R

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RD For Accelerating Structures

• H. Padamsee

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TESLA
Copper, 53 cm, rf 11.4 GHz
Niobium, one meter length, rf 1.3 GHz
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Substantial progress in SC gradients
9-cell
2002
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60 meters of structure
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Why do RF Losses increase above Eacc 20 MV/m?
Q
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Single Cells
Bake 140 C
30 MV/m
Chemical Polish
100 mm
Bake 140 C
Electropolish
New Loss Mechanism
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Need to UnderstandSC materials and surface
science

• Why do rf losses increase with surface roughness?
• Why do rf losses decrease with simple baking of
  140 C and 48 hours
• Why does sc quench below rf critical field 50
  MV/m?

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Possible Benefits from higher Q

• Double luminosity with 1.6 x AC power (eg double
  rep rate, increase Q to 5 x 1010
• Decrease number of klystrons from 680 to 330 by
  doubling rf pulse length, increase Q to 5 x1010
  gt increase reliability
• Reduce AC power of 800 GeV TESLA at 35 MV/m

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Q Improvements ?
2 K
BCS Contribution still important
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T 2 K, Rs 14 nW, Q 2 x1010
TESLA Q 1010
Lower mean free path, Q 3 x1010
Lower Temperature
T 1.8 K, Q 5 x 1010
T 1.7 K, Q 1.2 x10 11
T 1.5 K , Q 4.3 x1011
Need to Shield Earths magnetic field to 0.5
mOersted How to achieve this in an accelerator
cryostat?
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(No Transcript)
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Lower CostOld vs. New Fabrication Methods
Ebeam Welding
Half-Cells
Spinning
Hydroforming
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Better Material Control Methods Needed for 20,000
cavities
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1.8 meter 11.4 GHz
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J. Wang
1995
Eacc 65-75 MV/m
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SLAC DS2 1.8 m long structure - Cell 8,
Downstream Side AFTER RF
R. Kirby/SLAC
Individual sparks
Sustained spark
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Pits are harmful to small iris
G. Loew and J. Wang
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1.8 meter 11.4 GHz
1 meter TESLA Standing Wave
20 cm Standing Wave Cavity Vg 0
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Vg 3 - 5
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Juicy RD topics in physics of RF breakdown

• What is the trigger?
• field emission from microparticles? gas
  evolution from ion bomardment?
• How to keep the overall emission current low at
  operating gradient?
• How to get low spark rate ?

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DC Field Emission
Nb
50 - 175 MV/m
Digital Video of Spark
1 mm
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CERN Diamond Machined Copper cathode, HPR No
intentional contaminants
Auger later
?
20 um
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Sustained Spark
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SEM
50 microns
20 microns
Copper
170 MV/m
Sustained Breakdown
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A Model How Field Emission Leads to Voltage
Breakdown
Gas Distribution - starts by surface
desorption -or melting of emitter tips - builds
up by ion bombardment - may be ultimately
dominated by cathode material
Electrons
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1.25 RF Periods Later
Electrons
Ions
2 RF Periods Later
Ions
50 microns
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Electric Field
Field Enhancement
2.25 RF Periods
Emission Current Blow-up
Many Questions Remain How does the plasma ball
move around when fed with stored energy?
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Compare Nb/Cu with Bulk Nb
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Nb-3-Sn - Best Performance
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HTS - YBCOLow Field High Field Surface
Resistance
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MgB2
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A Comparison of Superconductors _at_ 1 GHz
X
MgB2 - estimate
R 4 X 10-8
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Linac Cavity Input Coupler TTF3
The TTF3 coupler
tested to 1.8 MW peak, 1.3 ms pulse length, 4.68
kW average power (TW) coupling is adjustable
from Qext 106 to Qext 2?107
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NLC Couplers100 MWLots of sparking near
coupler cellIs it the same?
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Linac Cavity Input Coupler TTF4
The TTF4 coupler is supposed to be
multipactor-free