Post on 30-Nov-2020
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
Short overview of TEUFEL-project
ELAN-meeting may 2004 – Frascati (I)
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
Contents
Overview of TEUFEL project at TwentePhoto cathode research Recent experienceOutlook
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
Overview FEL
LINAC
Wiggler
Drive laserDrive laser
Electron beam
FEL light
Photo cathode preparation chamber
Photo cathode
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
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
≤
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
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
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
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
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
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]
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
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
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
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
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
LANL linac and preparation chamber
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
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
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
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
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
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
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
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
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
Photo cathode research
experience
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
The preparation chamber
Vacuum connection to Linac4 containers
TeCsKSb
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
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)
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
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)
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
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)
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
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)
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
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
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
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
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
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)
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
Subsequent cathodes are different
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
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?
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
Possible improvements
some speculations
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
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
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
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
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
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
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
Scottky barrier height
Electrons with energy > SBH will overcome the potential barrier. Other electrons will be back scattered.
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
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
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
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.
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
Related items: Short pulses for PLWF
TiSaf laserShort pulses few tens femto secondsUse of metal photo cathodes for Laser Plasma Wake-Field accelerators
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
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
Jeroen W.J. Verschuur (TNW-LF)University of Twente
Technische Universiteit Eindhoven
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?…