Particle-in-Cell Modeling of Rf Breakdown in Accelerating Structures and Waveguides Valery...
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Transcript of Particle-in-Cell Modeling of Rf Breakdown in Accelerating Structures and Waveguides Valery...
Particle-in-Cell Modeling of Rf Breakdown in Accelerating
Structures and Waveguides
Valery Dolgashev,
SLAC National Accelerator Laboratory
Breakdown physics workshop,May 6th-7th, 2010,
CERN
Some of the results were published in
• Valery A. Dolgashev, Sami G. Tantawi, “RF Breakdown in X-band waveguides,” Proceedings of EPAC 2002, Paris, France, pp. 2139-2141
• Valery A. Dolgashev, Sami G. Tantawi, “Simulations of Currents in X-band accelerator structures using 2D and 3D particle-in-cell code,” SLAC-PUB-8866, Proceedings of the 2001 Particle Accelerator Conference, June 18-22, Chicago, Illinois. pp. 3807-3809.
• V.A. Dolgashev, T.O. Raubenheimer, “Simulation of RF Breakdown Effects on NLC Beam,” SLAC-PUB-10668, Proceedings of LINAC 2004, Lübeck, Germany.
• Karl L. F. Bane, Valery A. Dolgashev, Tor Raubenheimer, Gennady V. Stupakov, and Juhao Wu, “Dark currents and their effect on the primary beam in an X-band linac,” Phys. Rev. ST Accel. Beams 8, 064401 (2005) [11 pages]
Outline• Properties of rf breakdown in waveguides and
traveling wave (TW) accelerating structures• PIC model, based on “cathode spot”• Waveguides• Traveling Wave structures
– Ion current dependence– Beam pipe current mystery– Absorbed power
• Beam kick due to RF breakdown in TW structure
Properties of RF Breakdown in
Waveguides and Traveling Wave
Structures
Geometries
Low magnetic field waveguide, height 10 mm High magnetic field waveguide, height 1.3 mm
• The peak electric field surface area equal that of the low magnetic field waveguide
• For a given input power both waveguide have the same peak electric field — 80 MV/m at 100 MW of rf power
• Ratio between magnetic field at peak field between both guides = 21
Sami Tantawi
Electric field Magnetic field
Low magnetic field waveguide
High magnetic field waveguide
Field Distribution
Sami Tantawi
-750 -500 -250 0 250 500 750 1000
Time [ns]
20
40
60
80
100
120r
ew
oP
W]
[M34
Incident
Transmitted
Reflected
RF signals of breakdown
Breakdown event in waveguide, absorbed 30% energy and up to 80% power
~40 ns
Sami Tantawi
900 1000 1100 1200 1300 14000
20
40
60
900 1000 1100 1200 1300 14000
5
10
15
20
25
Measurements of a Breakdown event in TW structure, up to 80% power absorbed
RF breakdown in TW structure
Reflected Pulse
Transmitted Pulse
Time (ns)
Pow
er (
MW
)Po
wer
(M
W)
Chris Adolphsen
• Complete shut-off of transmitted power• Time constant of the power shut-off 20-200ns• Absorbed power 0-80% • Spectral lines of the light are mostly from neutral
copper atoms (waveguide breakdown)
Main Features of RF breakdown in TW structures and waveguides
3D PIC simulation of breakdown in waveguide
1. Model geometry 2. Physical model 3. Space charge limited emission of electrons only4. Space charge limited emission of electrons and copper
ion beam5. Space charge limited emission of electrons, copper ion
beam and neutral gas
3D geometry of the low rf magnetic field waveguide
y-z plane x-z plane
Physical model of breakdown
• Space charge limited emission of electrons• Copper ions • Neutral copper gas
3D PIC simulation of breakdown in waveguide
3D PIC simulation of breakdown in waveguide
Spot size 1.6x1.6mm, space charge limited emission of electrons
Projection of phase space on the x-z plane
Model• Space charge limited emission of electrons
3D PIC simulation of breakdown in waveguide
Spot size 1.6x1.6mm, space charge limited emission of electrons,
average current 40 A
Projection of phase space on the z-γ plane
3D PIC simulation of breakdown in waveguide
0 20 40 60 80 100 1200
50
100
inputreflectedtransmitted
Time [nsec]
Pow
er [
MW
]
Emission spot 4x4 mm, space charge limited emission of electrons, input power 80 MW, breakdown at 2 ns
• In order to significantly disrupt RF power spot size should be > 2cm2
