CONFINEMENT IMPROVEMENT DUE TO AMBIPOLAR RADIAL ELECTRIC-FIELD IN HELIOTRON TORSATRON PLASMAS

According to the neoclassical ripple transport, the positive radial el ectric field state of E(r) > 0 is favorable to obtain a good confineme nt compared to the negative radial electric field state of E(r) < 0. T he radial electric field depends sensitively on the density and the ch aracteristic length of density gradient, and a relatively flat density profile is desirable to minimize a heating power to obtain E(r) > 0. Our transport code study shows that gross confinement time is improved about seven times compared to the LHD scaling when E(r) > 0 is achiev ed in the whole plasma region. Without control to keep the density pro file relatively flat the confinement improvement reduces to about a fa ctor of two compared to the LHD scaling. Heating power required to ach ieve E(r) > 0 in the whole plasma region is evaluated. Enhancement of the neoclassical ripple transport with increase of helical magnetic fi eld ripple is favorable to reduce the heating power to achieve E(r) > 0 in the whole plasma region.