Photocathode analysis and characterization at DESY S. Lederer, I. Hansen, H.-H. Sahling, S. Schreiber (DESY, Germany) L. Monaco, D. Sertore, P. Michelato, (LASA, INFN - Milano, Italy)

Outline Standard Cathode analysis SEM/EDX XPS SAFEM

Standard Cathode analysis: cw mode Quantum Efficiency (QE) measurements in cw mode QE after preparation / after usage QE versus photon energy 𝑄𝐸 𝐸 𝑝ℎ = 𝐴 1 𝐸 𝑝ℎ − 𝑊 1 𝑚 1 + 𝐴 2 𝐸 𝑝ℎ − 𝑊 2 𝑚 2 𝑊 𝑖 = 𝐸 𝑔 𝑖 + 𝐸 𝑎 𝑖 Preparation recipe results in W2 = 3.5 - 3.6 eV Theory [1]: 3.5 eV New preparation technique from LASA => no low energy part [1] R. Powel et al., Phys. Rev. B 8 (1973), 3987.

Standard Cathode analysis: pulsed mode QE measurements in pulsed mode Measured inside RF-gun QE for different operational conditions Different laser spot sizes at cathode Different accelerating fields QE-maps

Standard Cathode analysis: pulsed mode automated procedure for measuring charge vs. laser energy dependence Measured at nominal phase (38 deg w.r.t. zero crossing), not maximum charge QE = 9.2 % @ 262 nm space charge effect charge trend at low charge fitted

Standard Cathode analysis: QE vs. field QE @ zero gradient = 11.2 % W = EG+EA = 3.5 eV b = 4.7 56 days of operation @ FLASH cathode #77.2 QE @ zero gradient = 4.5 % W = EG+EA = 3.8 eV = 12.7 QE decreased EG+EA increased field enhancement increased

Standard Cathode analysis: QE-maps Investigations on homogeneity of electron emission #613.1 operated at PITZ 60 MV/m, 600 bunches approx. 1 nC, 10 Hz – less than a week – possible life time problem!!

SEM/EDX: Cathode #114.2 Photo taken in transport box QE-map High QE sopt Missing part of the film!?

SEM/EDX: Cathode #114.2 Laser position Suspicious spot Traces from further measurements SEM Laser position Suspicious spot

SEM/EDX: Cathode #114.2 SEM SEM Simple elemental mapping Mo Te Cs

XPS Collaboration with HZB Set-up for handling cathodes one HZB experimental chambers #23.3, used @ PITZ #44.3, O2 poisoned unused #92.1, F poisoned #500.4, exposed to bad vacuum, unused

XPS Leybold UHV XPS-system X-ray source RQ 20/38 with Al-Ka and Mg-Ka Load-lock for connection of cathode boxes Caused by the design of the half-sphere energy analyzer the cathodes have to be rotated by 90 deg in the UHV Magnetic coupled rotator for the whole carrier Some minor changes scheduled -> fully operational in a couple of month

SAFEM Scanning Anode Field Emission Microscope Cathode handling as in RF-gun (including RF-contact spring) Scanning performed by three UHV motors, moving the tip Fields up to 200 MV/m Studies on field assisted photoemission possible Should finally serve as QA tool: Each cathode to be investigated for field emitters after production, since dark current is severe issue for long RF-pulse operation which even can prevent operation Currently in commissioning

Cathode preparation @ DESY After commissioning and training - system and operator now in production mode 10 cathodes produced for FLASH and PITZ average QE

Thanks for your attention!