mul$pac$ng*in*rf*structure** · 2015. 3. 5. · mul$pac$ng*in*rf*structure**...
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Mul$pac$ng in RF Structure
Tianhuan Luo, CBP LBNL USPAS “Fundamental Accelerator Physics”, June 2014
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Secondary Electron Emission • Secondary emission is a phenomenon where
primary incident par$cles of sufficient energy, when hiLng a surface or passing through some material, induce the emission of secondary par$cles.
• For secondary electron emission: the number of secondary electrons emiPed per incident par$cle is called secondary emission yield (SEY).
Primary
Secondary
SEY depends on the material and surface condi$on.
When SEY > 1, more secondary electron emiPed out than primary electron.
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• Three Steps of Secondary electron emission: – Produc$on of internal secondary electrons by kine$c impact of the
primary electrons. – transport of the internal secondary electrons through the sample bulk
toward the surface – escape of the electrons through the solid-‐vacuum interface.
Secondary Electron Emission
Most internal secondary electrons is generated at the end of its penetra$ng path.
The higher the primary E, the deeper the penetra$ng depth, thus harder for the internal secondary electron to escape the surface.
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Mul$pac$ng • Mul$pac$ng : mul$pling impacts
RF Source
A parallel plates connected to a RF power source.
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Mul$pac$ng • Mul$pac$ng : mul$pling impacts
RF Source
A parallel plates connected to a RF power source.
E_impact gives SEY=2
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Mul$pac$ng • Mul$pac$ng : mul$pling impacts
RF Source
A parallel plates connected to a RF power source.
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Mul$pac$ng • Mul$pac$ng (MP) is a resonant RF electron discharge in vacuum with electron mul$plica$on due to the secondary electron re-‐emission process.
• To have mul$pac$ng one need the occurrence of two condi$ons: – electron synchroniza$on with the RF fields: the $me interval between two impacts is an integer of half RF period.
– electron mul$plica$on via secondary: SEY >1 (even slightly), with right impact energy.
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Solve MP Analy$cally For simple geometry, some$mes with proper approxima$on, we can solve the resonant trajectory of MP analy$cally.
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Solve MP Numerically • Numerical simula$on tools: FishPac (2D), CST Par$cle Studio (3D), SLAC ACE3P (3D).
• Solve the RF field in the structure. • Track electrons’ mo$on in the $me varying RF field, adding any external sta$c E or B field, and iden$fy the resonant trajectories and impact energy.
• With the material SEY informa$on, one can inden$fy the mul$pac$ng. Enhancement Counter:
(E, )
€
EC = δ1 *δ2 * ...*δm
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Solve MP Numerically
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Possible Damage from MP • Hea$ng. • Out gassing: vacuum loss. • Change the coupling: power
reflec$on.
• Energy absorp$on: can’t increase the voltage.
• Sparking, RF breakdown. • Severe damage of cavity,
unrecoverable.
• CW and superconduc$ng RF cavity are especially vulnerable to MP.
11
Yellow Mag I eq. 0.391T/Unit, Red – radia$on mRem/hr Green – vacuum In Torr. E-‐7, The X axis is $me in sec.
RF cavity test at Fermilab MTA, from Al More$i
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How to Suppress MP • Improve the RF structure design. • Add external sta$c E or B field. • Cavity surface treatment: baking, Argon Glow Discharge, TiN coa$ng etc, to lower the surface SEY.