Fault Detection System for ICRF Transmission Line
Kenji SAITOa, Tetsuo SEKIa, Hiroshi KASAHARAa, Ryosuke SEKIa,
Shuji KAMIOa, Goro NOMURAa, and Takashi MUTOHb
aNational Institute for Fusion Science, National Institutes of Natural Sciences Toki, Gifu, 509-5292, JapanbChubu University, Kasugai, Aichi 487-8501, Japan
Korea-Japan Workshop on Physics and Technology of Heating and Current Drive
December 14th-16th, 2016
PAL Administration building, Pohang, Korea
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Contents
1. Introduction
・ICRF system in LHD
・Problems
2. Principle and setup of Fault Detection System
3. Calibration method
4. FDS simulation
・Single-probe method
・Double-probe method
5. Test results
・Calibration
・Breakdown simulation
・Self-oscillation simulation
6. Summary
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1. Introduction - ICRF system and problems
ICRF system in LHD
・The impedance matching device is located near LHD because the shorter distance
between the ICRF antenna and the matching device is better to reduce the power loss.
・As a result, distance of transmission line between the FPA (Final Power Amplifier) and
the impedance matching device reaches more than 100m.
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Directional
coupler
ICRF
antenna
Impedance
matching device
FPA DPA
Dummy load
IPACoaxial
switch
Amplifiers
Signal
generator
~
> 100 m
Devices are connected with coaxial lines
Breakdowns in transmission line
Breakdown between FPA and
impedance matching device Breakdown at a coaxial switch
・These severe damages occurred only in the transmission line between FPA and impedance
matching device.
・RF should be turned off before severe damage, especially in long pulse operation.
・Long distance and complex route make it difficult to monitor the entire line temperature between
matching device and FPA in order to detect a breakdown.
・It is also difficult to detect the breakdown with reflection rise because it may be caused by the
normal change in antenna-plasma coupling.
A new detection method is needed.
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Self-oscillation ・Amplifiers sometimes self-oscillate.
・This must be stopped immediately to avoid damage on ICRF devices.
・Self-oscillation cannot be stopped by shutting down of input RF into FPA.
・It can be stopped only by dropping bias voltages on plate (anode) and screen grid.
Problems
2. Principle and setup of Fault Detection System
To solve these problems, Fault Detection System (FDS) for ICRF
transmission line was developed based on SMAD in JET.
SMAD in JET ITER-like ICRF antenna
In SMAD, 4 signals (V1,V2,V3+ and V3
-) are numerically processed with
calculated S-matrix to generate arc signal.
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M. Vrancken et al., Fusion Engineering and Design 84 (2009) 1953-1960.
elimination of
Vf2, Vr2
・ (combined signal = fault signal) is kept zero even if load is changed.
・faultchange in S-matrixfinite
2r
1f
2221
1211
2f
1r
V
V
SS
SS
V
V
r2rf2f2 Vc+VcV
0)VS
c
S
S(c-}V
S
Sc-)
S
SS-(S{c-V r1
12
r
12
22ff1
12
11r
12
221121f2
0vvv rf
r1
12
r
12
22f2r
f1
12
11r
12
221121f2f
)VS
c
S
S(c-cv
}VS
Sc-)
S
SS-(S{c-cv
vf, vr: adjusted fwd and ref signalv=c2V2
2
1
2221
1211
2
1
a
a
SS
SS
b
b
(S-matrix is determined only by structure and frequency.)
a1,2: incident wave voltage
b1,2: outgoing wave voltage
S-matrix
6/13
Fault Detection System (FDS) for ICRF transmission line
v vrvf
voltage
probesICRF
antenna
impedance
matching device
FPA
combiners
variable attenuators
variable phase shifters
attenuator
directional
couplers
port 1port 2
B A
combiner
a1=Vf1
b1=Vr1
a2=Vr2
b2=Vf2
・Three signals vf, vr and v are combined with power combiners to generate fault
signal .
・Band pass filter was inserted in the line of signal in order to cut RF noise
from plasma.
・Due to long distance between directional coupler and voltage probe, there is
a time lag between signals. Therefore instantaneous turn on or off induce
finite at this timing.
Low pass filter at the DC signal of is necessary.
・If RF was turned-off but fault is still detected, then high voltages on FPA will be
dropped because FDS identified it as a self-oscillation.
FDS controller for two lines
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Band pass
filter
Power
detectorLow pass
filterAmplifier
comparator
ADC
DC signalRF signal
For the instantaneous turn-off
In the case of extremely large
For the calculation of normalized
with square root of |v|2+|vf|2+|vr|
2
3. Calibration method
2,1r2,1f2,12,1 vvv
rrff v'v,v'v
'v'vv'
'v'vv'
2r2f22
1r1f11
In this calibration, is one of measured values. If it is not zero, adjustment is
conducted. Therefore, will converge to zero in arbitrary impedance by repeating
this procedure several times.
The expected voltages of combined signals is written as follows:
where the output impedance was changed twice with the impedance matching
device as indicated by the suffixes 1 and 2.
First, vf, vr and are measured, and v is deduced:
In order to achieve =0 in normal
condition with arbitrary load, 4
parameters must be adjusted.
Then the phase shifters and attenuators are adjusted by and .
1)vvvv/()vv(
1)vvvv/()vv(
2f1r2r1f21f12f
2f1r2r1f21r12r
0'2,1
suffixes 1 and 2 two different output impedances
8/13
v vrvf
combiners
variable attenuators
variable phase shifters
Fromdirectional couplers
Fromvoltage probes
vf, vr: before adjustment
vf’, vr’: after adjustment
Forward and reflected waves should be adjusted in order to cancel the combined
voltage in arbitrary output impedance with phase shifters and attenuators as
4. FDS simulation
・Simulated with ideal transmission line at f=38.47MHz.
