Short overview of TEUFEL-project...Photo cathode preparation chamber Photo cathode Jeroen W.J....

Jeroen W.J. Verschuur (TNW-LF)University of Twente

Technische Universiteit Eindhoven

Short overview of TEUFEL-project

ELAN-meeting may 2004 – Frascati (I)



Contents

Overview of TEUFEL project at TwentePhoto cathode research Recent experienceOutlook



Overview FEL

LINAC

Wiggler

Drive laserDrive laser

Electron beam

FEL light

Photo cathode preparation chamber

Photo cathode



Electron beam and undulator

βγλπε r

uKN22

≤

3

3

GAINγ

JNu∝

( )22 1

2Ku

r +=γλλ

High current density since gain is proportional to itGood overlap between electron beam and optical beam

Matched electron beam:

Gaussian optical beamwith confocal parameter equal to undulator length

21

222

0

=

Kw uβγελ

π

uur

Nw λλπ

=202

Resonance condition

Undulator

322 βγπε uL

≤



Laser system for photo-cathode

Outputbeam

cw mode-lockedNd:YLF laser

cubepolariserλ/2-plate

Faradayrotator lenses

λ/2-plate

cubepolariserλ/4

plate

10xocular

10xocular

single modefiber

double-passamplifier

Faradayrotator

cubepolariser double-pass

amplifierFaradayrotator

cube polariser

second harmoniccrystal

acoustoopticslicer

fourth harmoniccrystal

Macro-pulse time 15 µsRepetition frequency 10 HzMicro-pulse time 20 psWavelength 527 nmEnergy per micro-pulse 5 µJAmplitude stability < 1%Phase stability < 1 ps



Principle of operation with numbers

RF-linac(: :)

wiggler resonator spectrometer

michelsoninterferometer

detector

gated camera

E = 3.1 - 6.5 MeVδE < 0.4 %I < 400 Aε < 10 π mm mrad

λ = 25 mmB = 0.7 TN = 50

‘waveguide’ structurehole couplingL = 1835 - 1842 mm

electrons

OTR-screen

FEL-light

transport+diagnostics

( )γλs

λw K= +

phv evc ev

−−

2

122

4th harmonic ofmodelockedNd:YLF laser onCs2Te photocathode

12.31 ns12.31 ns

1844.9 mm

11 µs train20 ps



Wavelength spectrum

0 2000 4000 60000.0

0.5

1.0

1.5

2.0

2.5

λ = 241.4 µm

λ = 252.2 µm

λ = 263.2 µm

Inte

nsity

[arb

. uni

ts]

Wavenumber [1/m]



Time structure

600

500

400

300

200

100

0

Vde

tect

or[x

10-6

V]

4003002001000-100t [ns]

250

200

150

100

50

0

Ibeam

[A]

saturation within 20 micro pulses E = 6 MeV



TEUFEL project parameters

RF-photo-cathode LINAC Race-Track-MicrotronElectron energy E = 6 MeV (γ = 12.74) E = 25 MeV (γ = 49.9) repetition frequency 81.25 MHzmicropulse 25 ps 25 psmacropulse 18 µs 18 µsUndulator• wavelength λw = 25 mm λw = 25 mm• field B = 0.67 T B = 0.67 T• number of periods Nw = 50 Nw = 50light transport waveguide free spaceresonator waveguide (hole coupling) Gaussian



LANL linac and preparation chamber



In the Lab

Einj = 5 MeVIinj = 250 Af = 10 Hzfmicro = 81 MHzfRF = 1.3 GHz

Eacc= 25 MeVIacc = 100 A9 orbits

Nu = 50

λu = 2.5 cm



Research environment

University of TwenteInfra-structureStudents

Technical University of EindhovenInfra structurestudents

STWStimulation of research on interface between university and industry

NCLR b.v.Company on the interface between university and industry



Policy of STW

Finance and stimulate high quality researchVia proposalsBudget M€ 40

Promote the application of the results of this researchVia user committee’s

Transfer of knowledgeUsers’ committeeOption/Right of first refusalLicense/know-how agreement



Embedding in Laser Physics Group

Free electron lasersCompton FELCherencov FEL

Laser wake field projectColaboration with Technical University of Eindhoven and Rijnhuizen FOM institute1,5 cell photo cathode injector for plasma channel

Wake field mechanism with collection mechanism

In Mesa+ lab a lot of deposition techniques available



Photo cathode research

experience



The preparation chamber

Vacuum connection to Linac4 containers

TeCsKSb



QE during evaporation

Prior to this data, Te was deposited during 17 minutes.The photo-emissive compound is either K2Te3 or K2Te2.or KTe.The shape of the curve is typical of tellurium-based cathodes.

0 5 10 15 20 250

2

4

6

8

10

Start K evaporation SP 6 %

MP 7.5 %

QE

at 2

59 n

m (%

)

Evaporation time (min)



Cs deposition

Cs-K-Te has a maximum QE of 23 % at 259 nm.Increase in the QE after removal of the boat s due to the shadow of the boat on the cathode.

