ESS-Bilbao Initiative Workshop. Overview of cryo-modules for proton accelerators

19 March 2009 Bilbao

overview of cryomodules

for proton accelerators

Paolo PieriniINFN Sezione di Milano

Laboratorio Acceleratori e Superconduttività [email protected]

March 19 2009 essbilbao initiative workshop - Paolo Pierini 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


SRF cavities and ancillaries - 1

• 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– …

cavities and ancillaries design are chosen on the basis of a complex optimization that depends on:


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


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


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...)


• 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!

cryogenic distribution system functions


• 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

cryogenic distribution system design


supports

the cryomodule environment: a“cartoon” view

all “spurious” sources of heat 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)

RFcavities RF penetrations

to He production and distribution

system

2 K

5-8 K

40-80 K

these are the accelerator active devices with tight alignment

constraints for beam “quality”


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

– 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

thc

ch

TTT

QW η⋅−⋅=


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, …

40 K to 80 K 5 K to 8 K 2 KTemperature level Temperature level Temperature level(module) (module) (module)

Temp in (K) 40,00 5,0 2,4Press in (bar) 16,0 5,0 1,2Enthalpy in (J/g) 223,8 14,7 4,383Entropy in (J/gK) 15,3 3,9 1,862Temp out (K) 80,00 8,0 2,0Press out (bar) 14,0 4,0 saturated vaporEnthalpy out (J/g) 432,5 46,7 25,04Entropy out (J/gK) 19,2 9,1 12,58

]J/g[]g/s[]W[ hmP flowremoved ∆=@2 K 20 J/g latent heat


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)


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


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


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


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


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

• structural support of the cold mass– different thermal contractions of

materials– precise alignment capabilities and

reproducibility with thermal cycling

12 m, 38” diameter, string of 8 cavities and magnet

cavity size


TESLA/ILC/(XFEL) modular cryogenic concept

• each module contains all cryo piping– each cavity tank in module

connected to two phase line– vapor is collected from 2 phase

line once per module in the GRP

• several modules are connected in strings– single two phase line along the

string– a JT valve once per string fills

two phase line via subcooled 2.2 K line

• strings are connected into units– each unit is fed by a single

cryogenic plant

modules without with withoutquad quad quad

RF unit (lengths in meters) 12.652 12.652 12.652three modules

RF unit RF unit RF unit RF unit end boxstring (vacuum length) 37.956 37.956 37.956 37.956 2.500

twelve modules plus string end box

string string string stringpossible segmentation unit 154.324 154.324 154.324 154.324

48 modules (segmentation box is the same as string end box (2.5 m) and all contain vacuum breaks)

service service box end segment segment segment segment box end

Cryogenic Unit 2.500 617.296 617.296 617.296 614.796 2.500

(16 strings) (1 cryogenic unit = 192 modules = 4 segments*48 CM with string end boxes plus service boxes.)

2471.7 meters

ILC scheme for segmentation and distribution

unit length limited by size of cryo plantneeded (25 kW equivalent at 4.5 K seems

max reasonable extrapolation of 18 kW LHC)

Cryo-string Cryo-string Cryo-string Cryo-string

Pumping return

Sub-cooled LHe supply

5 K supply

8 K return

50 K supply

75 K return

Cryogenic distribution box

Cryo-unit

Line D

Line E

Line F

Line C

Line A

Line B


schematicallyA

ll lin

es in

mod

ule

inner shield

outer shield

GRP

subcooled forward line


cryostrings in TTF&FLASH


The cross section

Shield gas feeding

Heliumtank

Couplerport

Thermalshields

Slidingsupport

HeliumGRP

Cryogenicsupport

Two phaseflow

Pressurizedhelium feeding

Shield gas feeding

Heliumtank

Couplerport

Thermalshields

Slidingsupport

HeliumGRP

Cryogenicsupport

Two phaseflow


Heliumtank

Couplerport

Thermalshields

Slidingsupport

HeliumGRP

Cryogenicsupport

Two phaseflow


Low thermal conduction composite supports

cavity

RF Penetration

(large because of pressure drop, usedas structural backbone)


three generations of cryomodules in TTF

��

1�2 Simplification of fabrication (tolerances), assembling & alignment strategy2�3 Longitudinal references, redistribution of cross section (42”�38”)


from prototype to Cry 3

Extensive FEA modeling (ANSYS™) of the cryomodule

– Transient thermal analysis during 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

