S. Lederer & S. Schreiber (DESY Hamburg) L. Monaco & D. Sertore (INFN-LASA, Milano) Studies on the performance of Cs 2 Te photocathodes at FLASH and the European XFEL European Workshop on Photocathodes for Particle Accelerator Applications STFC Daresbury Laboratory 6-8 June 2016

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 2 Outline > FLASH and European XFEL > Photocathode preparation > Cathode performance at FLASH  Life time  Dark current  Homogeneity of electron emission > First experience from European XFEL > Charge spikes

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 3 FLASH Variable gap Undulators

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 4 European XFEL Experiments 3.3 km SASE1 SASE3 SASE2 5 MeV150 MeV600 MeV2 GeV17.5 GeV RF Gun Bunch Compression Main Linac Collimation Beam Distribution Undulators Photon Beamlines Injector

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 5 FLASH and European XFEL FLASHEuropean XFEL RF-gun1½ cell 1.3 GHz normal conducting Gradient50 MV/m60 MV/m RF-pulse length800 μs650 μs Rep. rate10 Hz Temporal laser profileGaussian 7 psFlat top 2/20\2* Bunch charge20 pC.. 5 nC20 pC.. 1 nC Bunches per macropulseUp to 800Up to 2700 Emittance< 1.5 mm 1 nC< 1.0 mm 1 nC * start-up is gaussian

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 6 Photocathode preparation

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 7 > System comparable to the INFN-LASA Milano one > Base pressure < mbar > Mo-Plugs  Forged base material  Roughness on cylindrical part for RF-contact R a ~ 100 nm – already after machining  Front surface polished to R a ~ 20 nm > Preparation recipe is based on the INF-LASA Milano one  Plug 450 degC  Plug temp. during preparation 120 degC  First 10 nm 1 nm/min  Then Cs 1 nm/min until QE reaches maximum  Constantly illuminated at 254 nm Photocathode preparation

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 8 Photocathode preparation Te Cs Te Cs

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 9 Photocathode preparation > For each prepared cathode cw measurements  Spectral response – QE vs. photon energy  254 nm

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 10 Cathode transport > All cathode systems are compatible  INFN-LASA preparation system  DESY preparation system  FLASH cathode system  European XFEL cathode system > Cathodes are transported from system to system in battery buffered UHV-chambers

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 11 Cathode lifetime at FLASH

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 12 Pulsed QE measurements QE = 9.2 % space charge effect > automated procedure for measuring charge vs. laser energy dependence > 38 deg w.r.t. zero crossing, not maximum charge nor nominal operation phase charge trend at low charge fitted

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 13 Cathode lifetime at FLASH > Cathode #618.3: in use 21-Nov-2013 to 4-Feb-2015: 439 days in operation > Total charge extracted: 3.2 C > Cathode exchange not driven by too low QE

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 14 Cathode lifetime at FLASH > Cathode #618.3: in use 21-Nov-2013 to 4-Feb-2015: 439 days in operation > Total charge extracted: 3.2 C > Cathode exchange not driven by too low QE Low QE because Operation with leaky RF-window (10 -8 mbar l/s) Used for conditioning the RF-gun with new RF-window →Exposed to bad vacuum for a long time

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 15 Cathode lifetime at FLASH > Cathode #73.3: in use since 4-Feb-2015 (489 days up to now) > Total charge extracted up to now: 7.4 C

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 16 Homogeneity of electron emission over time

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 17 Cathode homogeneity at FLASH, cathode #618.3 > Laser spot size 100 μm > Step size 85 μm > Laser is moved over cathode and vacuum mirror Measured points Contour plot used as standard visualization

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 18 Cathode homogeneity at FLASH, cathode # QE in the center of the cathode, where the laser impinges, changes different from the rest of the cathode

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 19 Cathode #618.3 before and after usage > Photos of the Cs 2 Te film > Illumination different for both photos -> color seems to be different Right after preparation After usage Laser position during operation

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 20 Cathode homogeneity at FLASH, cathode #73.3 > As for cathode #618.3: QE in the center of the cathode, where the laser impinges, changes different from the rest of the cathode. > Initially QE drops on the area where the laser hits > QE slightly increases at the edges of the standard emission area > QE in the center is even higher than the at the rest Laser strongly misaligned on vacuum mirror

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 21 Dark current at FLASH

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 22 Dark current > Severe issue for accelerators with long RF-pulses > Losses may cause  Heat-up of 3.9 GHz modules at FLASH and European XFEL (aperture is smaller)  Damages of electronic components due to created radiation  Damages of Undulators due to created radiation  Activation of beam line components > Goal: Keep the photocathodes as particle free as possible  Only particle free cleaned components used (transport boxes, cathode exchange system, preparation system)  BUT: caused by the handling of the photocathodes particles are created (movement of carrier, cathode insertion into and re-movement from carrier, cathode insertion into the RF-gun →Limit the cathode actions to the absolute minimum

