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Status of the Photocathode R&Dfor ATF RFgun
N.Terunuma (KEK)
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Why we use Cs2Te photocathode?
Pure metal cathode.Q.E. 10-5 ~ 10-4
Cs2Te Q.E. ~ 10-1 , a few %
OthersNeed more study.
We prepared the Cs2Te formation system and the Cathode loadlock system for ATF.
For ATF, Shorter preparation time for the beam operation.--> Preparation system connected to the RFGUN.
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RFGUNCavity
Cathode loading mover
RFGUN
Laser
Cathode Load-lock System
Cleaning&Pickup Storage
Cathodeformation
Xe light
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Quantum EfficiencyQ.E.= Number of electrons / Number of photons
Offline measurementXe lamp with monocrometer~266nm.Measure the current on cathode.
Online measurementRFGUN operation, Laser~266nm.Extracted beam intensity detected by faraday cup.
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Cathode and Cs-Te sources
Te
Cs
Evaporation SystemCathode Plug (Cu)
15mm
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Half cell endplate with Cu plug
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Argon ion bombardment
WhenNew cathode is installed.Clean the used cathode.Sputter depth ~ 50nm .
Step 1: Cleaning
Ion Source
Cleaningchamber
Formation: Step 1, Cleaning
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Te is heated by a Tungsten filament.We can control the Te thickness.
Step 2: Evaporation of Te
Formation: Step 2, Te evaporation
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Cs is evaporated on the Te film and its thickness is controlled by monitoring the Q.E. with Xe system.
Step 3: Evaporation of Cs
Formation: Step 3, Cs evaporation
Cathode holder with Mask 8mm
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R&DFor the ATF operation…
Prepare the new cathode on Monday.Beam Test: a week or more.Check Q.E. by calibrated lights (Xe,266nm) every a few days.
For the basic study… in shutdown weeks.
This year, we had two months beam operation.Two Cu plugs Two Mo plugs
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QE of Cu#1 Cathode
ATF
1.3x10-6 Pa at RFGUN
Q.E.0.4%
Yellow area: RF on
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Cu cathode after the Beam TestTwo cathode plugs were used for a month operation.
Cu#1 cathode (left)Used for the cavity aging.Many RF breakdowns.
Cu#2 cathode (right)Used stable laser operation.
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Mo Cathode
Two plugs ware used for a month operation.Their surface were polished as a mirror.
Small damage was found after the beam operation.
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Q.E. of Cs2Te on Mo plugs
• Q.E. before RF power is supplied reaches > 15%.
• Q.E. dropped quickly when RF power was supplied.
• It shows stable Q.E. after RF power supplied.
• Cs2Te on Molybdenum plugs shows higher Q.E. ~3% than that on Copper plugs ~0.4%.
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Dark vs.
Dark Current
Power (MW)
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SummaryWe had two months beam operation.
Cs2Te on Cu plugsQ.E. ~0.4%.Damages on the Cu surface.
Cs2Te on Mo plugsQ.E. ~3%. Stable.Small damages compared to Cu case were observed.
Next Establish the formation method and its reproducibility.Check Q.E. under longer beam operations.