1. CERN, 6 May 2010M. ParalievSlide 1/16 PAUL SCHERRER INSTITUTE SwissFEL project Experimental Study of DLC Coated Electrodes for Pulsed Electron Gun SwissFEL project – 4MeV test stand Presented by Martin Paraliev Paul Scherrer Institute, Switzerland

2. CERN, 6 May 2010M. ParalievSlide 2/16 PAUL SCHERRER INSTITUTE SwissFEL project 4MeV Test Stand Overview 500kV pulse generator Vacuum chamber with pulsed accelerating diode Two cell 1.5GHz RF cavity Focusing solenoids Diagnostic screens Emittance monitor (pepper pot, slits) Quadrupole magnets Dipole magnet Beam dumps with faraday caps 5 degree of freedom mover Laser table Diagnostic screens BPMs 5.43 m Clean cubicle and air filter 3D CAD model of 4MeV test stand

3. CERN, 6 May 2010M. ParalievSlide 3/16 PAUL SCHERRER INSTITUTE SwissFEL project High Gradient Accelerating Diode System parameters  Max accel. diode voltage - 500kV  Diode pulse length FLHM – 250ns  Two cell RF cavity 1.5GHz  Max RF power - 5MW  RF pulse length – 5us  Beam energy - 4MeV  Rep. rate - 10Hz  Laser pulse length – 10ps  Laser wave length – 262, 266nm  Max laser pulse energy – 250uJ Features  Variable anode cathode distance  Adjustable cathode position  Exchangeable electrodes  Differential vacuum system  Bolts-free vacuum chamber  Scintillator based dark current monitoring system e- beam UV laser Cathode Anode Vacuum chamber Differential vacuum  Accelerating diode cross section RF cavity

4. CERN, 6 May 2010M. ParalievSlide 4/16 PAUL SCHERRER INSTITUTE SwissFEL project Diode Accelerating Voltage and HG Test Procedure Diode acceleration voltage is asymmetric oscillatory pulse produced by Tesla-like transformer. Laser pulse for photo emission is short (10ps FWHM) with respect to the oscillating accelerating voltage and it arrives at the first negative maximum - quasi DC acceleration. The scintillator registers RF cavity X-ray activity. It is used, as well, to detect parasitic e - emission during HG test. In case of breakdown or dark current, distinctive pulses appear, synchronized with the high voltage waveform. HG test procedure consists of three phases: I const gap, II const gradient and III const voltage Scintillator signal copies the filling of RF cavity  Accelerating voltage, laser pulse and scintillator signal waveforms Laser pulse Diode voltage e- emission High Gradient test procedure  Phase I Gap 1mm Phase II Grad 50MV/m Phase III Voltage 350kV

5. CERN, 6 May 2010M. ParalievSlide 5/16 PAUL SCHERRER INSTITUTE SwissFEL project Metal electrodes Different metals with different surface finish were tested for vacuum isolation. Surface finish appeared to be very important for vacuum breakdown performance of the electrodes. Hand polishing gave the best results. Further improvement of polishing did not give improvement in breakdown strength. Thanks to E. Kirk and S. Spielmann-Jaggi  Polished st. steel electrode surface under scanning electron microscope  Typical surface roughness (2D mapping)  Line height profile 0.5 mm A B A B

6. CERN, 6 May 2010M. ParalievSlide 6/16 PAUL SCHERRER INSTITUTE SwissFEL project Bare Metal Electrodes  There is some correlation between the material tensile strength and electrical vacuum insulation capability.  In the chart, for sputtered molybdenum, the bulk value of tensile strength is indicated.  Different metals polish differently and this made breakdown comparison difficult  Breakdown of a polished metal surface (bulk) did not exceed 150MV/m  Breakdown surface E field for different metal electrodes (polished). * 2um molybdenum layer was sputtered on a polished st. steel surface Hand polishing companies comparison (stainless steel) 

