1. Thermal Emittance Measurement at PITZ
Jang-Hui Han for PITZ Collaboration
4th October 2006
High QE Workshop at LASA

2. Thermal Emittance Measurement
Contents
Introduction
Measurement procedure
Single slit scan
Quadruple scan
Analysis of the measurement
Schottky model
Contributions to the thermal emittance measurement
Summary
Thermal Emittance Measurement

3. Thermal Emittance Measurementx
x′ = px/pz
two-dimensional elliptical phase space area occupied particle beam
According to Liouville’s theorem, the beam emittance is invariant of the particle motion
 Good indicator of the beam quality
In linear accelerator (linac), the phase space area is not elliptical.
Therefore, the rms emittance is useful to define the beam emittance
In linac, the beam is accelerated through the accelerator.
So, the transverse divergence is scaled with the longitudinal momentum.
In this case, the normalized emittance is invariant.
Thermal Emittance Measurement

4. Thermal Emittance Measurement
Initial emittance of the beam,
which is configured during the beam emission x x′ e-
drive-laser pulse
Thermal Emittance Measurement

5. Influence to the enormal
enormal  (ethermal2 + esc2 + erf2 + ewake2 + …)½
 sets the lower emittance limit of an electron source
At the FLASH injector, enormal ~ 2 mm mrad & ethermal (< 1 mm mrad) not critical contribution
At the European XFEL injector, enormal ~ 1 mm mrad & ethermal (< 1 mm mrad) most critical contribution
Thermal Emittance Measurement

6. Measurement Procedure
Thermal Emittance Measurement

7. Thermal Emittance Measurement
ethermal Vs. Beam Size
Thermal Emittance Measurement

8. Ek of emitted electrons
0.55 eV e- e-
ph = 4.75 eV
Ek of emitted electrons
= (max DOS in the CB)  Ek e- e- e-
Thermal Emittance Measurement

9. Effective electron affinity in RF guns
electron affinity increase due to surface contamination
maximum density state
(0.75 eV from CBM)
kinetic energy
of emitted electron
electron affinity decrease due to the Schottky effect
Potential barrier decrease
by the electric field
0.2 eV
(CBM)
Electron affinity variation results in
1. Bunch charge increase
Kinetic energy increase of emitted electrons (thermal emittance)
 : surface contamination factor
ph: field enhancement factor
Eemit: electric field at emission
Thermal Emittance Measurement

10. Thermal Emittance Measurement
Schottky effect: bunch charge
From W. E. Spicer, PR (1958)
 = 2.2, ph = 4
G = 11.7,  = 0.59
From C. I. Coleman,
Appl. Optics 17, 1789 (1978)
E (MV/m)
QE (relative)
 E = Emax sin 
Thermal Emittance Measurement

11. Analysis of the measurement
laser spot size ~ 0.55 mm
bunch charge ~ 3 pC
~ 0.68 mm mrad
(rrms = 0.55 mm)
at PITZ1 & VUV-FEL (TTF2) operating condition
Eemit ~ sin 35 x 42 (MV/m)
~ 24 MV/m
discrepancy
~ 34%
European XFEL
Eemit ~ sin 44 x 60 (MV/m)
~ 42 MV/m
~ 0.60 mm mrad
(rrms = 0.45 mm)
 E = Emax sin 
Thermal Emittance Measurement

12. Dependence on cathodes
cathode #43.2, Nov.2004, QE ~ 3%
#43.2
cathode #61.1, Apr.2004, QE ~ 1%
#61.1
Thermal emittance changes depending on the surface chemistry and geometry.
Thermal Emittance Measurement

13. Thermal Emittance Measurement
Summary & Outlook
Thermal emittance might be the most crucial contribution to the emittance for the European X-ray FEL.
Detailed information is missing for the thermal emittance, i.e. surface properties, of the Cs2Te photocathode
A more systematic study will be planned at PITZ in the beginning of next year.
Thermal Emittance Measurement