Current profile control is one of the key issue in high performance, long pulse tokamak operations, where Reversed Shear (RS) plasmas tend to substantially evolve according to the penetration of inductive currents into a core region and to the distribution of non-inductive bootstrap currents and off-axis LHCD [1, 2]. A stable starting-up particularly requires careful discharge controls to obtain steady RS target plasmas under certain restrictions imposed by MHD and control aspects. Hence it follows that the optimization of current ramping-up scenarios is required to attain high performance tokamak operations.
To investigate the details of the current profile behavior during ramping-up phase, we utilized Tokamak Simulation Code (TSC) [3] with a simple ITB (Internal Transport Barrier) model newly installed [4]. Two types of the ITB, i.e. weak (parabolic type) and strong (box type) ITBs [5], were taken into consideration by prescribing the functional form of plasma pressure profile, which model the ITB strength and width. Radial location of the ITB foot, however, was continually adapted in accordance with the movement of the radius of a magnetic shear reversal that was monitored throughout TSC simulations. The profile effect of the ITB-generated bootstrap currents on the ramping-up dynamics was studied from a point of view of stable starting-up and sustaining long pulse operation. This paper reports the simulation results on the optimization of current ramp scenarios of ITER-FEAT Scenario 4 (9 MA with weak RS) [6]. That is, how to establish RS plasmas with the radial location of the ITB extended as large as possible, how to find the stable path to a low q regime without collapses at a low ramping-up rate and how to avoid Current Hole-like, strong ITB.

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