WSHQE - PowerPoint PPT Presentation
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WSHQE
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roughly half of the predicted emittance of the XFEL injector ... longitudinal flat-top with 2 ps rise time. incl. thermal emittance. K. Floettmann. WSHQE, OCT. ... – PowerPoint PPT presentation
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Title: WSHQE
1
Photo cathode requirements for the European XFEL
WSHQE Oct. 5, 2006 Klaus Floettmann
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QE and Lifetime requirements
- A lifetime of some month with a
- QE gt 0.5 is required.
- This is achieved with Cs2Te Cathodes e.g. at
Flash. - Can we go home now?
3
Problem I
- roughly half of the predicted emittance of the
XFEL injector is due to the thermal emittance of
the cathode
4
Development of the transverse Emittance
- 1nC charge
- uniform transverse distribution
- longitudinal flat-top with 2 ps rise time
- incl. thermal emittance
5
Slice Emittance at the Injector exit
emittance in longitudinal slices of the bunch
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Slice Emittance at the Injector exit
w.o. thermal emittance
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- The measured thermal emittance is even higher
than expected from simple theoretical predictions
(Schottky effect, cathode roughness) - How can we reduce the thermal emittance?
- reduce cathode roughness
- control the work function
- ?
8
Problem II
- dark current presents a sever operational problem
for the XFEL injector
9
Dark Current extrapolation to 60 MV/m
no conditioning effect taken into account
10
- A large fraction of the dark current is generated
at or near the cathode. Collimation at low
energies is very difficult because the dark
current has similar properties as the beam - How can we reduce the dark current generation
- reduce cathode roughness
- reduce number of emitters by better cleaning
techniques - ?
11
Dry Ice Cleaning Principle
cold Nitrogen
CO2
12
CO2 Nozzle
13
- No field emission was observed up to 90 MV/m on
dry ice cleaned Cu samples in field emission
scans. - Dry ice cleaning of the gun cavity is in
preparation. - Dry Ice Cleaning of the cathode plugs should be
tested next. - To monitor the deposition of field emitters
during the cathode preparation process a field
emission scanning apparatus for the cathodes will
be designed and build in collaboration with the
University of Wuppertal.
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