Braid-cooled Cry 1 - 1997


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


cooldown behavior

• Fairly sophisticated non linear transient FEM models – reproduce with good accuracy

the cooldown behavior– assess max thermal gradients

and stresses during transients– allow to identify suitable

cooldown rates to keep thermal stresses below safe limits 0

30

60

90

120

150

180

210

240

270

300

0 5 10 15 20 25 30 35 40 45 50 55 60

Time (h)

Tem

pera

ture

(K

)

T out (CMTB)T in (CMTB)Delta T (CMTB)T in (ANSYS)T out (ANSYS)Delta T (ANSYS)

0

30

60

90

120

150

180

210

240

270

300

0 5 10 15 20 25 30 35 40 45 50 55 60

Time (h)

Tem

pera

ture

(K

)

T in (CMTB)T out (CMTB)Delta T (CMTB)DeltaT (ANSYS)T in (ANSYS)T out (ANSYS)

5 K shield

70 K shield

comparison FEM with CMTB cooldown


linac performances, low static load budget

~ 70 W ~ 13 W < 3.5 W


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!)


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


different design: SNS cryomodule

cryo distribution feed/end boxes


SNS He flowHe Supply 5 K, 3 bar

He Return

2 K

50 K Shield/thermalization

Coupler and flange thermalization with 4.5 K flow

Counterflow HEX

Cry

o lin

es

outs

ide

mod

ule


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


design for shipment (TJNAF to ORNL)

g/2

5 g

4 g

spaceframe concept


Around the cold mass

Tank

50 K thermal shield

Magnetic shields

Vacuum chamberEnd Plate

• 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


Alignment strategy

• 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.

• 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.


Project-X baseline cryogenics

• 2-phase He at 4.5 K• Strings are fed in parallel

– first string SC solenoids, warm RF– second string SSR/TSR modules

• Cryomodules are fed in series

• Revised TESLA cryo string concept

• 2 phase He line at 2 K

– concurrent liquid supply and vapor return flow in the string

• Double thermal shielding in strings to limit radiation flow at 2 K


Item Static Dynamic Static Dynamic Static Dynamic Static DynamicWRF Solenoid 19 - - 42 99 - - 536 -SSR1 2 - - 42 1 - - 1003 2SSR2 3 - - 62 10 - - 1279 8TSR 7 - - 93 50 - - 1965 40S-ILC 7 27 17 - - 69 18 517 477ILC-1 9 35 43 - - 105 47 727 1,226ILC-2 28 110 133 - - 328 146 2,260 3,813SCB, End Boxes, etc 1 50 - 100 - - - 500 -

Auxiliary Load 1 - - - - - - 1000 -

222 193 338 160 502 211 9787 5566

4.5K 5K 40K or 80K

Project X ICD

25 MV/m, 1.5 msec, 5Hz, 20 mA, 1.25 FT Qty

[# ]

Heat Load2K

29.9 Design Capacity, [kW] 0.8 1.0 1.4 Estimated, [W]

Plug Power, [MW] 2.38.24.5K Eqv [kW]

Project-X head load table


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


HINS - SSR1 conceptual cryomodule layoutstring on strongback, dressed, aligned, shielded

vessel replicates assembly table supports


strongback concept

Support lugs

Support post pockets


spoke/solenoid mounting scheme

Analysis of the strongbackdeflections unders dead loadswith support optimization


Vacuum vessel with internal strongback supports


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 independentlyfed 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


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


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


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


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

Technology

ESS-Bilbao Initiative Workshop. Overview of cryo-modules for proton accelerators