DC breakdown experimentsM.Taborelli, S.Calatroni, A.Descoeudres, Y.Levinsen, J.Kovermann, W.WuenschCERN

Ranking of materials

Cathode mechanism

Field emission

Residual gas effects

Time delay

Breakdown rate

Motivation for DC experiment:

-understanding breakdown mechanism in simpler system and simpler infrastructure than RF:

many testsreproducibility checkvarious materialschange parameters…….

-what can be transferred to RF? - see from the results if the mechanisms are

plausible also for RF-…obviously no B-field here

DC breakdown setup

DC spark test in UHV

HV

C

Hemispherical tip (2.3 mm diam) and flat sample, same

material for both

In UHV (10-9 mbar), baked system

Max voltage 12KV, typical gap 20-30 μm, and spark energy 1J Charge applied on capacitor step

by step until breakdown occurs.

Breakdown detected with current pulse or/and with charge

remaining on the capacitor

28nF

I probe

Q

Q initial

Eb

Q r

emai

ning

aft

er 2

s

nb of breakdown

Eb

Conditioning curves of various metals

OFEgraphite

Other materials : Alloys

• Others : – Cu + 500µm Cr coating (≈ Cr)– Mo + 2µm DLC coating (low Eb)

Al15

316LNTungsten carbide composite

C15000

Ranking of materials with respect to breakdown field

Also to be considered:► conditioning speed (depends

on material treatment, here all were just cleaned by

detergents/solvents as for UHV parts)

► ranking of Cu, W, Mo is as in RF (at high breakdown rate, 30GHz)

material “erosion”: for Ti, V and Cr the gap must be often readjusted

0 25 50 75 100 125 150 175 200 2250

100

200

300

400

500

600

700

800

900

1000

1100

1200

Ebr

eakd

own [

MV

/m]

Number of Sparks

Titanium (Ti) Tip - Tungsten (W) Sample

Ti W

0 10 20 30 40 50 60 70 80 90 100 1100

200

400

600

800

1000

1200

Ebr

eakd

own [

MV

/m]

Number of Sparks

Tungsten (W) Tip - Titanium (Ti) Sample

W Ti

Cathode limited breakdown field

Exchanging the materials of tip (anode) and sample (cathode) shows that the breakdown field is cathode limited

Evidence for field emission current before sparkingHere the capacitor is discharged through field emission current from the sample

Measured FE (far from breakdown field) (field enhancement β=17)

200MV/m

ln [

I/E

2]

0 100 200 300 400 5000

50

100

150

200

250

300

350 breakdown

Ef [

MV

/m]

Ei [MV/m]

NB: at higher fields emission from hot tips can be thermo-ionic

2700 2750 2800 2850 2900 29504,0x10-10

4,1x10-10

4,2x10-10

4,3x10-10

4,4x10-10

4,5x10-10

4,6x10-10

Ion

Cur

rent

[A

]

Relative Time [sec]

0 500 1000 1500 2700 2800 2900 30000

1x10-9

2x10-9

3x10-9

4x10-9

5x10-9

6x10-9

7x10-9

8x10-9

2,0x10-8

4,0x10-8

6,0x10-8

8,0x10-8

1,0x10-7

1,2x10-7

Pre

ssur

e H

2 [m

bar]

389,341 MV/m

372,037 MV/m

341,755 MV/mIon

Cur

rent

[A

]

Relative Time [sec]

Hydrogen GasDegassing during and before spark

Gradual increase of pressure burst (H2 and CO mainly) with increasing field before sparking

Consistent with increasing FE current for increasing field

Emitters-Typical measured β are in the range 10-100.

for cylinder: β = (h/r) + 2-No sharp features seen in SEM images of DC samples, either the tips are very small, or they are there only present when the field is applied, or the apparent β is not due to geometry

β=20

RF, Mo structure 30GHz

estimated beta, from geometry

100 μm

Cones observed after high power RF tests on Mo and Ti, not on Cu

Simulated Cu tip evolution

tip evolution on Cu in 2ns, 800K

Time for “diffusion smoothing” of the tip down to a flat monolayer on the surface

Simulation with applied field is in progress. (K.Nordlund, Uni Helsinki, Finland within the CLIC collaboration)

(K.Nordlund et al,J.Phys: Cond. Matt. 16, 2995, 2004)

Surface migration, macroscopic approach for a solid tip

Barbour et al.Phys. Rev. 117, 1452 (1960)

R

dz Without field the tip “dulls”dz/dt ~ D0exp(-Q/kT)

R3

The field stabilizes the tip

dz/dt ~ (1-k ε0R E2) dz/dtE=0E

= surface energyQ= activation energy of surface diffusion

k0.5-1

=(900 MV/m)-2 for W, R=50nm=(650 MV/m)-2 for Cu, R=50nm

Dyke et al. J.Appl.Phys. 31, 790, (1960)

W

Effect of various gases

-for inert gases (Ar) there is no effect at least up to 10-5 mbar-for reactive gases (air, O2, CO ) the breakdown field is lowered

for prolonged exposure and sparking-no effect for Cu in the above studied pressure range for air and CO

Molybdenum

R.Hackman et al, J.Appl.Phys. 46, 629, 1975

Indeed it was already known…

it needs 10-2 mbar of gas to favor breakdown for small gap geometry

V [KV]gap 0.13mm

Needs 10-2 mbarair pressure to have an effect

Which mechanism could provide the gas to initiate breakdown (and form a plasma)?

