crab cavity cryostat fabrication and challenges

The HiLumi LHC Design Study (a sub-system of HL-LHC) is co-funded by the European Commission within the Framework Programme 7 Capacities Specific Programme, Grant Agreement 284404.

Crab Cavity Cryostat Fabrication and Challenges

Tom Peterson (FNAL)FNAL-LHC Crab Cavity Engineering Meeting

14 Dec 2012

LHC Crab Cavity Cryostat, 14 Dec 2012

Crab cavity cryostat discussion• Fabrication of the different types of cryostat

and their advantage and disadvantages should be reviewed.

• Challenges in the design choice, fabrication steps, integration along with feasibility of a two cavity cryostat within the specified scheduled should be reviewed and mitigation should be discussed.

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Final crab cavity system requirements • Definition of requirements is still in progress

• Elements of these requirements include: • Interfaces to the LHC accelerator system, cryogenic system, and tunnel

infrastructure• Thermal conditions, cavity temperature, intercept temperatures, heat loads • Cavity arrangement, supporting structure and possibility for alignment,

beam-beam spacing and allowance for two beams, how many cavities per cryostat? 2

• Constraints from vertical and horizontal crabbing schemes RF coupler and HOM/LOM configurations and constraints

• Tuner configurations and constraints • Instrumentation requirements • Cryostat, piping, and helium vessel safety, code compliance requirements

from final design.3


Given the final system requirements above, how may the prototype differ from the final design?

• Cavity support structure, same as final design? YES

• Arrangement of multiple cavities. Same total number of cavities? NO

• Provisions in the prototype for one beam only? PERHAPS

• Cryostat vacuum vessel differences SIGNIFICANT

• RF couplers and HOM/LOM configuration and orientation through the cryostat SIMILAR to final design

• Special instrumentation not in the final cryostat Cryogenic connections, interfaces to infrastructure may differ YES, but not yet defined

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Peak warm pressure• Compressor suction set pressure

– 1.2 bar (to avoid subatmospheric excursions) • Control margin

– +/- 0.2 bar • Relief set pressure margin

– + 0.3 bar above control excursions (a judgment here, would like 0.5 bar)

• Suction relief set pressure – 1.7 bar (from 1.2 + 0.2 + 0.3 bar above)

• Pressure drop from far helium vessel to relief – + 0.1 bar (needs to be determined for specific system, but

probably low for the low flow in the warm situations) • Peak warm pressure

– 1.8 bar (note that 0.5 bar set pressure margin, which would be better ==> 2.0 bar peak warm pressure)

• Conclusion: 2.0 bar warm MAWP is a practical lower limit

ILC presentation – Tom Peterson

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Cold peak pressures - 1

• Loss of vacuum to air – “Safety Aspects for the LHe Cryostats and LHe

Containers,” by W. Lehman and G. Zahn, ICEC7, London, 1978

• “3.8 W/sq.cm. for an uninsulated tank of a bath cryostat”• “0.6 W/sq.cm. for the superinsulated tank of a bath cryostat”

– “Loss of cavity vacuum experiment at CEBAF,” by M. Wiseman, et. al., 1993 CEC, Advances Vol. 39A, pg 997.

• Maximum sustained heat flux of 2.0 W/sq.cm. – LEP tests and others have given comparable (2.0 to 3.8

W/sq.cm.) or lower heat fluxes – Film boiling of helium with 60 K surface is about 2.5

W/sq.cm.


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• Input parameters – Heat flux as limited by

• Rate of air inleak • Surface heat transfer

– Total surface area involved • Can be limited by vacuum breaks, fast valves

– Initial conditions • Note that 4.5 K just after filling (as opposed to 2 K when the

large, low pressure volume acts as a buffer) is the worst case!– Pipe diameters out to the helium vent – Overall distances and pipe lengths out to the helium

vent • A finer degree of segmentation can reduce pipe diameter

requirements


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• Relief pressure will be suction relief set pressure (for example, 1.7 bar)

• Heat flux of 10’s of KW to liquid helium• Mass flows of many kg/sec • Pressure drops to vent may result in peak

pressures of 2.5 bar to 4 bar locally • Maintaining a low peak pressure (e.g., 2.5

bar) requires larger piping and/or shorter vent path lengths


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Conclusion for MAWP

• “Maximum Allowable Working Pressure” (MAWP) – The pressure which we declare on our engineering

notes will be the maximum the vessel will see – Relief valves and vent piping are sized to prevent

pressure exceeding this value • 2 bar differential pressure warm (minimum!)