7. CERN, 6 May 2010M. ParalievSlide 7/16 PAUL SCHERRER INSTITUTE SwissFEL project Diamond Like Carbon a-C:H (DLC) Using Plasma Assisted Chemical Vapor Deposition (PACVD) process it is possible to deposit hydrogenated amorphous DLC (a-C:H) with tailored properties (thickness and conductivity) on virtually any type of metal surface (www.bekaert.com). Later, DLC coatings deposited by other processes were tested as well. Features:  Smooth and stable surface  Mechanical properties comparable to these of diamond  Unique electrical properties  Intact DLC surface type PSI 080815-UF Thanks to E. Kirk  Destroyed DLC surface (same type).

8. CERN, 6 May 2010M. ParalievSlide 8/16 PAUL SCHERRER INSTITUTE SwissFEL project Thickness Process Conductivity DLC Base DLC – parametric study The following DLC parameters were explored: Coating thickness Coating electrical resistivity (DLC type) Base metal type (internal stress, adhesion) Base metal surface roughness Process (& companies) 2um hydrogenated amorphous DLC (a-C:H) coating gave the best performance – note the correlation with hardness Larger base surface roughness gave lower breakdown strength  Breakdown strength vs DLC thickness - st. steel, Cu, bronze, Bekaert  Breakdown strength vs DLC type ( resistivity) - st. steel, 2um, Bekaert Stainless steel only Doped DLC (a-C:H, a-m) DLC (a-C:H) Doped Dylyn (a-C:H, a-Si:O, a-m) Coating type:

9. CERN, 6 May 2010M. ParalievSlide 9/16 PAUL SCHERRER INSTITUTE SwissFEL project DLC – parametric study Residual stress in the deposited layer and coating adhesion are expected to have influence on vacuum breakdown performance. Three different base metals were used in order to explore that. In certain occasions, the sample breaks down at low gradient unexpectedly (“sudden dead”). In the beginning, surface charging due to occasional laser illumination without accelerating voltage was suspected. Later experiments did not support this idea. Now, these breakdowns are attributed to defects in the coating layer. Copper results are higher because some of the samples were not tested until breakdown (saved for e - beam experiments) Thickness Process Conductivity DLC Base  Breakdown strength (2um DLC) vs process (companies)  Breakdown strength vs base metal (2um, Bekaert) PACVD IBSD Probably due to coating defects

10. CERN, 6 May 2010M. ParalievSlide 10/16 PAUL SCHERRER INSTITUTE SwissFEL project DLC (a-C:H) – photo emission  DLC coating structure is complex – hard to determine the exact emission process [1].  DLC and Diamond Like Nanocomposite (DLN) properties are not well defined since they depend on the sp2/sp3 bonding ratio (graphite/diamond) and doping levels [2].  Typical DLC layer structure (PSI 080815-UF) DLC Ti DLN Vacuum Base metal (Cu) Base metal (Cu) 2um 0.4um 0.2um Two possible electron photoemission mechanisms are possible: > Emission form DLC valence band > Electron injection in DLC conduction band at Metal-DLC interface  2um DLC Quantum efficiency (PSI 080815-UF) compared to photoemission from Cu-like metal [3] [1] J. Robertson, “Field emission from carbon systems”, Mat. Res. Soc. Symp. Proc. Vol. 62, 2000 [2] A. Wisitsorat, “Micropatterned diamond vacuum field emission devices”, PhD thesis, Nashville, TN, 2002 [3] D.H. Dowell et al. “In situ cleaning of metal cathodes using a hydrogen ion beam”, Phys. Rev. ST Accel. Beams 9, 063502 (2006)  266nm transmission through 1um DLC layer. 2um DLC - 25% UV transmission Factor of 5 lower ! Metal-DLC interface field is reduced with  (  = 4) ~10pC 32uJ ~56pC 185uJ Cu-like metal W = 4.6eV Cu-like metal x 5%

