Advances in potential formation and findings in sheared radial electric-field effects on turbulence and loss suppression in GAMMA 10

Full text: Following the Lyon IAEA Conference, (1) a factor of two progress in the formation of ion confining potential heights in comparison to those attained 1992-2002 is achieved for tandem-mirror plasmas in the hot-ion mode with ion temperatures of several keV. For producing such high potentials, merely a 20% increase in plug ECH powers for the confining potential formation is applied as compared before. (2) The advance in the potential formation gives bases for a finding of the remarkable effects of radially produced shear of electric fields, dE(r)/dr, on the suppression of not only coherent drift waves but turbulence- like fluctuations for the first time in GAMMA 10. Such a shear [dE(r)/dr] effect on the central-cell plasmas is highlighted visually by x-ray tomography diagnostics; that is, spatially and temporally fluctuated vortex-like structures are clearly observed in plasmas produced by ICH alone [having a quite weak dE(r)/dr]. However, during the application of plug ECH into the ICH plasmas, an associated potential rise produces a stronger dE(r)/dr [=several 10kV/m2]. The disappearance of the turbulent vortices on the basis of such a high-potential formation due to ECH is of remarkable importance for plasma confinement improvement. In fact, the associated temperature rise and non-ambipolar loss suppression are observed. From the viewpoints of both (i) a conventional idea of higher and better potential confinement in the axial direction [i.e. E(z) effects] and (ii) the present new finding of a turbulent vortex disappearance due to a strong radial electric shear [i.e. dE(r)/dr effects] in the transverse direction, simultaneously, such a high potential formation is one of the most essential key issues. (3) For the physics interpretations and control of such potential [or the associated E(r) shear] formation, the validity of our proposed theory of the potential formation is extendedly tested under the conditions with auxiliary heatings. The data described above are well plotted on the extended surfaces calculated from our consolidated theory of the strong ECH theory (plateau formation) with Pastukhov's theory on energy confinement. The validity of the extension of our proposed physics mechanism encourages the future extendable scalability of potential formation having prospective simultaneous E(z) and E(r) shear effects on confinement improvements.(author)