Photocathodes FLASH: Quantum Efficiency (QE)

Transcription

1 Photocathodes FLASH: Quantum Efficiency (QE) L. Monaco, D. Sertore, P. Michelato J. H. Han, S. Schreiber Work supported by the European Community (contract number RII3-CT-4-568) /8

2 Main Topics Overview Photocathode Production Production at LASA Transportation to DESY Database CW QE measurements (Hg lamp) Experimental set-up Results of measurements at FLASH Pulsed QE measurements Laser energy calibration Measurements on different cathodes Results QE maps Conclusions /8

3 Cathode in the RF Gun Photocathode inserted into the gun backplane 3/8

4 Quantum Efficiency QE(%).5*Q(nC)/E(µJ) The design asks for 7 nc/sec QE required for FLASH: >.5 % to keep the laser in a reasonable limit: within an average power of ~W Design of present laser accounts for QE=.5% with an overhead of a factor of 4 and has an average power of W (IR) Cs Te cathodes found to be the best choice 4/8

5 Photocathode Production: Preparation Chamber Photocathodes are LASA on Mo plugs under UHV condition. Transport Box Source holder Preparation LASA UHV Vacuum System - base pressure - mbar 6 sources slot available Te sources out of % pure element Cs sources from SAES High pressure Hg lamp and interference filter for online monitoring of QE during production Masking system 5 x UHV transport box Mo Plug Masking Masking 5/8

6 Production: from Mo plugs... Polishing procedure before after. Milling and/or lathing of the plug from the rod (arc cast / sintered). Buffer Chemical Polishing (BCP) 3. Polished to optical finishing (roughness about nm) 4. Reflectivity measurement to check optical polishing Rough Surface 8 6 ArcCast R = % reduced dark current by an one order of magnitude Optical Finished Sintered R = % 4 6/ Reflectivity [%]

7 Production:...to photocathodes growth Cs Te photocathode receipe: during the evaporation, the plug is heated to C. The dimension of the film is determined by a circular masking (the actual one is 5mm diameter) first, a thin layer of nm of Te is produced then Cs is evaporated at a rate of nm/min during the deposition, the film is illuminated with UV (λ=54 nm) of a Hg-lamp to monitor the quantum efficiency. the evaporation is stopped, when the QE is at maximum. Active Area 5 mm Different stoichiometric compounds form during Cs deposition till the correct Cs/Te ratio is reached corresponding to the QE maximum After min of Te deposition After 45 min of Cs deposition 7/8

8 Production: diagnostic on photocathodes The photoemissive properties of produced cathodes are checked performing spectral response measurements and QE maps (also at different wavelengths). QE 54nm (Hg lamp, interferential filter, mm spot diameter) QE map:73-i(54nm) of cathode #73. (3-3-4), +/-4mm, step.5mm Spectral response (Hg lamp, interferential filters, 3mm spot diameter) QE (%).E+.E+.E+.E-.E-.E-3.E-4 Just after the deposition Photon Energy (ev) d=5mm QE map:73-i(54nm) of cathode #73. (3-3-4), +/-4mm, step.5mm /8

9 Production: from LASA to FLASH and PITZ Transport box Produced cathodes, are loaded in the transport box and shipped to FLASH or PITZ keeping the UHV condition. The box is then connected to the RF gun. Since 998, we have shipped to TTF phase I, FLASH and PITZ: 49 x Cs Te x KCsTe 5 x Mo Total transfers from LASA: 5 RF gun and FLASH linac 9/8

10 Production:The Photocathode Database Many of the data relative to photocathodes (production, operation, lifetimes) and transport box are stored in the photocathode database whose WEB-interface is available at: The database keeps track of the photocathodes in the different transport boxes and in the different labs (TTF, PITZ and LASA). /8

11 CW QE measurements: Experimental set-up The experimental set-up for the CW QE measurements is mainly composed by: towards the RF gun a high pressure Hg lamp towards the exchange chamber Interferential filters (39nm, 54nm, 97nm, 334nm) Pico-Amperemeter cathodes cathode carrier. na 3 V picoammeter neutral density filters lens interferential filter Hg lamp Power energy meter Neutral density filters Optical components ( lens, mirror, pin-holes) transport box anode viewport pin-hole power head n W power meter condenser pin-hole /8

12 CW QE measurements: Results DESY on March 3 6 Data have been fitted to evaluate: the 6nm and Eg+Ea Cathode Dep. data QE@54nm (LASA) Operation lifetimes QE@54nm (DESY) QE@6nm (DESY) Eg+Ea (ev) Mar-5 7.9% 86.64%.79% Mar-5 9.% 66.44%.33% Sep-4 7.% 6.%.5% 4.57 Cathode 73. Cathode 7. Cathode 3. Cathode Eg+Ea =4.654eV - QE (@6 nm) =.7873% - Slope =.47 Experimental Fit QE (@6 nm) =.79% Cathode 7. - Eg+Ea =4.68eV - QE (@6 nm) =.3676% - Slope =.4 - QE (@6 nm) =.33% Cathode 3. - Eg+Ea =4.57eV - QE (@6 nm) =.5369% - Slope =.6575 Experimental Fit - QE (@6 nm) =.5% QE [%] - QE [%] QE [%] Experimental Fit photon energy [ev] photon energy [ev] / photon energy [ev]