11. CERN, 6 May 2010M. ParalievSlide 11/16 PAUL SCHERRER INSTITUTE SwissFEL project “Hollow” cathode geometry High breakdown strength of DLC coated electrodes gave the opportunity to develop so called “hollow” cathode geometry for testing different photo-emitting materials and Field Emitting Arrays (FEAs). It decreases the breakdown probability reducing sample’s area exposed to high E field. The edges of the sample are covered by small lip that makes electrical contact to the sample front surface. In addition, electric field lines in proximity to the emission surface are deformed due to concave electrode profile. It provides electro-static e - beam focusing where electrons have small kinetic energy and the beam is prone to space charge degradation. DLC coated surface Sample e - beam Hollow cathode cross-section Electrostatic simulation of the field in the accelerating diode. Diode gap 15mm Electric field distribution along the acceleration path Anode surface Hollow cathode surface Emission surface Hollow cathode Anode e - beam Electric field is about 50% of the max acceleration field due to cathode recess screening effect.

12. CERN, 6 May 2010M. ParalievSlide 12/16 PAUL SCHERRER INSTITUTE SwissFEL project Photoemission from other materials Photoemission from different cathode inserts was studied. A “standard” procedure was established in order to compare the QE. The samples were irradiated with 6ps (rms) long UV laser pulse (266nm). Accelerating gap and accelerating voltage are varied: gap range from 5.4mm to 6.6mm and voltage range from 315kV to 385kV Thanks to F. Le Pimpec, R. Ganter,  Quantum efficiency comparison of different metal photo-cathodes vs extraction electric field. The samples are hand polished in air using sand paper and abrasive pastes. The last polishing stage is repeated before putting the samples in the test chamber (to reduce the surface exposure to air) Dry ice blasting is used to clean the surface before installation. No further in-vacuum preparation is applied.

13. CERN, 6 May 2010M. ParalievSlide 13/16 PAUL SCHERRER INSTITUTE SwissFEL project 500kV pulser Conditioning chamber Nanosecond driver and FEA integration Fast driver circuit and low impedance contact system was developed to drive the FEA gate. FEA parameters: FEA capacitance 1.3nF FEA diameter 2mm Number of tips40 000 Gate pulse duration 15ns FWHM Emitted current duration 5ns FWHM  Gate voltage dummy FEA chip  Emitted current (conditioning chamber) Hollow cathode DLC coating FEA chip Spring loaded contact Low inductance connection 5ns

14. CERN, 6 May 2010M. ParalievSlide 14/16 PAUL SCHERRER INSTITUTE SwissFEL project Gated FEA in high gradient Achieved up to now (only two FEA tested): Max gradient* 30MV/m (230kV, 1pC) Max beam energy* 300keV (11MV/m, 1.5pC) Max emitted charge >10pC (9MV/m, 250keV) + Stable emission pattern - Not good emission homogeneity *Not limiting values (up to our knowledge - record values) FEA e - beam focused FEA imaging FEA V-A emission characteristic

15. CERN, 6 May 2010M. ParalievSlide 15/16 PAUL SCHERRER INSTITUTE SwissFEL project Outlook Hydrogenated amorphous DLC (a-C:H) coating has exceptionally good vacuum breakdown performance for short damped oscillatory pulses.  Max surface gradient >300MV/m @ 1mm  Photo-emission at >150MV/m @ 2mm  No dark current is detected  Stable operation Surface breakdown field surplus, due to DLC coating, makes possible to do additional field shaping.  Hollow cathode geometry Testing of variety of photocathode materials and FEAs was possible due to DLC coated electrodes.  Different material QE evaluation  Max extracted charge (metal insert) >200pC  FEA integration in high gradient environment

16. CERN, 6 May 2010M. ParalievSlide 16/16 PAUL SCHERRER INSTITUTE SwissFEL project Thank you for your attention! Project team in 4MeV test bunker - some time ago...