dark current at the xfel injector · jang.hui.han@desy.de xfel beam dynamics meeting, 19 june 2006...
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jang.hui.han@desy.de XFEL Beam Dynamics Meeting, 19 June 2006
Dark current at the XFEL injector
Jang-Hui HanDESY, MPY
•Observations at PITZ and FLASH•Estimation for the European XFEL• Ideas to reduce dark current at the gun
jang.hui.han@desy.de XFEL Beam Dynamics Meeting, 19 June 2006
DC at FLASH
250 m
2nd bunch compressor
bypass
6 undulator modules
collimator
M1 M2 M3 M4 M5 M6 M7
1st bunch compressor
RF gun
laser
4 MeV 130 MeV 350 MeV 450… 800 MeV
RF pulse lengths (flat region)Gun: 70 µsAcc. Modules: 100 µs
jang.hui.han@desy.de XFEL Beam Dynamics Meeting, 19 June 2006
0
1
2
3
4
5
20 30 40 50 60max rf field at the cathode (MV/m)
dark
cur
rent
(mA
) measured on 9 Nov. 2004
fit with extrapolation
measurement at PITZ
fit with extrapolation
)/exp(.FE ECECI 2
521 −=
Estimation of dark current for the XFEL
European XFEL gun
Measurement with FC at 0.78 m downstream from cathode
Dark current might be more serious problem at the Euro-XFEL
jang.hui.han@desy.de XFEL Beam Dynamics Meeting, 19 June 2006
Dark current reduction1. Suppressing field emission
– Improved surface preparation– Generation of new field emitter
jang.hui.han@desy.de XFEL Beam Dynamics Meeting, 19 June 2006
Dark current reduction1. Suppressing field emission
− Improved surface preparation− Generation of new field emitter
jang.hui.han@desy.de XFEL Beam Dynamics Meeting, 19 June 2006
Dark current reduction
1. Suppressing field emission− Improved surface preparation− Generation of new field emitter
2. Lowering RF gradient at the cathode area− Lower amount of field emission− Beam quality degradation
3. Applying collimators
jang.hui.han@desy.de XFEL Beam Dynamics Meeting, 19 June 2006
Collimator
6 undulator modulesM1 M2 M3 M4 M5 M6 M7RF gun
laser
Geometrical collimator (different size)Energy collimator (different energy distribution)
Energy collimator
jang.hui.han@desy.de XFEL Beam Dynamics Meeting, 19 June 2006
Parameter summary
<2 mm mrademittancebeam
~ on crestACC1 φ
16 MV/mEmax
2.48 mentranceACC1
gun
laser
38°φemit
~42 MV/mEcath
0.55 eV (assumed)Ek
rt
6~7 ps GaussianLt
~1 mm XYrms
0.276 mSol. position
0.165 TBmax
FLASH (measure)
jang.hui.han@desy.de XFEL Beam Dynamics Meeting, 19 June 2006
DC trajectories
Astra simulation at 40 MV/m gradient and 300 A main solenoid current
The maxima of the rf field strength can be the major source of dark current
(a)
(b) (c) (d)
jang.hui.han@desy.de XFEL Beam Dynamics Meeting, 19 June 2006
DC trajectories
Local maxima
jang.hui.han@desy.de XFEL Beam Dynamics Meeting, 19 June 2006
DC trajectories
Local maxima
front view in the cathode
chamber
side viewin the gun cavity
jang.hui.han@desy.de XFEL Beam Dynamics Meeting, 19 June 2006
DC Image analysis
jang.hui.han@desy.de XFEL Beam Dynamics Meeting, 19 June 2006
without beam
The green points are the electrons from the boarder of Cs2Te.The orange points are the electrons from the edge of Mo plug.
DC Image analysis
jang.hui.han@desy.de XFEL Beam Dynamics Meeting, 19 June 2006
with beam
The green points are the electrons from the boarder of Cs2Te.The orange points are the electrons from the edge of Mo plug.The red points are the electron beams.
