Photo cathode requirements for the European XFEL
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Photo cathode requirements for the European XFEL. WSHQE Oct. 5, 2006 Klaus Floettmann. A lifetime of some month with a QE > 0.5% is required. This is achieved with Cs 2 Te Cathodes e.g. at Flash. Can we go home now? . QE and Lifetime requirements.

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Slide1 l.jpg

Photo cathode requirements for the European XFEL

WSHQE

Oct. 5, 2006

Klaus Floettmann

WSHQE, OCT. 5, 2006


Qe and lifetime requirements l.jpg

A lifetime of some month with a

QE > 0.5% is required.

This is achieved with Cs2Te Cathodes e.g. at Flash.

Can we go home now?

QE and Lifetime requirements

WSHQE, OCT. 5, 2006


Problem i l.jpg

roughly half of the predicted emittance of the XFEL injector is due to the thermal emittance of the cathode

Problem I:

WSHQE, OCT. 5, 2006


Development of the transverse emittance l.jpg
Development of the transverse Emittance is due to the thermal emittance of the cathode

  • 1nC charge

  • uniform transverse distribution

  • longitudinal flat-top with 2 ps rise time

  • incl. thermal emittance

WSHQE, OCT. 5, 2006


Slice emittance at the injector exit l.jpg
Slice Emittance at the Injector exit is due to the thermal emittance of the cathode

emittance in longitudinal slices of the bunch

WSHQE, OCT. 5, 2006


Slice emittance at the injector exit6 l.jpg
Slice Emittance at the Injector exit is due to the thermal emittance of the cathode

w.o. thermal emittance

WSHQE, OCT. 5, 2006


Slide7 l.jpg

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

…?

WSHQE, OCT. 5, 2006


Problem ii l.jpg

dark current presents a sever operational problem for the XFEL injector

Problem II:

WSHQE, OCT. 5, 2006


Dark current extrapolation to 60 mv m l.jpg
Dark Current extrapolation to 60 MV/m XFEL injector

no conditioning effect taken into account

WSHQE, OCT. 5, 2006


Slide10 l.jpg

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

…?

WSHQE, OCT. 5, 2006


Dry ice cleaning principle l.jpg
Dry Ice Cleaning Principle the cathode. Collimation at low energies is very difficult because the dark current has similar properties as the beam

cold Nitrogen

CO2

WSHQE, OCT. 5, 2006


Co2 nozzle l.jpg
CO2 Nozzle the cathode. Collimation at low energies is very difficult because the dark current has similar properties as the beam

WSHQE, OCT. 5, 2006


Slide13 l.jpg

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.

WSHQE, OCT. 5, 2006


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