13 CW QE measurements: Data Analysis CW data analysis Fitting of the spectral response [ h ( )] m QE = A ν E G + E A where A is a constant, E G and E A are energy gap and electron affinity. Cathode Eg+Ea =4.654eV - QE (@6 nm) =.7873% - Slope =.47 Experimental Fit QE (@6 nm) =.79% An example is given for the analysis of the CW QE data for cathode 73.. In this case: QE [%] - - Eg+Ea = 4.65 ev m =.4-3 Cathode # photon energy [ev] 3/8 (for a fresh Cs Te cathode we tipically have Eg+Ea = 3.5eV)

14 Pulsed QE measurements: laser energy calibration experimental set-up The laser energy transmission (from the laser hut to the tunnel) has been evaluated for different iris diameters (3.5mm,.mm and.6mm) and different energies. The laser energy has been measured using a Pyroelectric gauge (Joulemeter), varying the laser energy using the variable attenuator (λ/ wave plate + polarizer). λ/ plate Joulemeter polarizer movable mirror mirror 4/8

15 Pulsed QE measurements: laser beamline transmission analysis The QE measurement procedure uses the laser energy measured on the laser table Transmission to the vacuum window is regularly measured Transmission of the vacuum window (9 %) and reflectivity of the vacuum laser mirror (9 %) are accounted for Energy [μj] O:\TESLA\TTF\QE\3-3-6\6-3-3T493-QE.dat.5 Laser Room Attenuator Step O:\TESLA\TTF\QE\energy calibration\cal-pyro-336_65iris.txt Mean trasnmission:7.78 [%]above4steps.5.8 Tunnel Energy [μj] iris = 3.5 mm as an example: sin fit sin fit Attenuator Step Laser room / tunnel sin (Laser room) / sin (tunnel) Mean trasnmission:4.868 [%]above4steps Attenuator Step iris = Attenuator Step 5/8 Laser energy is measured as a function of the variable attenuator setting fitted by sin to evaluate the transmission Iris = 3.5 mm Iris =. mm Iris =.6 mm (σ)

16 Pulsed QE measurements: laser beam line transmission measurements The laser beamline transmission has been evaluated four times (from March to August 6) to take care of changes in the optical transmission path. Iris Φ (mm) Iris (step) Date (tunnel file) Date (laser room file) Transmission Used Mar-6 -Mar-6 3. % -Mar-6 -Mar not done not done % Mar-6 3-Mar-6 7. % Mar-6 3-Mar % Mar-6 3-Mar-6.85% not done not done June-6 6-June %.6 88 not done not done - From March to 3 March From 3 March to 6 June From 6 June to 7 Aug not done not done - From Aug-6 7-Aug % August till now.6 88 not done not done - 6/8

17 Pulsed QE measurements: measurement analysis The QE measurement is done following this procedure:. Measurement of the charge (toroid T, Q[C]).Measurement of the laser energy (laser hut) E 3.Calculation of the energy on the cathode E cath [J] using transmission (considering the losses due to the vacuum window and mirror) 3 [ ] QE % = nel nph = [ ] Eph[ ev] E [ J] Q C cath 6 nm: QE(%).5*Q(nC)/E(µJ) QE =3.% O:\TESLA\TTF\QE\4-3-6\6-3-4T88-QE.dat.5 The QE value is then obtained fitting the charge low charge to be sure not to be affected by the space charge Charge (nc).5 space charge effect.5 The relative and systematic error are in the order of %. 7/ Laser Energy (uj) The systematic error is mainly due to the uncertainty of identifying the linear part for the fit and due to the transmission measurement uncertainty

18 Pulsed QE measurements: cathode lifetime QE of cathodes are measured frequently within months. Example: cathode 7. and 73.. We define the end of lifetime when the QE reaches.5 % The CW QE of cathode 73. is compared with the pulsed QE measured the same day. The difference may be explained considering the increase of the charge due to the field enhancement. All cathodes show a drop of the QE over time, with different characteristics. QE (%) QE (%) Nov-5 4-Dec-5 3-Jan End of lifetime QE <.5% date End of lifetime QE <.5% 3-Mar-6 9-Mar-6 5-Mar-6 3-Mar-6 6-Apr-6 8/8 date 7. mean 7. CW_7. -Feb-6 4-Mar-6 3-Apr mean 73. CW_73.

19 Pulsed QE measurements: drop of QE with time We can relate the drop of QE with the vacuum condition in the RF gun. As an example, early 6, the RF gun has been operated with 3 μs long RF pulses. Up to this, the pulse length was restricted to 7 μs. During the long pulse operation period, the pressure increased from 5 7 mbar to mbar. This coincides with the drop of QE of cathode 73.. QE (%) Mar-6 9-Mar-6 5-Mar-6 3-Mar-6 6-Apr-6 date 9/8 End of lifetime QE <.5% 73. mean 73. CW_73.

20 Pulsed QE measurements: cathode 78. Referring to cathode 78., several measurements have been done during about 3 months (period: April, 9 to July, ). Also this cathode shows a drop of the QE vs. time. 35 long pulse operation (increase of vacuum) different growth of the cathode during deposition damaging due to dark current coming from ACC CW_78. mean just after the deposition QE (%) during operation 3-Apr-6 3-Apr-6 3-May-6 -Jun-6 -Jun-6 -Jul-6 date /8

21 QE (%) Comparison between: Pulsed QE and CW QE measurements 7. mean 7. CW_7. The pulsed QE measurements of cathode 7. and 73. have been compared with the CW QE λ = 6nm, evaluated from the spectral response mean 73. CW_73. 4-Nov-5 4-Dec-5 3-Jan-6 -Feb-6 4-Mar-6 3-Apr-6 date CW 6nm The CW QE respect to the pulsed QE value is lower: QE (%) Mar-6 9-Mar-6 5-Mar-6 3-Mar-6 6-Apr-6 date this can be due to the high accelerating field on the cathode in pulsed QE measurements. /8

22 Pulsed QE measurements: QE vs. phase laser/rf gun Measurements have been performed on two cathodes varying the laser/rf gun phase. For cathodes 7. and 78., the measured 7 deg is higher respect to the one 38 deg cathode #7. cathode # QE (%) iris = 3.5mm iris = mm mean QE (a.u.) RF phase 9 iris = 3.5mm iris = mm mean RF phase /8

23 Pulsed QE measurements: analysis () RF data analysis QE enhancement given acc. gradient E acc and phase φ with a given laser energy without space charge QE = A hν ( E + E ) G A + q e q e β Eacc sin 4 π ε ( φ) m where E acc is the accelerating field, φ is the phase RF/laser, β is geometric enhancing factor Using the values calculated before for A, E G +E A and m, the geometric enhancing factor results: β= QE [%].6.4. with E acc = 4.9 MV/m and the phase φ = 38 from the experimental measurement..8 3/8 cathode Eacc [MV/m]

24 RF data analysis Laser spot profile influence.5 Pulsed QE measurements: analysis () given E acc and φ, at different laser energies Space charge forces have to be taken into account and depends on the laser transverse profile. Square profile Gaussian profile.5.5 Laser Beam Transverse Profile Laser spot radius Laser spot radius Charge.5 q c E acc_gun φ π, π r 8 beam nc Extracted Charge vs. Laser Energy Charge.5 q c E acc_gun φ π, π r 8 beam nc Laser Energy Laser Energy 4/8

25 Pulsed QE measurements: Comments to the analysis The influence of the laser spot profile mainly affects the shape of the charge vs. laser energy curves. With this simple model, we can explain the shape of the curve and some of the asymptotic values. It would be very helpful to have CW QE and pulsed QE measurements in the same day (QE constant) to further study the model. 3 QE :3.3[%]; spot diameter:3.8586[mm]; laser sigma:.99[mm] Example for cathode Laser spot/iris diameter = 3.5mm. Extrapolated spot size = 3.8mm. QE from the linear fit = 3.% QE from this analysis = 3.3% Charge [nc].5 Experimental Fit Laser energy [μj] 5/8

26 Pulsed QE measurements: QE map () QE maps by scanning a small laser spot over the cathode tiny iris =.6mm (σ), step size.3 mm. Map of the charge emitted from the cathode moving the iris only. cathode 73. Map of charge emitted from the cathode moving iris and mirror together. The photoemissive layer is 5 mm in diameter. Well reproduced, center position: (-.,-.) mm. 5mm diameter of the photoemissive layer 6/8

27 Pulsed QE measurements: QE map () QE maps cathode 77., used to center the laser beam on the cathode. QE maps before alignment QE map of cathode 77. after centering of the laser beam 7/8

28 Conclusion CW QE measurements: Experimental set-up in the tunnel The CW QE of 3 cathodes has been FLASH Pulsed QE measurements: Laser beamline transmission calibration QE vs. time and vs. RF phases Analysis of the pulsed QE measurements: QE maps E acc, RF phase, etc. Tool to check the centering between the laser spot and the photoemissive film For the future On-line measurements of the laser beamline transmission 8/8