• Fast transient process ~ns• ~50% of emitted current returns back to the emitting spot
Result
3D PIC simulation of breakdown in waveguide
3D PIC simulation of breakdown in waveguide
Model
• Space charge limited emission of electrons• Copper ion beam with current needed to disrupt transmitted
power
Spot size 1.6x1.6mm, copper ion current ~8A
Fast electron motion, projection of phase space on the x-z plane
3D PIC simulation of breakdown in waveguide
Spot size 1.6x1.6mm, copper ion current ~8A
Fast electron motion, projection of phase space on the z-γ plane
3D PIC simulation of breakdown in waveguide
Low magnetic field waveguide, spot size 1.6x1.6mm, copper ion current ~8A
Electron - ion motion, projection of phase space on the x-z plane
3D PIC simulation of breakdown in waveguide
High rf magnetic field waveguide, spot size 0.6x0.6mm, copper ion current ~35A
Spot size 1.6x1.6mm, copper ion current ~8A
Slow ion motion, projection of phase space on the z-γ plane
3D PIC simulation of breakdown in waveguide
0 10 20 30 40 50 60 70 800
50
100
inputreflectedtransmitted
Time [nsec]
Pow
er [
MW
]
Spot size 1.6x1.6mm, copper ion current ~8A
Input, reflected and transmitted power vs. time
3D PIC simulation of breakdown in waveguide
Spot size 1.6x1.6mm, copper ion current ~8A
Emitted electron current vs. time
3D PIC simulation of breakdown in waveguide
Spot size 1.6x1.6mm, copper ion current ~8A
Electron current destroyed at the emission spot
Power of electrons destroyed at the emission
spot
3D PIC simulation of breakdown in waveguide
3D PIC simulation of breakdown in waveguide
0
20
40
60
80
100
120
0 50 100 150 200 250 300 350
inputreflectedtransmitted
time [ns]
0
20
40
60
80
100
120
0 50 100 150 200 250 300 350
inputreflectedtransmitted
time [ns]
Measurements, 24 April 2001,18:13:40, shot 45
3D PIC simulations, 4x4 mm emitting spot, electron current 7kA, copper ion current 30A
V.Dolgashev, S. Tantawi
• Ions cross the waveguide in ~30 ns• Time constant of the power shut-off 10-20 ns• Ion current determines electron current by compensating
space charge of electrons• Oscillation of transmitted and reflected power determined by
ion-electron density ~ 10-40 ns• ~80% of emitted current returns back to the emitting spot• Maximum absorbed power 50%
Result
3D PIC simulation of breakdown in waveguide
Model• Space charge limited emission of electrons• Copper ion beam with current needed to disrupt transmitted
power• Drag associated with presence of neutral copper ions
3D PIC simulation of breakdown in waveguide
• Maximum absorbed power up to 75%• Ion-electron oscillation damped
Result
Transmitted powerInput - reflected power
Higher power absorption
Traveling wave accelerating structures
3D PIC model based on properties of “cathode spot”
• Matched traveling wave structure with coaxial couplers
• Emission of ion beam with predetermined current from small spot on iris
• Space charge limited electron current from the same iris
Ion current dependence
Procedure: Increase ion current until transmitted power completely shuts off
3D PIC simulations, T20VG5, 5 A ion current, cell breakdown, 5 cell structure, spot ~2mm2
V.A.Dolgashev, 6 December 02
3D PIC simulations, T20VG5, 5 A ion current, 5 cell structure, cell breakdown, spot ~2mm2
V.A.Dolgashev, 6 December 02
3D PIC simulations, T20VG5, 5 A ion current, cell breakdown, 5 cell structure, spot ~2mm2
V.A.Dolgashev, 6 December 02
rf Emitted currents
Beam pipe currents Back-bombardment currentsV.A.Dolgashev, 6 December 02
3D PIC simulations, T20VG5, cell breakdown, 5 A ion current, 5 cell structure, spot ~2mm2
0 20 40 60 800
2
4
6
8
ElectronsIons
Time [ns]
Cur
rent
, ion
s [A
], e
lect
rons
[kA
]
0 20 40 60 800
5
10
15
20
ElectronsIons
Time [ns]
Cur
rent
, ion
s [A
], e
lect
rons
[kA
]
0 20 40 60 800
10
20
30
40
50
60
70
80
InputTransmittedReflected
Time [ns]
Pow
er [
MW
]
0 20 40 60 8050
0
50
100
InputOutput
Time [ns]
Cur
rent
, ele
ctro
ns [
A]
3D PIC simulations, T20VG5, coupler breakdown, 10 A ion current, 5 cell structure, spot ~2mm2
V.A.Dolgashev, 6 December 02
