Preliminary Results of PHIN Run Summer 2014
Christoph Hessler, Eric Chevallay, Steffen Doebert, Valentin Fedosseev, Irene Martini, Mikhail Martyanov
CLIC/CTF3 experimental verification meeting11.11.2014
C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 2
Initial Situation: Results 2012
Very good lifetime measurements of Cs3Sb cathodes in 2012:
11.11.2014
2.3 nC, 350 ns, l=524 nm
1/e lifetime 168 h
… even for operation with 2.3 nC and 1.2 µs:
2.3 nC, 1200 ns, l=524 nm
1/e lifetime 135 h
C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 3
Initial Situation: Results 2011/2012
Compared with previous measurements under worse vacuum conditions lifetime could be drastically improved for Cs2Te:
… and for Cs3Sb:
11.11.2014
C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 4
Initial Situation: Results 2013
Therefore vacuum conditions were further improved in 2013 by additng an additional NEG pump:
Dynamic vacuum level: 7e-10 mbar → < 2e-10 mbar Unfortunately, due to many problems (“unknown” beam instrumentation,
low initial QE, fast QE decrease, QE jumps, 24h drifts, etc.) no comparable lifetime measurement could be performed:
11.11.2014
C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 5
Plans for 2014
Lifetime studies with Cs2Te cathode under improved vacuum conditions
Long-term measurement with Cs2Te under nominal operating conditions (2.3 nC, 1.2 µs)
Lifetime studies with Cs3Sb cathodes and green laser light under improved vacuum conditions.
RF lifetime measurements of Cs3Sb cathodes
Dark current studies Emittance measurement with low intensity beam to investigate PHIN’s
suitability for AWAKE QE measurement of copper cathode for defining QE requirements for
AWAKE
11.11.2014
C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 6
Layout of PHIN
11.11.2014
FCT: Fast current transformerVM: Vacuum mirror SM: Steering magnet BPM: Beam position monitorMSM: Multi-slit Mask OTR: Optical transition radiation screenMTV: Gated cameras SD: Segmented dump FC: Faraday cup
C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 711.11.2014
Changes in the Setup
Faraday cup (FC) has been reconnected to vacuum system (no vacuum window) and additional ion pump was installed.
Old fast current transformer (FCT) has been reinstalled. A second new FCT has been installed in front of FC. Both BTV screens (CT.MTV0265 and CT.MTV0305) were equipped with
notch filters for 532 nm to avoid detection of reflected green light. Virtual cathode camera has been replaced. New motorized mirror has been installed with a better repeatability. But also a new problem occurred: During bake-out the vacuum window
in spectrometer line broke.→ Beam line was vented at high temperature. Gun was not baked and
stayed under vacuum.→ Window has been replaced by blind flange.→ Segmented dump was not usable.→ Baking was redone.
C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 8
Photocathodes Used during PHIN Run
11.11.2014
Number Material Age QE in DC gun
QE in PHIN
#198 Cs2Te New cathode, produced 05.03.2014
14.8% after production
~10%
#199 Cs3Sb New cathode, produced 27.5.2014
5.2% after production
4.9%
#200 Cs3Sb New cathode, produced 7.8.2014
5.5% after production
3.9%
6A56 Cu Copper plug (Diamond powder polished) used for RF conditioning
N/A 3e-4
This year the initial QE of Cs2Te and Cs3Sb cathodes in PHIN was in reasonable agreement with the measurements in the DC gun.
QE of Cu cathode was too high compared with best literature values (1.4e-4) . Maybe contaminated with Cs.
