Photocathode activity for. P. Michelato

Transcription

1 Photocathode activity for FLASH and PITZ P. Michelato INFN Milano LASA

2 Our Production at glance 5 Oct 6 49 Cs Te (37+1) KCsTe ( + ) 5 Mo (11+14)

3 Main Topics Preparation System Plug Preparation Photocathode Deposition Photocathode th Diagnostics After Use Analysis R&D

4 Preparation System Transport Box Preparation Chamber UHV Vacuum System (base pressure 1-1 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 Source holder and Masking Masking MaskingSystem

5 Mo Plug Preparation The Mo plugs, after proper machining to design specifications with either a lathing or a milling machine, are cleaned with a Buffered Chemical Polishing. The plugs are then polished to optical finishing with two different procedures: Manual Silicon Carbide based abrasive papers Different clothes with diamond suspensions spensions (3μm.1μm) Automatic Special paper ( diamond embedded disc ) with diamond suspension (6μm) Different clothes with diamond suspensions (3μm.1μm) The automatic procedure ensures planar photocathode surfaces. Sintered Arc-cast 7 Before and After Optical Polishing 8

6 Plug Roughness and Dark Current The main advantage of the optical finishing of the photocathodes is the reduced dark current (one order of magnitude). The two polishing techniques on plugs for Cs Te cathodes give similar reflectivity values. Rough Surface Optical Finished No significant reflectivity difference between Arc-cast and sintered Mo..736 μm 1 8 Mo 543 nm ArcCast R = % Sintered R = % Reflectivity [%]

7 Cs Te Photocathode Deposition Cs Te Mo Cs/Te b).6 c) 1. d) 1.9 e).5 Different stoichiometric compounds form during Cs deposition till the correct Cs/Te ratio is reached corresponding to the QE maximum. The Mo plug prevents diffusion of the film. Estimated photocathode typical thickness is -3 nm. A circular masking system shapes the photoemissive layer and center it on the plug. The actual masking diameter is 5 mm. Active Area A. di Bona et al., JAP8(1996)34-13 mm -1 mm - 5 mm 5 mm

8 Photocathode Diagnostics QE 54 nm We check routinely : QE and QE uniformity Color and color uniformity Spectral response (QE vs. Photon Energy) Contour 1, 1, 1 Spectral Response Normalized QE,8,6,4,, 4 X Axis (mm) Y axis (mm) 1 QE [%] TTF&PITZ Laser λ =6 nm.1.1 Experimental Data Photon Enegy [ev]

9 MultiWavelengths QE maps 39 nm 54 nm 97 nm QE map: nm of cathode #73.1 (4-3-4), +/-4mm, step.5mm QE map:73-1i(54nm) of cathode #73.1 (3-3-4), +/-4mm, step.5mm QE map: nm of cathode #73.1 (4-3-4), +/-5mm, step.5mm The MultiWavelength Maps allow to check the cathode uniformity. Moreover since: QE (E ph -(E g +E a )) m nm QE map: nm of cathode #73.1 (3-3-4), +/-5mm, step.5mm where E ph is the photon energy, E g the energy gap and E a the electron affinity, we might calculate the E g +E a of the photocathode. This parameter influences the emission properties, the darkcurrent and the thermal emittance nm QE map: nm of cathode #73.1 (3-3-4), +/-5mm, step.5mm

10 E g +E a map Molybdenum E g +E a Cs Te The cathode show a clear region of low E g+e a in correspondence with the photoemissive film. Irregularities in the map can be correlated to region of unexpected behaviour of the cathode

11 Color Evolution during preparation We are interested to film colors and their uniformities a) h) since they carry information on film optical properties and thickness. Pictures show the evolution of the film during the different phases of the photocathode growing process. h) e) After 1 min of Te deposition (end of Te deposition) From further analysis, we expect information on the optical properties and thickness of the photoemissive film. After 45 min of Cs deposition After 65 min of Cs deposition (cathode finished)

12 Color Uniformity Investigation A Puzzle

13 QE LASA DC QE LASA before and after use at FLASH or PITZ.

14 QE FLASH/PITZ not recently updated d Pulsed QE FLASH/PITZ.

15 43. QE Statistics 14 1 QE = 9.19%+ 3.4% Photocathode QE distribution for the samples sent to FLASH and PITZ Samples QE [%] Different cathodes produced on the same plug QE [%] 8 6 4

16 After Transportation Analysis Cathode 1. was produced in year. 1 8 It was delivered and stored in the transport box for 3 years without being used in any gun. After such long period, it was measured at LASA showing good uniformity in color and QE, and QE values similar to the ones after production QE map after production Optical image of the cathode after 3 years QE map after 3 years

