Crab Cavity
Cryostat
Fabrication and
Challenges
Tom Peterson (FNAL)
FNAL-LHC Crab Cavity Engineering Meeting
14 Dec 2012
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 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.
LHC Crab Cavity Cryostat, 14 Dec 2012
2
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.
LHC Crab Cavity Cryostat, 14 Dec 2012
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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
LHC Crab Cavity Cryostat, 14 Dec 2012
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ILC presentation – Tom Peterson
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
LHC Crab Cavity Cryostat, 14 Dec 2012
5
Cold
peak
pressures
1
ILC presentation – Tom Peterson
• 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.
LHC Crab Cavity Cryostat, 14 Dec 2012
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ILC presentation – Tom Peterson
Cold peak pressures - 2
• 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
LHC Crab Cavity Cryostat, 14 Dec 2012
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Cold
peak
pressures
3
ILC presentation – Tom Peterson
• 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
LHC Crab Cavity Cryostat, 14 Dec 2012
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ILC presentation – Tom Peterson
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
LHC Crab Cavity Cryostat, 14 Dec 2012
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Presentation to DOE, 21 Sep 2011
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
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LHC Crab Cavity Cryostat, 14 Dec 2012
Presentation to DOE, 21 Sep 2011
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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LHC Crab Cavity Cryostat, 14 Dec 2012
Presentation to DOE, 21 Sep 2011
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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LHC Crab Cavity Cryostat, 14 Dec 2012
Presentation to DOE, 21 Sep 2011
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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LHC Crab Cavity Cryostat, 14 Dec 2012
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
LHC Crab Cavity Cryostat, 14 Dec 2012
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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
LHC Crab Cavity Cryostat, 14 Dec 2012
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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
LHC Crab Cavity Cryostat, 14 Dec 2012
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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
LHC Crab Cavity Cryostat, 14 Dec 2012
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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
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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
LHC Crab Cavity Cryostat, 14 Dec 2012
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Saclay/Fermilab Capture Cavity
LHC Crab Cavity Cryostat, 14 Dec 2012
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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
LHC Crab Cavity Cryostat, 14 Dec 2012
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Horizontal Test Cryostat
LHC Crab Cavity Cryostat, 14 Dec 2012
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Horizontal
test
cryostat
LHC Crab Cavity Cryostat, 14 Dec 2012
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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
LHC Crab Cavity Cryostat, 14 Dec 2012
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LHC Crab Cavity Cryostat, 14 Dec 2012
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Design Approach – Cryomodule Schematic
Shrikant Pattalwar
4K GHe RETURN
2K (GHe) RETURN
2K HEX and a valve
box could be a part
of the module ??
Outer Vacuum
Chamber
Magnetic shield
4K PRE-COOL
4K (LHe) SUPPLY
40 K FORWARD
60 K RETURN
THERMAL SHIELD 40K TO 60K
TWO PHASE LINE
5K THERMAL INTERCEPTS
Shrikant Pattalwar Hi-Lumi Crab Cavity Engineering Meeting Dec 13-14, 2012LHC
Fermi
LabCavity Cryostat, 14 Dec 2012
Crab
26
Design Approach – Conceptual Model
Shrikant Pattalwar
1000mm
2160mm
420mm
1000mm
413mm
194 mm
Low order
High order mode
mode coupler
coupler
RF input
coupler
Triple tube cavity
support system
SPS by-pass line
Shrikant Pattalwar Hi-Lumi Crab Cavity Engineering Meeting Dec 13-14, 2012LHC
Fermi
LabCavity Cryostat, 14 Dec 2012
Crab
27
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
LHC Crab Cavity Cryostat, 14 Dec 2012
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LHC Crab Cavity Cryostat, 14 Dec 2012
Schedule
2013
Eric Montesinos
2014
2015
February
•
•
•
•
•
•
•
•
•
•
•
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 !)
2016
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
LHC Crab Cavity Cryostat, 14 Dec 2012
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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 SM18
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