Ultra High Vacuum Transport System for Hi h Q Effi i High uantum Efficiency Photocathodes ·...
Transcript of Ultra High Vacuum Transport System for Hi h Q Effi i High uantum Efficiency Photocathodes ·...
Ultra High Vacuum
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gTransport System for
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11 High Quantum Efficiency Photocathodes
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D. Sertore, P. Michelato, L. Monaco
Photocathodes
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SAES GettersSAES Getters
High QE Photocathodes in RF Gunsg• High QE photocathodes are the laser stimulated emitters in high brightness electron sources.
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d h h d h b h
Courtesy K. Floettmann (DESY)
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Requirements for photocathode operation in RF Gunsoperation in RF Guns
• The use in RF Gun requires also
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– QE uniformity
– Low dark current
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– Stable operation along the train
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– Fast response time
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• The photocathode sensitivity to gas exposition requires UHV conditions.
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Multialkali Photocathodes• Antimonied (Cs3Sb, KCsSb, (Cs)NaKSb, etc)
– High QE with visible light
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BUT …. very sensitive to gas pollution
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gas pollution
• Telluride (Cs2Te, KCsTe, K2Te)
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– High QE, sensitive only to UV light but more robust.
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The INFN – DESY Cathode Systemy• In 1998, a split INFN – DESY preparation‐transfer system was
designed and built. The preparation chamber in Milano and th t f t th t DESY H b
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the transfer to the gun at DESY Hamburg.
Transport Box
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PreparationChamber
RF Gun @ FLASH linac
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Transfer Chamber
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The Photocathode Databasehttp://wwwlasa.mi.infn.it/ttfcathodes/
The database tracks photocathode performances in the different transport boxes and in the different labs (FLASH, PITZ and LASA).
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We have produced up to now 111 Cs2Te and 2 KCsTe photocathodes.
Operative lifetime now larger than 90 daysPh h d Hi
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11 Photocathode HistoryProduced Cathodes
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Operation Lifetime
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INFN Photocathode SystemsyPreparation Systems Transfer System
INFN Milano – LASA DESY ‐ HH
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DESY – HH DESY ‐ PITZ
Fermilab – Lab7 Fermilab –NML
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11 Fermilab‐A0 LBNL
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Transport Systemp y• Cathodes are transported under UHV condition from INFN Milano to the Transfer Systems.
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• A carriage holding up to five cathodes is loaded in the transport system.
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• A battery powered ion getter pump (60 l s‐1) keeps the necessary UHV conditions during transport.
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Transport boxCathode Loading
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Limit of the present Transport SystemTransport System
• Since the vacuum level is guaranteed by the ion
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g ypump, any power failure could damage the transported cathodes.
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11 • The system is heavy due to the ion pump and its power supply.
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• Given the high voltage necessary to power the ion getter pump, the system cannot be transport by
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What What is is a getter a getter ??ggA substance that removesremoves molecules from the gas phase by a
chemical reaction on its active surface
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chemical reaction on its active surface
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G d d b ll i i l iGetters are produced by alloying reactive metals in vacuum
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Non Evaporable Getter (NEG) pump General FeaturesGeneral Features
• No evaporation required (it is different from TSP);
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No evaporation required (it is different from TSP);
• NEG must be heated under vacuum (“ACTIVATION”) (St172 500°C 1h )
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• After activation, the NEG removes gases at room
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temperature without power (surface adsorption)
• The NEG pump can also be used hot (250‐300°C) to
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p p ( )assist the bake out process (surface+bulk adsorption)
• Many re‐activations possible (>100)• Many re‐activations possible (>100)
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Non Evaporable Getter (NEG) pump General Features contGeneral Features cont.
• High pumping speed for active gases (H2, H2O, O2, CO2 CO N )
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CO, N2)
• Constant speed in UHV‐XHV
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11 • High capacity in a very compact size
• Vibration‐free, light weight, very clean
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• Extremely small power consumption during activation (e.g. 1hr@ ~ 35W for Capacitorr D 100)
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• No interference with magnetic/electric fields
• No pressure limitation (10‐14 torr Benvenuti et al )No pressure limitation (10 torr, Benvenuti et al.)
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Capacitorr D‐100• Large pumping speed in a
compact design
Pumping
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speed (l/s)
H2 100H2O 80
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• Lightweight: only ~ 300 g
2
CO 60
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New pumping system designp p g y g• Varian Ion Getter Pump 20 ls‐1 (mounted horizontally on top)
• Triax Cold Cathode Gauge (lower range 10‐11 mbar)
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• SAES NEG Pump D100 (mounted bottom)
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Ion Triax
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Getter Pump
C.C.