17 Operation Effect on Plugs We have observed damages on the plug of used cathodes. The damages are located in the spring region of the plug. Sputtered material are clearly visible. Cathode #8.1 Cathode #47.3

18 RF spring contact Two different spring types have been used, in the same insert! The insert was designed for a WELDED Watch bend type spring. Watch bend tipe CuBe hard, silver coated spring Difficult to weld (becomes hard) Critical number of convolutions Cantend coil spring CuBe uncoated. Available coated with Ag, Au etc Welded by the manufacturer

19 Spring, a dark current source? Pictures taken at PITZ, PITZ in the same condition of gradient and focalization, of Mo (up, #56.) and CsTe (down, #54.) cathodes. The images g looks similar except p for the inner ring! g We observe: Three circular regions of different intensities Clear flares are visible #56. (Mo) The flares spirals from the center outwards due to the large energy spread of the emitted electrons traveling in the solenoid magnetic fields. The flares seem to originate from the outer and middle rings. The outer flares have been attributed, in the past, to dark current coming from the region between the plug and the gun body, where the RF contact spring is located. #54 (C #54. (CsTe) T )

20 Dark current investigation We developed a simple computational model to reproduce the dark current patterns at the screen position particles generated only on the plug border 5 azymuthal angles, in order to mimic the spring convolutions (~8 in the present design). Increasing the number of azymuthal positions results in a more defined outer ring image. the main features of the images are reproduced the plug border region, where the RF spring is located, could be responsible of the dark current emission. as previous case plus one hot spot of electrons to artificially enhanced the distribution. two bright spots are visible and a light blue flare crosses the whole image. Since many particles have been used for the hot spot generation, the underlining structure is fainter then before

21 R&D on Photocathodes Optical Properties Thermal Emittance Photoemission Model DC Gun Test Stand

22 Cs Te Optical Properties 1 Vacuum.8 ivity.6 Thin Film Cs Te Reflecti.4 R S Substrate Mo. S. Bettoni, Thesis R P Incidence Angle [ ] n k σ [nm] Thickness [nm] σ = roughness parameter From the depth profile we measured about 3 nm Mo Mo+Te Mo+Cs Te

23 a. u. Thermal Emittance by TOF Angle resolved photoemission spectra θ Raw Spectra V=.45V (sn),,4,6,8 1 1, energy (ev) Angle Integrated Electron velocity distribution resolved in angle and energy imation (mm mrad) The ermal Emittance Esti th harmonic (λ = 64 nm) ε th =.5 ±.1 mm mrad for 1 mm rms spot radius 5 4 th th harmonic 45 th th harmonic Bias Voltage (V) 5 th harmonic (λ = 11 nm) ε th = 7.7 ± 1.1 mm mrad for 1 mm rms spot radius

24 Vacuum Modello della fotoemissione di Spicer Photoemission Model This model is based on a simple implementation of Spicer s Three Step Model I Vacuum Vacuum This model, once developed and I R test, will be used to study CsTe spectra and estimate thermal emittance and key parameters Sample Sample Sample

25 DC Gun Test Stand Dark current test Field enhanced photoemission Transfer System HV Extractor

26 Conclusions The photocathodes produced so far have performed as requested. An R&D activity it is in progress to both investigate sources of dark current and reduce them. An improvement of the reproducibility is required.

27 Photocathode FLASH: Quantum Efficiency (QE) L. Monaco Work supported by the European Community (contract number RII3-CT-4-568)

28 Main Topics Overview Photocathode Production Production at LASA and transportation The Photocathode 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

29 Quantum Efficiency For the FLASH laser (λ = 6nm) QE(%).5*Q(nC)/E(µJ) The design asks for 7 nc/s 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

30 Photocathode Production: Preparation Chamber Photocathodes are LASA on Mo plugs under UHV condition. Transport Box Source holder Preparation LASA Masking UHV Vacuum System - base pressure 1-1 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

31 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, 1mm spot diameter) QE map:73-1i(54nm) of cathode #73.1 (3-3-4) 4), +/-4mm /4mm, step.5mm 8 1.E+ 1.E+1 1.E+ Spectral response 6 (Hg lamp, interferential filters, 5 4 3mm spot diameter) QE (%) 1.E-1 1.E- 1.E-3 Just after the deposition QE map:73-1i(54nm) of cathode #73.1 (3-3-4), +/-4mm, step.5mm E Photon Energy (ev) d=5mm

32 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 1998, 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

33 Cathode in the RF Gun Photocathode inserted into the gun backplane

34 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).