To get 10-2 mbar in the tip-plane space : 106-108 atoms of Cu

Vapor pressure of hot tip of 100 μm2 surface (Pvap and conductance through the a spot) at Tm: 103 atoms of Cu melting temperature is not sufficient

Thermal desorption of 1ML of adsorbates: 109 moleculesElectron stimulated desorption (from anode), with 1mA FE:

108 moleculesthe last two would be less relevant after conditioning

Sublimation of a cylinder tip of 100 nm diameter and β=30:109 Cu atoms

…how?

Field enhancement on the tip by ionized gas in front and field induced atom desorption ? Particle in cell calculations by R.Schneider Max-Planck Inst. Greifswald, Germany and Uni Helsinki Finland in progress within CLIC collaboration

Copper, in less than 100 ns, with 20 μm electrode distance

V

1) 2)

power supply(up to 12 kV)

28 nF

UHV

VHV probe current probe

delay

Time delay for breakdown

Delayed breakdowns

Immediatebreakdowns

“avg.” 119 ns,but resolution is of the order of 100ns

avg.1.17 ms

Histogram of delays

Mo

Similar to RF pulse length range

Distribution of delays

Much slowerthan usual RF case

2 mechanisms of breakdown

Delay times for different materials

Cu Ta Mo SS

fraction R of delayed breakdowns (excluding conditioning phase) increases with the average breakdown field

R = 0.07 R = 0.29 R = 0.76 R = 0.83

Eb = 170 MV/m Eb = 300 MV/m Eb = 430 MV/m Eb = 900 MV/m

• It is important to know when it breaks down, but also at which field it can be safely used

• measured by applying/removing the field and monitoring y/n breakdown with voltage probe

no breakdown

breakdown

Measurement of the breakdown rate (BDR)

The present setup is limited to a breakdown probability of about 10-4, for reasonable measuring times

often grouped

Breakdown rate vs field : RF (30 GHz)

different materials give

different slopes

from S. Doebert

Cu 70ns

Mo 80ns

for Cu

for Mo

BDR ~ E30

BDR ~ E20

With exponential law With power law (BDR=0 @E=0)

different materials give different exponents

Breakdown rate vs field : DC

NB: RF data are displayed vs surface

field

Ranking of slopes of BDR opposite to RF case

for Cu

for Mo

BDR ~ E10-15

BDR ~ E30-35

Breakdown rate vs normalized field

Idea of the normalization : ‘how many decades of BDR do we gain if we decrease the max. field by X%’

DC RF

Cu 10 - 15 30

Mo 30 - 35 20

Conclusions

-cathode limited breakdown resistance

-field emission as precursor

-time lags indicate two mechanisms

-time lags compatible with RF

G.Arnau-Izquierdo,S.Calatroni, S.Heikkinen,H.Neupert, T.Ramsvik,S.Sgobba,CLIC study team

Acknowledgments

Gas effect, chemical: oxygen or air exposure of Mo during breakdown

A B

0 200 400 6000

100

200

300

400

500

600

Number of Sparks

10-910-810-710-6

0 200 400 600 800 1000 1200 1400 16000

100

200

300

400

500

600

Ebr

eakd

own [

MV

/m]

Number of Sparks

10-910-810-710-6

Pre

ssur

e [m

bar]

A prolonged exposure to 10-6 mbar range produced surface oxidation and lowers the breakdown field: similarly part of the initial conditioning process is also removal of the oxide

air10-6

X-ray Photo Emission Spectroscopy

238 236 234 232 230 228 226

0

5k

10k

15k

20k

25k

30k

35k

40k

45k

50k

55k

60k

65k

I

II

III

MoIV+ Mo0

Inte

nsi

ty [

a.u

.]

Binding Energy [eV]

Mo0

After prolonged breakdown in O2 10-6 mbaroxidized again

sputter cleaned + 1h ambientair

initial stateoxidized

Conditioning of Mo is removal of oxide

Region of sparkmetallic

Fast conditioning: heat-treated Mo

(to reach 400 MV/m)

~ 60 sparks ~ 15 sparks ~ 12 sparks ~ 10 sparks

In UHV oven , ex situ treatment

and e-beam ex situ heating: immediate conditioning

0 20 40 60 80 100 120 140 1600

100

200

300

400

500

600

Ebr

eakd

own

[MV/

m]

Number of Sparks

600

500

400

300

200

100

0

Eb[M

V/m

Number of sparks

0 20 40 60 80 100 120No significant change of saturated breakdown field !

Consistent with the cathode dominated scenarioThe precursor to breakdown is possibly FE current reaching a threshold value (which can be field dependent)

Breakdown initiated by field emission

E [

MV

/m]

for

10

-7 A

FE c

urr

ent

Eb

reakd

ow

n [

MV

/m]

Which tip size can melt in such a short time through FE currents?

10-4

10-3

10-2

10-1

100

101

102

10-12

10-10

10-8

10-6

10-4

0.01

1

101 102 103 104 105

Parameters to attain the melting point of the tipof a Cu cylinder of given radius and =30

I peak [A]

P peak [W]time constant [sec]

Energy [J]

radius [nm]

E>Erunaway

IFE

tmelting

Select β

Calculation as in Williams et al J.Phys D5, 280 (1972)

Tips which can heat so fast are very small, below 50 nm diam