losses
• Thus, long cryomodules, containing many cavities (and
the necessary beam focusing elements) are preferred,
and they should be cryogenically connected, to form cryo-
strings, in order to minimize the number of cryogenic
feeds
– Limit to each cryomodule unit is set by fabrication (and cost)
issues, module handling, and capabilities to provide and
guarantee alignment
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16. consequences/II
• The cryogenic distribution for the cryo-string is integrated
into the cryomodule, again to minimize static losses
– several cryogenic circuits running along the cold mass to provide
the coolant for the cavities and for the heat interception at several
temperatures
• To take out the RF power dissipated along the long cryo-
string formed by many cryomodules connected together a
large mass flow of 2 K He gas is needed, leading to a
large diameter He Gas Return Pipe (HeGRP) to reduce
the pressure drop
– This pipe was made large and stiff enough so that it can act as the
main structural backbone for the module cold mass
• cavities (and magnet package) can be supported by the HeGRP
• The HeGRP (and the whole cold mass) hangs from the vacuum
vessel by means of low thermal conduction composite suspension
posts
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17. the TESLA module provides
• cryogenic environment for the cold
mass operation
– cavities/magnets in their vessels filled
with sub atmospheric He at 2 K
– contains He coolant distribution lines at
required temperatures
– collect large flow of return gas from the
module string without pressure increase
– Low losses penetrations for RF,
cryogenics and instrumentation
• shield “parasitical” heat transfer
– double thermal shield
cavity
• structural support of the cold mass
size
– different thermal contractions of
materials 12 m, 38” diameter, string of
8 cavities and magnet
– precise alignment capabilities and
reproducibility with thermal cycling
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18. TESLA/ILC/(XFEL) modular cryogenic concept
ILC scheme for segmentation and distribution
• each module contains all cryo modules without with without
quad quad quad
piping RF unit (lengths in meters) 12.652 12.652 12.652
three modules
– each cavity tank in module
connected to two phase line RF unit RF unit RF unit RF unit end box
37.956 37.956 37.956 37.956 2.500
string (vacuum length)
twelve modules plus string end box
– vapor is collected from 2 phase
line once per module in the string string string string
154.324 154.324 154.324 154.324
possible segmentation unit
GRP 48 modules
(segmentation box is the same as string end
• several modules are connected box (2.5 m) and all contain vacuum breaks)
in strings service service
box end segment segment segment segment box end
Cryogenic Unit
– single two phase line along the 2.500 617.296 617.296 617.296 614.796 2.500
(16 strings) (1 cryogenic unit = 192 modules = 4 segments*48 CM
string with string end boxes plus service boxes.)
2471.7 meters
– a JT valve once per string fills unit length limited by size of cryo plant
needed (25 kW equivalent at 4.5 K seems
two phase line via subcooled Cryogenic
max reasonable extrapolation of 18 kW LHC) distribution box
2.2 K line Line F 75 K return
• strings are connected into units
Line E 50 K supply
Line D 8 K return
Line C 5 K supply
– each unit is fed by a single Line A Sub-cooled LHe supply
cryogenic plant
Line B Pumping return
Cryo-string Cryo-string
Cryo-string
Cryo-string
Cryo-unit
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19. schematically
outer shield
All lines in module
inner shield
subcooled forward line
GRP
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20. cryostrings in TTF&FLASH
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21. The cross section
Low thermal conduction
composite supports
Cryogenic
Pressurized
support
helium feeding
Helium
GRP
Shield gas
(large because of
feeding
pressure drop, used
as structural backbone)
Thermal
shields
cavity
Two phase
RF Penetration flow
Sliding
Coupler
support
Helium
port
tank
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22. three generations of cryomodules in TTF
1 2 Simplification of fabrication (tolerances), assembling & alignment strategy
2 3 Longitudinal references, redistribution of cross section (42” 38”)
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23. from prototype to Cry 3
Extensive FEA modeling (ANSYS™)
of the cryomodule
– Transient thermal analysis during
Braid-cooled Cry 1 - 1997
cooldown/warmup cycles,
– Coupled structural/thermal
simulations
– Full nonlinear material properties
Detailed sub-modeling and testing of
new components
– Finger-welding for stress-relief
– Cryogenic tests of the sliding
supports
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24. Cold mass alignment strategy
• The Helium Gas Return Pipe (HeGRP) is the system
backbone
– 3 Taylor-Hobson spheres are aligned wrt the HeGRP axis, as
defined by the machined interconnecting edge flanges
• Cavities are aligned and transferred to the T-H spheres
• Cavity (and Quad) sliding planes are parallel to the
HeGRP axis by machining (milling machine)
– Longitudinal cavity movement is not affecting alignment
– Sliding supports and invar rod preserve the alignment while
disconnecting the cavities from the huge SS HeGRP contraction
• 36 mm over the 12 m module length cooling from 300 K to 2 K
• Variation of axis distances by differential contraction are
fully predictable and taken into account
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25. cooldown behavior
300
T in (CMTB)
270 T out (CMTB)
Delta T (CMTB)
240 DeltaT (ANSYS)
T in (ANSYS)
210
T out (ANSYS)
Temperature (K)
180
150
120
70 K shield
90
60
30
0
0 5 10 15 20 25 30 35 40 45 50 55 60
Time (h)
comparison FEM with CMTB cooldown
• Fairly sophisticated non linear 300
T out (CMTB)
transient FEM models 270 T in (CMTB)
Delta T (CMTB)
240
T in (ANSYS)
– reproduce with good accuracy T out (ANSYS)
210
Delta T (ANSYS)
the cooldown behavior
Temperature (K)
180
– assess max thermal gradients 150
and stresses during transients
120
5 K shield
90
– allow to identify suitable 60
cooldown rates to keep thermal 30
stresses below safe limits 0
0 5 10 15 20 25 30 35 40 45 50 55 60
Time (h)
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26. linac performances, low static load budget
~ 70 W ~ 13 W < 3.5 W
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27. proven design, still few details to clean up
• XFEL introduced small enhancements
– quad sliding fixture (as for cavities)
– better heat sinking (all coupler sinking style)
– cables, cabling, connectors and feed-through
– module interconnection: vacuum vessel sealing, pipe welds, etc.
– coupler provisional fixtures and assembly
– preparing large production at qualified industries
• important actions for ILC
– move quadrupole to center (vibrations)
– short cavity design (decrease cutoff tube)
– cavity interconnections: flanges and bellows (Reliability)
– fast tuner (need coaxial so that filling factor can be further
increased!)
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28. TESLA cryomodule concept summary
positive
• very low static losses
• very good filling factor: best real estate gradient
• low cost per meter in term both of fabrication and assembly
project dependent
• long cavity strings, few warm to cold transitions
• large gas return pipe inside the cryomodule
cavities and quads position controlled at ± 300 µm (rms)
•
• reliability and redundancy for longer MTTR (mean time to repair)
• lateral access and cold window natural for the coupler
negative
• Long MTTR in case of non scheduled repair
• Moderate (± 1 mm) coupler flexibility required
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29. different design: SNS cryomodule
cryo distribution feed/end boxes
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30. SNS He flow
He Supply 5 K, 3 bar
outside module
He Return
Cryo lines
Coupler and flange thermalization with 4.5 K flow
2K Counterflow HEX
50 K Shield/thermalization
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31. design rationales
• Fast module exchange and independent
cryogenics (bayonet connections)
1 day exchange
2K production in CM
• Warm quad doublet
Moderate filling factor
• Designed for shipment
800 km from TJNAF to ORNL
• No need to achieve small static losses
single thermal shield
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32. design for shipment (TJNAF to ORNL)
4g
5g
g/2
spaceframe concept
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33. Around the cold mass
• Helium to cool the SRF linac is provided by the central helium liquefier
• He from (8 kW) 4.5K cold box sent through cryogenic transfer lines to the
cryomodules
• Joule Thomson valves on the cryomodules produce 2.1 K (0.041 bar) LHe for
cavity cooling, and 4.5 K He for fundamental power coupler cooling
• boil-off goes to four cold-compressors recompressing the stream to 1.05 bar
and 30 K for counter-flow cooling in the 4.5K cold box
Magnetic shields
50 K thermal shield
Vacuum chamber Tank
End Plate
March 19 2009 essbilbao initiative workshop - Paolo Pierini 33

