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