JangHui Han, DESY

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Half Cell Length Optimisation of Photocathode RF
Gun
Jang-Hui Han, DESY

• Cell length optimisation
• for lowest transverse emittance
• Consideration on dark current from gun

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RF Gun Schematic
1.5 cell L-band RF gun (1.3 GHz)
Photocathode
Electron beam
Bucking solenoid
Coaxial RF coupler
Main solenoid
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Phase Dependence of Ebeam
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Average Beam Energy
45 MV/m
40 MV/m
At cathode 0 MeV At the first iris
2 MeV At the exit 4.5 MeV
Most important for beam quality
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Thermal Emittance

• Measurement with
• laser temporal distribution
• 3 ps rms Gaussian
• laser spot size 0.55 mm
• bunch charge 3 pC

? E Emax sin ?
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?trans Vs Half Cell Length
ASTRA simulation with 50000 macro particles
45 MV/m at cathode
Half cell length
longitudinal (fixed)
2 ps rise/fall 20 ps fwhm
transverse
radius variable
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?trans Vs Half Cell Length
40 MV/m at cathode
50 MV/m at cathode
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?trans Vs Half Cell Length
55 MV/m at cathode
60 MV/m at cathode
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Gun (50 MV/m) Linac
Thermal emittance (?therm 0.5 mm mrad)
included assuming Ekin 0.55 eV
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Gun (50 MV/m) Linac
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Emission Phase Variation
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Field Emission (Dark Current)
Dark current after gun
Field emission Vs RF phase
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Dark Current Source
(c)
(d)
(b)
(a)
The maxima of the rf field strength can be the
major source of dark current
ASTRA simulation at 40 MV/m max field and 300 A
main solenoid current
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Dark Current Beam Momentum
measurement at PITZ
simulation with ASTRA
PITZ Photoinjector Test Facility at
DESY, Zeuthen
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Dark Current at 60 MV/m
Dark current at the entrance of the 1st module
dark current reduced by 63
half cell length 65 ? 75 mm
Very small overlap in the momentum spectra
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Dark Current (Simulation)
Dark current starts from the cathode area (2 mm
rms) ASTRA simulation with 100 000
macro-particles Particle tracked up to 10 m
downstream including aperture
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Summary

• Beam transverse emittance calculated for cavities
  with different half cell length.
• Transverse emittance shows no big difference for
  64 70 mm first cell length.
• With increasing first cell length, dark current
  separated from beam.

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RF Field Profile
Calculated by Microwavestudio
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Dark Current Beam Images
y ??3 mm
drive-laser
? 16 mm
Dark current image
? 5 mm
y ?0 mm
laser spot (0.44 mm rms)
Mo
y ?2 mm
Cs2Te
y
mirror scan
x
Electron beam movement on the dark current image
By the influence of the solenoid field the
trajectory of the beam rotates by 90?.
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Bunch Charge Vs RF Phase
Operating condition for min. ?transverse ? max.
momentum
Electron bunch is emitted with a combination of
several factors

• Space charge force dominated emission
  longitudinal laser profile
• Schottky effect dominated emission
• Beam dynamics after the emission and aperture

ltGun parametersgt Bunch charge 1 nC at 35? Laser
parameter temporal 20 ps flat top
transverse 0.55 mm rms RF gradient 45
MV/m Cathode Cs2Te with 60 nm Main solenoid 320
A Bucking solenoid 24 A