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NEG Pumped19/5/2011 14
Test with ONLY 20 l/s ion getter pumpg p p
• Bakeout:– Temperature of the system
250
300
10-3
10-2
10-1
100
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Temperature of the system raised to 200 °C (cyan line)
– Temperature of the Ion Pump raised to 250 °C (green line)
150
200
perature [°C]
10-7
10-6
10-5
10-4
sure [m
bar]
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(green line)
– Pumping during bake out with a TMP (blue line)
– Overall bake out about 7 50
100 Temperature Master (°C) Temperature Chamber (°C) Temperature Ion Pump (°C) Pressure TMP (mbar)P P i ( b )
Temp
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10-11
10-10
10-9
10-8
Press
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1E‐6
days
• During cooldown, when at 200 °C, Ion Getter P it h d O
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Pressure Penning (mbar)
Time (Days)
10-13
10-12
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1E‐8
1E‐7
e P
enni
ng (m
bar)
Pump was switched On
• Final pressure high 10‐10 mbar Ion Getter Pump OFF
0 6 12 18 24 301E‐10
1E‐9
Pres
sure
Time (Hours)
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Test with D100 pumpp p
– Venting the system with N2 and then at air for pump installation– Bake out:
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Bake out:• Temperature of the chamber raised to 200 °C • Temperature of the Ion Pump raised to 250 °C • Pumping during bake out with a TMP
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Pumping during bake out with a TMP • Overall bake out about 4 days. Pressure at TMP during process at least one order of magnitude lower.
– During cooldown, when at 200 °C, Ion Getter Pump was
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switched On– At T=100 °C Ion pump Off and NEG Activation. Pumping with
TMP
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– After Activation Ion Pump On again– Final Pressure in low 10‐11 mbar
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Test with D100 pump
250
300
10-5
10-4
p pNEG Activation
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200
250
C]
10-6
10
r]
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150
mperature [°C
10-7
essure [m
bar
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100Tem
Temperature Master (°C) Temperature Chamber (°C)
10-9
10-8 Pre
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0 1 2 3 4 5 6 70
50 Temperature Ion Pump (°C) Pressure TMP (mbar) Pressure Penning (mbar)
10-10
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0 1 2 3 4 5 6 7
Time (Days)19/5/2011 17
D100 only long term stabilityy g y1E-7
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1E-8
mba
r) Ion Getter Pump Switched Off
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Penn
ing
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Pre
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P
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0 7 14 21 351E-12
Time (Days)
“Fast” bakeout
• Open transport system for loading cathodes
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• Bake out:– Temperature of the chamber raised to 200 °C
T t f th I P i d t 250 °C
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– Pumping during bake out with a TMP
– First NEG activation at 200 °C to help in the bakeout process
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– Second NEG activation at 120 °C
– Overall bakeout about 2 days
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“Fast” bakeout
250
300
10‐5
10‐4
NEG Activation
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200
250
)
10‐6
10
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150
perature (°C)
10‐7
ure (m
bar)
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100Temp
10‐8
Temperature Master (°C)
Press
Two days bakeout
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50
10
10‐9
Temperature Master ( C) Temperature Ion Pump (°C) Temperature Chamber (°C) Pressure Fore Vacuum (mbar)
0 14 28 42 56 700
Time (Hours)
10‐10
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Long term stability
10-8
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Spikes are due to carriage movements
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10-8
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(mba
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0 7 14 21 28 35 42 49 56 6310-11
0 7 14 21 28 35 42 49 56 63
Time (Days)19/5/2011 21
QE measurements
1.0E+2• QE response at diff l h
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1.0E+0
1.0E+1different wavelengths has been measured for five months
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1.0E‐2
1.0E‐1
E (%)
for five months without any variation
• No cathode material
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1.0E‐4
1.0E‐3QE
18‐Nov‐1023‐Nov‐1029‐Dec‐1020‐Jan‐1122‐Feb‐118‐Mar‐1123 M 11
No cathode material modification.
• Photoemissive
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1.0E‐5
2.0 3.0 4.0 5.0 6.0Ph E ( V)
23‐Mar‐115‐Apr‐11
Photoemissiveproperties perfectly maintained.
Photon Energy (eV)
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Conclusions and Outlook
• The new pumping configuration has proven to be very effective
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effective• The overall weight of the system has been reduce• The system is no more subject to power failure problems
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y j p p• In few weeks, the transport box will fly to Lawrence
Berkely National Laboratory in San Francisco
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• The low vacuum level reached is suitable for antimoniedphotocathodes too
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remove also the 20 l s‐1 ion getter pump
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