F. Durodié 4-Aug-2005JET-EP ITER-Like Antenna / CCFW39 / JET
Review of the Status of theArc Detection Issues and Proposals
for theJET-EP ITER-like ICRH Antenna
4 August 2005 – LPP-ERM/KMS
Presented by F. Durodié
F. Durodié 4-Aug-2005JET-EP ITER-Like Antenna / CCFW39 / JET
Outline
• The simulation technique / tools• Arc Simulation and Locations• Visibility of Arcs• Alternative detection techniques tried
– Mode of Operation
– Path
– Pattern
– Signal Consistency
– (Visible) Light
• Summary
F. Durodié 4-Aug-2005JET-EP ITER-Like Antenna / CCFW39 / JET
Matching and Arc Simulation for JET-EP• Full circuit model using Antenna input impedance matrix, Z, from MWS
• L/H transitions and ELMs are currently simulated by scaling the antenna input impedance matrix, Z :
Service stub and 2nd (corrective) matching stage
3dB
Spl
itter
3dB
Com
bine
r
30 30
(2MW)
(2MW)
Low impedance quarter wave (43 MHz) transformer
1 of 2 toroidal coupled halves shown
(4MW)
simulation
Eq
uiv
ale
nt D
iele
ctric
MW
S
****
**
coupling)ondependancereactance(
ELM
modeH
modeL
40
875.0
75.0
1
Im1
Re
ZiZZ
* arcs
F. Durodié 4-Aug-2005JET-EP ITER-Like Antenna / CCFW39 / JET
• Circuit model and control algorithm is implemented in the Simulink environment
• Control algorithm (present status):
separate error signals, (Ci), evaluated from the measured admittance at the T-junction , YT, for each RDL :
ZMatch : impedance at T-junction to match RDL to
gref : offset from perfect match to cover the elmvariation
: ad hoc parameter added to rotate controlsignals to compensate effects of cross-coupling
the rate of change of the capacitance is proportionalto the amplitude of the error signals :
clipped to maximum speed (200pF/s)zero or reduced speed during ELMy phase
Matching Simulations for JET-EP (cont’d)
refTMatchi gYZeCiC 21
gref=1
VSWR=1.5 contourduring ELM
VSWR=1.5 contourduring base load
F. Durodié 4-Aug-2005JET-EP ITER-Like Antenna / CCFW39 / JET
Matching Simulations (cont’d)• At present it has not been possible to feedback control the 8 capacitors of the 4 cross-
coupled RDLs.– Runaway of one or more capacitors to their end of ranges.
• Using the toroidal symmetry is has been possible to feedback control 4 capacitors of one toroidal half and copy their position to the other half
– Assessment of the impact of non-idealities is ongoing(e.g. systematic mis-position one or two capacitors by O(1)pF : appears acceptable)
– Home-in from 5pF away from the solution appears to be possible across the frequency band [30-55 MHz]
C5-8 are copied from C1-4
VSWR afterhybrid splitter
1.2
1.0
Power loss inhybrid splitterdummy load [MW]
0
0.25
ELM-index
0
6
F. Durodié 4-Aug-2005JET-EP ITER-Like Antenna / CCFW39 / JET
Arc Simulation and Locations
• Arcs are simulated by adding a 20nH inductance in parallel between two conductors at t = tarc.– The build up of the arc as well as the transients in the circuit are not modeled– 20nH is “guesstimated” from the size of the arc channel (R. Goulding, I. Monakhov).
• Arc locations shown for RDL P. Similar locations for RDL Q.
