(IT/P2-9) ITER limiters moveable during plasma discharge and optimization of ferromagnetic inserts to minimize toroidal field ripple

K. Ioki1), V. Chuyanov1), F. Elio1), D. Garshuka2), E. Lamzin2), M. Morimoto1), M. Shimada3), M. Sugihara3), A. Teraswas1), Yu. Utin1), X. Wang1)
1) ITER International Team, Garching, ITER
2) SINTEZ, St. Petersburg, Russia
3) ITER International Team Naka, Japan

Abstract.  Two important design updates have been made in the ITER VV and in-vessel components recently. One is the introduction of limiters moveable during a plasma discharge, and the other is optimization of the ferromagnetic insert configuration to minimize the toroidal field ripple. In the new limiter concept, the limiters are retracted by 8 cm during the plasma flat top phase in the divertor configuration. This concept gives important advantages: (i) the particle and heat loads due to disruptions, ELMs and blobs on the limiters will be mitigated approximately by a factor 1.5 or more; (ii) the gap between the plasma and the ICRH antenna can be reduced to improve the coupling of the ICRH power, with the ICRH antenna in a protected position flush with the FW. A flexible support of thin plates is used for the limiter system and there is no sliding support inside the vacuum, to keep the reliability of the system. Driving mechanisms are also located outside the vacuum boundary. During the plasma shutdown phase, the limiters are again moved to the same location as that during the startup phase. The time scale of the movement is 5 s. The limiter position and angles can be precisely adjusted between plasma shots. The location and the filling factor of the ferromagnetic inserts have been optimized based on the field ripple calculation. The ferromagnetic inserts are located exactly periodically in the toroidal direction. The ferromagnetic inserts have previously not been planned to be installed in the outboard midplane region between equatorial ports due to irregularity of tangential ports for NB injection. The result is a relatively large ripple (1%) in a limited region of the plasma, which nevertheless seems acceptable from the plasma performance viewpoint. However, toroidal field flux lines fluctuate 10 mm due to the large ripple in the FW region. To avoid problems due to the large TF flux line fluctuation, additional ferromagnetic inserts are now planned to be installed in the equatorial port region. The additional inserts near the NB ports are not identical with those between regular equatorial ports. Detailed analysis and assessment will be reported in the paper.

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