Experimental Study of DLC Coated Electrodes for Pulsed Electron Gun
SwissFEL project – 4MeV test stand
Presented by Martin Paraliev
Paul Scherrer Institute, Switzerland Paul Scherrer Institute, Switzerland
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
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 e- beam UV laser UV laser Cathode
Cathode
Anode Anode
Vacuum chamber Vacuum chamber Differential
vacuum Differential
vacuum
Accelerating diode cross section
RF cavity RF cavity
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
HV test procedure
0 50 100 150 200 250 300 350 400 450 500
Time Voltage, kV Gradient, MV/m
0 1 2 3 4 5 6 7 8 9 10
Gap, mm
Voltage Gradient Gap
Laser pulse Diode voltage e- emission
High Gradient test procedure
Phase I Gap 1mm
Phase II Grad 50MV/m
Phase III Voltage 350kV
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
Breakdown E field and tensile strength
0 50 100 150 200 250
Bronze Copper St. steel Molybdenum *
Breakdown field, MV/m
0 320 640 960 1280 1600
Tensile strength, MPa
Hand polishing
0 20 40 60 80 100 120 140 160
In-house Auchlin SA Pilz AG
Com panies
Breakdown field, MV/m
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)
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).
Coating types comparison (Bekaert 2um)
0 50 100 150 200 250 300 350
5.0E+04 5.0E+07 5.0E+12
Resistivity, Ohm.cm Breakdown E field, MV/m
0 3 6 9 12 15 18 21
Micro hardness, GPa DLC thickness comparison (Bekaert)
0 50 100 150 200 250 300 350
1 2 4
DLC thickness, um
Breakdown E field, MV/m
Thick ness
Pro cess Conductivity
DLCDLC
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:
Process comparison
0 50 100 150 200 250 300 350
Bekaert bronze
Bekaert st. seel
PlascoTec st. steel
OerlikonBalzers st. steel
Fraunhofer st. steel Companies
Breakdown E field, MV/m
Base metal comparison
0 50 100 150 200 250 300 350
Bronze Copper St. steel
Base material
Breakdown E field, MV/m
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)
Thick ness
Pro cess Conductivity
DLCDLC
Base
Breakdown strength (2um DLC) vs process (companies)
Breakdown strength vs base metal (2um, Bekaert)
PACVD PACVD PACVD PACVD IBSD
Probably due to coating defects
40%
50%
60%
70%
80%
Bekaert, PSI 080815-HR
Bekaert, PSI 080815-RG
Bekaert, PSI 080815-UF
Fraunhofer
PlasmaConsult
Oerlikon Balzers
DLC type
Transmission of 1um DLC Photoemission Quantum Efficiency (262nm)
1.E-07 1.E-06 1.E-05 1.E-04
0 50 100 150 200
E field, MV/m
QE
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) DLCDLC
TiTi DLNDLN VacuumVacuum 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 ε(ε= 4)
~10pC 32uJ ~56pC 185uJ Cu-like metal W = 4.6eV
Cu-like metal x 5%
“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.
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.
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 tips 40 000
Gate pulse duration 15ns FWHM Emitted current duration 5ns FWHM
Voltage over FEA 1nF (150V charge)
-200 -150 -100 -50 0 50
0 50 100 150 200
Tim e, ns
U, V
FEA emitted current
-250 -200 -150 -100 -50 0 50
0 10 20 30 40 50 60 70 80 90 100 Time, ns
Current, uA
Uch = 117V
Gate voltage dummy FEA chip Emitted current (conditioning chamber)
Hollow cathode DLC coating
FEA chip Spring loaded
contact Low inductance
connection
5ns
Em itted current vs Ug
0 500 1000 1500 2000
50 55 60 65 70 75 80
Ug, V
Emitted current, uA
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
