at Helmholtz-Zentrum Berlin S.Schubert │susanne.schubert@helmholtz-berlin.de P3 workshop, Cornell Oct. 2012 Growth and Operation of Pb/Nb photocathodes in SRF gun at Helmholtz-Zentrum Berlin

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, 013502 (2008)

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

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 100-200 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

Witness sample – SEM measurements Only a small part of the sample was Irradiated 2 4 3 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

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

Laser-Cleaning in the Gun The laser energy density was increased in 7 steps from 0.045 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 9·10-5. 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

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

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

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

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

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!

New gun and beamline with Pb cathode