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Breakdown Rate Dependence on Gradient and Pulse Heating in Single Cell Cavities and TD18. Faya Wang, Chris Nantista and Chris Adolphsen May 1, 2010. LC Breakdown Limits. - PowerPoint PPT Presentation
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Breakdown Rate Dependence on Gradient and Pulse Heating in Single
Cell Cavities and TD18
Faya Wang, Chris Nantista and
Chris Adolphsen
May 1, 2010
LC Breakdown Limits• For NLC, the breakdown rate limit was chosen so that the 2% pool of spare rf units
would rarely (once a year) be depleted assuming a 10 second recovery period after each breakdown. Operation with a 10 second recovery period has been demonstrated but longer times (30-100 sec) are typically used since the structure monitoring system has a 30 second sampling period. For 60 Hz operation at NLCTA, the breakdown rate limit translates to 0.1 per hour with the nominal 60 cm long structure design. This choice is somewhat soft in that a four-times higher rate would still provide 99% full-energy availability assuming a 5 second recovery time, which has also been demonstrated.
• For reference, at the 0.1 per hour limit, a breakdown would occur in one of the ~ 20,000 NLC X-Band structures once every 120 pulses. Such breakdowns will degrade the luminosity from that pulse, but the beam kicks from the breakdown fields should not inhibit beam operation.
• To 'translate' this limit, for one bkd in 10 hours in a 0.6 m structure at 60 Hz, the rate is 4.6e-7/pulse/structure or 7.7e-7/pulse/m. For CLIC 3-TeV, this same limit gives 1 bkd per 40 pulses, but because of the smaller beam emittance (and hence a smaller collimator aperture), the kicks are more likely to hit the collimators. The bkd kick distribution measured at NLCTA has values as large as 30 keV/c - this level is < 1/10 of the amount needed to hit the collimators at NLC.
98 100 102 104 106 108 110 11210
-7
10-6
10-5
10-4
10-3
Gradient (MV/m)
BD
R (
1/p
uls
e/m
)
500 hr
250 hr
1400 hr
900 hr
1200 hr
TD18 700 hr
Breakdown Rates: T18 and TD18
Pulse width 230ns, Green line for TD18, Others for T18
Single Cell SW Cavity1C-SW-A3.75-T2.60-Cu6N-KEK
Parameters Unit Value
FrequencyGHz
11.427 (Nitrogen, 20 oC)
Cells 1+matching cell + mode launcher
Q (loaded) 4660
Coupling 0.97
Iris Thickness T mm 2.6
Iris Dia. a / % 14.4
Phase Advance Per Cell deg 180
Es/Ea 2.03
Maximum surface electric field for 10 MW
MV/m 399
Maximum surface magnetic field for 10 MW
A/m 6.7e5
Peak pulse heating for 1 μs pulse with flat field of 100 MV/m
oC 24
0 500 1000 1500 20000
5
10
15
20
25
30
35
Time (ns)
Inp
ut p
ow
er
of s
tru
ctu
re (
arb
.u.)
Pre Pulse After Pulse
Main Pulse
Input RF Pulse Maximized initial pulse to reduce fill time
30 35 40 45 50 55 60 65 70110
120
130
140
150
160
170
180
Pulse heating (oC)
Gra
die
nt f
or
flat t
op
of 1
50
ns
(MV
/m)
Constant pulse heating test (45oC)
Constant gradient test (135 MV/m)
Constant gradient test (145 MV/m)
Constant gradient test (151MV/m)
Gradient and pulse heating for normal runing
Measurement Points: Vary Either Pulse Heating or Gradient
0 500 1000 1500 20000
5
10
15
20
25
30
Time (ns)
Ca
vity
Inp
ut P
ow
er
(arb
.u.)
0 500 1000 1500 20000
5
10
15
20
25
30
Time (ns)
Ca
vity
Re
flect
ed
Po
we
r (a
rb.u
.)
0 500 1000 1500 20000
10
20
30
40
50
60
70
Time (ns)
Pu
lse
He
atin
g (
oC
)
Low Pulse HeatingMiddle Pulse HeatingHigh Pulse Heating
Input Power
Reflected Power
Peak Pulse Heating
Breakdown Study with Constant Gradient but Different Pulse Heating from the Pre-Fill ‘Warm-up’
35 40 45 50 55 60 6510
-1
100
101
102
Peak Pulse Heating ( oC)
Bre
akd
ow
n R
ate
(1
/hr)
135 MV/m151 MV/m
1st Test of 145 MV/m
2nd Test of 145 MV/m
Breakdown Rate for Fixed Gradient
0 500 1000 1500 20000
10
20
30
40
50
Time (ns)
Pu
lse
He
atin
g (o
C)
Breakdown Study with Constant Pulse Heating
0 500 1000 1500 20000
5
10
15
20
25
30
Time (ns)
Inp
ut p
ow
er
(arb
.u.)
