Observations of differences in the spatial-profile symmetry-of ion-confining potentials in the cases with and without the central-cell density increase during an ion-plugging period

Abstract

Temporal evolution of radial profiles of end-loss-ion currents, ion temperatures, and ion-confining potentials φc is investigated in the GAMMA 10 tandem mirror by the use of our newly designed electrostatic ion-energy-spectrometer arrays. In tandem-mirror experiments, plasma-confining potentials produced by electron-cyclotron heatings (ECH’s) play one of the most critical roles in the improvement of simple-mirror plasma confinement. For the observations of these confining potentials, the development of ion-diagnostics equipment providing clear energy spectra without disturbances from ECH produced energetic electrons is of essential importance, since conventional multigrided Faraday cup signals are seriously influenced by such incident electrons. Each spectrometer unit in our developed array has a specific structure with obliquely placed multiple grids with respect to the direction of the ambient lines of magnetic force for plasma confinement. The relation between the spatial profiles of end-loss-ion fluxes IELA and those of φc is investigated by the use of these newly designed ion-energy-spectrometer arrays installed in both end regions of GAMMA 10. The first remarkable finding of axisymmetric profiles of φc having a good correlation with axisymmetric plugging distributions in IELA along with the central-cell density nc increase is made; while, the contrary result without nc increase even during a plugging period is found with asymmetric profiles of φc with asymmetric plugging profiles in IELA. In the article, detailed data comparison between the two is reported by the use of each single-plasma discharge with the developed ion-spectrometer arrays.