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

Initial Situation: Results 2012  Very good lifetime measurements of Cs 3 Sb cathodes in 2012: C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov2 2.3 nC, 350 ns,  =524 nm 1/e lifetime 168 h  … even for operation with 2.3 nC and 1.2 µs: 2.3 nC, 1200 ns,  =524 nm 1/e lifetime 135 h

Initial Situation: Results 2011/2012  Compared with previous measurements under worse vacuum conditions lifetime could be drastically improved for Cs 2 Te:  … and for Cs 3 Sb: C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov3

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: C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov4

Plans for 2014  Lifetime studies with Cs 2 Te cathode under improved vacuum conditions  Long-term measurement with Cs 2 Te under nominal operating conditions (2.3 nC, 1.2 µs)  Lifetime studies with Cs 3 Sb cathodes and green laser light under improved vacuum conditions.  RF lifetime measurements of Cs 3 Sb 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 C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov5

Layout of PHIN C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov6 FCT: Fast current transformer VM: Vacuum mirror SM: Steering magnet BPM: Beam position monitor MSM: Multi-slit Mask OTR: Optical transition radiation screen MTV: Gated cameras SD: Segmented dump FC: Faraday cup

C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov7 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.

Photocathodes Used during PHIN Run C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov8 NumberMaterialAgeQE in DC gun QE in PHIN #198Cs 2 TeNew cathode, produced % after production ~10% #199Cs 3 SbNew cathode, produced % after production 4.9% #200Cs 3 SbNew cathode, produced % after production 3.9% 6A56CuCopper plug (Diamond powder polished) used for RF conditioning N/A3e-4  This year the initial QE of Cs 2 Te and Cs 3 Sb 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

Improvement of Vacuum in PHIN C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov9 1/e lifetime 185 h 1 nC, 800 ns,  =524 nm, Cs 3 Sb 1/e lifetime 26 h 1 nC, 800 ns, =262 nm, Cs 3 Sb 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 /e lifetime n/a due to different problems during PHIN run

Lifetime Measurement with Cs 2 Te Cathode  Under improved vacuum conditions:  Double exponential fit represents well the data  Lifetime improved from 250 h to 300 h  Cs 2 Te is not ultra-sensitive against non-optimal vacuum conditions C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov10 Dynamic pressure: 3e-10 mbar 1.5e-9 mbar nC, 350 ns

Lifetime Measurement with Cs 2 Te Cathode  Under nominal operation conditions (2.3 nC, 1.2 µs)  Strong pressure increase. Heating of Faraday cup?  1/e lifetime still 55 h C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov11

Lifetime Measurement with Cs 3 Sb 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 C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov12 Dynamic pressure: e-10 mbar ~9e-10 mbar 2.3 nC, 350 ns,  Cs 3 Sb 1/e lifetime 168 h 2.3 nC, 350 ns,  Cs 3 Sb

Lifetime Measurement with Cs 3 Sb Cathodes  Under improved vacuum conditions:  Despite better vacuum level the lifetime is significantly shorter.  Strong QE decrease started after a phase jump C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov13 Dynamic pressure: 2.3e-10 mbar 1/e lifetime 185 h 7e-10 mbar 1 nC, 800 ns,  Cs 3 Sb

RF Lifetime of Cs 3 Sb Cathodes C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov14 Fresh cathode Cathode #200 (Cs 3 Sb) Used cathode Cathode #199 (Cs 3 Sb)  Fast and slow decay as seen with beam?  In both cases longer lifetimes as with beam.

Possible Explanations  RGA spectrum:  CO 2 is known to have a negative effect on K 2 CsSb [1]  Source of CO 2 ? Bake-out accident? C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov p(total)2.70E E-10 p(H 2 )2.46E E-11 p(H 2 O)1.23E E-11 p(CO/N 2 )2.09E E-11 p(CO 2 )1.55E E-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)

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 C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov16

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 C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov nC, 350 ns,  Cs 3 Sb nC, 800 ns,  Cs 3 Sb 2014

 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.  Cs 3 Sb cathodes (  ~2 eV) produce higher dark current than Cs 2 Te (  ~3.5 eV). Dark Current Measurements C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov18

Emittance Measurements for AWAKE Laser beam size: ~ 1 mm sigma, charge 0.2, 0.7, 1.0 nC, energy 5.5 – 6 MeV C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov19 Normalized emittance for 0.2 nC: 3.2 mm mrad ( big errors !) Charge dependence is roughly sqrt as it should be E n (0.2 nC): 3.2 mm mrad E n (0.7 nC): 4.6 mm mrad E n (1 nC): 5.5 mm mrad

Outlook: Plans for Laser C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov20  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.

Outlook: Plans for Photoemission Lab C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov21  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.

Outlook: Plans and Ideas for PHIN C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov22  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 500 MHz corresponds to a 1.6 µs 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 Cs 3 Sb and Cs 2 Te  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. K 2 CsSb)  Study the effect of surface roughness on cathode lifetime (Electro-polished cathode plugs)  Many interesting ideas for a further PHIN run in 2015!

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 C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov23

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! C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov24