Preliminary Results of PHIN Run Summer 2014 Christoph Hessler, Eric Chevallay, Steffen Doebert, Valentin Fedosseev, Irene Martini, Mikhail Martyanov CLIC/CTF3.

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:

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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:

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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:

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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

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C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov 6

Layout of PHIN

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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

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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

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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

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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

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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.

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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.

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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

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RF Lifetime of Cs3Sb Cathodes

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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?

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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

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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.

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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

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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

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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

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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.

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Outlook: Plans for Photoemission Lab

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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.

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Outlook: Plans and Ideas for PHIN

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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.

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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!

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