(EX/P1-8) ITB-events and their Triggers in T-10 and JT-60U

S.V. Neudatchin1), T. Takizuka2), Y. Sakamoto2), N. Hayashi2), A Isayama2), A. Ya. Kislov1), S.V. Krylov1), Yu. D. Pavlov1), H. Shirai2), A.A. Borshegovskii1)
 
1) Nuclear Fusion Institute RRC Kurchatov Institute, Moscow, Russian Federation
2) Japan Atomic Energy Agency, Naka Fusion Institute, Naka 311-0193, Japan

Abstract.  Non-local transport bifurcations inside and around ITB (abrupt variations of transport in a ms timescale within 30-40% of minor radius) were found in various JT-60U reverse shear (RS) and NrS plasmas and called ITB-events. The abrupt reduction of transport in central part of plasma column often interrupts slow diffusive inward cold pulse propagation (CPP) in T-10. CPP is created by cut-off of the off-axis ECRH. This phenomenon may be called ITB-event as well. In the present paper, we report analysis of ITB-events, which have been observed at T-10 under various plasma parameters regularly. In many cases transport is halved within 40% of minor radius, which correlates with the appearance of q=1 at r=0 (calculated value). In some cases, ITB-events are also observed during CPP created by the reduction of ECRH power. In this case sawteeth do not appear in new steady-state, and q=1 should not exist at the time of ITB-event. In any case, it is not clear why the appearance of q=1 at r=0 is able to create the abrupt and non-local reduction of heat transport. In JT-60U low-heated RS plasmas, ITB-events are connected with crossing of qmin = 3.5,3,2.5 values. Internal MHD-activity n=1 has been reported earlier as ITB-events trigger in JT-60U. The fishbone activity in ASDEX-U and coupling of edge and core MHD-activity (between q=4 and q=2) at JET assist ITB formation. In addition, the present paper shows the new MHD triggers of ITB-events in JT-60U. ITB-event is triggered by series of small internal disruptions probably associated with q=2.5 surface in RS plasmas. ITB-event occurs in ms timescale correlation with the start of ELMs series that create enhanced level of Hα in high-βp NrS shot. The total heat flux reduces abruptly in the zone between r/a=0.3 and 0.7 (by 3 times at the reduction maximum). The calculated radial electric field (with assumed neoclassical poloidal rotation) does not vary at ITB-events. The ms correlation between the start of ELMs series and the above-mentioned reduction of the heat flux gives possibility to control the ITB formation immediately and non-locally by inducing the ELM-like MHD activity. The new examples of ITB-triggers presented above highlight the importance of further systematical study of ITB triggering with internal and external MHD-activity. The success should bring new knobs for ITER scenario .

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