Engineering Peer Review Reportof the

Prototype LCLS-II Cryomodules

June/July 2014

Introduction

SLAC in partnership with Fermilab, Jefferson Lab, Lawrence Berkeley Lab, Cornell, and Argonne is in the process of designing LCLS-II. It is a CW 4 GeV electron linear accelerator whose primary accelerating structures are 8-cavity Superconducting RF modules resonating at 1.3 GHz.

Figure 1: LCLS-II Layout

The Cryomodule (CM) design is strongly based upon that for the European XFEL now under construction. The basic design is thus of the TESLA design. By virtue of operating in continuous wave (CW) rather than pulsed mode some design changes are necessary. The current plan calls for two prototype modules to be built and evaluated prior to the fabrication of 32 ‘Production’ modules. While considered prototype or ‘pre-production’ modules it is intended that the prototypes will meet design specifications and be installed as part of the full Linac. Fermilab and Jefferson lab are responsible for assembly and testing with Fermilab taking a lead role in the design effort.

Figure 2: LCLS-II Prototype Cryomodule preliminary design – Front and Sectional Views

Engineering Peer Review

An Engineering Peer Review of the prototype 1.3 GHz cryomodules to be constructed by Fermilab for the LCLS-II project at SLAC took place on Friday, 6 June 2014, at Fermilab. Five presentations were made:

Welcome/Introduction/Agenda/Plans (C. Ginsburg) Cryomodule Requirements (J. Theilacker) Cryomodule thermal/cryogenic design (T. Peterson, J. Kaluzny) Cryomodule mechanical layout (Y. Orlov) Near term work plan and work tasks (T. Peterson)

Review panel members were Joel Fuerst/Argonne, Elvin Harms/Fermilab AD (chair), Jerry Makara/Fermilab AD, John Mammosser/Jefferson Lab, and Tom Nicol/Fermilab TD.

Other participants included staff from Fermilab, Jefferson Lab, and SLAC with some connecting remotely.

The Charge to the committee was to focus on technical issues, primarily considering the cryostat thermal and mechanical design. Topics not addressed in detail: dressed cavity, tuner, RF power input coupler, HOM couplers, magnetic shielding, magnet, BPM, HOM absorber. These items may enter the discussion as they impact integrated cryomodule design. The goal is to inform everyone of the status of the work and receive comments on the following topics:

The Design team is to be commended on a tremendous effort in a short time to reach the present state of design. The strategy of adopting the TESLA/XFEL concept with minimal changes is working.

Design overview -- Do you see risks, technical issues that should have more attention? Are the requirements well defined?

FindingsIt was difficult to fully determine the requirements as they appear to be scattered across several documents which are not uniformly up to date (Seismic Design Spec, for example). Six Physics Requirements Documents reference the cryomodule, and three Engineering Notes are being drafted. The source material exists but is distributed among various requirements docs.  Milestones for completion of documents was described as ‘lagging’ in one presentation. The engineering specification document for the cryomodule is a work in progress (since approved).  This specification will take some time to generate and approve and could pose a schedule risk.

The main areas of risk lie with the changes to the cryomodule design from the existing XFEL design as well as the unique installation into SLAC tunnel. Understanding how these changes affect the system design is important to reduce risk. Questions that came out in the discussions were:

Welding of the helium circuit JT connections, what type weld is this (ie full penetration?) and can it be done reliably?

Is the DESY alignment data reliable? How has microphonics changed due to the changes to the 2-phase line Can the connections of modules in the tunnel be reliably done with the

module placement in the tunnel? Will there be a need to access the wall side of the cryomodule during its

use? Due to the tunnel slope will the recovery of the cavity at the low liquid

level end of the module be an operational problem? New tuner design which will need thorough testing

The impact of some known requirements (for example the SLAC seismic design specifications) are not fully understood.  This should be captured in the risk matrix if not already present.

Seismic requirements must be addressed in CM design, based on SLAC standards and documented in an engineering note.  Previous studies for KEK and the ILC should provide initial design verification but must be verified against SLAC requirements.

