PHYSICS ISSUES OF HIGH BOOTSTRAP CURRENT TOKAMAKS

Physics issues of a tokamak plasma with a hollow current profile produ ced by a large bootstrap current are discussed based on experiments in JT-60U. An internal transport barrier for both ions and electrons was obtained just inside the radius of zero magnetic shear in JT-60U. Ana lysis of the toroidal ITG microinstability by toroidal particle simula tion shows that weak and negative shear reduces the toroidal coupling and suppresses the ITG mode. A hard beta limit was observed in JT-60U negative shear experiments. Ideal MHD mode analysis shows that the n = 1 pressure-driven kink mode is a plausible candidate. One of the meth ods to improve the beta limit against the kink mode is to widen the ne gative shear region, which can induce a broader pressure profile resul ting in a higher beta limit. The TAE mode for the hollow current profi le is less unstable than that for the monotonic current profile. The r eason is that the continuum gaps near the zero shear region are not al igned when the radius of q(min) is close to the region of high del n(c ). Finally, a method for stable start-up for a plasma with a hollow cu rrent profile is described, and stable sustainment of a steady-state p lasma with high bootstrap current is discussed.