Short overview of TEUFEL-project ELAN-meeting may 2004 Frascati (I)
Contents Overview of TEUFEL project at Twente Photo cathode research Recent experience Outlook
Overview FEL Drive laser Photo cathode Drive laser Electron beam Wiggler FEL light LINAC Photo cathode preparation chamber
Electron beam and undulator High current density since gain is proportional to it Good overlap between electron beam and optical beam GAIN 3 N u 3 J γ Matched electron beam: Gaussian optical beam with confocal parameter equal to undulator length Resonance condition 2 βγελu w0 = π 2 2K 2πw λ r λ λ = 2 γ Undulator 2 0 = N u u 1 r + 2 λ u 2 ( K ) 1 2 π ε 2 2 ε KN u Lu π 2 2 βγ 3 λr βγ
cw mode-locked Nd:YLF laser λ/2-plate Laser system for photo-cathode cube polariser Faraday rotator lenses λ/2-plate 10x ocular single mode fiber 10x ocular λ/4 plate cube polariser acousto optic slicer cube polariser cube polariser second harmonic crystal Faraday rotator Faraday rotator fourth harmonic crystal double-pass amplifier double-pass amplifier Output beam Macro-pulse time 15 µs Repetition frequency 10 Hz Micro-pulse time 20 ps Wavelength 527 nm Energy per micro-pulse 5 µj Amplitude stability < 1% Phase stability < 1 ps
Principle of operation with numbers 4 th harmonic of modelocked Nd:YLF laser on Cs 2 Te photocathode ( 2 1 ) λ s = λ w 2 + 2γ K v c ph v v e e michelson interferometer detector RF-linac transport + diagnostics (: :) wiggler resonator spectrometer FEL-light E = 3.1-6.5 MeV δe < 0.4 % I < 400 A ε < 10 π mm mrad 11 µs train 1844.9 mm λ = 25 mm B = 0.7 T N = 50 12.31 12.31 ns ns waveguide structure hole coupling L = 1835-1842 mm 20 ps OTR-screen electrons gated camera
Wavelength spectrum 2.5 Intensity [arb. units] 2.0 1.5 1.0 0.5 λ = 263.2 µm λ = 252.2 µm λ = 241.4 µm 0.0 0 2000 4000 6000 Wavenumber [1/m]
Time structure 600 250 V detector [x10-6 V] 500 400 300 200 200 150 100 I beam [A] 100 50 0 0-100 0 100 t [ns] 200 300 400 saturation within 20 micro pulses E = 6 MeV
TEUFEL project parameters RF-photo-cathode LINAC Race-Track-Microtron Electron energy E = 6 MeV (γ = 12.74) E = 25 MeV (γ = 49.9) repetition frequency 81.25 MHz micropulse 25 ps 25 ps macropulse 18 µs 18 µs Undulator wavelength λ w = 25 mm λ w = 25 mm field B = 0.67 T B = 0.67 T number of periods N w = 50 N w = 50 light transport waveguide free space resonator waveguide (hole coupling) Gaussian
LANL linac and preparation chamber
In the Lab E inj = 5 MeV I inj = 250 A f = 10 Hz f micro = 81 MHz f RF = 1.3 GHz E acc = 25 MeV I acc = 100 A 9 orbits N u = 50 λ u = 2.5 cm
Infra-structure Students Research environment Technical University of Eindhoven Infra structure students STW Stimulation 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 research Via proposals Budget M 40 Promote the application of the results of this research Via user committee s Transfer of knowledge Users committee Option/Right of first refusal License/know-how agreement
Free electron lasers Compton FEL Cherencov FEL Embedding in Laser Physics Group Laser wake field project Colaboration with Technical University of Eindhoven and Rijnhuizen FOM institute 1,5 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 Linac 4 containers Te Cs K Sb
QE during evaporation QE at 259 nm (%) 10 8 6 4 MP 7.5 % Start K evaporation SP 6 % 2 0 0 5 10 15 20 25 Evaporation time (min) Prior to this data, Te was deposited during 17 minutes. The photo-emissive compound is either K 2 Te 3 or K 2 Te 2.or KTe. The shape of the curve is typical of tellurium-based cathodes.
