1. at Helmholtz-Zentrum Berlin
S.Schubert
P3 workshop, Cornell Oct. 2012
Growth and Operation of Pb/Nb photocathodes in SRF gun
at Helmholtz-Zentrum Berlin

2. q.e. enhancement through laser cleaning
Q.E. values before and after irradiation
Q.E. of Pb films prepared by various deposition techniques
[Sme08] Smedley, J. et al., Phys. Rev. ST Accel. Beams 11, (2008)

3. The first Srf injector at HZB
1.5 cell srf photoinjector with lead spot as photocathode
arc-deposited at the back-wall of the cavity

4. Witness samples – PES Measurements
oxidized lead
View into the PES chamber
the alignment laser shows were the cleaning laser will hit the sample
As witness sample were
chosen lead-films of about
nm thickness arc-
deposited on polished Mo.
The cleaning was performed
by means of a KrF Excimer
laser at 248 nm with a puls
length of 5 ns.
Pb 5d area
a) before laser treatment
b) After 10 min irradiation at a fluence of 0.32 mJ/mm2
S.Schubert, Proceedings of IPAC12, TUPPD050

5. Witness sample – SEM measurements
Only a small part of the sample was
Irradiated 2
Laser hit the sample between point 1 and 2
SEM pic were taken from all over the sample revealing a continuous
Pb cover
No changes in the surface morphology were
observed – one could interpret that the
particles are smoother, meaning no sharp
edges
Top: before; buttom: after irradiation

6. Injector testing setup
bending magnet
injector
view screen
laser in
cryo vessel
Berlin setup and beam parameters
(measured)
T. Kamps et al., Proceedings of IPAC11, MOODA03

7. Laser-Cleaning in the Gun
The laser energy density was increased in 7 steps from mJ/mm2 to 0.23 mJ/mm2.
The QE map was recorded by measuring the photoemission current as the laser spot was scanned across the back wall of the cavity.
Prior to the laser cleaning the maximum QE of 3.6·10-5 was registered in the center of the photocathode. After the final laser cleaning with 0.23 mJ/mm2 the quantum efficiency in the center of the cathode increases to 9· Meaning a quantum efficiency improvement of a factor of 2.5 which is still 5times higher than the Nb q.e..
0.045 mJ/mm2
0.087 mJ/mm2
Barday, R., et al. Proceedings of PST 11

8. Gun Performance after Cleaning
Irregular emission from non uniform cathode area : hot spots
Pattern can be reproduced with Astra-simulations:
Astra input: simulated rough surface (placing random dropplets of different size
with local q.e. variations
View screen pattern
Astra simulations
Völker, J. et al., Proceedings of IPAC12, TUPPD051

9. Current Activity
Cavity is equipped with input coupler and valve and is now installed in the Bessy test facility.
courtesy of J. Sekutowizc

10. Current activity: Pb cathode
As in the first run a Pb spot was arc-deposited onto the back wall of the cavity. Unfortunately the spot was removed at the high pressure rinsing step. Due to time schedule it was decided to drill a hole in the back wall to insert the coated plug later.
Left: outer back wall of cavity
Right: unpolished plugs as installed for the first tests
courtesy of J. Sekutowizc

11. Current Activity: Field at the cathode
Back wall Ez
E=1.25Ez
Pb spot
Electric field on edge of the plug is 25% higher than in the center of the Pb spot.
For the recessed plug (~200µm) radial component of the electric field may act as an additional focusing force. We plan to study this within next few weeks.
courtesy of J. Sekutowizc

12. Institutions and People involved + Thanks
TJNAF:
P. Kneisel
G. Ciovati
………..
NCNS:
R. Nietubyć
J. Lorkiewicz
R. Mirowski
T. Wasiewicz
…………..
HZB:
T. Kamps
J. Knobloch
A. Jankowiak
O. Kugeler
W. Anders
R. Barday
A. Neumann
T. Quast
DESY:
D. Klinke
A. Schmidt
A. Poerschemann
J. Iversen
X. Singer
W. Singer
K. Floettmann
J. Sekutowicz
HZR:
J. Teichert
THANKS FOR YOUR ATTENTION!

13. New gun and beamline with Pb cathode