F. Durodié 4-Aug-2005JET-EP ITER-Like Antenna / CCFW39 / JET
Modeling of the Binocular – Low Z VTL - Transition
Lvtl= 1.765m (1.735 - 1.785m) Zvtl= 9.5
L30= 0.450m
Lbridge= 0.100mZbridge= 20
F. Durodié 4-Aug-2005JET-EP ITER-Like Antenna / CCFW39 / JET
0.2
0.4
0.6
0.8
1
30
210
60
240
90
270
120
300
150
330
180 0
0 10 20 30 40 50 60 70 80 90 1000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
magnitude Sij
frequency [MHz]
MWS Model Fit
Lvtl 1.715 m (1.765 m)
Zvtl 9.0 Ohm (9.5 Ohm)
L30 0.540 m (0.450 m)
Lbridge 0.14 m (0.10 m)
Zbridge 20 Ohm
S11
S12
S23
S22
S22S11
S12
S23
F. Durodié 4-Aug-2005JET-EP ITER-Like Antenna / CCFW39 / JET
0.2
0.4
0.6
0.8
1
30
210
60
240
90
270
120
300
150
330
180 0
MWS model of Arc
S11
S22
S12
S23
• Work is still ongoing :results presented hereafter are with a 20 nH arc on a 0-dimension junction
1mm wire connecting the side of the binocular to the outer vtl
F. Durodié 4-Aug-2005JET-EP ITER-Like Antenna / CCFW39 / JET
Visibility of Arcs• Arcs that can be easily detected by looking at the VSWR from the RDLs
(after the second stage matching) :
– Feeders (Straps) (VSWR > 10)
– Vacuum Window (VSWR > 10)
– Low impedance VTL about >50cm away from the junction (VSWR > 4)
P
QR
S
Distance from junction [m]
F. Durodié 4-Aug-2005JET-EP ITER-Like Antenna / CCFW39 / JET
Visibility of Arcs• Arcs that cannot be detected by looking at the VSWR from the RDLs :
– Within 50cm from the T-junction• The impedance of 20nH is similar in magnitude to the impedance at the T-junction
– Series arc inside some capacitors• Which branch depends on frequency and chosen matching solution
Arc to ground at T-junction (cfr. Tore Supra)
Series arc in capacitor on inductive branch
L to H
L to H ELM
ELM
Arc
Arc
Time expanded
F. Durodié 4-Aug-2005JET-EP ITER-Like Antenna / CCFW39 / JET
Alternative Mode of Operation
• Change of Match Point : Raise the T-junction impedance and try and compensate the loss of elm-resilience using the hybrid couplers
– arc at one of the two T-junctions involved has a clear signature, unfortunately an arc the T-junction on the other branch is much less visible (VSWR change comparable to ELMs) and can only be detected by comparison with the change of VSWRs in other RDLs.
– The effect of off-setting the match point on the voltages and capacitor ranges has still to be assessed across the frequency band (30-55MHz) but for the case studied at 43MHz the voltages increase to close to 50 kV and appear to require a larger spread in capacitor settings.
F. Durodié 4-Aug-2005JET-EP ITER-Like Antenna / CCFW39 / JET
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1Power in Splitter Loads [MW]
simulation time [s]
PQ
(re
d o)
RS
(bl
ue *
)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.81
2
3
4
5
6
7VSWR
simulation time [s]
P (
r) Q
(g)
R (
b) S
(m)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.81
1.2
1.4
1.6
1.8
2
2.2
2.4
2.6
2.8
3VSWR PQ RS
simulation time [s]
PQ
(re
d o)
RS
(bl
ue *
)
Arc Detection : Normal Match Point
• F = 43 MHz
• Zmatch = 3 – j0.6
Arc P : RDL VSWRs
Arc P : Pwr spl. load
Arc P : VSWR Spl. Hybrid
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80
5
10
15
20
25
30
35
40
45
50Vc 1/2/3/4 [kV]
simulation time [s]
C1
(r)
C2(
g) C
3 (b
) C
4(m
)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8100
110
120
130
140
150
160
170
180
190
200Capacitor Values [pF]
simulation time [s]
C1/
5 (r
ed o
) C
2/6
(blu
e *)
C3/
7 (r
ed o
--)
C4/
8 (b
lue
* --
)
Vc
C [pF]
F. Durodié 4-Aug-2005JET-EP ITER-Like Antenna / CCFW39 / JET
Arc Detection : Change of Match Point
• F = 43 MHz
• Zmatch = 7
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80
5
10
15
20
25
30
35
40
45
50Vc 1/2/3/4 [kV]
simulation time [s]
C1
(r)
C2(
g) C
3 (b
) C
4(m
)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8100
110
120
130
140
150
160
170
180
190
200Capacitor Values [pF]
simulation time [s]
C1/
5 (r
ed o
) C
2/6
(blu
e *)
C3/
7 (r
ed o
--)
C4/
8 (b
lue
* --
)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.81
2
3
4
5
6
7VSWR
simulation time [s]
P (
r) Q
(g)
R (
b) S
(m)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Power in Splitter Loads [MW]
simulation time [s]
PQ
(re
d o)
RS
(bl
ue *
)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.81
1.2
1.4
1.6
1.8
2
2.2
2.4
2.6
2.8
3VSWR PQ RS
simulation time [s]
PQ
(re
d o)
RS
(bl
ue *
)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Power in Splitter Loads [MW]
simulation time [s]
PQ
(re
d o)
RS
(bl
ue *
)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.81
2
3
4
5
6
7VSWR
simulation time [s]
P (
r) Q
(g)
R (
b) S
(m)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.81
1.2
1.4
1.6
1.8
2
2.2
2.4
2.6
2.8
3VSWR PQ RS
simulation time [s]
PQ
(re
d o)
RS
(bl
ue *
)
Arc P : RDL VSWRs
Arc P : Pwr spl. load
Arc P : VSWR Spl. Hybrid
Arc Q : RDL VSWRs
Arc Q : Pwr spl. load
Arc Q : VSWR Spl. Hybrid
Vc
C [pF]
F. Durodié 4-Aug-2005JET-EP ITER-Like Antenna / CCFW39 / JET
Observation of the ELM/Arc Variations on the VSWR• Looking at the difference in “path” followed by the reflection coefficient due
an ELM or an Arc– Assumes that the arc inductance during the set-up of the arc varies from a large value
down to the 20 nH.
