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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