Experimental Study of DLC Coated Electrodes for Pulsed Electron Gun

1. Experimental Study of DLC Coated Electrodes for Pulsed Electron Gun SwissFEL project – 4MeV test stand Paul Scherrer Institute, Switzerland Presented by Martin Paraliev

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

3. 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 Differential vacuum High Gradient Accelerating Diode RF cavity Anode Cathode e- beam UVlaser Vacuum chamber  Accelerating diode cross section

4. 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 Diode Accelerating Voltage and HG Test Procedure e- emission Diode voltage Scintillator signal copies the filling of RF cavity Laser pulse Phase II Grad 50MV/m Phase III Voltage 350kV Phase I Gap 1mm  Accelerating voltage, laser pulse and scintillator signal waveforms High Gradient test procedure 

5. 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. Metal electrodes A B 0.5 mm  Typical surface roughness (2D mapping) A B  Line height profile  Polished st. steel electrode surface under scanning electron microscope Thanks to E. Kirk and S. Spielmann-Jaggi

6. 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. 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 Diamond Like Carbon a-C:H (DLC)  Intact DLC surface type PSI 080815-UF  Destroyed DLC surface (same type). Thanks to E. Kirk

8. Thickness Conductivity DLC Process Base 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 DLC – parametric study Coating type: Stainless steel only Doped Dylyn (a-C:H, a-Si:O, a-m) DLC (a-C:H) Doped DLC (a-C:H, a-m) • Breakdown strength vs DLC thickness - st. steel, Cu, bronze, Bekaert • Breakdown strength vs DLC type ( resistivity) - st. steel, 2um, Bekaert

9. DLC – parametric study Thickness Conductivity DLC Process Base 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) IBSD PACVD PACVD PACVD PACVD Probably due to coating defects • Breakdown strength (2um DLC) vs process (companies) • Breakdown strength vs base metal (2um, Bekaert)

10. Base metal (Cu) Ti DLN Vacuum DLC • 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]. Two possible electron photoemission mechanisms are possible: > Emission form DLC valence band > Electron injection in DLC conduction band at Metal-DLC interface 2um DLC - 25% UV transmission Factor of 5 lower! DLC (a-C:H) – photo emission Cu-like metal W = 4.6eV Cu-like metal x 5% ~56pC 185uJ ~10pC 32uJ Metal-DLC interface field is reduced with  ( = 4)  266nmtransmission through 1um DLC layer.  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) 0.4um 2um 0.2um  Typical DLC layer structure (PSI 080815-UF)

11. “Hollow” cathode geometry Anode e- beam Hollow cathode DLC coated surface e- beam Sample 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. Hollow cathode cross-section Hollow cathode surface Anodesurface Electrostatic simulation of the field in the accelerating diode. Diode gap 15mm Emission surface Electric field is about 50% of the max acceleration field due to cathode recess screening effect. Electric field distribution along the acceleration path

12. 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 Photoemission from other materials 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. • Quantum efficiency comparison of different metal photo-cathodes vs extraction electric field. Thanks to F. Le Pimpec, R. Ganter,

13. Nanosecond driver and FEA integration DLC coating Hollow cathode 500kV pulser Spring loaded contact Low inductance connection Conditioning chamber FEA chip 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 tips 40 000 Gate pulse duration 15ns FWHM Emitted current duration 5ns FWHM 5ns Emitted current (conditioning chamber)  Gate voltage dummy FEA chip

14. 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) FEAs can be used in High Gradient environment FEA V-A emission characteristic FEA e- beam focused FEA imaging

15. 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 Outlook

16. Thank you for your attention! Project team in 4MeV test bunker - some time ago...