– Helium space to cavity vacuum – Helium space to insulating vacuum

• 2.5 bar to 4 bar differential pressure cold – Helium space to cavity vacuum – Helium space to insulating vacuum – Higher (closer to 4 bar) is better in allowing smaller

diameter and longer pipes to vent valves


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Safety/compliance issue

• In the U.S., Europe, and Japan, these helium containers and part or all of the RF cavity fall under the scope of the local and national pressure vessel rules.

• Thus, while used for its superconducting properties, niobium ends up also being treated as a material for pressure vessels.

• For various reasons, it is not possible to completely follow all the rules of the pressure vessel codes for most of these SRF helium vessel designs

Presentation to DOE, 21 Sep 2011

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Issues for code compliance• Cavity design that satisfies level of safety

equivalent to that of a consensus pressure vessel code is affected by» use of the non-code material (niobium), » complex forming and joining processes, » a shape that is determined entirely by cavity RF

performance, » a thickness driven by the cost and availability of niobium

sheet, » and a possibly complex series of chemical and thermal

treatments. • Difficulties emerge pressure vessel code

compliance in various areas » Material not approved by the pressure vessel code » Loadings other than pressure

• Thermal contraction • Tuning

» Geometries not covered by rules » Weld configurations difficult to inspect


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General solution• In applying ASME code procedures, key elements

demonstrating the required level of design safety are » the establishment of a maximum allowable stress » And for external pressure design, an accurate

approximation to the true stress strain curve• Apply the ASME Boiler and Pressure Vessel Code

as completely as practical » Exceptions to the code may remain » We have to show the risk is minimal

• Satisfy the requirement for a level of safety greater than or equal to that afforded by ASME code.

• Fermilab, Brookhaven, Jefferson Lab, Argonne Lab, and others in the U.S. have taken a similar approach


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Conclusions• Niobium, niobium-titanium, electron beam

welding, and other features required for the proper function of superconducting RF cavities create problems with respect to pressure vessel codes in all regions of the world

• With significant effort, laboratories have found various ways to provide levels of safety equivalent to the applicable code rules

• These methods involve taking some very conservatively low values for niobium yield strength due to heat treatments and uncertainty, and doing analysis and quality assurance inspections in accordance with code rules as much as possible

• Treating the vacuum vessel as the primary containment volume or excluding the niobium material from the pressure boundary definition may be feasible in some cases


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Cryomodule requirements -- major components • Dressed RF cavities • RF power input couplers • One intermediate temperature thermal shield • Cryogenic valves

• 2.0 K liquid level control valve • Cool-down/warm-up valve • 5 K thermal intercept flow control valve

• Pipe and cavity support structure • Instrumentation -- RF, pressure, temperature, etc. • Heat exchanger for 4.5 K to 2.2 K precooling of the liquid

supply flow

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Cryomodule requirements -- major interfaces • Bayonet (or other style) connections for helium supply and return • Vacuum vessel support structure • Beam tube connections at the cryomodule ends

• Double vacuum valves • Guard vacuum pumping • Thermal intercepts • Allowance for thermal contraction

• RF waveguide to input couplers • Instrumentation connectors on the vacuum shell • Alignment fiducials on the vacuum shell with reference to cavity

positions. • Vacuum system for pumping insulating vacuum

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Design considerations• Cooling arrangement for integration into cryo system

• Pipe sizes for steady-state and emergency venting

• Pressure stability factors • Liquid volume, vapor volume, liquid-vapor surface area as buffers for

pressure change • Evaporation or condensation rates with pressure change

• Updated heat load estimates

• Options for handling 4.5 K (or perhaps 5 K - 8 K) thermal intercept flow

• Alignment and support stability

• Thermal contraction and fixed points with closed ends

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Cryomodule Pipe Sizing Criteria• Heat transport from cavity to 2-phase pipe

• 1 - 1.4 Watt/sq.cm. is a conservative rule for a vertical pipe (less heat flux with non-vertical connection to helium vessel, analysis for tight spaces)

• Two phase pipe size • 5 meters/sec vapor “speed limit” over liquid • Not smaller than nozzle from helium vessel

• Gas return pipe (also serves as the support pipe in TESLA-style CM)• Pressure drop < 10% of total pressure in normal operation• Support structure considerations

