1. 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)

2. Outline
Standard Cathode analysis
SEM/EDX
XPS
SAFEM

3. 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 = 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.

4. 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

5. 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

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

7. 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!!

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

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

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

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

12. 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

13. 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

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

15. Thanks for your attention!