Cs deposition QE at 259 nm (%) 20 15 10 5 Start K deposition End K deposition; start Cs deposition After removal of Cs boat 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 0 10 20 30 40 50 Evaporation time (minutes)
QE as function of photon energy Quantum Efficiency (%) 10 1 0.1 0.01 1E-3 Cs 2 Te K-Te Cs-K-Te 2.5 3.0 3.5 4.0 4.5 laser 5.0 Photon Energy (ev) Threshold energy for Cs 2 Te 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.
QE as function of usage 16 Cs 2 Te 14 K-Te Cs-K-Te 12 QE at 259 nm (%) 10 8 6 4 2 0 0 5 10 15 Operation time (h)
Comparison of properties of various cathodes Photon energy (ev) CsK 2 Sb Cs 2 Te K-Te Cs-K-Te 2.35 4.7 4.7 4.7 QE (%) 3.8 12.0 8.1 23.4 Lifetime (hours) 2 11 11.9 0.5 / 12.0 Threshold 2.1 3.8 4.5-5.0 4.0-4.5
Recent experience Photo cathodes were not optimized for maximum QE Te on Mo; 25-30 min @ 120 o C Cs until max photo current @ 120 o C QE between 4 8 % Depends strongly on usage and history Life time 6 48 hour Depends strongly on usage and history Rejuvenation Cs @ 120 o C max 7-8 cycles Cleaning Number of photo cathode cleanings > 600 o C max 7-8 cleanings corresponds to about 50 cathodes (~1200h) Mechanical Cleaning Total reset of photo cathode
Degradation depends on usage Low current < 150 A Life time: several hours No visible change of photo cathode surface Rejuvenation possible High current > 150 A Life time: a few tens minutes Clear observable change in of photo cathode surface Ablation or structure change? Degradation only significant at spot of illumination Rejuvenation not possible New cathode with reduced Te evaporation time (15-20 min)
Subsequent cathodes are different
Conclusions Operational life time reasonable for low current Still unclear mechanism of degradation Need better stability, special for high current Degradation at high current is different for that at low current drawn Need better analysis of photo cathode during degradation Clarify drop in QE at high current drawn Phase change of material? Ablation of material?
Possible improvements some speculations
CsTe CsTe is a good base material Good quantum efficiency Relative easy to make However Sensitive to contamination Relative short life time at high current (illumination) Performance at high current still poor e-bunch time greater than 1 ps
Protection of photo cathode Add protective layer onto photo cathode Preliminairy research done Some promising research on carbon layers Layer will reduce the QE, however not to unacceptable levels Still a lot of questions Growth of layer: new deposition processes for single layer Pulsed laser vapor deposition
Shielding by several metals Pd Co Cr Ti The absorbed intensity by Cs2Te ~82.8 % ~69.42 % ~68.73 % ~74.7 % Q.E ~10.5 % ~8.9 % ~8.7 % ~9.5 %
Scottky barrier height Electrons with energy > SBH will overcome the potential barrier. Other electrons will be back scattered.
EA (Cs2Te) = 0.2 ev; Eph = 4.79 ev; Work function and Scottky barrier Pd Co Cr Ti Work function (ev) 5 5 4.4 4.1 SBH 4.8 4.8 4.2 3.9 (ev)
Make the protection layer Pulsed Laser Vapor Deposition What needs to be done Deposit atoms before previous deposited ones form islands Precursor gas technique Use surface and precursor gas as a kind of catalyst to force single atom layer Likely to destroy layer Measure the effect on the QE for various materials Test the stability for low and high current in the Linac Additional Metal cathodes for short pulse generation in injector for Plasma Laser Wakefield accelerator.
TiSaf laser Related items: Short pulses for PLWF Short pulses few tens femto seconds Use 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 time Initial QE might be important for level of stabilization of QE at low levels Need better control on fabrication Thickness control during evaporation or evaporation rate monitoring Calibrate evaporation process Need more information on degradation process Shoot with laser with low RF field Apply RF without illumination of photo cathode Monitor 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 cause Illumination is key issue Temperature effect?