WG 1: High gradient R

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WG 1 High gradient RD Beyond Field Emission
(Conveners H. Padamsee, L.Lilje, J. Mammosser )

• Load
• Report/discuss about means to gain high yield in
  cavity gradients.
• Propose coordinated global effort for making
  progress on the gradient frontier.
• Sessions Monday 13.30 1700 (3.5 hours)
• Tuesday 9 12.30 (3.5 hours)
• General comments for the WG
• Exclude field emission related topics (unless
  time left over)
• Jlab and recent DESY 9-cell data show Quench
  occurrence is now greater than FE, thanks to new
  rinsing techniques (ultrasonic degreasing,
  ethanol rinsing) see Jlab pie-chart DESY
  ethanol rinse
• Focus on quench limitation
• Exclude field emission induced quench
• Quench is another strong reason for the spread in
  gradients
• See Zanon and ACCEL cavity quench data

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Jlab Ultrasonic Degreasing
Total 43 vertical tests
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Development of Field Emission since Jan
06compiled by D.Reschke
DESY

• - Analysis of 1. Q(E)-results only EP cavities
  (all tests, not preparations)

Ethanol rinsing
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Probability of Quench OnlyDESY 9-cell Cavities
(EPcavities only )Quench Distribution Compiled
by H.Padamsee from DESY Data Base on Cavities
Z83-Z111, Z50, and A16, A63, A70 A81
Gradient Spread Due to Quench
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Session Monday 13.30 1700 (3.5 hours)

• I. (0.75 hour) Agenda and Introductory Remarks
  H. Padamsee
• II. (1 hr) Review data on quench occurrence,
  spread in quench fields for 9-cells
• Short Talks to present data, followed by
  discussions
• Zanon cavities, Accel cavities (D.Reschke)
• A6, A7, A8, AES cavities (Geng)
• Cornell A8, A9 cavities (H Padamsee)
• (Make a histogram (s) of quench fields for
  summary talk)
• III. (1 hr) Review examples of defects found with
  thermometry/dissections/analysis
• Cornell, Jlab, DESY, KEK (P. Kneisel,
  D.Reschke, KEK person)
• IV. (0.25 hr) Review data on RRR vs weld chamber
  pressure (DESY, FNAL) W. Singer
• V. (0.5 hr) Material defects
• Review examples of defects found by eddy current
  scanning (DESY, FNAL)
• A. Brinkmann, W. Singer

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Tuesday 9 12.30 (3.5 hours)
I. (0.5 hr) Suggestions for other defect
possibilities Nb-S (Saito/Morozumi), Nb-S
(Further surface studies, H. Padamsee) II. (0.5
hr) Make a list other possible sources of quench
with evidence to support these possible
sources III.(2.5 hr) Propose coordinated
global effort for understanding and avoiding
quench A. 9 cell studies proposals Study
quench field changes with light EP (already
proposed under S0) Study quench field changes
above 30 MV/m with HF rinse or oxipolishing
Baking benefit remains, does quench originate
from first 10 20 nm? Need full scale
thermometry to track changes in quench location
from test to tests (overlap with WG2) B.
Single cell cavity tests with thermometry Quench
field changes and location changes with mild
baking C. Sample studies proposals Any other
agenda items?
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Introductory Remarks- Outline

• What we think we know
• Review theory of quench
• Quench field vs normal conducting defect size,
  defect resistance and RRR
• Thermal model calculations results
• Comparison of theory to EP/BCP quench field data
• Some puzzles and possible explanationsopen to
  discussion
• Review existing knowledge about defects that heat
  up or quench
• Gather more defect examples today
• What we dont know and would like to know
• Classify types of defects
• Weld beads, weld contamination, material
  inclusions, large particles (gt 10 mm) pits with
  sharp edges, chemical stains
• Does quench field and location change with ?
• Test to test, Mild bake, light EP?
• Other topics

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Thermal Breakdown - Quench

• Very strong heating observed over extended areas.
• Often centered on a hotspot.

Temperature map shows defect heating Below Quench
Strong heating over large are during quench
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Examples of Defect Heating Below Quench and Quench

• Show Temperature Map Movie

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Theory of thermal breakdown

• TB is a premature quench of the superconducting
  state.
• A normal conducting spot triggers quench when it
  heats the neighboring Nb above Tc.

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An Ideal Picture
Analytical Model Result
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Simulation

• Take into account temperature dependent thermal
  conductivity, kapitza conductance, BCS surface
  resistance

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Early Thermal Model Simulation for Quench (HP -
1982)
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For NC defects
The benefit of higher RRR diminishes as surface
magnetic field approaches the RF critical
magnetic field (40 MV/m)
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Variable Mesh Simulation (Reschke - 1995)
To get Eacc 35 MV/m with RRR 300 defect size
needs to get down to below 20 mm Assuming a
normal conducting defect And no local field
enhancment
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Compare with Quench Data

• Compare thermal model predictions with quench
  field data to estimate range of defect sizes, if
  NC
• BCP and EP cases
• Saclay/KEK
• DESY
• Jlab
• BCP comparisons above 25 MV/m are complicated by
  high field Q-slope heating and induced quench

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KEK/Saclay (2000) Data Single Cell BCP/EP ,
Quench Field vs RRR

• There is a large scatter in defect properties
• BCP results suggest that single cell cavities
  have typical defects of 100 200 mm diameter
• if characterized as NC regions and
• if there is no local field enhancement
• EP shows higher quench fields for same RRR

100 mm diameter
200 mm Diameter defect
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DESY Data - All BCP (Singer- post 2005)
200mm NC defect
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DESY BCP/EP Data (Lutz -2007) Recent CEBAF BCP
data (see next slide)
CEBAF Refurbished Cavities (gt30) BCP
200 micron NC defect Thermal Model Predictions
High Field Q-slope Range
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Recent Data from Jlab Re-furbished Cavities
Hpk/Eacc 47 Oe/MV/m Average 15 2 MV/m
convert to 17 2 (normalized Hpk to
Tesla-shape)
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Why does EP give higher ( x 2) quench field than
BCP for the same RRR?

• Eacc 17 MV/m (BCP) gt gt30 MV/m (EP) _at_ RRR
  250
• Removal of high field Q-slope allows E gt 25 MV/m
• But high field Q-slope does not start in BCP
  cavities till Eacc gt 20 MV/m
• Field enhancement at grain boundary steps on BCP
  surface enhances local field at defect?

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200 mm defect
Possible Explanation for Higher Quench Fields
With EP
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(No Transcript)
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Review Known Defect Examples

• Inclusion
• Large particle
• Pit with sharp edge
• Chemical stain
• Weld beads
• Collect more examples in WG1

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Dissection
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Typical Defects
CERN
CERN
Cornell
Cornell
0.1 0.2 mm size defects cause Quench
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Temperature Map and Defect
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Field emission prevented reaching Quench Estimate
Eac 12 13 MV/m
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Famous, but one and only Ta defectDESY
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End