Breakdown field, MV/m

High Gradient Electrodes for LEG

Electrode

Electrode surface surface preparation preparation

Sladjana Ivkovic

Hand Polishing

Surface finish appeared to be very important for vacuum breakdown performance of the electrodes.

Hand polishing gave the best results.

External companies: Buob, Auchlin SA, Pilz AG (30 electrodes) – reproducibility, difficult to control the process, expensive

Developed in-house polishing technology

Anode

Hollow cathode St. steel cathode

Insert

over 240 electrodes from stainless steel, copper and bronze inserts

single tips

single tip

Ø 45mm Ø 39mm

Ø 45mm

Abrasive paper 120 – 3000

320(400), 600, 800, 1200, 1500, 2500(3000)

Polishing disk

Polishing compound LUXOR 0.1 – 3 m

Materials for Polishing

Side

Diamond suspension

Polycrystalline, oil/water soluble

Cleanroom wipers

Polishing machine

St.steel surface:

after abrasive paper 400

St. steel fine polished

St.steel surface:

after abrasive paper 1200

Copper fine polished Bronze fine polished

Typical SEM picture of polished St.

steel electrode surface by Auchlin

There are clearly defects on the surface

(embedded particles, inclusions, voids) but it is hard to correlate them with a breakdown location.

Thanks to E. Kirk

Polished surface

Surface roughness of stainless steel electrodes

2D mapping

∆ Hpp =72nm Hrms = 16nm Ra = 13nm Line height profile

1.5 mm long Ra = 12.6nm Rp-p = 72.3nm Rrms = 16.2nm There are not enough samples measured to get a correlation between vacuum breakdown voltage and surface roughness on a micro scale.

Cathode M5 (SS 316L) 9.09.2008. Pilz AG: pair V11/V12 (Implant) 29.12.2008.

Ra

= 5.3nm Ra

= 8.6nm

Line height profile

0.15 mm long

Breakdown of

Bare Electrodes

Polishing comparison (Bare Stainless steel)

Hand polishing

0 20 40 60 80 100 120 140 160

In-house Auchlin SA Pilz AG

Com panies

B re a k d o w n f ie ld , M V /m

Breakdown E field and tensile strength

0 20 40 60 80 100 120 140 160

St. steel Decolletage Implant

Breakdown field, MV/m

0 320 640 960 1280

Tensile strength, MPa

Implant X2CrNiMo 18-15-3 1.4441 AISI ~316LVM

Decolletage X2CrNiMo 17-12-2+S+Cu AISI ~316L

Stainless steel X2CrNiMo 18-14-3 1.4435 AISI 316L

Comparison of stainless steel types (cut from round bars)

In-house polishing Auchlin polishing

1.4435

Maximum achieved surface gradients for polished metal electrodes

There is some correlation between the material tensile strength (hardness) and electrical vacuum insulation capability.

Different metals are polished differently and this made breakdown comparison difficult.

Breakdown of a polished metal surface did not exceed 150MV/m.

Breakdown surface E field for different metal electrodes (polished).

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

* - Sputtered 2um molybdenum on polished stainless steel

DLC Coating

deposition

Features:

Thin, chemically inert

Low surface roughness - smooth

Mechanical properties comparable to these of diamond Unique electrical properties

Intact DLC surface type under scanning electron microscope

Thanks to E. Kirk

Typical architecture (Bekaert) :

Ti adhesion layer : 100 nm to 600 nm DLN layer : 100 nm to 600 nm

DLC layer : 1µm to 10µm

Ti layer is optional, to enhance adhesion.

Typical architecture (Bekaert)

In many cases a tuned multilayer architecture is used to optimize

performance towards the application.

• Coating thickness (1, 2 and 4 m)

• Coating electrical resistivity (5.0E+04, 5.0E+07 and 5.0E+12 cm)

• Base metal - internal stress, adhesion (St. steel, copper and bronze)

• Base metal surface roughness (fine and coarse polish of SS)

• Process – company :

Bekaert (www.bekaert.com)

Plasma Assisted Chemical Vapour Deposition (PACVD)

OerlikonBalsers (www.oerlikon.com/balsers/ch)

Plasma Assisted Chemical Vapour Deposition (PACVD)

PlascoTec (PlasmaConsult) (www.plasmaconsult.de)

Plasma Assisted Chemical Vapor Deposition (PACVD )

Fraunhofer IWS (www.iws.fraunhofer.de )

Ion Beam Sputter Deposition (IBSD)

Breakdown strength vs DLC resistivity St. steel with 2um coating thickness

Breakdown E field vs Resistivity (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

