Experimental Study of DLC Coated Electrodes for Pulsed Electron Gun

Transcription

1 Experimental Study of DLC Coated Electrodes for Pulsed Electron Gun 4MeV test stand Paul Scherrer Institute, Switzerland Presented by Martin Paraliev Slide 1/16

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

3 High Gradient Accelerating Diode Differential vacuum Vacuum chamber Accelerating diode cross section Anode Anode Cathode RF cavity e- beam UV laser System parameters Max accel. diode voltage - 5kV Diode pulse length FLHM 25ns Two cell RF cavity 1.5GHz Max RF power - 5MW RF pulse length 5us Beam energy - 4MeV Rep. rate - 1Hz Laser pulse length 1ps Laser wave length 262, 266nm Max laser pulse energy 25uJ Features Variable anode cathode distance Adjustable cathode position Exchangeable electrodes Differential vacuum system Bolts-free vacuum chamber Scintillator based dark current monitoring system Slide 3/16

4 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 (1ps FWHM) with respect to the oscillating accelerating voltage and it arrives at the first negative maximum - quasi DC acceleration. Diode voltage Scintillator signal copies the filling of RF cavity e- emission Laser pulse 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 HV test procedure Phase I Gap 1mm Phase II Grad 5MV/m Phase III Voltage 35kV Accelerating voltage, laser pulse and scintillator signal waveforms Voltage, kv Gradient, MV/m Voltage Gradient Gap Gap, mm 5 1 High Gradient test procedure Time Slide 4/16

5 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. A B.5 mm Typical surface roughness (2D mapping) A B Polished st. steel electrode surface under scanning electron microscope Slide 5/16 Line height profile Thanks to E. Kirk and S. Spielmann-Jaggi

6 Bare Metal Electrodes Breakd own field, M V/m Breakdown E field and tensile strength Bronze Copper St. steel Molybdenum * T ensile strength, M Pa 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 15MV/m Hand polishing 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) Breakdown field, MV/m In-house Auchlin SA Pilz AG Companies Slide 6/16

7 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 ( 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 8815-UF Destroyed DLC surface (same type). Slide 7/16 Thanks to E. Kirk

8 DLC parametric study Thickness Conductivity The following DLC parameters were explored: Coating thickness Coating electrical resistivity (DLC type) DLC Base metal type (internal stress, adhesion) Base metal surface roughness Process (& companies) Base Process 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 E field, MV/m DLC thickness comparison (Bekaert) Stainless steel only Coating type: Breakdown E field, MV/m Coating types comparison (Bekaert 2um) Doped DLC (a-c:h, a-m) DLC (a-c:h) Doped Dylyn (a-c:h, a-si:o, a-m) 5.E+4 5.E+7 5.E Micro hardness, GPa DLC thickness, um Resistivity, Ohm.cm Breakdown strength vs DLC thickness - st. steel, Cu, bronze, Bekaert Slide 8/16 Breakdown strength vs DLC type ( resistivity) - st. steel, 2um, Bekaert

9 DLC parametric study Thickness Conductivity 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. Base DLC Process 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) Process comparison Base metal comparison 35 PACVD PACVD PACVD PACVD IBSD 35 Breakdown E field, MV/m Breakdown E field, MV/m Probably due to coating defects Bekaert bronze Bekaert st. seel PlascoTec st. steel OerlikonBalzers st. steel Fraunhofer st. steel Bronze Copper St. steel Companies Base material Breakdown strength (2um DLC) vs process (companies) Slide 9/16 Breakdown strength vs base metal (2um, Bekaert)

10 Transmission of 1um DLC 8% 7% 6% 5% 4% Bekaert, PSI 8815-HR Bekaert, PSI 8815-RG PAUL SCHERRER INSTITUTE Bekaert, PSI 8815-UF 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]. Two possible electron photoemission mechanisms are possible: > Emission form DLC valence band > Electron injection in DLC conduction band at Metal-DLC interface Fraunhofer DLC type PlasmaConsult Oerlikon Balzers 266nm transmission through 1um DLC layer. QE 1.E-4 1.E-5 1.E-6 Photoemission Quantum Efficiency (262nm) Cu-like metal W = 4.6eV Cu-like metal x 5% ~1pC 32uJ 1.E E field, MV/m Metal-DLC interface field is reduced with ε (ε = 4) 2um DLC - 25% UV transmission Factor of 5 lower! ~56pC 185uJ 2um DLC Quantum efficiency (PSI 8815-UF) compared to photoemission from Cu-like metal [3] Base metal (Cu) Ti Typical DLC layer structure (PSI 8815-UF) DLN.4um.2um DLC 2um Vacuum Slide 1/16 [1] J. Robertson, Field emission from carbon systems, Mat. Res. Soc. Symp. Proc. Vol. 62, 2 [2] A. Wisitsorat, Micropatterned diamond vacuum field emission devices, PhD thesis, Nashville, TN, 22 [3] D.H. Dowell et al. In situ cleaning of metal cathodes using a hydrogen ion beam, Phys. Rev. ST Accel. Beams 9, 6352 (26)

11 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 e - beam Anode Hollow cathode Electrostatic simulation of the field in the accelerating diode. 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. e - beam DLC coated surface Sample Hollow cathode cross-section Hollow cathode surface Diode gap 15mm Anode surface Electric field is about 5% of the max acceleration field due to cathode recess screening effect. Emission surface Electric field distribution along the acceleration path Slide 11/16

12 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 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. Slide 12/16 Thanks to F. Le Pimpec, R. Ganter,

13 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 tips 4 Gate pulse duration 15ns FWHM Emitted current duration 5ns FWHM Hollow cathode DLC coating 5kV pulser Spring loaded Low inductance contact Conditioning chamber connection FEA chip Voltage over FEA 1nF (15V charge) FEA emitted current U, V Time, ns Current, ua ns Uch = 117V Time, ns Gate voltage dummy FEA chip Slide 13/16 Emitted current (conditioning chamber)

14 Gated FEA in high gradient Achieved up to now (only two FEA tested): Max gradient* 3MV/m (23kV, 1pC) Max beam energy* 3keV (11MV/m, 1.5pC) Max emitted charge >1pC (9MV/m, 25keV) + Stable emission pattern - Not good emission homogeneity *Not limiting values (up to our knowledge - record values) Emitted current vs Ug 2 Emitted current, ua Ug, V FEA e - beam focused FEA imaging Slide 14/16 FEA V-A emission characteristic

15 Outlook Hydrogenated amorphous DLC (a-c:h) coating has exceptionally good vacuum breakdown performance for short damped oscillatory pulses. Max surface gradient 1mm Photo-emission at 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) >2pC FEA integration in high gradient environment Slide 15/16

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