Test for AWAKE
C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 9
Improvement of Vacuum in PHIN
11.11.2014
1/e lifetime 185 h
1 nC, 800 ns, l=524 nm, Cs3Sb
1/e lifetime 26 h
1 nC, 800 ns, l=262 nm, Cs3Sb
7e-10 mbar
1.3e-10 mbar
Dynamic vacuum level:4e-9 mbar
Static vacuum level:2.2e-10 mbar
March 2012March 2011
Activation of NEG chamber
around gun
Installation of additional
NEG pump
<2e-10 mbar
2.4e-11 mbar
July 2013
1/e lifetime n/adue to differentproblems duringPHIN run
C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 10
Lifetime Measurement with Cs2Te Cathode
Under improved vacuum conditions:
Double exponential fit represents well the data Lifetime improved from 250 h to 300 h Cs2Te is not ultra-sensitive against non-optimal vacuum conditions
11.11.2014
Dynamic pressure: 3e-10 mbar 1.5e-9 mbar
2014 2011
2.3 nC, 350 ns2.3 nC, 350 ns
C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 11
Lifetime Measurement with Cs2Te Cathode
Under nominal operation conditions (2.3 nC, 1.2 µs)
Strong pressure increase. Heating of Faraday cup? 1/e lifetime still 55 h
11.11.2014
C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 12
Lifetime Measurement with Cs3Sb Cathodes
Under improved vacuum conditions
Data can be partially fitted with a double exponential curve, with similar lifetime as 2012, however, measurement time is too short for reliable fit.
Klystron trip and phase jump changed slope drastically.
11.11.2014
Dynamic pressure: 2.5 - 5e-10 mbar ~9e-10 mbar
2.3 nC, 350 ns, Cs3Sb
1/e lifetime 168 h
2.3 nC, 350 ns, Cs3Sb
2014
2012
C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 13
Lifetime Measurement with Cs3Sb Cathodes
Under improved vacuum conditions:
Despite better vacuum level the lifetime is significantly shorter. Strong QE decrease started after a phase jump.
11.11.2014
Dynamic pressure: 2.3e-10 mbar
1/e lifetime 185 h
7e-10 mbar
1 nC, 800 ns, Cs3Sb 1 nC, 800 ns, Cs3Sb
2014 2012
C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 14
RF Lifetime of Cs3Sb Cathodes
11.11.2014
Fresh cathodeCathode #200 (Cs3Sb)
Used cathodeCathode #199 (Cs3Sb)
Fast and slow decay as seen with beam? In both cases longer lifetimes as with beam.
C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 15
Possible Explanations
RGA spectrum:
CO2 is known to have a negative effect on K2CsSb [1]
Source of CO2? Bake-out accident?
11.11.2014
2011 2014p(total) 2.70E-10 1.20E-10p(H2) 2.46E-10 7.33E-11p(H2O) 1.23E-12 1.53E-11p(CO/N2) 2.09E-11 1.87E-11p(CO2) 1.55E-12 1.27E-11
Rough estimate of partial pressure of residual gas components:
[1] A. Di Bona et al., Nucl. Instr. Meth. In Phys. Res. A 385 (1997) 385-390
C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 16
Possible Explanations
Could be breakdowns be a reason for the shorter lifetime?
In 2014 not more breakdowns than in 2012 Periods with fast QE decrease had only few breakdowns in 2014 Maybe more large breakdowns in 2014
11.11.2014
C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 17
Possible Explanations
Could the phase jumps be a reason for the shorter lifetime?
Phase jump of ~220 deg, once per day in average Phase jumps can clearly change the slope of the curve. … but not every phase jump does it. Klystron phase was stable, phase jump must have occured either in
the LLRF for the laser or the laser / synchronisation electronics. Further tests planned.
11.11.2014
2.3 nC, 350 ns, Cs3Sb
2014
1 nC, 800 ns, Cs3Sb
2014
C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 18
New, more precise dark current measurements have been performed:
Field emission contribution from gun cavity (Cu) and cathode. The low dark current measured with copper confirmed that the major
contribution is coming for the cathode. Cs3Sb cathodes (F~2 eV) produce higher dark current than Cs2Te (F~3.5
eV).