34. Alignment strategy
• indexing off of the beamline
flanges at either end of each
cavity
• Nitronic support rods used to
move the cavity into alignment
• targets on rods on two sides of
each flange.
• cavity string is supported by the spaceframe
• each target sighted along a line between set monuments (2 ends and sides)
• the nitronic rods are adjusted until all the targets are within 0.5 mm of the line
set by the monuments
• cavity string in the vacuum vessel: the alignment is verified and transferred
(fiducialized) to the shell of the vacuum vessel.
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35. Project-X baseline cryogenics
• 2-phase He at 4.5 K • Revised TESLA cryo string concept
• Strings are fed in parallel • 2 phase He line at 2 K
– first string SC solenoids, warm RF
– concurrent liquid supply and vapor
– second string SSR/TSR modules
return flow in the string
• Cryomodules are fed in series
• Double thermal shielding in strings
to limit radiation flow at 2 K
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36. Project-X head load table
Project X ICD
25 MV/m, 1.5 msec, Heat Load
Qty
5Hz, 20 mA, 1.25 FT 2K 4.5K 5K 40K or 80K
[# ]
Item Static Dynamic Static Dynamic Static Dynamic Static Dynamic
WRF Solenoid 19 42 99 536 -
- - - -
SSR1 2 42 1 1003 2
- - - -
SSR2 3 62 10 1279 8
- - - -
TSR 7 93 50 1965 40
- - - -
S-ILC 7 27 17 69 18 517 477
- -
ILC-1 9 35 43 105 47 727 1,226
- -
ILC-2 28 110 133 328 146 2,260 3,813
- -
SCB, End Boxes, etc 1 50 100 500
- - - - -
Auxiliary Load 1 - 1000
- - - - - -
Estimated, [W] 222 193 338 160 502 211 9787 5566
Design Capacity, [kW] 0.8 1.0 1.4 29.9
4.5K Eqv [kW] 8.2
Plug Power, [MW] 2.3
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37. Project-X cryo r&d plan
• cryo distribution and segmentation
– study existing cryomodules thermal cycling experience
– stationary, transient, fault, maintenance and commissioning
scenarios
– component over pressure protection study
– define cryogenic string size limits and segments
– liquid helium level control strategy development
– development of tunnel ODH mitigation strategy
• capital and operational cost optimization
– lifecycle cost optimization & Cryogenic Plant Cycle
– heat shields operating parameter optimization
• heat load analysis
– static and dynamic loads analysis for components/sub systems
– define overcapacity and uncertainty factors
– fault scenarios heat flux study
March 19 2009 essbilbao initiative workshop - Paolo Pierini 37