High gradientMiddle gradientLow gradient
0 500 1000 1500 20000
2
4
6
8
10
12
14
Time (ns)
Re
flect
ed
po
we
r (a
rb.u
.)
0 500 1000 1500 20000
50
100
150
Time (ns)
Gra
die
nt (
MV
/m)
115 120 125 130 135 1400
0.5
1
1.5
2
2.5
3
3.5
4
Flat top gradient (MV/m)
Bre
akd
ow
n r
ate
(1
/hr)
First TestSecond TestThird Test
Flat top = 160 ns
Increase much less than expectation of (139/119)^25 ~ 50
Breakdown Rate for Fixed Peak Pulse Heating
130 135 140 145 150 155 16010
-1
100
101
102
Gradient (MV/m)
Bre
akd
wo
n R
ate
(1
/hr)
40 45 50 55 6010
-1
100
101
102
Peak Pulse Heating ( oC)
Bre
akd
wo
n R
ate
(1
/hr)
Breakdown Rate with Varying Gradient and Pulse Heating
Blue for the 1st test and Red for the 2nd test.
0 500 1000 1500 20000
10
20
30
40
Time (ns)
Pow
er (
arb.
u.)
Damage to Single Cell Irises
V Dolgashev, L Laurent
a [mm] 4.06 3.36 2.66
a/(%) 15.4 12.8 10.1
d [mm] 2.794 2.054 1.314
e 1.21 1.18 1.15
f [GHz] 11.424 11.423 11.424
Q(Cu) 5098 5364 5458
vg/c [%] 2.25 1.47 0.87
r’/Q [LinacΩ/m] 10195 12560 15034
Es/Ea 1.97 1.88 1.88
Hs/Ea [mA/V] 5.85 5.2 4.85
Sc/Ea2 [mA/V] 0.52 0.39 0.3
For regular cellsP = 59.8 MW for <G> = 100
MV/mActive Length = 15.8 cm
First Middle Last Cell
TW Structure with Damping (TD18)
0 100 200 300 400 500 600 700 800 900 10000
5
10
15
20
25
n
|Ez|
[kV
/m]
Last Cell Surface Magnetic Field
T18 and TD18 BDR Pulse Heating Dependence
20 30 40 50 60 70 80 90 10010
-7
10-6
10-5
10-4
10-3
Peak Pulse Heating at Last Cell (K)
BD
R (
1/p
uls
e/m
)
T18, 900 hrsTD18, 100 MV/m @ 100ns,870hrsTD18, 100MV/m@ 150ns, 960hrs TD18, 100MV/m@200ns, 750hrsTD18, 115MV/m@150ns, 550hrsTD18, 120MV/m@150ns, 570hrsTD18, 100MV/m@230ns, 700hrsTD18, 105MV/m@230ns, 680hrsTD18, 100MV/m@50ns
110 MV/m
Pulse Heating BDR Test
0 500 1000 1500 2000-10
0
10
20
30
40
50
60
70
Time (ns)
Po
we
r to
Str
uct
ure
(M
W)
150ns - normal150ns - pre heating
0 500 1000 1500 20000
20
40
60
80
Time (ns)
Pe
ak
Pu
lse
He
atin
g in
La
st C
ell
(K)
100 MV/m, 200 ns100 MV/m, 150 ns100 MV/m, 150 ns pre-heating
60 65 70 750.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
2.2
2.4
Peak Pulse Heating in Last Cell (K)
BD
R (
1/h
ou
r)
100MV/m@150ns100MV/m@150ns + pre-heating100MV/m@200ns
A Coaxial Two Mode Cavity is Being Designed to Study E and B Effects Somewhat Orthogonally
TEM3 TE011
|Hs||Es|
A coaxial cavity resonant with 11.424 GHz TEM3 and TE011 would be excited by two rf sources, one coupling to each mode.
The high E field on the center conductor is determined solely by the TEM3 excitation, with the peaks at the zero points of the H field.
Adding TE011 increases the H field, preferentially around the central E field lobe.
|Hs||Es|Center conductor:r = 0.635 cm
C. Nantista
Latest Design
TEM3
TE01
WR90
0.592”
11.42375 GHz
11.4243 GHz
Es Bs
Pulse Heating Summary• All NLC structures and undamped CLIC structures operate with
pulse heating below 50 degC – do not believe it impacts BDR at this level as damped and undamped NLC structures performed the same despite the 20-40 degC variation in heating (should recheck with T18)
• TD18 and single cell BDR show clear pulse heating dependence above 50 degC
– Since TD18 differs from T18 mainly in the outer cell wall, indicates that heating (or B field) ~ 0.5 cm from irises has an influence on breakdown at the irises (as did the sharp-edge coupler heating in earlier NLC structures)
• Pre-heating tests at constant gradient confirm the pulse heating sensitivity
– However, longer exposure to moderate gradient (half of max) during the pre-heating period could be a factor