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 23 > Dark current images on YAG screen at gun exit > Green marker corresponds to dark current collimator position and roughly to the emissive Cs 2 Te film, orange marker used as reference in the past > Some emitters slightly change Dark current at FLASH – cathode # Cathode removed from RF-gun and re-inserted for reactivation of TSP →No significant change of dark current (this time) > Amount of dark current measured with FC constant between 5 and 6  A

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 24 Dark current at FLASH > Some emitters are independent from the cathode → RF-gun related (marked red in the images) > Comparison of dark current images important to identify problems with the RF-gun itself cathode # cathode #618.3

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 25 First experience from European XFEL

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 26 First experience from European XFEL > Cathode #680.1 in operation since

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 27 > Cathode #680.1 > Procedure for measurement still under development > As for FLASH homogeneity of electron emission influenced by the drive laser First experience from European XFEL

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 28 Charge spike for fresh Cs 2 Te cathode

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 29 Emission issues (of fresh cathode, #73.3) > Laser pulse trains measured by fast UV photodiodes on scope > Both UV-pulse trains are flat Laser 1 on scope (UV-pulse train is green, n.b. ripple comes from scope not laser) Laser 2 on scope (UV-pulse train is blue)

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 30 Emission issues (of fresh cathode, #73.3) > Even though the laser 2 UV-pulse is flat the emitted charge shows a spike in the beginning Charge measured by toroid at gun exit (resolution 3 pC) Laser 2 only on cathode

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 31 Emission issues (of fresh cathode, #73.3) > Even though the laser 1 UV-pulse is flat the emitted charge shows a spike in the beginning Charge measured by toroid at gun exit (resolution 3 pC) Laser 1 only on cathode

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 32 Emission issues (of fresh cathode, #73.3) > Laser2 30 bunches, μs > Laser1 90 bunches, μs Charge emitted by laser 2 spiky Charge emitted by laser 1 flat

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 33 Emission issues (of fresh cathode, #73.3) > After a certain delay between the laser pulses both emitted charge distributions spiky Charge emitted by laser2 spiky Charge emitted by laser1 spiky

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 34 Emission issues (of fresh cathode, #73.3) intensity dependence > Decreasing to energy of laser 2 changes the charge distribution from spiky to flat while the charge distribution emitted by laser 1 changes the other way around Charge emitted by laser 2 Charge emitted by laser 1 Laser 1 kept constant, laser2 energy varied

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 35 Emission issues (of fresh cathode, #73.3) field dependence > RF-pulse starts at 730 μs -> photoemission caused by laser1 before RF-pulse > Laser1, 760 μs, 90 bunches; Laser2, 700 μs, off > Charge distribution spiky Charge emitted by laser1

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 36 > RF-pulse starts at 730 μs -> photoemission caused by laser1 before RF-pulse > Laser1 760 μs, 90 bunches; Laser2 700 μs, 30 bunches > Charge distribution spiky → field required to produce the spike Emission issues (of fresh cathode, #73.3) field dependence Charge emitted by laser1

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 37 Emission issues (of fresh cathode, #73.3) field dependence > For lower fields on the cathode the charge distribution becomes flat > Laser2 μs > Laser1 μs > Below 4 MW it was necessary to go into gun-mode, so number of pulses had to be decreased

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 38 Summary > Currently no life time issues of Cs 2 Te at FLASH  Last two cathode operated for more than 430 days  No cathode exchange because of too low QE since years > During the actual commissioning phase of the European XFEL no life time issues of Cs 2 Te photocathodes > Lifetime at standard operation of the European XFEL unclear and might by an issue  1 nC per bunch operation would yield 16 C/week ! > Dark current crucial issue  Even good cathodes from the PE point of view can be unusable > Not understood charge spikes for fresh photocathode observed

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 39 Cathode homogeneity at FLASH, cathode #618.3 > Since during QE-map measurements the laser is moved over the photocathode and the in-coupling mirror -> check if observations are really related to photocathode > Vacuum mirror does not produce the features in the QE-maps! Cathode rotated by 180 °

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 40 Emission issues (of fresh cathode, #73.3) > Laser2  s > Laser1  s > At 1.28 MW both charge profiles are flat > Even an increase in laser energy (Laser2) only increases the total amount of charge but does not yield a spike in the distribution

S. Lederer European Workshop on Photocathodes for Particle Accelerator Applications, Daresbury, UK Page 41 > Dark current images on YAG screen at gun exit > Amount of dark current measured with FC constant between 5 and 6  A > the orange marker for reference Dark current at FLASH – cathode # Nov Sep Aug May Jan-2014