Effect of RF MagneticFields and Input

Power on RFBreakdown Limit

V.A. Dolgashev

SLAC

ContributorsC. Adolphsen, S. Döbert,

C. Nantista, R. Miller,

R. Ruth, S. Tantawi,

J. Wang and

P. Wilson

“Everybody knows”• Breakdown limit depend only on maximum

surface electric field

• Breakdown limit set by microscopic cleanliness ofthe metal surface: no dust – no breakdowns

• Breakdown trigger is a “field emitter” that heatedand evaporated by dark current ohmic losses

We found that:• Breakdown limit depend more on input rf

power then on maximum electric field• Breakdown limit is reproducible from

structures of the geometries• RF magnetic field could be more damaging

then electric fields

OUTLINE

• High power RF conditioning• Breakdown limits

–RF magnetic fields:•couplers problem and solution•stainless steel particle

–Input power

High powerconditioning

ofTW53VG3MC

170 ns

100 ns 400 ns240 ns

V.Dolgashev, C. Adolphsen, 10 February 03

50 ns

High powerconditioning of

H60vg3_6c up to10 Feb 03

170 ns

100 ns

400 ns

240 ns

V.Dolgashev, C. Adolphsen, 10 February 03

No Jumps

Jumps

High powerconditioning ofH90vg3 up to

10 Feb 03

250 ns170 ns 400 ns

V.Dolgashev, C. Adolphsen, 10 February 03

Conditioning Observations• During conditioning the limiting gradient increases until final gradient

is reached.

• Variations in surface processing obviously effect speed of the

conditioning (sometimes by factors of 10) but weakly effect final

gradient.

• Vacuum pressures up to 10-7 Torr do not seem to affect breakdown

rate.

• At each stage of processing the breakdown rate increases with

increasing pulse width.

• The breakdown rate grows exponentially with the power. On a linear

scale, it appears like the breakdown rate has a threshold.

• The amplitude of dark currents does not depend on pulse length (if no

breakdowns during measurements)

RF breakdownsand rf magnetic

fields

TW structure: Breakdowns duringprocessing and steady-state run

Fra

ctio

nal m

issi

ng r

f en

ergy

C. Adolphsen

C. Adolphsen

Juwen Wang

Coupler geometry

76 _m radii

Typical edge damage on thecell side of the Horns

R. Kirby et al.

Typical edge damage on thewaveguide side of the Horns

R. Kirby et al.

Surface electric field distribution, max. field in thecoupler cell 140 MV/m, power 48 MW

Surface magnetic field distribution,field on a flat part of the coupler iris~0.28 MA/m

Mesh

H [

A/m

]

s [mm]Surface magnetic field

Field distribution in T53VG3 coupler

~130o C

~60o C

Breakdown rate vs. pulse temperature rise

SW structure breakdown rate vs. pulse temperature riseSW565, input rf pulses with different shape and amplitude

SEM pictures of X-band klystron output iris

Horn-BeamSide-Dwn-30x

Calculated temperature rise for 50 MW,

1.2 _s (~180 deg. C) and 2.42 _s (~270 deg. C)

Bob Kirby, Daryl Sprehn, Chris Pearson

Surface field in input coupler of H60VG3

Surface electric field on theedge of the waveguide to coupler iris,iris rounding 100 _m, Emax ~12 MV/mat 73 MW of input power

(~13 MV/m for 76 _m)

Surface magnetic field,Hmax ~1. MA/m (rounding 100 _m) at 73MWof input power (~1.1 MA/m for 76 _m)

H [

A/m

]

s [mm]

s [mm]

E [

V/m

]

Newcouplers

Surface magnetic field

Maximum (without 2D correction)

Hs~1.4 MA/mMaximumHs~0.6 MA/m

H [

A/m

]

s [mm]s [mm]

H [

A/m

]

Surface electric field for max. Es ~200 MV/m

“Fat lips” combined with “race-track”

Fat-lip coupler for SW structure

~30o C

TM01 Mode Launcher Coupler

WR90

WC90

beampipe

Pulse

heating~3°C

C. Nantista and S. Tantawi

Waveguide coupler

Stainless steelparticle on 2bcell diameter

F. Le Pimpec et al.

Stainless steel particle in H90vg3

F. Le Pimpec et al.

Metal ball on surface of first cell of H60VG3, ball radius 100_m, 20_mdeep in outer wall, ~70 MV/m accelerating gradient

Surface magneticfield, maximumfield 0.6 MA/m H

[A

/m]

s [mm]

0 10 20 30 40 500

20

40

60

80

100

Depth [um]

Temperature [deg.C]

Pulsed heating, 0.6 MA/m, 400 ns,maximum temperature ~100o C.

