cryomodule maintenance workshop - vacuum seal issues...• indium –large seal at bottom flange of...

Chris ComptonCavity Fabrication Group Leader

Cryomodule Maintenance Workshop -Vacuum Seal Issues

, Slide 2

• Vacuum seal design for FRIB SRF cavity applications

• Seal Designs

• Flange Materials

• Gasket Materials

• Issues and concerns addressed – to date

• Tolerance control

• Material brittleness concerns

• Cavity assemble steps

• Etching environment concerns

• Thermal cycling

• Reliability, breakdown, and reuse (indium related)

• Threading techniques

• Mitigation research

• European Conflat design

• Thread mill techniques

• LCF gasket

Outline

C. Compton, March 2014 Workshop on Cryomodule Maintenance - 21

, Slide 3

• Vacuum Seal Types/Designs

• Conflat – beamline and RF interfaces

• Indium – large seal at bottom flange of QWR

• Through and blind tapped bolting configurations

• Flange Materials

• Niobium-Titanium (45/55) – Conflat flanges

• Titanium flange with Niobium insert (QWR bottom flange)

• Gasket Materials

• Copper gaskets (standard sizes), procured off the shelve

• Indium (indium set with mold tool)

SRF Cavity Sealing Designs for FRIB


, Slide 4

• Tolerance concerns

• Knife-edge profile tolerance spread

• Material concerns

• Machining, surface finish

• Brittle at Cryo temperatures

• Cavity assembly – welding sequence

• Warping of flange from welding

• Etching environment

• Surface pitting from acid vapor

• Thermal cycling, differential shrinkage

• Three material is seal – Nb-Ti, Copper, Stainless Steel

• Reliability, breakdown, and reuse (indium related)

• Knife-edge rolling

• Surface finish

• Residual indium

• Thread loading in blind-tapped holes

• Stripping and gulling

What Design Concerns Using Nb-Ti Conflat?


, Slide 5

• Tolerance concerns

• Leak hunting study – Bricker (TN)

• Systematically measured and leak check several vendor cavities

• Material concerns

• Machining, surface finish

• Practice speeds with internal and external shops

• Brittle at Cryo temperatures

• Test results from Japan

• Discussion with NIST/FSU on cryogenic impact testing

• Charpy Impact tests conducted at MSU (TN)

• Cavity assembly – welding sequence

• Knife-edge profile machined after EB welding

• Etching environment concerns

• Etching study conducted by MSU – Ian Malloch (TN)

Design Concerns Addressed in Nb-Ti Conflats - 1

* TN = Tech Note drafted


, Slide 6

• Charpy measurements to validate cryogenic behavior

Is Nb-Ti Brittle Under Cryogenic Operation?

Sample Material Temperature Impact Energy (J)

Al #1 Room 37.96

Al #2 77 K 39.32

Al #3 77 K 39.32

Nb-Ti #1 Room 172.19

Nb-Ti #2 Room 170.83

Nb-Ti #3 77 K 28.47

Nb-Ti 77 K 25.76

Charpy Impact Test Equipment at MSU

Charpy Impact Results at MSU


, Slide 7

• Thermal cycling differential shrinkage

• Low profile prototype testing (Rania Oweiss, Andy Rauch) (TN)

• Repeatability study (Nick Putman) (TN)

• Flange acceptance testing (Steve Bricker) (TN)

• Used Copper gasket inspection (Di Kang) (TN)

• Longevity concerns (knife-edge rolling/surface finish)

• Knife-edge profile study – knife-edge degradation documented (QA trouble

report)

• Thread fabrication concerns

• Thread fabrication study – threads failure documented (QA trouble report)

• Thread loading in blind-tapped holes

• Fabricate test flanges with blind, thread mill, tapped holes (planned)

Design Concerns Addressed in Nb-Ti Conflats - 2

* TN = Tech Note drafted


, Slide 8

• Conflat knife-edge profile design and testing

• Thread milling fabrication technique

• Indium free sealing of QWR bottom flange

What Additional Mitigation Strategies are being Researched?


Can the Conflat Profile be Optimized for Nb-Ti? [1]

, Slide 9

• Conflat knife-edge profile was studied to improve reliability

• American standard compared to European standard

• Validation plan initiated

• Several flanges fabricated

• Flanges are assembled, evacuated, leak checked, and thermal cycled

• Knife-edge profile measured/photographed before and after every cycle

American Tip radius

(mm)

Counterslope

angle

(degrees)

A 0.050 5

B 0.060 5

C 0.065 0

D 0.215 0

European E 0.105 40

F 0.108 40

G 0.114 40

H 0.097 10

I 0.105 10

J 0.116 10

Standard Conflat Design Parameters

List of accept Conflat parameters for

American and Europe flanges


, Slide 10



, Slide 11


1 2 3 4 5 6

American A1 0.028 0.029 0.02945 0.02966 0.0299 0.029667

American A2 0.028 0.028 0.02908 0.02978 0.02974 0.0299

European F1 0.025 0.025 0.02558 0.02547 0.02591 0.026433

European F2 0.025 0.025 0.02551 0.02557 0.025933

0.022

0.023

0.024

0.025

0.026

0.027

0.028

0.029

0.03

0.031

Def

orm

atio

n (

Inch

es)

Trial

Knife Edge Deformation

American A1

American A2

European F1

European F2


, Slide 12

• Several flanges received with threads not within

specifications

• Go/NoGo gauge failures

• Bolt gulling during assembly

• Discussions with experts in the field

• Roll tapping/forming not good for titanium alloys

• Thread milling recommendation, tool spec

• Validation plan

• Fabricate sample holes (complete)

• Starting hole size

• Speeds and feeds

• Fabricate several holes continuously (complete)

• Tooling dulling

• Material build up

• Fabricate test flange with blind tapped holes

(ongoing)

• Test flange fabricated with Euro Conflat

profile, using thread mill technique

What is the Best Practice for Thread Fabrication in Titanium?

Thread Mill Prototyping Test Piece


, Slide 13

• Can the indium seal be eliminated from QWR bottom flange?

• Go to Conflat style sealing

• Large diameter in titanium

• Proof-of-principle at 13.25” in titanium

• Propose new gasket design

• MO seal developed at KEK

• Testing initiated as part of NC50 research

Can FRIB Eliminate Indium?

Prototype Testing at MSU using MO Seal

Gasket Sealing Concept, MO Seal


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