Dark Current Measurements
11.11.2014
C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 19
Emittance Measurements for AWAKE
Laser beam size: ~ 1 mm sigma, charge 0.2, 0.7, 1.0 nC, energy 5.5 – 6 MeV
11.11.2014
50 100 150 200 250
50
100
150
200
250
300
350
-4 -2 0 2 4 6-1.6
-1.55
-1.5
-1.45
-1.4
-1.35
-1.3x 10
5
Normalized emittance for 0.2 nC: 3.2 mm mrad ( big errors !)
0 0.2 0.4 0.6 0.8 1 1.20
1
2
3
4
5
6
Charge (nC)
Em
itta
nce
no
rm (
mm
mra
d)
Charge dependence is roughly sqrt as it should be
En(0.2 nC): 3.2 mm mradEn(0.7 nC): 4.6 mm mradEn(1 nC): 5.5 mm mrad
C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 20
Outlook: Plans for Laser
11.11.2014
Installation of new 500 MHz fiber front end with CLIC specs for PHIN and total decoupling from CALIFES laser system.
Delivery promised for end of this year, however there are already large delays. Stability studies with new front end with improved stability. Studies of heat-load effects and thermal lensing in laser rods at 50 Hz rep rate. In parallel further studies on new harmonics generation schemes with multiple
crystals to solve problem with UV generation for 140 µs long trains.
C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 21
Outlook: Plans for Photoemission Lab
11.11.2014
Surface analysis of photocathode materials with XPS and their impact on the cathode performance in collaboration with TE/VSC. New UHV transfer arm was built
Problems with the XPS apparatus, but we hope that they are solved now. First measurement with Cu plug successfully performed yesterday. Possible collaboration with Daresbury Lab in the field of photocathode surface analysis.
Modification of cathode preparation system for three components cathodes. Continue high-charge studies (with different cathode layer thicknesses). Plans for upgrade of cathode preparation system with load-lock system is on
hold as long as the longer term future of photocathode research at CERN is not clear.
C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 22
Outlook: Plans and Ideas for PHIN
11.11.2014
Original plan: Studies with 5 µs long pulse trains (max klystron pulse length) and 5 Hz repetition rate.→ Be window needed→ Long and probably painful RF conditioning required
However, due to new laser front end the bunch spacing will be 3 times longer. In terms of charge a 5 µs pulse @ 500 MHz corresponds to a 1.6 µs pulse @ 1.5 GHz. This is only slightly more than the present nominal intensity and it is doubtful that we can learn much of this measurement.→ not foreseen anymore, unless there are other strong arguments
Study of impact of the longer bunch spacing on the photocathode lifetime→ Measurements with 1.05 µs (=3*350 ns) and 2.3 nC with Cs3Sb and Cs2Te
Impact of longer bunch spacing on beam dynamics? Study of the charge stability with the new laser system. Study of performance of three components cathodes in PHIN (e.g. K2CsSb)
Study the effect of surface roughness on cathode lifetime (Electro-polished cathode plugs)
Many interesting ideas for a further PHIN run in 2015!
C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 23
Prerequisites for next PHIN Run
Successful installation of new laser front end. Synchronisation of the 500 MHz oscillator to the 3 GHz
gun (Preserving the synchronisation for CALIFES). PHIN laser amps must be available and working: Pumping
diode stack burned recently. Investigations on-going at manufacturer. Might be due to corrosion. If this is the case there is a high probability that other diode stacks will have the same problem at some point.
Replacement of broken vacuum window in spectrometer line.
No further modification of vacuum system (Be window) needed.
11.11.2014
C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 24
Acknowledgement
Controls: Mark Butcher, Mathieu Donze, Alessandro Masi, Christophe Mitifiot
Beam instrumentation: Thibaut Lefevre, Stephane Burger Vacuum: Berthold Jenninger, Esa Paju RF: Stephane Curt, Luca Timeo Wilfrid Farabolini CTF3 operators XPS studies: Holger Neupert, Elise Usureau Collaborators at LAL and IAP-RAS … and many others
… and thank you for your attention!
11.11.2014