1. M Nightingale/A WaldenArc Detection Meeting - JET - 4 August 2005 ARC DETECTION FOR THE EP ANTENNA - THE OPERATOR’S PERSPECTIVE Mark Nightingale, Tony Walden, Trevor Blackman and Margaret Graham UKAEA JET 4 th August 2005

2. M Nightingale/A WaldenArc Detection Meeting - JET - 4 August 2005 Introduction The EFDA Leader for JET has requested that the ITER-like antenna project implement a means of arc detection that can protect against the recently identified arcs that are capable of severe damage lead to increases in VSWR that lie below the trip level. The Operator needs to agree any proposed system to (a) ensure that it is compatible with the safe operation of JET, (b) agree any need for changes to existing JET systems, and (c) agree the cost that JOC may need to deploy as its part of any implementation. The Operator is also keen to identify whether any arc detection schemes beyond those based upon VSWR trip could be beneficial for the existing A2 antennas. Recent Damage to the VTL of one of the A2 Antennas

3. M Nightingale/A WaldenArc Detection Meeting - JET - 4 August 2005 Planned Layout of the JET ICRH Systems from 2006 Antenna C Antenna D Generator C Generator D External Conjugate-T 3 dB couplers JET-EP Antenna Antenna A Antenna B Generator A Generator B

4. M Nightingale/A WaldenArc Detection Meeting - JET - 4 August 2005 Options considered so far : –VSWR –Voltage Probes –Current Balance –Optical –Sub-Harmonic Arcs to and between screen bars not covered. Optical detection the only viable system for the screen bars? Potential Arc Detection Options

5. M Nightingale/A WaldenArc Detection Meeting - JET - 4 August 2005 Existing JET system developed and used on the A0, A1 and A2 antennas. For the EP antenna, the trip system will be as follows: –Hard-wired VSWR protection is fitted to each Amplifier Output Transmission Line (before the 3dB combiner) via the ATM electronics. –Additional VSWR protection will be provided using directional couplers on the antenna feeds. –Trip response curves to be decided - but simple to change. –50-60  s response time (20-30  s detection + 20-30  s turn off) –The trip level should not be set below 1.5 as the VSWR is expected to rise to this level during ELM’s. Recent experience at Tore Supra and ORNL, backed up by modelling at ERM, has shown that some arcs - in particular close to the T-point and capacitor internal breakdown - cannot be detected by this method. VSWR Trip

6. M Nightingale/A WaldenArc Detection Meeting - JET - 4 August 2005 Voltage Probes Voltage probes will be installed at the strap end of each capacitor (two, for redundancy); primarily to protect against capacitor over-voltage. The Operator is already providing signal processing electronics in J1H for the generation of amplitude and phase signals for each probe supplied in parallel to CODAS and to a dedicated PC for matching control. To use the probes as part of an arc detection system: –New electronics will be required, fed from a new parallel feed from the processing electronics. –Need to compare ratios of strap voltages. –Possible need for rate of change detection. –Feasibility may depend on signal to noise levels.

7. M Nightingale/A WaldenArc Detection Meeting - JET - 4 August 2005 Current Balance The ERM proposal to combine output from the voltage probes and the forward/reflected power monitoring has been reviewed at conceptual design level by the Operator with the following conclusions: – New electronics required. –JET’s digital magnitude and phase information make a DSP-based solution relatively simple. –A detailed calculation requirement yet to be done, but 10-20  s sensing time seems possible. –Careful attention to system reliability and self-checking. –DSP electronics could combine current balance, voltage probes and VSWR detection in one system (plus others?) –Again, signal to noise level may be critical. –Practicability of look up tables needs verification (how large? how accurate? number of frequencies? etc.)

8. M Nightingale/A WaldenArc Detection Meeting - JET - 4 August 2005 Optical Limited to ex-vessel sensing? It is assumed here that the project team will provide the following items for the optical system proposed by ERM for the EP antenna: –Internal mirrors –Imaging system, optical fibre and detector (including power supplies). Signal and noise levels unknown. JET spectroscopists may provide useful input. Propose to fit one system on the test phase and, if promising, on JET. Full implementation depends on measured viability. The requirements for the Operator are unclear at present.

9. M Nightingale/A WaldenArc Detection Meeting - JET - 4 August 2005 Arc detection based upon a sub- harmonic system will require: –direct feed from voltage probes. –New electronics. Signal and noise levels highly speculative. JET have information from shot #63412 on the D2 antenna. Propose to fit one system on the test phase and, if promising, on JET. Full implementation depends on measured viability. Sub-Harmonic Detection #63412 D2 trip OTL reflected voltage and spectrum Arc/trip ‘signature’ ? Trip

10. M Nightingale/A WaldenArc Detection Meeting - JET - 4 August 2005 Conclusions Various arc detection options could, in principle, be implemented from the Operator’s perspective. The proposal to diagnose current balance at the T-point was presented to the Operator over the last few weeks, and some conceptual assessment of the Operator requirements has been made. This suggests that the response time is possibly feasible, but can only be confirmed on completion of detailed design. Proposed test of optical and sub-harmonic systems on the JET antenna.