DC Image analysis
jang.hui.han@desy.de XFEL Beam Dynamics Meeting, 19 June 2006
Field emission Vs. emission phase
( ) ( )[ ]θβθβ sin /expsin max.
maxFE ECECI 252
1 −=
jang.hui.han@desy.de XFEL Beam Dynamics Meeting, 19 June 2006
0.0
1.0
2.0
3.0
4.0
5.0
0 30 60 90 120 150 180
rf phase (degree)
kine
tic e
nerg
y (M
eV) .
0
10
20
30
40
50
rf fi
eld
at c
atho
de (M
V/m
)
Field emission Vs. emission phase(FLASH)
jang.hui.han@desy.de XFEL Beam Dynamics Meeting, 19 June 2006
Momentum distribution after gun (measurement at PITZ)
measurement
simulation
jang.hui.han@desy.de XFEL Beam Dynamics Meeting, 19 June 2006
XY size of beam & dark current (FLASH)
jang.hui.han@desy.de XFEL Beam Dynamics Meeting, 19 June 2006
Gun for PITZ and FLASH
F = 1300.1361 MHz, Q = 26754
60 MV/m
45°
jang.hui.han@desy.de XFEL Beam Dynamics Meeting, 19 June 2006
Parameter summary
0.7 mm mrad<2 mm mrademittancebeam
-16°~ on crestACC1 φ
21.5 MV/m16 MV/mEmax
3.2 m2.48 mentranceACC1
gun
laser
46°38°φemit
60 MV/m~42 MV/mEcath
0.55 eV0.55 eVEk
2rt
206~7 ps GaussianLt
0.44 mm~1 mmXYrms
0.276 m0.276 mSol. position
0.225 T0.165 TBmax
XFEL (original)FLASH (measure)
jang.hui.han@desy.de XFEL Beam Dynamics Meeting, 19 June 2006
XY size of beam & dark current (XFEL, original)
jang.hui.han@desy.de XFEL Beam Dynamics Meeting, 19 June 2006
Momentum of beam & dark current (XFEL, original)
jang.hui.han@desy.de XFEL Beam Dynamics Meeting, 19 June 2006
F = 1300.0112 MHz, Q = 27621
New design for the XFEL
30°
jang.hui.han@desy.de XFEL Beam Dynamics Meeting, 19 June 2006
Parameter summary
0.7 mm mrad0.7 mm mrad<2 mm mrademittancebeam
-16°-16°~ on crestACC1 φ
20 MV/m21.5 MV/m16 MV/mEmax
2.4 m3.2 m2.4 mentranceACC1
gun
laser
30°46°38°φemit
60 MV/m60 MV/m~42 MV/mEcath
0.55 eV0.55 eV0.55 eVEk (assumed)
22rt
23.5206~7 ps GaussianLt
0.55 mm0.44 mm~1 mmXYrms
0.286 m0.276 m0.276 mSol. position
0.233 T0.225 T0.165 TBmax
XFEL (new)XFEL (original)FLASH (measure)
jang.hui.han@desy.de XFEL Beam Dynamics Meeting, 19 June 2006
Electron acceleration at half cellPresent gun New design
Half cell Half cell
Lower gradient at the cathodes, but even more acceleration at the half cellà Able to get a low emittance!
jang.hui.han@desy.de XFEL Beam Dynamics Meeting, 19 June 2006
Beam at the new design gun
0
3
6
9
12
15
0 1 2 3 4
z (m)
(mm
mra
d) o
r siz
e (m
m)
rms sizeemittance
ε
jang.hui.han@desy.de XFEL Beam Dynamics Meeting, 19 June 2006
XY size of beam & dark current (XFEL, new design)
jang.hui.han@desy.de XFEL Beam Dynamics Meeting, 19 June 2006
Momentum of beam & dark current (XFEL, new design)
jang.hui.han@desy.de XFEL Beam Dynamics Meeting, 19 June 2006
Conclusion and outlook
•With enlarging the half cell length, the momentum distribution of beams and dark current can be separated.
•Further optimization of the cell length ratio and machine parameters are necessary.
•Find optimum position and size of collimators including the first accelerator module
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