3D PIC simulations, T20VG5, coupler breakdown, 10 A ion current, 5 cell structure, spot ~2mm2
V.A.Dolgashev, 6 December 02
rf Emitted currents
Beam pipe currents Back-bombardment currentsV.A.Dolgashev, 6 December 02
3D PIC simulations, T20VG5, coupler breakdown, 10 A ion current, 5 cell structure, spot ~2mm2
0 10 20 30 40 50 600
5
10
15
ElectronsIons
Time [ns]
Cur
rent
, ion
s [A
], e
lect
rons
[kA
]
0 10 20 30 40 50 600
10
20
30
40
50
60
ElectronsIons
Time [ns]
Cur
rent
, ion
s [A
], e
lect
rons
[kA
]
0 10 20 30 40 50 6050
0
50
100
150
InputOutput
Time [ns]
Cur
rent
, ele
ctro
ns [
A]
0 10 20 30 40 50 600
10
20
30
40
50
60
70
80
InputTransmittedReflected
Time [ns]
Pow
er [
MW
]
3D PIC simulations, T20VG5, coupler breakdown, spot ~2mm2 , ion current 20 A
V.A.Dolgashev, 6 December 02
3D PIC simulations, T20VG5, coupler breakdown, spot ~2mm2 , ion current 20A
V.A.Dolgashev, 6 December 02
3D PIC simulations, T20VG5, coupler breakdown, spot ~2mm2 , ion current ~20 A
V.A.Dolgashev, 6 December 02
rf Emitted currents
Beam pipe currents Back-bombardment currentsV.A.Dolgashev, 6 December 02
3D PIC simulations, T20VG5, coupler breakdown, spot ~2mm2 , ion current ~20A
0 10 20 30 40 500
10
20
30
ElectronsIons
Time [ns]
Cur
rent
, ion
s [A
], e
lect
rons
[kA
]
0 10 20 30 40 500
10
20
30
40
50
60
ElectronsIons
Time [ns]
Cur
rent
, ion
s [A
], e
lect
rons
[kA
]
0 10 20 30 40 500
10
20
30
40
50
60
70
80
InputTransmittedReflected
Time [ns]
Pow
er [
MW
]
0 10 20 30 40 50100
0
100
200
InputOutput
Time [ns]
Cur
rent
, ele
ctro
ns [
A]
Mystery of small beam pipe currents:
Beam currents through output pipes during breakdown are
small ~100 mA, while currents in the cell are ~10 kA.
Why output current are only ~0.001% of cell currents?
V.A.Dolgashev, 6 December 02
3D PIC simulations, T20VG5, coupler breakdown, spot ~4mm2
V.A.Dolgashev, 6 December 02
3D PIC simulations, T20VG5, coupler breakdown, spot ~4mm2
rf Emitted currents
Beam pipe currents Back-bombardment currentsV.A.Dolgashev, 6 December 02
0 20 40 60 80 100 1200
10
20
30
ElectronsIons
Time [ns]
Cur
rent
, ion
s [A
], e
lect
rons
[kA
]
0 20 40 60 80 100 1200
10
20
30
40
50
60
ElectronsIons
Time [ns]
Cur
rent
, ion
s [A
], e
lect
rons
[kA
]
0 20 40 60 80 100 1200
10
20
30
40
50
60
70
80
InputTransmittedReflected
Time [ns]
Pow
er [
MW
]
0 20 40 60 80 100 12050
0
50
100
150
InputOutput
Time [ns]
Cur
rent
, ele
ctro
ns [
A]
Beam kick due to rf breakdown
This work we did with Juhao Wu
Breakdown simulation in single-cell TW structure,emission from downstream side of the first iris
(cell breakdown)
Breakdown currents and beam
RF characteristics, cell breakdown
Horizontal kick, cell breakdown, on axis
0 2 4 6 8 10 12 14 16 18 20200
100
0
100
200
phi = 0phi = Piaccleration /10
Time [ns]
Kic
k [k
V]
Horizontal kick, cell breakdown, on axis
11 11.1 11.2 11.3 11.4 11.5 11.6 11.7 11.8 11.9200
100
0
100
200
phi = 0phi = Piaccleration /10
Time [ns]
Kic
k [k
V]
Horizontal kick, coupler breakdown, on axis
0 2 4 6 8 10 12 14 16 18 20200
100
0
100
200
phi = 0phi = Pi
Time [ns]
Kick
[kV
]
Horizontal and vertical kicks, coupler breakdown, on axis
12 12.1 12.2 12.3 12.4 12.5 12.6 12.7 12.8 12.9200
100
0
100
200
phi = 0phi = Piaccleration /10
Time [ns]
Kick
[kV
]
SUMMARY• Model of “plasma spot” with ion current of ~30 A reproduces rf
breakdown signals for “soft event” in waveguide with ~1 cm height.
• Same model with ion current of ~20 A reproduces rf breakdown signals for “soft event”(~25% of input power absorbed in steady-state breakdown) in traveling wave structure
• Breakdown can potentially kick beam ~100 kV transversely, the kick strongly depends on accelerating rf phase
• To explain “hard events” with absorption of more then 25% of input power and extremely small beam pipe currents model need additional assumptions: for example drag and scattering for electrons on neutral copper gas or something else
• This simple model can’t predict power shutoff in narrow (~1mm height) waveguide, need additional assumption about expansion of the ion-emitting spot