• Loss of vacuum venting P < cold MAWP at cavity • Heat flux as much as 4 W/cm2 on low-T bare metal surfaces • Path includes nozzle from helium vessel, 2-phase pipe, may include gas

return pipe, and any external vent lines

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Cryostat design options • Use existing designs to the extent possible • Two cavities, R&D nature of test

• Provide relatively easy access to cavities, tuner, input coupler, HOM couplers

• Several examples of such cryostats exist • 1 – capture cavity • 2 – horizontal test cryostats • 3 – various top-loading designs

LHC Crab Cavity Cryostat, 14 Dec 2012 18

Cryostat design option examples – 1 • Capture Cavity

• Single cavity in a cryostat • Tension rod support • Flanged vacuum shell heads • Short cryostat allows attachment of tension

rods to vacuum shell and transfer of load to the rods after insertion of cavity on simple tooling

• Similar to what we saw from Niowave


Saclay/Fermilab Capture Cavity


Cryostat design option examples – 2 • Horizontal test cryostat (similar to “Chechia” at

DESY) • Single cavity in a cryostat • Support post and frame beneath cavity • Cavity rolls into position for ease of frequent

changes • Flanged and hinged vacuum shell heads


Horizontal Test Cryostat


Horizontal test cryostat


Cryostat design option examples – 3 • Top-loading cryostat

• Argonne, Triumf, Daresbury, and others • Rectangular sides • Structures hung from top plate • This design has some attractive features given

the physical constraints of the SPS tunnel location and the R&D nature of these first tests


Design Approach – Cryomodule Schematic

Shrikant Pattalwar Hi-Lumi Crab Cavity Engineering Meeting Dec 13-14, 2012 Fermi Lab

TWO PHASE LINE

60 K RETURN40 K FORWARD

4K (LHe) SUPPLY

2K (GHe) RETURN4K GHe RETURN

THERMAL SHIELD 40K TO 60K

5K THERMAL INTERCEPTS

4K P

RE-C

OO

L

2K HEX and a valve box could be a part of the module ??

Magnetic shieldOuter Vacuum Chamber

Shrikant Pattalwar


Design Approach – Conceptual Model

Shrikant Pattalwar Hi-Lumi Crab Cavity Engineering Meeting Dec 13-14, 2012 Fermi Lab

1000mm2160mm

1000

mm

Triple tube cavity support system

High order mode coupler

Low order mode coupler

RF input coupler

SPS by-pass line

413mm

420mm

194 mm

Shrikant Pattalwar


Schedule• Design process for new cryostat or cryogenic

box is typically 2 to 3 years • After a complete definition of requirements,

details of associated components (e.g., tuner, input coupler) are known, conceptual design (first year or more) . . .

• ~1 yr engineering and design/drafting • ~1 yr procurement and fabrication


Schedule

• Cavities to be installed in SPS in December 2015• Cryostat fully tested Q3-2015• Cryostat fully dressed Q2-015• Couplers available for cryostat Q1-2015• Couplers RF processed 50 kW SW cw all phases Q4-2014• Couplers assembled in clean room Q2-2014 onto test box• Special processes FPC + Test Box (Cleaning, Brazing, EB welding,

Gold plating, Ti coating) completed Q1-2014• All couplers + Test Box parts machined Q4-2013• All raw material delivered Q2-2013• All raw material ordered Q1-2013• (common) Coupler design completed February 2013 (+ Test Box !)

2013 2014 2015 2016


February

Eric Montesinos

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SM18 tests• Important to verify as much as possible before

installation in SPS • Leak tight cold • Heat loads • RF performance of power couplers and cavities

• Compatibility for cryogenic connections between SM18 and SPS • 2 K – 4 K heat exchanger, valves • Opportunity also to verify SPS flexible connections

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Conclusions • Three cryostats, one for each cavity type

• Each will contain two cavities of the same type • Tight thermal constraints for SPS operation • Tight mechanical constraints for SPS installation

• Plus motion requirement

• Vertical cavity tests • Cryostat test at SM18 before tunnel installation

• Verify thermal performance within SM18 constraints • Verify, as-installed in cryomodule, coupler and cavity RF

performance at SM1831

crab cavity cryostat fabrication and challenges

Documents

cavity temperature

cryostat similar

final system requirements

bar needs

bar abovepressure

bar set pressure margin

bar warm mawp

cavity support structure