0 10 20 30 40 500

5

10

15

20After removalof Cs boat

End K deposition;start Cs deposition

Start K deposition

QE

at 2

59 n

m (%

)

Evaporation time (minutes)



QE as function of photon energy

Threshold energy for Cs2Te is 3.85 eV.The data suggests for K-Te threshold energy between 4.0 - 4.5 eV, and for Cs-K-Te between 3.5 - 4.0 eV.

2.5 3.0 3.5 4.0 4.5 laser 5.0

1E-3

0.01

0.1

1

10 Cs2Te K-Te Cs-K-Te

Qua

ntum

Effi

cien

cy (%

)

Photon Energy (eV)



QE as function of usage

0 5 10 150

2

4

6

8

10

12

14

16 Cs2Te K-Te Cs-K-Te

QE

at 2

59 n

m (%

)

Operation time (h)



Comparison of properties of various cathodes

CsK2Sb Cs2Te K-Te Cs-K-Te

Photon energy(eV)

2.35 4.7 4.7 4.7

QE (%) 3.8 12.0 8.1 23.4Lifetime (hours) 2 11 11.9 0.5 / 12.0Threshold 2.1 3.8 4.5 - 5.0 4.0 - 4.5



Recent experience

Photo cathodes were not optimized for maximum QETe on Mo; 25-30 min @ 120 oCCs until max photo current @ 120 oC

QE between 4 – 8 %Depends strongly on usage and history

Life time 6 – 48 hourDepends strongly on usage and history

RejuvenationCs @ 120 oCmax 7-8 cycles

CleaningNumber of photo cathode cleanings > 600 oCmax 7-8 cleanings – corresponds to about 50 cathodes (~1200h)

Mechanical CleaningTotal reset of photo cathode



Degradation depends on usage

Low current < 150 ALife time: several hoursNo visible change of photo cathode surfaceRejuvenation possible

High current > 150 ALife time: a few tens minutesClear observable change in of photo cathode surface

Ablation or structure change?Degradation only significant at spot of illumination

Rejuvenation not possibleNew cathode with reduced Te evaporation time (15-20 min)



Subsequent cathodes are different



Conclusions

Operational life time reasonable for low currentStill unclear mechanism of degradation

Need better stability, special for high currentDegradation at high current is different for that at low current drawn

Need better analysis of photo cathode during degradationClarify drop in QE at high current drawn

Phase change of material?Ablation of material?



Possible improvements

some speculations



CsTe

CsTe is a good base materialGood quantum efficiencyRelative easy to make

HoweverSensitive to contaminationRelative short life time at high current (illumination)Performance at high current still poore-bunch time greater than 1 ps



Protection of photo cathode

Add protective layer onto photo cathodePreliminairy research done

Some promising research on carbon layersLayer will reduce the QE, however not to unacceptable levels

Still a lot of questionsGrowth of layer: new deposition processes for single layer

Pulsed laser vapor deposition



Shielding by several metals

~74.7 % ~68.73 % ~69.42 % ~82.8 % The absorbed intensity by Cs2Te

~9.5 % ~8.7 % ~8.9 % ~10.5 % Q.E

TiCrCoPd



Scottky barrier height

Electrons with energy > SBH will overcome the potential barrier. Other electrons will be back scattered.



Work function and Scottky barrier

EA (Cs2Te) = 0.2 eV;Eph = 4.79 eV;

3.94.24.84.8SBH (eV)

4.14.455Work function (eV)

TiCrCoPd



What needs to be done

Make the protection layerPulsed Laser Vapor Deposition

Deposit atoms before previous deposited ones form islandsPrecursor gas technique

Use surface and precursor gas as a kind of catalyst to force single atom layerLikely to destroy layer

Measure the effect on the QE for various materialsTest the stability for low and high current in the LinacAdditional

Metal cathodes for short pulse generation in injector for Plasma Laser Wakefield accelerator.



Related items: Short pulses for PLWF

TiSaf laserShort pulses few tens femto secondsUse of metal photo cathodes for Laser Plasma Wake-Field accelerators



Summary of discussion after presentation

Our photo cathodes have a shorter life time compared to other institutes.

Michelato’s cathodes for DESY have a significant longer life timeInitial QE might be important for level of stabilization of QE at low levels

Need better control on fabricationThickness control during evaporation or evaporation rate monitoringCalibrate evaporation process

Need more information on degradation processShoot with laser with low RF fieldApply RF without illumination of photo cathodeMonitor vacuum pressure during beam

Advice: do not go into the trouble of protective layers



Additional comment

During high beam current fast degradation of only the illuminated part of photo cathode:

Vacuum conditions are not likely to be primary causeIllumination is key issue

Temperature effect?…

Short overview of TEUFEL-project...Photo cathode preparation chamber Photo cathode Jeroen W.J....

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Transcript of Short overview of TEUFEL-project...Photo cathode preparation chamber Photo cathode Jeroen W.J....