35 CW QE measurements: Experimental setup The experimental set-up for the CW QE measurements is mainly composed by: towards the RF gun A high pressure Hg lamp Interferential filters (39nm, 54nm, 97nm, 334nm) Picoammeter Power energy meter cathodes towards the exchange chamber cathode carrier 1. na 3 V picoammeter neutral density filters interferential filter lens Hg lamp Neutral density filters Optical components (1 lens, 1 mirror, pin-holes) transport box anode viewport pin-hole power head n W power meter condenser pin-hole

36 CW QE measurements: Results DESY on March 31 6 Data have been fitted to evaluate: the 6nm and Eg+Ea Cathode Dep. data QE@54nm Operation QE@54nm QE@6nm Eg+Ea (LASA) lifetimes (DESY) (DESY) (ev) Mar-5 7.9% %.79% Mar-5 9.% %.33% Sep-4 7.% 161.%.15% Cathode 73.1 Cathode 7.1 Cathode Cathode Eg+Ea =4.1654eV - QE (@6 nm) =.7873% - Slope = Experimental Fit QE (@6 nm) =.79% 1 Cathode Eg+Ea =4.168eV - QE (@6 nm) =.3676% - Slope = QE (@6 nm) =.33% Cathode 3. - Eg+Ea =4.157eV - QE (@6 nm) =.15369% - Slope = Experimental Fit 1-1 QE (@6 nm) =.15% QE [%] 1-1 QE [%] QE [%] Experimental Fit photon energy [ev] photon energy [ev] photon energy [ev]

37 CW QE measurements: Data Analysis CW data analysis Fitting of the spectral response = A [ h ν ( E E ) ] m QE E G + E A where A is a constant, E G and E A are energy gap and electron affinity. 1 1 Cathode Eg+Ea =4.1654eV - QE (@6 nm) =.7873% - Slope =1.417 QE [%] Experimental Fit QE (@6 nm) =.79% An example is given for the analysis of the CW QE data for cathode In this case: 1 - Eg+Ea = ev Cathode # photon energy [ev]

38 CW QE measurements: Data Analysis 1 1 QE (%) 1.1 Just after.1 the deposition Photon energy (ev) 1.E+ Powel (*) 1.E+1 QE (%) 1.E+ 1.E-1 1.E- 1.E-3 1.E-4 just after the deposition after 61h and 55min after 114h and 4min after 46 and min (*)R. A. Powell, W. E. Spicer, G. B. Fisher, P. Gregory, Photoemission studies of cesium telluride, PRB 8 (9), p (1973) A fresh cathode shows a Eg+Ea of about 3.5 ev. Several measurements of the spectral response have been performed to study the Eg+Ea increase vs. time. 1.E Photon Energy (ev)

39 Pulsed QE measurements: laser energy calibration experimental set-up λ/ plate polarizer movable mirror The laser energy transmission (from the laser hut to the tunnel) has been evaluated for different iris diameters (3.5mm,.mm and.16mm) and different energies. Joulemeter mirror The laser energy has been measured using a Pyroelectric gauge (Joulemeter), varying the laser energy using the variable attenuator (λ/ wave plate + polarizer).

40 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 i of the vacuum laser mirror (9 %) are accounted for iris = 3.5 mm as an example: O:\TESLA\TTF\QE\31-3-6\6-3-31T1493-QE.dat.5 Laser Room sin fit Energy [μj] Attenuator Step Laser room / tunnel Mean trasnmission: [%]above4steps.15 O:\TESLA\TTF\QE\energy calibration\cal-pyro-3136_1651iris.txt Mean trasnmission: [%]above4steps.5.18 Tunnel.4 sin fit.175 Energy [μj] Attenuator Step nel) sin (Laser room) / sin (tun Attenuator Step iris i = Attenuator Step 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 =.16 mm (σ)

41 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 (motor Date (tunnel Date (laser room Transmission Used steps) file) file) Mar-6 1-Mar % From 1 March Mar-6 1-Mar % to 31 March not done not done Mar-6 31-Mar % From 31 March Mar-6 31-Mar % to 6 June Mar-6 31-Mar-6.85% not done not done - From 6 June to June-6 6-June % 7 Aug not done not done not done not done - From 7 August Aug-6 7-Aug % till now not done not done -

42 Pulsed QE measurements: measurement analysis The QE measurement is done following this procedure: 1. Measurement of the charge (toroid T1, Q[C]).Measurement of the laser energy (laser hut) E 3.Calculation of the laser energy on the cathode E cath [J] using transmission i (considering i the losses due to the vacuum window and mirror) 3 [ ] QE % = nel nph 1 = [ ] Eph[ ev] E [ J] Q C cath 1 6 nm: QE(%).5*Q(nC)/E(µJ) QE =3.1% O:\TESLA\TTF\QE\14-3-6\6-3-14T 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 Charg ge (nc) space charge effect The relative and systematic errors are in the order of % 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

