1. F. Durodié4-Aug-2005JET-EP ITER-Like Antenna / CCFW39 / JET Review of the Status of the Arc Detection Issues and Proposals for the JET-EP ITER-like ICRH Antenna 4 August 2005 – LPP-ERM/KMS Presented by F. Durodié

2. F. Durodié4-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

3. F. Durodié4-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 2 nd (corrective) matching stage 3dB Splitter 3dB Combiner 30  (2MW) Low impedance quarter wave (43 MHz) transformer 1 of 2 toroidal coupled halves shown (4MW) simulation Equivalent Dielectric MWS * * * * * * * arcs

4. F. Durodié4-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,  (C i ), evaluated from the measured admittance at the T-junction, Y T, for each RDL : Z Match : impedance at T-junction to match RDL to g ref :offset from perfect match to cover the elm variation  :ad hoc parameter added to rotate control signals to compensate effects of cross-coupling the rate of change of the capacitance is proportional to 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) g ref =1 VSWR=1.5 contour during ELM VSWR=1.5 contour during base load

5. F. Durodié4-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] C 5-8 are copied from C 1-4 VSWR after hybrid splitter 1.2 1.0 Power loss in hybrid splitter dummy load [MW] 0 0.25 ELM-index 0 6

6. F. Durodié4-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 = t arc. –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.

7. F. Durodié4-Aug-2005JET-EP ITER-Like Antenna / CCFW39 / JET Modeling of the Binocular – Low Z VTL - Transition L vtl = 1.765m (1.735 - 1.785m) Z vtl = 9.5  L 30 = 0.450m L bridge = 0.100m Z bridge = 20 

8. F. Durodié4-Aug-2005JET-EP ITER-Like Antenna / CCFW39 / JET MWS Model Fit L vtl 1.715 m(1.765 m) Z vtl 9.0 Ohm(9.5 Ohm) L 30 0.540 m(0.450 m) L bridge 0.14 m(0.10 m) Z bridge 20 Ohm S 11 S 12 S 23 S 22 S 11 S 12 S 23

9. F. Durodié4-Aug-2005JET-EP ITER-Like Antenna / CCFW39 / JET MWS model of Arc S 11 S 22 S 12 S 23 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

10. F. Durodié4-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 Q R S Distance from junction [m]

11. F. Durodié4-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 ELM Arc Time expanded

12. F. Durodié4-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.

13. F. Durodié4-Aug-2005JET-EP ITER-Like Antenna / CCFW39 / JET Arc Detection : Normal Match Point F = 43 MHz Z match = 3 – j0.6  Arc P : RDL VSWRs Arc P : Pwr spl. load Arc P : VSWR Spl. Hybrid VcVc C [pF]

14. F. Durodié4-Aug-2005JET-EP ITER-Like Antenna / CCFW39 / JET Arc Detection : Change of Match Point F = 43 MHz Z match = 7  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 VcVc C [pF]

15. F. Durodié4-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) in case of arcs @ junctions P or Q. Only a small part of the trajectories is used by real ELMs or arcs, making the discrimination of ELMs and arcs very difficult, even in the absence of noise!

16. F. Durodié4-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

17. F. Durodié4-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 V c,i V c,j V +,k V -,k S V +,i and V +,j are not measured but the equations can be rewritten using V c,i and V c,j In general all internal signals (V,I) and linear combinations thereof can be expressed as a linear combination of V +,k, V -,k, V c,i, and V c,j : e.g. the sum of the currents flowing in the binocular

18. F. Durodié4-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 < 10  s cycle time ? –Process cycle : DAQ capacitor position Get coeff. from look-up table DAQ RF signals -> get Real and Imaginary parts Compute error signal

19. F. Durodié4-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.

20. F. Durodié4-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

21. F. Durodié4-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)