– Requires fast DAQ !
Trajectories of reflection coefficient for both ELMs (blue) and arcs(red) incase of arcs @ junctions P or Q.
Only a small part of the trajectories is used by real ELMs or arcs, makingthe discrimination of ELMs and arcs very difficult, even in the absence ofnoise!
F. Durodié 4-Aug-2005JET-EP ITER-Like Antenna / CCFW39 / JET
Observation of the Voltage Patterns• Pattern of voltages on the array are different for ELMs and Arcs• Allows detection of series arcs inside capacitors
F. Durodié 4-Aug-2005JET-EP ITER-Like Antenna / CCFW39 / JET
• In normal operation (no arc at the T-junction) the S-matrix between the measured points (capacitor voltage probes / directional couplers on the APTL) is known.
• All measured signals must fit the circuit described by this S-matrix independently of the feeding and loading (ELMs) of the circuit on the other side of the measured locations.
• If the measured signals do not fit the S-matrix assumed, then this can only be due to an error in the assumed S-matrix due to e.g. an arc.
Signal Consistency
Vc,i
Vc,j
V+,k V-,k
S
j
i
k
j
i
k
V
V
V
S
V
V
V
,
,
,
,
,
, V+,i and V+,j are not measuredbut the equations can be rewrittenusing Vc,i and Vc,j
jcjicikkk VcVcVcV ,,,,
In general all internal signals (V,I) and linear combinations thereof can be expressed as a linear combination of V+,k, V-,k, Vc,i, and Vc,j : e.g. the sum of the currents flowing in the binocular
F. Durodié 4-Aug-2005JET-EP ITER-Like Antenna / CCFW39 / JET
Signal Consistency• Clear arc error signal
– Arc on T-junction P during ELMs– 5% gaussian noise added– Real voltage probe characteristic has been
taken into account (approx. 10% effect of T-junction voltage).
• Modest computations required
– Coefficients need to be computed and stored [need coefficient as function of the capacitor positions (and frequency)].
– Real time digital processing : present day technology should allow for < 10s cycle time ?
– Process cycle :• DAQ capacitor position• Get coeff. from look-up table• DAQ RF signals -> get Real and Imaginary parts• Compute error signal
jcjicikkk VcVcVcV ,,,,
F. Durodié 4-Aug-2005JET-EP ITER-Like Antenna / CCFW39 / JET
Detection of (Visible) Light
• Light travels in the gap between the inner and outer VTL and is reflected using mirrors mounted on the square flange through the pumping/mounting port flange.
• The light from the plasma is (partially) shadowed by the straps and tomb-stone.
F. Durodié 4-Aug-2005JET-EP ITER-Like Antenna / CCFW39 / JET
Detection of (Visible) Light
• Just a concept lay-out at present– Can be retro-fitted and mounted on the Inner-VTL Square flange
– ? Timeliness w/r to the assembly of the inner-VTLs and the availability of DO resources
F. Durodié 4-Aug-2005JET-EP ITER-Like Antenna / CCFW39 / JET
Summary• Consolidation of the “Arc” impedance model is ongoing
• A combination of three techniques of analyzing the presently measured RF signals allows in principle to detect all arcs :
– VSWR : Straps and Feeders, Vacuum Window,Low impedance VTL 50cm away from the T-junction
– Signal Consistency : Arcs near the T-Junction, Capacitor Series Arcs
– Voltage Patterns : Capacitor Series Arcs(could be part of Signal Consistency hardware or separate)
• Optical observation of light emitted from the arc in the VTL region could be retro-fitted during the assembly of the inner VTL :
– A minimum of interfaces between the mirrors and the Square flange need to be prepared during the assembly of the inner-VTLs (e.g. bolting holes or welded mounting plates)
– It is unclear at present if stray light from the plasma could hamper the observation of the arc
• There is not much time left to implement the proposals– Only concepts at present
– On testbed : VSWR + light observed should be sufficient (no plasma / no ELMs)
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