Effect of Magnetic Field Structure on Electron Internal Transport Barrier and Its Role for the Barrier Formation in Heliotron J

An effect of magnetic field configuration to form an electron internal transport barrier (eITB) and a role in forming the eITB in the helical plasma are discussed. In Heliotron J experiments, the eITB formation is determined not only by the neoclassical transport to produce the positive radial electric field but also by the existence of low-order rational surface. The low-order rational surface expands the improved confinement region generated by the eITB, and the power threshold to form the eITB for the plasma without the eITB is also reduced. Because a magnetic island can be formed on this rational surface, it is necessary to consider the effect of the magnetic island on the eITB formation.