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Commissioning results of the SRF gun with Pb cathode R. Barday 14.09.11. Energy Recovery Linac BERLinPro. 32 m. Beam dump. Main linac. Merger. SRF Gun. Booster. 6 m. demonstration of the feasibility to use ERL technology for future synchrotron light sources. Undulator. 25 m. - PowerPoint PPT Presentation
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XIV Workshop on Polarized Sources, Targets and Polarimetry
Commissioning results of the SRF gun with Pb cathodeR. Barday14.09.11
XIV Workshop on Polarized Sources, Targets and Polarimetry
Energy Recovery Linac BERLinPro
14.09.2011 2
Beam energy 50 MeV
Average current 100 mA
Bunch charge 77 pC
Normalized emittance <1 mm mrad
Repetition rate 1.3 GHz
32 m
25 m
6 m
Undulator
Main linacSRF GunBooster
MergerBeam dump
final beam parameters are determined by the performance of the electron source: SRF gun
demonstration of the feasibility to use ERL technology for future synchrotron light sources
XIV Workshop on Polarized Sources, Targets and Polarimetry
Electron source
14.09.2011 3
Parameter Stage A Stage B Stage CGoal Beam
demonstratorBrightness R&D
gunProduction
gun
Cathode Pb CsK2Sb CsK2Sb
Laser wavelength 258 nm 526 nm 526 mn
Theoretical thermal emittance in terms of laser spot
0.8 µm/mm 0.4 µm/mm 0.4 µm/mm
Repetition rate 8 kHz 52 MHz 1.3 GHz
Laser pulse length (FWHM)
(2-3) ps* <20 ps <20 ps
Laser pulse shape Gauss Gauss Gauss Flat-top*
Bunch charge 6 pC 77 pC 77 pC
Average beam current
50 nA 50 µA4 mA in 400 µs
macropulse
100 mA
Development of the SRF gun in a staged approach
Stage A SC cathode on the back wall of the SC cavityPb: robust, fast response timeWork function ~(4-5) eV→→ UV laser• emission from a few atomic layers• high reflectivityQE ~ few 10-4: too low to produce high average currentHigh peak current, comparable to the BERLinPro requirementsStage BImplementation of NC cathode with high QE at VIS in the SRF gunStage CHigh power operation
XIV Workshop on Polarized Sources, Targets and Polarimetry
Gun cavity with diagnostics beam line
14.09.2011 4
few hunderd nm thick Pb cathode film
plasma arc deposition
R. Nietubyc
first electron beam 21st of April 2011
C ath o d e
YA G -S creenF arad ay C u p B e am
D u m p
B eam D u m p
T H z -P o rtB e n d in g M ag n e t
YA G -S cre en
YA G -S creen
0 1 2 3 4
S o le n o id
Cryom odule Warm partD riv e L ase r P o rt
S tr ip lin e
IC T
D o se m e te r
m
S R F C av ity
e
XIV Workshop on Polarized Sources, Targets and Polarimetry
Vacuum system
14.09.2011 5
Particle free assemblyBeam line is not yet baked out. TSP is not yet activatedPlaned for October 2011…
ok for Pb cathode, cavity, bad for CsK2Sb cathode
P<1E-8 mbar
C ath o d e1 5 0 l/s IG P7 5 l/s IG P
0 1 2 3 4
Cryom odule W arm part
2 0 l/s IG P
m
S R F C av ity
1 5 0 l/s IG P
7 5 l/s IG P
T S P1 5 0 l/s IG P
M asssp ec tro m e te r
v acu u mg au g e
XIV Workshop on Polarized Sources, Targets and Polarimetry
QE distribution
14.09.2011 6
NNQE e
QEmetal<10-4
Plaser~1.5 mW → Ie≤50 nA
phot
ocat
hode
e
Schottky effect (field dependent electron emission)• describes the lowering of the electron affinity (work function)
• leads to an encreased electron emission (QE)
Eph 0phase=25 degr.
XIV Workshop on Polarized Sources, Targets and Polarimetry
QE distribution
14.09.2011 7
QEmax ~ 3.6*10-5
NNQE e
QEmetal<10-4
Plaser~1.5 mW → Ie≤50 nA
phot
ocat
hode
e
Schottky effect (field dependent electron emission)• describes the lowering of the electron affinity (work function)
• leads to an encreased electron emission (QE)
Eph 0
XIV Workshop on Polarized Sources, Targets and Polarimetry
In situ laser cleaning
14.09.2011 8
KrF excimer laser (Xantos XS)=248 nm (~5 eV)5 ns pulse duration (FWHM)Repetition rate 500 HzCathode surface was irradiated for 10 minutes
Laser cleaning removes the contaminations from the surface.Laser cleaning improve QE in terms of absolute value and uniformity.BUT! High laser density can damage the photocathode/substrate.7 laser cleaning runs with laser density between 0.045 mJ/mm2 and 0.23 mJ/mm2 were performed at HZB.
