F. Durodié4-Aug-2005JET-EP ITER-Like Antenna / CCFW39 / JET Review of the Status of the Arc...

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


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


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


• 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


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


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.


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


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


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


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]


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


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.


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]


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]


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!


Observation of the Voltage Patterns• Pattern of voltages on the array are different for ELMs and Arcs• Allows detection of series arcs inside capacitors


• 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


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 ,,,,


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.


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


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)

F. Durodié4-Aug-2005JET-EP ITER-Like Antenna / CCFW39 / JET Review of the Status of the Arc...

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