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Dark Current Simulation for the T18vg2.6 Structure. Zenghai Li , Arno Candel, Lixin Ge SLAC National Accelerator Laboratory CLIC Workshop, Oct 14, 2009, CERN. * Work supported by U.S. DOE ASCR, BES & HEP Divisions under contract DE-AC02-76SF00515. Outline. T18vg2.6 Dark Current Simulation - PowerPoint PPT Presentation
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Zenghai Li Oct 13, 2009
Dark Current Simulation for the T18vg2.6 Structure
Zenghai Li, Arno Candel, Lixin Ge
SLAC National Accelerator Laboratory
CLIC Workshop, Oct 14, 2009, CERN
* Work supported by U.S. DOE ASCR, BES & HEP Divisions under contract DE-AC02-76SF00515
Zenghai Li Oct 13, 2009
Outline
T18vg2.6 Dark Current Simulation
Dark current spectrum vs measurement
Field emitter modeling
- PIC simulation of emitter emission (Arno’s talk)
- Emitter heating due to emission current
- Emitter heating due to RF field enhancement
Zenghai Li Oct 13, 2009
T18vg2.6 Structure
• Structure being tested at KEK and SLAC• Simulation Code: (ACE3P)
• S3P - S-Parameter & Fields• Track3P - Particle Tracking
Zenghai Li Oct 13, 2009
T18 Structure Fields
Higher B field at the output end, not as significant as E field
Es Bs
Structure tapered: higher E fields at output end
RF fields obtained using S3P with surface lossS11=0.014; S22=0.032; S12=0.82
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
-0.05 0.00 0.05 0.10 0.15 0.20 0.25 0.30Z (m)
B_
su
rfa
ce
0.0E+00
5.0E+07
1.0E+08
1.5E+08
2.0E+08
2.5E+08
3.0E+08
3.5E+08
4.0E+08
-0.05 0.00 0.05 0.10 0.15 0.20 0.25 0.30z (m)
E_
su
rfa
ce
Zenghai Li Oct 13, 2009
Dark Current Simulation Using Track3P
Dark Current Simulation• Fowler-Nordheim field emission
• Secondary Electrons
• Analyze accumulated effects of DC current & power
Ee
EEJ
5.191053.6252.46
1054.1),(
Copper SEY
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 1000 2000 3000 4000incident electron energy [eV]
delta
Zenghai Li Oct 13, 2009
Dark Current Emitter Simulation
• Intercepted electrons - dark current heating on surface– Deposit energy into the wall results in surface heating
• Captured electrons: energy spectrum– Emitter (disk) location - energy– Emitter density on disk – amplitude
• Heating on dark current emitter– Due to emission current– Due to RF field enhancement on emitter
Emitted from iris #6
Zenghai Li Oct 13, 2009
Dark Current vs RF Heating
Dark Current Heating• Impact concentrated in high E region
around iris • Impact energy could be as high as a few
MeV• Depth of energy deposit ~ 1-2 hundred
microns • Significantly higher heating power at
output end
RF Pulse Heating• High on outer wall where E field is “low”. • Depth ~ skin depth• Temperature rise is around 250C at
100MV/m, 200ns pulse length• At Eacc=80 MV/m; (Hs/Ea~0.004),
Power_max=1.4 GW/m2
Dark Current Heating distribution
RF Heating distribution
Assumed emitters uniformly distributed. In reality, most likely clusters of emitters, result in local hot spots.
Zenghai Li Oct 13, 2009
High Power Test Data - Breakdown Distribution
0 5 10 15 200
5
10
15
20
Cell No.
Bre
akd
ow
n E
ven
ts: p
erc
en
t
1~201 Breakdown Events
y(x) = a x^na = 0.070631n = 1.8328R = 0.81556 (lin)
0 5 10 15 200
5
10
15
20
Cell No.
Bre
akd
ow
n E
ven
ts: p
erc
en
t
201~400 Breakdown Events
y(x) = a x^na = 0.72984n = 0.91333R = 0.90839 (lin)
0 5 10 15 200
5
10
15
20401~600 Breakdown Events
Cell No.
Bre
akd
ow
n E
ven
ts: p
erc
en
t y(x) = a x^na = 0.21073n = 1.4129R = 0.88267 (lin)
F. Wang
• Breakdown rate significantly higher at the output end
• Good correlation with field enhancement and dark current heating at the output end
KEK, Higo, Doebert
Red: real cell timingBlue: linear cell timing
Zenghai Li Oct 13, 2009
Dark Current Measurement & Simulation
Schematic of KEK high power test and dark current measurement
GVGVPM PMAM
Sl i t
FC
E
E
I P
Louni nel oad
Otsukal oad
DC
I PH
VAC
I nsulVAC
I PDC H
H
Vari an I P
187
KX03 425
FC
I P
Load
Vari anI P
Q-mass
WC WCFC
FC
VAC DC
230
Faraday Cup to measure dark current
dE/E filter
slitSimulation “schematics”
Dipole (dP/P)
T18_VG2.4_Disk_#2Dark current spectra measured 18 June 2009
0
0.2
0.4
0.6
0.8
1
1.2
0 5 10 15
T18_VG2.4_Disk #2Spectrum_vs_Width at 64MW
I_slit_peak [microA] 64MW, 252ns
I_slit_peak [microA] 113ns, 64MW
I_slit_peak [microA] 64MW, 331ns
I_slit peak [microA]
pc (MeV/c)
090618
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 5 10 15
T18_VG2.4_Disk #2Spectrum_vs_power at 252ns
I_slit_peak [microA] 64MW, 252ns
I_slit_peak [microA] 55MW, 252ns
I_slit_peak [microA] 70MW, 252ns
I_slit peak [microA]
pc (MeV/c)
090618
Dependence on power Dependence on width
Higo 090703Measurement Data at KEK (Higo)
Zenghai Li Oct 13, 2009
Dark Current Spectrum Comparison
Measured dark current energy spectrum at downstream
SimulationEacc=97MV/m.dE/E=0.1, zbp=2.9m
Differences?Measured dark current spectrum details would depend on the number of emitters on the disks
Zenghai Li Oct 13, 2009
Heating Due to Dark Current Impact ?