XIV Workshop on Polarized Sources, Targets and Polarimetry
In situ laser cleaning
14.09.2011 9
• first laser cleaning energy density~0.04 mJ/mm2
• second laser cleaning energy density~0.09 mJ/mm2
• third laser cleaning energy density~0.09 mJ/mm2
Pb cathode
start laser cleaning
XIV Workshop on Polarized Sources, Targets and Polarimetry
In situ laser cleaning
14.09.2011 10
• before laser cleaning QEmax~3.6E-5• first laser cleaning energy density~0.04 mJ/mm2
QEmax~4.8E-5 • second/third laser cleaning energy density~0.09 mJ/mm2
QEmax~4.7E-5 QEmax~30% ~factor 2.5
XIV Workshop on Polarized Sources, Targets and Polarimetry
In situ laser cleaning
14.09.2011 11
During 4th laser cleaning the left side of the cathode was cleaned with the energy density of 0.23 mJ/mm2
fluorescence Nb
XIV Workshop on Polarized Sources, Targets and Polarimetry
In situ laser cleaning
14.09.2011 12
trapped He was released during the laser cleaningTsurface>4K?
XIV Workshop on Polarized Sources, Targets and Polarimetry
In situ laser cleaning
14.09.2011 13
Metal cathodes installed in the rf guns typically have lower QE than the one of the test probe.QE of the cathode is 10 times higher than for cleaned NbImax=50 nA demonstratedQEmax=3.6E-5 16.06.11
QEmax=9.0E-5 06.07.11
J.Smedley, PRST-AB 11, 013502 (2008)
HZB data for QEmax
XIV Workshop on Polarized Sources, Targets and Polarimetry
-10 -8 -6 -4 -2 0 2 4 6 8 10-0,05
0,00
0,05
0,10
0,15
0,20
0,25
0,30
0,35
0,40
measurement ASTRA Ee<0.8 eV ASTRA Ee<0.01 eV
inte
nsity
[a.u
.]
time [ps]
Phase scan
14.09.2011 14
-8 -6 -4 -2 0 2
0,0
0,1
0,2
0,3
0,4
0,5
measurement ASTRA simulation
char
ge [p
C]
phase [degr.]
Epeak=12 MV/m
Epeak=18 MV/m
Epeak=18 MV/m
Epeak=18 MV/m; 10 degr. 0.45 pCFWHM~0.9 ps
tlaser(FWHM)=2ps
tFWHM=2.6 ps
XIV Workshop on Polarized Sources, Targets and Polarimetry
Phase scan
14.09.2011 15
To the first order approximation, QEmetal near photoemission threshold can be expressed as
20 EhQE ph
EhQE ph 0
XIV Workshop on Polarized Sources, Targets and Polarimetry
Dark current
14.09.2011 16
Time averaged field emission current
field enhancement factor: ideal surface field E is increased to a local microscopic field Em: =Em/E
E0
E0
oxides/carbides
EEAI
5.19
75.1
5.252.412 1053.6exp10107.55.0
Epeak=15 MV/m
Epeak=17 MV/memission around 900 launch phase
XIV Workshop on Polarized Sources, Targets and Polarimetry
Dark current
14.09.2011 17
Time averaged field emission current
field enhancement factor: ideal surface field E is increased to a local microscopic field Em: =Em/E
E0
E0
oxides/carbides
=540
=230=180
Fowler-Nordheim plots for1.6 cell SRF gun with Pb cathode
EEAI
5.19
75.1
5.252.412 1053.6exp10107.55.0
β was calculatedfor =4.0 eV
XIV Workshop on Polarized Sources, Targets and Polarimetry14.09.2011 18
This work is a collaborative effort by:
W. Anders, R. Barday, A. Jankowiak, T. Kamps, J. Knobloch, O. Kugeler, A. Matveenko, A. Neumann, T. Quast, J. Völker, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin, Germany
J. Sekutowicz, DESY, Hamburg, Germany
J. Smedley, BNL, Upton, Long Island, New York, USA
P. Kneisel, JLAB, Newport News, Virginia, USA
R. Nietubyc, The Andrzej Soltan Institute for Nuclear Studies, Swierk/Otwock, Poland
J. Teichert, HZDR, Dresden, Germany