38. HINS - SSR1 conceptual cryomodule layout
string on strongback, dressed, aligned, shielded
vessel replicates assembly table supports
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39. Support post pockets
strongback concept
Support lugs
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40. spoke/solenoid mounting scheme
Analysis of the strongback
deflections unders dead loads
with support optimization
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41. Vacuum vessel with internal strongback supports
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42. EUROTRANS prototype module
• short, single cavity module
under fabrication for the
European program on ADS
assisted nuclear waste
transmutation EUROTRANS
(CW)
– based on the SNS concept
of short independently
fed and rapidly
exchangeable units
– will be used for long
testing for the reliability
characterization of
components
• reliability/beam availability
is the key goal for ADS linacs,
rather than performance
INFN MI & IPN Orsay
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43. emerging issues
• pressure vessel regulation (in a EU contest)
– will big machines in the near future require formal certification of
components as pressure vessels?
• non standard materials, welds & T ranges, not in PV codes
– XFEL effort in collaboration with German TÜV
• “Crash tests” performed in Cryomodule Test Bench
– slow and fast loss of all vacuum spaces (coupler, iso, beam)
– very successful
• hydraulic testing of HeTank space at 1.43 MAWP=6 bar, according to
safety regulations
– although ok for beta=1 cavities, treacherous issue for low beta structures
• resolving issues of integrating different components contributed “in-
kind” from several partner into a single object
• worldwide approach from ILC GDE
– how can a truly worldwide project deal with many different
regulations across the three regions (Europe, Asia, America)
– also linked to “plug-compatibility” approach on components
March 19 2009 essbilbao initiative workshop - Paolo Pierini 43

44. XFEL crash tests
• No major damage
– cavities unchanged
• pressure behavior in
circuits confirmed
– beam pipe venting shows
that pressure drop needs
3.6 s to propagate to other
side of module - Good
March 19 2009 essbilbao initiative workshop - Paolo Pierini 44

45. trade offs & choices for cryomodule design
• Main decision: Filling factor vs. fast module exchange
– Linac length vs. availability/reliability concerns
– Real estate gradient is more strongly influenced by module length
constraints or cavity ancillaries than from intrinsic cavity
accelerating gradient
• Heat load balances and cryo system layout
– need in iterations to estabilish layout
• Can’t “buy” a single design, as it is
– Can surely transfer design ideas and subcomponents
• TESLA attractive for filling factor
• SNS for module exchange capabilities
• LEP has easy access to cold mass, but not compatible with 2 K
March 19 2009 essbilbao initiative workshop - Paolo Pierini 45

46. Acknowlegments
• I want to thank many colleagues, since I have been using
their material from privately and publicly available
presentations and tutorials, in particular (but not limited
to...)
• Tom Peterson, Arkadiy Klebaner, Tom Nicol,
Don Mitchell, Vittorio Parma, Joe Preble, ...
• Whole TTF/XFEL colleagues in DESY & INFN Milano
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