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

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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)

#123714@34.8s#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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