Formation of Internal Transport Barrier for Negative/Positive Magnetic Shear Profiles
The recent high fusion performance satisfying the break-even plasma condition (Exp. Rep. #39) was achieved in a negative magnetic shear discharge.
A remarkable feature of the negative shear discharge is that energy confinement is improved both for ions and electrons at relatively high densities. This confinement improvement is due to the formation of internal transport barrier (ITB) located near the minimum-q position.
Compared to a normal positive shear discharge, negative shear plasma has ITB at a larger minor radius and steeper temperature and density gradients. The velocity shear of the toroidal rotation is also larger for the case of negative shear.
A transport analysis shows that effective electron diffusivity at ITB is decreased by one order of magnitude in a negative shear plasma. Detailed analysis is in progress.
