Grigory Eremeev

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The Race For The Highest Gradient
Grigory Eremeev
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Outline - Cavities and Fields - Results -
Tricks of the Trade new shapes - Making of
theoretical niobium.
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Conducting cavities can be filled with
electromagnetic energy. This energy is then
transferred to charged particles flying along the
cavity axis.
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• Niobium best superconducting properties among
  all pure metals
• Tc 9.25 0K
• Hc 2000 Oe
• Rbcs .00001 mO at 2 0K

Nb Rs .00001 mO Cu Rs 10 mO ? Qnb 1011
up to Eacc 5107 Volts per meter
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Ideal
10
10
Q0
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Real
Real
Real
9
10
8
10
0
20
40
60
Eacc, MV/m
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Limitation
Thermal breakdown
High field Q-reduction
High-purity Nb, sheet inspection, CBP, etc
Electropolishing, low temperature baking,
Solution
Electron field emission
Electron multiplication
Quench
Limitation
High-pressure rinsing, cleanroom procedures
Further improved cavity shapes
Improved cavity shape
Solution
Courtesy of Matthias Liepe
High field Q-reduction
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Best cavities
We are still limited at about Eacc 40 MV/m for
standard elliptical-shape cavities.
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Tricks of the Trade New shapes
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Re-entrant cavities
Reduction of surface magnetic field by increasing
magnetic volume
Courtesy of Valery Shemelin
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Re-entrant cavity breaks the barrier in 2005 at
Cornell
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For the talk by J. Sekutowicz, SLAC, January 25,
2005
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For the talk by Takayuki Saeki, KEK
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Superconductivity above Hc
Tested at Cornell on March 14, 2007record still
holds
Hsurface 2065 Oe
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Another Trick
Making of Theoretical Niobium large grain and
single crystal
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Large grain cavities
In order to improve niobium surface it was
proposed to increase grain size, and to make
large grain or even single crystal cavities.
P. Kneisel et al., Single Crystal Workshop
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Exceptional smoothness
From paper by W. Singer et al., PAC2007
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We are looking for other tricks
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More tricks?
Next speaker will talk more about studies related
to high-field limitation of RF cavities.