The design shown for the magnet current leads is based on conduction-cooled leads being developed for SSR1. In that design though, there is a helium connection at the magnet end. With the LCLS-II conduction cooled magnet, there is no helium connection so the lead design might be able to be simplified to take advantage of that difference.

Comments It was stated most topics brought up at the review will be addressed soon either by analysis or testing and the answers were reasonable to the questions presented. However some areas may need deeper analysis or understanding.

Areas requiring more design attention include: Titanium-Stainless Steel transition joints in the helium vessel to 2-phase pipe

interface warm vs. cold alignment offsets mechanical stability of the beam pipe interconnect between cryomodules, and possible changes in microphonics due to capped 2-phase pipe.

There is an impression that procurement of the transition joints is just a matter of buying a few and testing them. It sounds like it might not be that straightforward, thus an adequate program for testing and qualification must be ensured.

With new Joule-Thomson (JT) valves located in each CM, design verification is needed on several aspects.  Fabrication technique needs to be verified, possibly with a mockup assembly, as welding access seems difficult in current location.  Consideration should be made of moving the JT valve to the Cryomodule end rather than present center design, as it may provide better access potential.  Modal analysis of the 2-phase system should be done to verify that the JT flow and associated flexhose connections do not generate microphonics beyond requirements of the Physics Requirements Document for SRF cavities.

Relief requirement methodology relating to heat flux for various failure scenarios must be agreed upon, preferably via an Engineering Note, as the overall system is affected, especially the Cryogenic Distribution System (CDS).  Beam tube relief requirements should also be addressed formally.  Discussion of these items should continue with AD/Cryogenics personnel as they affect design of CDS components.  Subsequent discussion will also be needed to address required pressure test methodology of various components during the installation process in the SLAC tunnel.

Insulating vacuum relief requirements must address both CGA liquid container inventory and Cryo Plant/Cryogenics Distribution System supply flows based on credible failure scenarios, based on lessons learned from LHC failure resulting in significant movement of magnets and such.

The CM P&ID needs to be reviewed such that it incorporates needed instrumentation for CM cool down and warm up limitations as defined in the CM Functional Requirement Document (LCLSII-2.5-FR-0053-R0), specifically relating to the 312 mm 2K Sub-Atm Return header.  Note that differences may occur between prototype and production CM’s.  Discussion of this should continue with AD/Cryogenics personnel to address various operating modes and relief requirements.

Interface connections of CM’s/Feed Cap/End Cap need several verifications.  An early mechanical mockup of such would go a long way verifying access to all connections from aisle only in the SLAC linac tunnel, as this may have significant impact on design based on installation requirements.  Lessons learned from DESY and Fermilab/NML(ASTA) experience should be incorporated as well, specifically relating to quality control of interface dimensions (x,y,z,theta), such that orbital welders can be utilized.

Significant discussion occurred relating to failure rate of stainless/titanium transition joints during thermal cycling.  This should be explored more formally to address risks associated with such numerous joints in CMs.  Additionally, CMs, and to a less extent

CDS, use numerous stainless/aluminum transition joints and failure history of such should also investigated.

The design seems good at this stage but with more work to be done. The following areas of the design were good choices:

Reliefs outside the cryomodule Tuner access port , usable for early failures and outside tunnel repairs Larger helium riser and 2-phase pipe Analysis seems appropriately conservative and a good approach

RecommendationsEnsure that there is a well-thought-out pressure test program to preclude any potential cavity damage due to testing high-pressure circuits connected to the cavity string.

Make sure that the cavity string is well shielded from potential magnetic materials in the cold assembly, e.g. invar tie rods and invar rods in the 2-phase pipe anchor system. Material compatibility deserves scrutiny.

Ensure that the maximum axial force on the cold string doesn’t exceed the capability of the support system.

Ensure that there is an adequate program for testing and qualifying Titanium to Stainless Steel joints.

Schedule. Do the next key milestones appear reasonable?

