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Presentation to CCIC: ITER-like JET ICRH Antenna Status of Arc Detection

Presentation to CCIC: ITER-like JET ICRH Antenna Status of Arc Detection. Mark Nightingale (on behalf of Frederic Durodie, Ernesto Lerche, Trevor Blackman, Mark Vrancken & Philippe Jacquet). Introduction. 2. C. 1. C. Strap 2. Strap 1.

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Presentation to CCIC: ITER-like JET ICRH Antenna Status of Arc Detection

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  1. Presentation to CCIC:ITER-like JET ICRH AntennaStatus of Arc Detection Mark Nightingale (on behalf of Frederic Durodie, Ernesto Lerche, Trevor Blackman, Mark Vrancken & Philippe Jacquet)

  2. Introduction 2 C 1 C Strap 2 Strap 1 • Key objective of the JET ITER-like antenna (ILA) is to couple power throughout large ELMs (ELM tolerance) by running the antenna in conjugate-T mode with a low (3W) impedance at the T-point. • Using a 3W conjugate-T introduces a risk: arcing close to the T-point does not result in high swings in VSWR on the incoming transmission lines. • Two new arc detection schemes have been fitted to the ILA. The Sub-harmonic Arc Detection (SHAD) system is not yet ready for operation; the Scattering Matrix Arc Detection (SMAD) system is almost ready for testing at 3W. • As neither system is operational, the ILA is being operated with 6W at the T-point; this means that all arcs should be detected by VSWR swings, but that the ILA is only tolerant to small ELMs. T-Point Conjugate-T Configuration

  3. SMAD From RF model K1, K2, K3 = function(Capa1, Capa2, freq) V1meas, V2meas, V-3meas, V+3meas: RF measurements

  4. Error Signals Computed by SMAD Verified 75500 4.0 3.0 2.0 1.0 0.0 Red, Blue, Green: SMAD Output Yellow: Modelled

  5. Typical Error Signals in H-mode Using TL Model Coefficients. 75940 75945

  6. Coefficient Minimization • Motivation: • SMAD error signals based on Transmission Line model somewhat high 30-40% •  Large ERRLIM (restricted arc detection range) • Possible solutions: •  ‘Running average’ of error signals in SMAD real-time calculations • (needs change in FPGA algorithm  MARCHTEC  >1month) •  Optimization of K coefficients based on experimental results • (needs change in K-tables  relatively easy  1 day) • Coefficient optimization: •  Minimization of the error signals by ‘fine tuning’ 3 (out of 4) SMAD coefficients •  Use as wide as possible experimental conditions • Advantages: •  Small ‘background’ error signals allow low ERRLIM values and increase range of arc detection •  Fast and easily ‘updatable’ when new conditions are investigated • Caution: •  Minimization based on finite parameter space •  New conditions (coupling, phasing, frequency) may need further fine-tuning to maintain operations.

  7. Results with Optimised Coefficients H H H RDL56 RDL12 RDL34 RDL78

  8. Shot 75876 Again with Optimized Coefficients. 75876

  9. Error Signals in H-mode with Optimized Coefficients 75940 75945

  10. SMAD Modelling I VSWR VSWR VSWR VSWR C C C C C C C C V V V V V V V V V V V V V V V V 78 12 34 56 56 56 34 78 12 12 34 78 1 3 6 7 8 1 5 4 5 2 2 4 8 7 6 3 E E 56 78 - + + - + + - - SMAD SMAD SMAD SMAD 78 56 34 12 E 34 20 nH arc moves along VTL of E12 0 1 E 12

  11. SMAD Modelling II VSWR VSWR VSWR VSWR 12 34 78 56 SMAD SMAD SMAD SMAD 34 56 78 12 42MHz, Z = 3 Ohm everywhere, 20nH arc on VTL of E12 only T SMAD insufficient SMAD trip limit mutual coupling does not affect SMAD VSWR insufficient VSWR trip limit mutual coupling does affect VSWR

  12. SHAD Status SHAD, RDL56 SHAD, RDL12 PILA (top row) • SHAD hardware tested and commissioned • Ongoing comparison between VSWR trips and SHAD events: • Some VSWR trips not correlated with SHAD events. Scope data (when available) during VSWR trips show no sub-harmonics signals in these cases). • Some VSWR trips are correlated with SHAD events: • On some events, RF signal measurements (fast oscilloscope) and spectral analysis show clear arc signature. • Most of these arcs detected by SHAD happened on RDL12 – we still need to understand why. • More systematic comparison with SMAD and fast signals in MTL/APTL to be carried out to further assess if recorded SHAD events (but with no scope measurements) are likely caused by arcing. • Plan to optimise system sensitivity - reduce detection threshold, but caveat of false trips? Spectral analysis of RF signal during SHAD event Philippe Jacquet (UKAEA) Pulse 75348

  13. Conclusions • VSWR trip systems not do not fully protect the JET ILA. • Testing underway on two JET systems: SHAD and SMAD. • General status: • SHAD: • Hardware operational. • Following a lack of trips, some trips now observed. Initial evidence of sub-harmonic signals under investigation. • SMAD: • Long period of commissioning almost complete. • SMAD performance reviewed last week by the JET MPWG. • Provisional recommendation to use SMAD to allow 3W operation (nominally full ELM tolerant) from next week.

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