Disk 10
Disk 17
• Dark Current Collimation By Disk Iris. • Some electrons have very high impact energies.
Zenghai Li Oct 13, 2009
Electron Impact Energy
Emitted from disk 3
Emitted from disk 17
Emitted from disk 10
Zenghai Li Oct 13, 2009
Impact Energy vs Emission Site Field
E field determined amount of emission
Zenghai Li Oct 13, 2009
Heating Due to Dark Current Impact• Field emission current density based on FN can be significant
- with beta=50, Eacc=100 MV/m,- Jpeak ~1013 A/m2
• Need to study effects of individual emitters - PIC simulation of initial emitter phase space
Emitter - micron or less in size
• MeV energy electrons• Larger spot and deep depth
localized heating may not be as significant
• Current may be much higher when breakdown is being developed? site of future emitter?
Sharon Lee ICSE2006
Colored by momentum
Zenghai Li Oct 13, 2009
“Modeling” Of Field Emitters
(C. Adolphsen)
G. A. Mesyats /P. Wilson
cone double tipasperities
• Calculate field enhancement beta of emitter protrusion• PIC simulation: FN + self consistent space charge effect
– Using a emitter shape with right beta vale (50 as measured)– Initial emitter phase space – impact heating distribution– Emission current - emission heating of emitter
Zenghai Li Oct 13, 2009
Single Tip: Beta vs Shape
Field contour plot
Field enhancement beta vs tip elongate ratio and tip length
Zenghai Li Oct 13, 2009
Double Tip: Beta vs Shape
base_height Base_r height2 base_r2 dztip beta6 3 5 1 2 236 3 5 0.5 2 54 5 2 5 1 2 23 5 4 5 1 2 215 4 5 1 4 275 4 5 0.5 4 52
Single-tip base_height base_r Beta
5 0.5 385 0.87 225 1 175 1.5 115 2 85 4 4
Zenghai Li Oct 13, 2009
Field Emission Heating
0 50 100 150 2000
5 1012
1 1013
1.5 1013
1.206 1013
0.127
j E sin phi
180
17010 phi
• Current is pulled out in ~ ± 40deg rf phase (FN model)• Field emission current density
– Jpeak ~1013 A/m2 with beta=50, Eacc=100 MV/m
– Need PIC to include space charge effects (Arno’s talk)
• This current produce heating on emitter
Zenghai Li Oct 13, 2009
Emitter Heating Due to RF
• Emitter protrusion can produce significant surface magnetic field enhancement
• May lead to higher local heating on emitter tip due to RF magnetic field
Zenghai Li Oct 13, 2009
Heating On Emitter
Both RF and Emission (field+thermal) contribute to emitter heating
– Larger cell iris – higher RF heating– Smaller iris (but high E) – higher
emission heating In high E region, strong E force pull
the tip outward– may result in development of
“sharp” emitters over time– lead to breakdown when dark
current and dark current heating exceed threshold
Zenghai Li Oct 13, 2009
NLC H60VG3S17 Structure
H60VG3S17 Gradient
0.0
20.0
40.0
60.0
80.0
100.0
120.0
140.0
160.0
180.0
0 20 40 60Cell number
G, E
s (G
_a
ve=
70
MV
/m)
H60VG3A17 DS Pulse Heating
0
5
10
15
20
25
0 20 40 60Cell number
dT
(d
eg
C)
Surface Field RF Heating
Peak Surface Field & Heating• Surface E field and RF heating higher at the output end• Most breakdowns in the front
Zenghai Li Oct 13, 2009
NLC H60VG3S17 Structure
• Surface field along disk contour• Disk 8 and 50 comparison (same acceleration gradient)
Zenghai Li Oct 13, 2009
Summary
Progress being made in simulating CLIC T18 structures using Track3P. Dark current spectrum compared with measurement – which may provide information of field emission conditions of disks
Both RF and field emission contribute to emitter heating– High temperature plus strong E force pulling (of emitter
tip) could lead to development of “sharp” emitters over time, may eventually reach breakdown threshold
Self consistent (space charge) emitter emission being performed using PIC to study emission heating
Surface field enhancement due emitter protrusion being calculated using Omega3P to study RF heating
Detailed of modeling of field emitters in progress