・Resistances Rb=10, 100, and 1000 were inserted at the position x.
・n/(|v|2+|vf|2+|vr|
2)0.5
・Voltage probe is at x=0.
・There exist insensitive regions around x=n/2.
・Voltage probes are at x=0 (probe A) and
x=-/4=-1.95m (probe B).
・Signal B was attenuated by -4.44dB and
phase was shifted by -90.0, then it was
combined with signal A. (v VfA+4VrA)
・The combined signal is used as a voltage
signal v.
・Insensitive regions disappeared between
voltage probe A and directional coupler.
Single-probe method Double-probe method
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-50
-40
-30
-20
-10
0
10
0 1 2 3 4 5 6 7 8
xscan_singledelta_n(dB)delta_n(dB)delta_n(dB)
L=0(H)
|n|
(dB
)
x (m)
100
10
1000
voltage probeFPA side
-50
-40
-30
-20
-10
0
10
-2 0 2 4 6 8
xscan_doubledelta_n(dB)delta_n(dB)delta_n(dB)
voltage probe B
voltage probe A
L=0(H)|
n|
(dB
)
x (m)
100
10
1000
FPA side
(Breakdown position)
(Norm
aliz
ed f
ault s
ignal)
5. Test results
・Impedance matching device was used to change the reflection ratio .
・ at C and E are used for the calibration.
・Adjustment was conducted 4 times in order to reduce .
・In the wide range of , |n| converged to nearly zero.
Calibration
・Test was conducted with low power (8.6mW)
・Signals were increased by amplifiers in this test.
・Double-probe method was used.Distance between voltage probes A and B: 6.09m.
Combination of signals: v VA+0.57e1.72jVB VfA+3.6e0.06jVrA
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-1
-0.5
0
0.5
1
-1 -0.5 0 0.5 1
較正_様々な反射率(ダブルプローブ法) Im(gamma_s) (ref=0.704)
Im(
)
Re()
A
B
C
D
E
C&E are used for calibration.
-50
-40
-30
-20
-10
0
10
0 1 2 3 4
較正収束(ダブルプローブ法)
ABCDE
|n| (d
B)
calibration trial number
Complex reflection ratios Reduction of |n| by the iterative calibration
Breakdown simulation
)LjR(250
50
Z2Z
Z
cbbc
c
・Voltage probe A-Contactor: 1.45m.
・Inductance Lc=1.4210-7H was deduced from || at Rb=0.
・Even when the reflection is small, FDS detects the fault.
Rb
(||=1.42Vrp/Vfp)
Wave forms at Rb=100 and
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Contactor
0
0.2
0.4
0.6
0.8
1
-40
-30
-20
-10
0
10
10 100 1000
Rbスキャン(ダブルプローブ法)
Vr/Vf/0.704
abs(gamma1)
delta_n(dB)
delta_n(dB)
delta_n_dB_min
||
|n | (d
B)
|n| (x=1.45 m)
Rb ()
RB=0での反射率が実験に合うようにインダクタンスを決めた。(L=1.42177e-7H)ブレークダウンポイントは上流電圧プローブの1.45m上流。
|n|min
|| -100
-50
0
50
100
delta_high_gain_ave64
delta_high_gain (ms)
delta_high_gain (ms)
am
pli
fie
d
(m
V)
Rb=100
Rb=∞
-100
-50
0
50
100
-30 -20 -10 0 10 20 30
Vf_Vr_ave64
Vf (mV)Vr (mV)Vf (mV)Vr (mV)
Vf a
nd
Vr (m
V)
time (ns)
Vr (Rb=100
Vr (Rb=∞)
Vf
Reflection ratio || and the intensity of normalized
combined signal |n| with various resistances Rb
Various resistance Rb was
attached at the port of contactor
located between voltage probe
and directional coupler.
(Zb: impedance of contactor with resistance)
Self-oscillation simulation
・Frequency of self-oscillation shifts by several 100kHz from original frequency
of 38.47MHz.
・Frequency shift changes S-matrix.
・|n| clearly increased by the sweeping of frequency.
・Self-oscillation is detectable and it will be stopped immediately.
Increase in |n| by change of frequency
measured in low power testSignal of self-oscillation detected
by magnetic probe in LHD
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-50
-40
-30
-20
-10
0
10
34 36 38 40 42
f_scan(ダブルプローブ法)normalized_delta(dB,smoothing=0.99,SG=-3dBm)B
|n| (d
B)
original
(38.47 MHz)
37.70 MHz
frequency (MHz)
10-13
10-11
10-9
10-7
10-5
37 37.5 38 38.5 39
自励周波数(SOFT)
psd(V^2/Hz)(t=34800.1356-34800.2356ms)B
ps
d (
a.u
.)
frequency (MHz)
#[email protected]#3(4.5U)が自励発振5.5Uの磁気プローブで計測
ori
gin
al fr
eq
ue
nc
y
(38
.47
MH
z)
shot 123714 t=34.8 s
self-oscillation
(37.70 MHz)
Self-oscillation detection is so far difficult since forward and
reflection power may be RF induced by other FPAs.
6. Summary
・FDS was developed for the ICRF transmission line in LHD.
・Attenuators and phase shifters were precisely adjusted by the iterative calibration.
・There is no effect of load variation on =0 in the case of no-fault.
・Self-oscillation is also detectable due to frequency-shift.
・Even if the reflection is small, fault can be detected.
・FDS will be utilized from the next ICRF plasma experiments in LHD.
(ICRF heating experiments are suspended at least for 3 years from 2017.)
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