FindingsThe schedule is ambitious, but not unrealistic. Resource availability is a nagging question

The schedule (final design review in 5 months, fabrication readiness review one month later) provides little or no time to perform significant development work or implement significant design changes. 

Comments The time between prototype and production – given the schedule – is very tight. It is surmised that small changes in design that might come out of the prototype magnet experience could be accommodated, but time might be too tight for big changes. Every effort should be made to make time to react to design changes if warranted.

Current schedules are very tight and human resources may not be sufficient to address the issues noted above.  Additionally, discussions are still ongoing relating to CM heat load effect on cryoplant capacity limitations as well as various operating modes required (e.g. static, static+RF, static+RF+beam, cooldown/warmup).  All these issues affect design of cyomodules thus discussions must continue with AD/Cryogenics personnel.

A major concern is the availability of resources. There remains much to do even for the prototype design including actual design work, mechanical analysis, folding in internal shipping studies and seismic studies at KEK, preparation of engineering notes, and detailed drawings, etc. More often than not these needs are underestimated and with the same resources being spread over many projects it’s even more challenging.

RecommendationsBe cognizant of potential design issues as early as possible in order to have time to react to and implement design changes for the production CM’s as warranted.

Provide realistic resource estimates and schedules considering both the obvious and ‘background’ work required to design, assemble, and test cryomodules.

Please comment on any open design issues which you see, concerns, or risks. Cryomodule placement in SLAC tunnel – height above floor and distance from ‘back wall’: what rationale went into these decisions. Have all factors been taken into account?

Tuner motor design – concerns were voiced about not just the tuner motor, but the piezo’s and especially the drive (screw) mechanism Recent developments in cryomodule design from XFEL or CM-2, for example, should be incorporated as practical.

The design seems good at this stage but with more work to be done. The following areas of the design were good choices:

Reliefs outside the cryomodule Tuner access port , usable for early failures and outside tunnel repairs Larger helium riser and 2-phase pipe

Analysis seems appropriately conservative and a good approach

The technical issues are for the most part well in hand although there are some elevated-risk subsystems which require testing and validation (magnetic shielding of Invar supports, SS/Ti transitions, slow tuner maintenance/access).

The helium line sizes within the cryomodule have been analyzed with respect to heat load, allowable pressure drop, etc. but the overall cryomodule “design margin” in terms of heat load was not clearly identified.

Recommendations:Review the risk matrix and update as necessary to capture uncertainties associated with incompletely defined requirements and open technical issues.

Analyze cryomodule performance over a range of expected cavity performance to understand the operating margin.  Compare this with the cryoplant and distribution system margins.

Ensure that there is an agreed-upon specification for shipping and handling design loads and make sure they have been incorporated into the analysis of the cryomodule/shipping frame assemblies.

Appendix AScope and Charge for the Review

Appendix BParticipants

Review CommitteeJoel Fuerst (ANL)Elvin Harms (Fermilab)Jerry Makara (Fermilab)John Mammosser (JLab) Tom Nicol (Fermilab)

PresentersCamille GinsburgJay TheilackerTom PetersonJosh KaluznyYuriy Orlov

Appendix DReview Agenda

Appendix ESupporting Documents

An extensive list of supplementary documents were made available to the review committee online: https://indico.fnal.gov/materialDisplay.py?materialId=0&confId=8598

Accelerator_Systems_to_Cryogenic_Systems-draft.docx Cryogenic_Distribution_System_ICD-draft.docx Cryomodule_FRS-draft.docx CryomoduleComparisonsIncludingLCLS-II.doc Draft_Schedule_for_LCLS-II_Preproduction_Module-v11.pdf LCLS-II-CM-orlov_F10009945---DWG1_copy.pdf LCLS-II-Cryomodule-P-ID.pdf LCLS-II-CryomoduleVolumes-18Apr2014.xls LCLS-II_Cryogenic_Heat_Load_22May2014.docx LCLScryoHeat-22May2014.xlsx LCLSII-4.5-EN-0186_CMdesign.docx LCLSII_CM_P&ID_legend.docx SCRF_1.3_GHz_Cryomodule-PRD.pdf