Metal ball on surface of first cell of H60VG3, ball radius 100 _m, 20 _m deep inouter wall, ~70 MV/m accelerating gradient

Surface electric field, maximumfield ~7 MV/m

E [

V/m

]

s [mm]

“Magnetic” breakdowns

• High rf magnetic fieldproduces damage

• High rf magnetic fields and>10 kV/cm electric fields

produce breakdowns

Input rf powerbreakdown limit:threshold behavior

Steffen Döbert

Breakdown rates for H90VG3 vs. pulse width

65 MV/m run

No clear hot spots

But typical front bias

0 10 20 30 40 50 600

5

10

15

cell number

Steffen Döbert

Breakdown locations - FXB003, 65 MV/m

0 5 10 15 20 25 300

10

20

30

40

50

60

70

80

90

0 5 10 15 20 25 300

5

10

15

20

25

30

Statistics of “breakdown chains” in H60VG3(6C)with 400 ns Pulses

Num

ber

of T

rips

(16

2 T

otal

)

Time Between Trips (Minutes)(Times > 30 Plotted at 30)

Num

ber

of T

rips

(11

4 T

otal

)

Time Between Trips (Minutes)(Times > 30 Plotted at 30)

65 MV/m 70 MV/m

“breakdown chains”

C. Adolphsen

Comparison of Power*sqrt(pulse widths) for3 accelerating structures

H60vg3_6c

H90vg3

T53vg3MC

Accelerating structures

“Prediction” of M2 structure processingwith “destruction limit” of 100 MW*sqrt(400ns)

Data: Rod Loewen, August, 2000

2000

1500

P [MW]*sqrt(pulse_length [ns])

Forward processing of M2 Structure

0 500 1000 15000

500

1000

1500

2000

2500

3000

Time [ns]

Power [MW]*sqrt(pulse width [ns])

Comparison of Power*sqrt(pulse widths) for accelerating structure and 2 copper waveguides

“destruction limit”400*pulse_width1/3

Low_magnetic

Accelerating structures Waveguides

High_magnetic

Behavior of the“breakdown chains” in

TW structures and Waveguidessuggests that while increasingpower and pulse width we hit a

some kind of threshold in amountof damage due to a breakdown.

And this damage preventsfurther conditioning of the

structures.

Comparison withDC breakdown:

Thresholdbehavior

Conditioning of molybdenumelectrodes at DC

V.Shirokov, Budker INP

Not processedConditioned,

no “breakdown chains”“Breakdown chains”Degraded

Conditioning of electrodes at DC

V.Shirokov, Budker INP

Breakdown voltage for stainless steel electrode vs. number ofbreakdown for different capacitors for different stored energy.

Breakdown electric filed vs. stored energy for different materials

Vol

tage

[kV

]

Breakdown number

Electric field [kV/cm]

Stor

ed e

nerg

y [J

]“breakdown chains”

conditioning

degradation

“Threshold” in amount of erosion duringvacuum arcs for copper electrodes

Vaporization

Ablation

IEEE, 1990

Summary on breakdown limitsBreakdown behavior of accelerating structures is very similar.The behavior is threshold-like: breakdown rate growsexponentially fast with input power and pulse width, wherecurve of constant breakdown rate is ~P*pulse_width0.32…0.5

This behavior suggests two limits:

RF magnetic field limit•Damage on sharp edges correlated with high magnetic fields

•Breakdowns need combination of high magnetic andmoderate electric fields

Input power limit•Breakdown limit in traveling wave structures and waveguides isweakly depend on peak surface electric field and possibly setby ablation threshold of the copper surface

What we can do?

Experiments on breakdown limit• Different geometries

– Compare SW and TW structures– Waveguides

• Different materials– Whole metal structures– Bi-metal structures– Alloys and metal compounds

Experiments on breakdown trigger• Pulse heating• Nature of dark currents• Different materials

Theoretical particle-thermo-mechanical models