Bekaert DLC coating

2um and DLC type (5.0E+07 m)

for best performance (n.b. correlation with hardness) Breakdown strength vs DLC thickness

St. steel, copper and bronze

DLC thickness comparison (Bekaert)

0 50 100 150 200 250 300 350

1 2 4

DLC thickness, um

Breakdown E field, MV/m

Stainless steel only

Doped DLC (a-C:H, a-m)

DLC (a-C:H)

Doped Dylyn (a-C:H, a-Si:O, a-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

Copper results are higher because some of the samples were not tested until breakdown (saved for other experiments)

Breakdown strength vs process ( all companies)

Bekaert coating structure for Bronze and st. steel is different. It is expected to have influence on the DLC layer internal stress and adhesion to the base metal.

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 large defects in the coating layer.

Breakdown strength vs base metal (2um, Bekaert)

Process comparison

0 50 100 150 200 250 300 350

Bekaert bronze

Bekaert st. seel

PlascoTec st. steel

OerlikonBalzers st. steel

Fraunhofer st. steel Process

E field, MV/m

PACVD PACVD PACVD PACVD IBSD

Probably due to coating defects

Top

Side

Annexe Thickness measurements with

FTIR (Fourier Transform Infrared) Spectroscopy (Bekaert)

0.93µm 0.92µm

0.97µm 0.97µm

0.91µm 0.96µm

0.94µm 0.90µm

Part ID11

1.47µm 1.58µm

1.45µm 1.65µm

1.46µm 1.67µm

1.53µm 1.56µm

Part F6

Side Top

FTIR involves a plastic-housed sensor, which is gently placed on the coated surface. It is based on interferometery that compares the returned or transmitted light energy of the IR spectrum to the emitted source spectrum. The result is a spectrum of the infrared energy absorbed by the sample.

DLC coating type on Si (Bekaert)

Stress in a thin film on a flexible substrate indicates a curvature of the substrate.

Curvature is measured using profilometry contact method.

Coating thickness is determined with the F20-NIR (Near- Infrared) device. Principe of working is based on

interference of different wavelengths opposed to the changing refractory index between substrate and coating.

Thickness of Si substrate is measured 1.42 m.

Residual compressive stress measured on Sample 1: 559 MPa, Sample 2: 815 MPa Calculated results according to Stoney’s equation:

Stoney’s equation:

Annexe Thickness measurements with FTIR (Bekaert)

Coating defects

Bekaert

Bekaert

(surface under scanning electron microscope)

Fraunhofer

Oerlikon Balsers

PlascoTec

Emitting Inserts

for Cathodes

For testing emitting surface and FEAs, designed a “hollow cathode” for rapid exchange of 14mm inserts.

Optimized DLC for Hollow Cathode

• Coating thickness: 2 m

• Coating electrical resistivity: 5.0E+07 cm

• Base metal: Implant 1.4441

• Polishing: In-house

• DLC Company: Bekaert

We can be certain they will operate at >100MV/m and >400kV

with minimal electron emission.

Stainless steel Bronze

Copper Aluminum Niobium Titanium * Molibdenum * TiVAl

Inconel

Aluminum-Lithium Magnesium

Yttrium

For testing different photo-emitting materials metal inserts (Ø=14mm) are polished in-house and installed inside the DLC coated hollow cathodes.

Metal inserts surface preparation

DLC coated surface

Sample e^-beam

Hollow cathode reduces the chance of breakdown of the metal inserts reducing its exposed area to high E field due to cathode recess screening effect.

Hollow cathode cross-section

Electric field is about 50%

of the max

acceleration field

Unpolished surface of Cu insert Unpolished surface of Al insert

Polished surface of Al insert

Polished surface of Cu insert

Burned DLC + Al insert Burned Cu insert

After some photo-emission experiments

Summary

In-house polishing technology for different metal surfaces is developed. Since August ‘08 we have polished more then 240 electrodes. There has been continuing improvement in preparation and use of electrodes in the LEG test stand.

Breakdown of a polished metal surface did not exceed 150MV/m, but the tests of DLC coated electrodes showed increase of breakdown field with a factor of 2. On some tests breakdown has occurred with gradient over 300MV/m.

DLC coated electrodes with different coating parameters were tested for vacuum

breakdown. DLC parameters are optimized for Hollow cathode in order to test flat-polished samples of different photo-emitting materials and field emitter array chips at high gradients.

The surface polishing of high grade stainless steel and DLC deposition has matured and when electrodes are installed, we can nearly certain they will operate >100MV/m and

>400kV with negligible electron emission.

Thank you for

your attention !