1LAPPD Team meeting 6/10/2010
Ossy Siegmund, Experimental Astrophysics Group,
Space Sciences Laboratory,
U. California at Berkeley
Bialkali Photocathode Development
GALEX M31
2LAPPD Team meeting 6/10/2010
LAPD Photocathode MilestonesLAPD Photocathode Milestones
• Systematic characterization of Photo-electron Emission (PE) properties of materials for photocathode development.
• Demonstration of an operational 8”-square photo-cathode with a viable path to QE ≥ 15% for wavelengths between 300 and 450 nm.
Associated Milestones - Dependencies:-
• Demonstration of the window-to-body seal solution.
• Design and costing of the vacuum-transfer/assembly facility for the 8”-square MCP module.
3LAPPD Team meeting 6/10/2010
Cathode Bandpass and WindowsCathode Bandpass and Windows
Acceptable cutoff range
Typical window transmission curvesNominal Cherenkov emissionspectrum compared with bialkali
4LAPPD Team meeting 6/10/2010
100 200 300 400 500 600 700 800
0,1
1
10
100
Na-K-Sb:Cs (S-20)
Wavelength [nm]
Cs3Sb (S-11)
Na2KSb
Rb2CsSb
K2Cs-Sb6*109Ohm/square
2*106Ohm/square
Bi-Alkali Cathode Characteristics
QE and resistivity for various bialkaliswe will use K2CsSb & Na2KSb
Lyashenko, Chicago 7/09
Cathode Noise vs Temp
5LAPPD Team meeting 6/10/2010
Photocathode Configuration Photocathode Configuration
Numerous processes affect the QE
Bialkali is a few 100Å thick,and is compatiblewith depositionas semitransparenton the window,but is very difficultto achieve as anopaque cathode on the MCPsurface.
6LAPPD Team meeting 6/10/2010
Work Flow Program for Bialkali Cathode Development
7LAPPD Team meeting 6/10/2010
Bialkali Photocathode QEBialkali Photocathode QE
Examples of SSL bialkali photocathode depositions with different wavelength optimizations (on fiber optics). PeakQEs 15% to 20% using Na2KSb.
0
0.2
0.4
0.6
0.8
1
1.2
350 400 450 500 550 600 650
AST (t)AST (d)STL02 (d)Cherenkov
Wavelength (nm)High efficiency Bialkali cathodes at Photonis. Clear room for improvementover standard bialkalis. Need to Establish enhancement techniques.
Schyns, Clermont 2010
8LAPPD Team meeting 6/10/2010
B33/B270 General Parameters
B33 Composition
Refractive index
@400nm
B33 1.47
Air ~1.0
Water ~1.32
The cathode substrate, window or window coating, affects the photocathode performance. Quartz, fiber optics, 7056 glass are common. Borofloat B33 Borosilicate is not, and also has Tin diffused into one side from the float process, so we we need to test this. B270 is a soda-lime Crown glass and should be OK.
Standard AR coating is bad for LAPD - most likely don’t needAR coating for water/B33 interface
9LAPPD Team meeting 6/10/2010
Compatibility and Transmittance of Windows
B33 Transmittance is typicalfor borosilicate glasses
B270 Transmittance isonly slightly worse than B33
Both materials have similar mechanical properties, strength,hardness, etc, BUT the thermal coeff’ts are not, 0.003”/100Cfor B33 and 0.008”/100C for B270 - not clear that this is an issue.
3.25 x 10-6/K
8.8 x 10-6/K
Thermal
Thermal
10LAPPD Team meeting 6/10/2010
Conductive Layer Properties for Windows
Braem, NIMA 2003
ITO is a possibility for the conductive underlayer,BUT - it might affect the cathode QE! So we can alsotry metal conductor stripes as we did for GALEX.
11LAPPD Team meeting 6/10/2010
Small Tube and Cathode Process Tank System
Small window cathode development, 33mm samples- Process samples to optimize QE and bandpass
-Na2KSb, K2CsSb cathodes- Use several substrate materials, SiO2, verify B33/B270- Test precursor/ITO/conductor underlayer for cathodes
Large size window cathode study, 8” windows.- Study source alkali design for large cathodes- Develop techniques to make larger area uniform QE- Optimize cathode QE levels- Test metal/ITO conductor underlayer for cathodes- Test metalization and sealing techniques
12LAPPD Team meeting 6/10/2010
33mm Cathode Test Samples
We have cut up one B33 and one B270 window to make 30ea 1.3”test samples each, also have 18ea fused silica as control samples.
Inconel annular electrodes were evaporated just as they would be for In seals
13LAPPD Team meeting 6/10/2010 13
Tube Lab, 33mm sample test/process station.
Small tank used to process alkali cathodes (33mm) and tubes of small area. Can take 4 samples/run. 1st run done, making mods for 2nd run.- Small sample test runs- Substrate material tests
14LAPPD Team meeting 6/10/2010
0
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350 400 450 500 550 600 650 700
Quartz, NaKSbB33-1, NaKSbB33-2, NaKSbB270, KCsSb
Wavelength (nm)
Initial Bialkali Trials on 33mm Sample Substrates
Trying to set up process parameters for comparative tests and optimizations.
Used quartz, B33 and B270 sequential depositions in one pumpdown.
Short process / thin cathode-ended up too blue / low QE.
-Had problems with K channels that precluded optimal process, mostly on
later depositions.
Initial cathode trials,
Process order
Plan for next run-Fix alkali dispensers, retool window holders-shoot all substrates in one go at same time-Add extra cycles to push peak redder-higher QE
15LAPPD Team meeting 6/10/2010
Cathode Process Test Tank Systemreplacement for 33mm chamber
Larger 16” flange tank, can take full size windows (8.7”) to test-Quantum efficiency-QE Uniformity-Seal tests
16LAPPD Team meeting 6/10/2010
Evaporator design modifications to Evaporator design modifications to accommodate 8.7” windowsaccommodate 8.7” windows
Design done for NiCr/Cu evaporation tooling for 8.7” windows
Currently OK to 5” Rotation stage for 8.7” windows
17LAPPD Team meeting 6/10/2010
Window Seal Development and Window Seal Development and Cathode Test “Diodes”Cathode Test “Diodes”
3” window test article on metal frame with Indium seal
Will metalize 8.7”window and produce an Will metalize 8.7”window and produce an Indium seal on a frame to test leak Indium seal on a frame to test leak
tightness of the seal.tightness of the seal.
3” ceramic body with strip anode, and metal frame with Indium seal
18LAPPD Team meeting 6/10/2010
8” Tube and Cathode Process Tank System
UHV 18” processChamber:-Initial use for cathode and seal development.Eventually use for complete tubeprocess task.
B33/B2708.66” windows- large area cathodes- uniformity tests- QE optimization
19LAPPD Team meeting 6/10/2010
8” Tube and Cathode Process Tank System
Large window cathode development, 8.66” square
- Implement alkali source design for large cathode areas- Develop wet cleaning and plasma cleaning processes- Establish metalization scheme- Test metal/ITO conductor underlayer for 8” cathodes- Implement AR coatings/treatment on both surfaces- Develop techniques to make 8” area uniform QE- Optimize cathode QE levels- Trial seals on 8.66” “frames”
- Then
- Make LAPD 8” tubes