43 Pulsed QE measurements: cathode lifetime 6 QE of cathodes are measured frequently within months. Example: cathode 7.1 and We define the end of lifetime when the QE reaches.5 % The CW QE of cathode 73.1 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. E (%) Q %) QE ( End of lifetime QE <.5% 14-Nov-5 14-Dec-5 13-Jan date End of lifetime QE <.5% 7.1 mean 7.1 CW_7.1 1-Feb-6 14-Mar-6 13-Apr mean 73.1 CW_ Mar-6 19-Mar-6 5-Mar-6 31-Mar-6 6-Apr-6 date

44 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 on period, the pressure increased from mbar to 1 1 mbar. This coincides with the drop of QE of cathode QE (%) End of lifetime QE <.5% 73.1 mean 73.1 CW_ Mar-6 19-Mar-6 5-Mar-6 31-Mar-6 6-Apr-6 date

45 Pulsed QE measurements: cathode 78.1 Referring to cathode 78.1, several measurements have been done during about 3 months (period: April, 19 to July, 11). long pulse operation (increase of vacuum, ion-back Also this cathode bombardment??) shows a drop of the QE vs. time. 35 different growth of the cathode during deposition damaging due to dark current coming from ACC CW_78.1 mean just after the deposition QE (%) during operation 3-Apr-6 3-Apr-6 13-May-6 -Jun-6 -Jun-6 1-Jul-6 date

46 QE (%) Comparison between: Pulsed QE and CW QE measurements 7.1 mean 7.1 CW_ Nov-5 14-Dec-5 13-Jan-6 1-Feb-6 14-Mar-6 13-Apr-6 date CW 6nm The CW QE respect to the pulsed QE value is lower: QE (%) The pulsed QE measurements of cathode 7.1 and 73.1 have been compared with the CW QE λ = 6nm, evaluated from the spectral response mean 73.1 CW_ Mar-6 19-Mar-6 5-Mar-6 31-Mar-6 6-Apr-6 date this can be due to the high h accelerating field on the cathode in pulsed QE measurements.

47 Pulsed QE measurements: QE vs. phase laser/rf gun Measurements have been performed on two cathodes varying the laser/rf gun phase. For cathodes 7.1 and 78.1, the measured 7 deg is higher respect to the one 38 deg..65 cathode # cathode # QE (% %) iris = 3.5mm iris = mm mean QE (a.u.) RF phase iris = 3.5mm iris = mm mean RF phase

48 Pulsed QE measurements: analysis (1) 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 E acc sin φ 4 π ε where E acc is the accelerating field, φ is the phase RF/laser, β is geometric enhancing factor m Using the values calculated before for A, E G +E A and m, the geometric enhancing factor results: β= 1 with E acc = 4.9 MV/m and the phase φ = 38 from the experimental measurement. QE [%] cathode Eacc [MV/m]

49 1.5 Pulsed QE measurements: analysis () RF data analysis Laser spot profile influence 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 1.5 Gaussian profile 1.5 Laser Beam Transverse Profile Laser spot radius Laser spot radius Charge q c1 E acc_gun φ π, 18 π r beam nc 1 Extracted Charge vs. Laser Energy Charge q c1 E acc_gun φ π, 18 π r beam nc Laser Energy Laser Energy

50 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. Charge [nc] QE :3.3[%]; spot diameter:3.8586[mm]; laser sigma:.199[mm] 3 Example for cathode Experimental Fit Laser energy [μj] Laser spot/iris diameter = 3.5mm. Extrapolated spot size = 3.8mm. QE from the linear fit = 3.1% QE from this analysis = 3.3%

51 Pulsed QE measurements: QE map (1) QE maps by scanning a small laser spot over the cathode tiny iris i =.16mm (σ), step size 3.3 mm. Map of the charge emitted from the cathode moving the iris only. cathode 73.1 Map of charge emitted from the cathode moving iis iris and mirror together. th The photoemissive layer is 5 mm in diameter. Well reproduced, d center position: (-.,-.) mm. 5mm diameter of the photoemissive layer

52 Conclusion CW QE measurements: Experimental set-up in the FLASH tunnel has been installed CW QE has been FLASH Pulsed QE measurements: Laser beamline transmission calibration QE vs. time and vs. RF phases Analysis of the pulsed QE measurements: E acc, RF phase, etc. QE maps Tool to check the centering between the laser spot and the photoemissive film For the future On-line measurements of the laser beamline transmission to continuously follow the cathode lifetime (helpful to decide when to change it, etc.)