Implementation of the asynchronous landing scheme in KSTAR plasma control system MinHo Woo 1) , H.Han 1) , S.H. Hahn 1) , J.Kim 1) , Y. S. Bae 1) , J. G. Bak 1) , M. L. Walker 2) , R. D. Johnson 2) , N. W. Eidietis 2) 1) National Fusion Research Institute, Daejeon 305-333, Korea 2) General Atomics, San Diego CA, U.S.A 1
Outline • Motivation • Conceptual design • Algorithm development • Model validation • Experiment • Conclusion and Future works 2
Motivation Consequences of the disruption Sudden thermal load to the first wall Electro-magnetic load on conducting wall Direct collision of run-away electrons to the wall Cause of the disruption MHD stability limit in high beta plasma Hardware failure Lose of the plasma control Mitigation and avoidance of the disruption Massive Gas Injection (MGI) ECCD injection to suppress MHD mode Asynchronous (soft) landing 2013 KSTAR conference, Lotte Buyeo Resort, February 26-27, 2013 3
Asynchronous landing What is asynchronous landing ? Switching the poloidal field coil control scheme into the pre-designed ramp down scenario at any time at any plasma current How asynchronous landing works ? Catch up disruption precursor or hardware fault Change target plasma current with specified ramp down rate Feed currents to each PF coil with given ramp rate Move the plasma inward with fixed plasma shape Advantages of the Asynchronous landing Reduce the plasma current before hitting the first wall Reduce the stored energy and eventually wall damage Same damage pattern for different shots Enable the tokamak to operate in more dangerous parameter regime without concerning wall damage 4
Conceptual Design • Severe fault Immediate termination Ip min fault Total MVA fault PS fault • Median fault Asynchronous landing PFC fault rtEFIT fault External fault … • Intended landing Locked mode coil IVC coil Zp estimator 5
Algorithm Development • Different types of asynchronous landing 1. Intended landing Monitor precursor signal of the MHD IVC or Zp to determine if plasma has lost control 2. By hardware faults External faults by ECH, NBI etc… PFC armor fault Ip control error faults • Flow of the sequence Different sequence has different phase for different category When asynchronous landing triggered sequence changes and so is phase 6
Algorithm Development • Block Diagram for Asynchronous landing 7
Model Validation • Plasma control simulator - simserver Closed loop simulator with plasma current and shape control with model based on real experiments Plasma shape Upper single null to circular Plasma current Ramp down from actual Ip Plasma position Move inward about 10cm Red line- before landing Black line – after landing Landing started at 5s 8
Experiment shot 8771 Asynchronous landing at 21.375s due to the NBI-1 failure at 21.125s Change of target Ip and actual Ip Change of the feed forward current Drop of the electron temperature Sudden increase in loop voltage Elongation recovers to 1.85 2013 KSTAR conference, Lotte Buyeo Resort, February 26-27, 2013 9
Experiment • Robustness of the asynchronous landing Landing by IVC Strong IVC oscillation before landing Strong IVC at 5s PF5 feed forward change from 5s 10
Experiment Landing by PFC fault High temperature of the Plasma facing component due to the previous experiment Fault received from external fault monitoring system Similar behavior to the IVC fault case 2013 KSTAR conference, Lotte Buyeo Resort, February 26-27, 2013 11
Conclusion and Future Works Conclusion • Asynchronous landing scheme is developed • The model is validated using plasma control simulator • It is applied real experiment in 2013 KSTAR campaign • Asynchronous landing works well in various situations Limitations and Future works • Changing feed forward PF current technique is very limited • Problematic when one of the PF coils are saturated • Sudden change in PF request needs to be improved • On-line feed forward scheme is under development • Extend its applicability to more general dangerous situations in KSTAR 12
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