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Enhanced performance and control issues in JT-60U long pulse discharges

Y Sakamoto and the JT-60 Team

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Recent experimental results are reported on control issues involved in long timescales and enhanced performance in JT-60U. The control issues in neoclassical tearing mode (NTM) suppression in the weak shear plasma regime include background optimization through decreasing βp(Lq/Lp) at the rational surface and active stabilization of NTMs using ECCD. By optimizing βp(Lq/Lp), a condition of βN ~ 2.5 was sustained for 10 times the current profile relaxation time and one of βN ~ 2.4 with qmin ~ 1.5 was sustained for 2.8 times the current profile relaxation time, with nearly full non-inductive current drive. In addition, a condition of βN ~ 3 was sustained for 5.5 s through stabilization of NTMs using ECCD, and an EC driven current nearly equal to the bootstrap current was required for complete stabilization. In the reversed shear plasma regime, the issue is the existence of the steady state solution with a large fBS value. By controlling the pressure gradient at the internal transport barrier through toroidal rotation to avoid the disruption, a large fBS value of approximately 75% was sustained for 2.7 times the current profile relaxation time, with nearly full non-inductive current drive, and a steady-state solution with a large fBS value is confirmed. The control issues for the edge pedestal and edge localized modes (ELMs) are control of the pedestal pressure and the energy loss through ELMs. The pedestal pressure increases by >40% through the change in toroidal rotation. The type of ELM can be controlled by toroidal rotation from type-I to grassy.


PACS

52.55.Fa Tokamaks, spherical tokamaks

52.55.Wq Current drive; helicity injection

52.40.Hf Plasma-material interactions; boundary layer effects

52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)

Subjects

Plasma physics

Dates

Issue 12B (December 2005)

Received 1 July 2005

Published 7 November 2005



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