1. Secondary Electron Emission in Photocathode RF Guns
Jang-Hui Han
4th October 2006
High QE Workshop at LASA

2. Secondary Electron Emission
Contents
Introduction
Secondary emission model
Detection of secondary electrons
Emission phase scan for bunch charge
Emission phase scan for bunch momentum
Multipacting at photocathode
Measurement at PITZ
Model simulation
Summary
Secondary Electron Emission

3. SE in photocathode RF guns
SEE during photoemission?
Multipacting?
Dark current emission?
No direct measurement data for Cs2Te
 indirect measurement under RF operation
Secondary Electron Emission

4. Secondary emission (SE)  process
When a primary electron strikes a solid material, it may penetrate the surface and generate secondary electrons.
From J. Cazaux, JAP89, 8265 (2001)
Energy (keV)
SEY
SE process has a great similarity to photoemission process.
 Good photo-emitters are good secondary electron emitters.
Secondary electron yield, SEY ()
From N. Hilleret et.al., EPAC2000 4
3.5 3
2.5 2
1.5 1
0.5
SEY
500
1000
1500
2000
Energy (eV)
Aluminum 99.5%
Titanium
Copper OFHC
Stainless steel
TiN
Secondary Electron Emission

5. Secondary emission (SE)  modeling
A SE model has been implemented into ASTRA
Secondary Electron Emission

6. Measurement setup (PITZ)
With changing the relative phase between the RF and the drive-laser, the emission phase of electron bunch is determined.
Secondary Electron Emission

7. Secondary Electron Emission
Qbunch & pmean vs. emit
Max. Qbunch ~ 5 pC
Max. RF field: ~22 MV/m (negligible dark current)
Laser profile: [temporal] ~2.3 ps rms Gaussian
[transverse] ~0.5 mm rms
(b)
(a)
(e)
(c)
(d)
Secondary Electron Emission

8. SE dependence on emission phase
 measurement points
 photoelectrons (simul.)
 secondary electrons (simul.)
Secondary Electron Emission

9. SE dependence on emission phase
measurement
ASTRA simulation
 photoelectrons
 secondary electrons
Secondary Electron Emission

10. Secondary Electron Emission
Multipacting: electron multiple impacting
Explosive increase of the number of electrons
Multipacting may cause RF power loss, lead to vacuum breakdown, and even damage the surface inside the cavity.
secondary electrons
Secondary Electron Emission

11. Observation at PITZ and TTF phase 1
This multipacting has not been observed with Mo cathodes except for the case of very bad vacuum in the gun cavity.
 Multipacting at the Cs2Te photocathode
From D. Setore et.al., FEL2000
RF forward power to the gun
signal from
the Faraday cup
multipacting peaks
multipacting peaks
PITZ
TTF phase1
Secondary Electron Emission

12. Secondary Electron Emission
DC and multipacting vs. max RF gradient
40 MV/m
33 MV/m
Dark current following
the Fowler-Nordeim relation
Multipacting peak
Independent of the gradient
Secondary Electron Emission

13. Secondary Electron Emission
Description of multipacting peak
RF forward
RF stored
dark current
multipacting
peak
starting of the multipacting peak
tdelay
RF stored
Emax
EMP
EMP: RF field when the multipacting starts
Emax: maximum field of the RF pulse
tdelay : the delay between the RF pulse and
the multipacting peak
 : fill/decay time of the RF field in the cavity
Secondary Electron Emission

14. Secondary Electron Emission
Delay time vs. max RF gradient
max RF gradient
The shape and height do not depend on the max RF gradeint
cathode
#60.1
#43.2
Meas. time
Mar.04
Sep.04
Apr.05
EMP (MV/m)
2.70
1.04
1.07
 (s)
2.80
2.83
RF measurement in the cavity: 2.78 (s)
Secondary Electron Emission

15. Dependence on solenoid field profile
main solenoid current
Strong dependence on the solenoid field profile
 Magnetic mirror configured by the solenoid field plays a crucial role in generation of the multipacting.
No multipacting region
Multipacting sometimes takes place in the region according to the cathode parameter and vacuum condition
Secondary Electron Emission

16. Secondary Electron Emission
ASTRA simulation of multiplication process
Number of secondary electrons per one seed electron
simulation conditions:
max = 20, Emax = 1 keV, s = 2.2
Imain = 400 A & Ibucking = 30.5 A
Estarting = 0.6 MV/m,  = 2.78 s
(1 RF cycle ~ 0.77 ns)
Secondary Electron Emission

17. Secondary Electron Emission
Summary
Secondary electron detected in photocathode RF guns at selected machine conditions
Multipacting explained by high SEY of the Cs2Te cathode
At the nominal operation condition of FLASH (1 nC, ~45 MV/m, ~ 38), no secondary electron generated during photoemission
No clear evidence of SE in dark current
Secondary Electron Emission