Electrostatic end plugging accompanied by a central-cell density increase in an axisymmetric tandem mirror

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Abstract
Electrostatic end plugging is observed in a completely axisymmetric, three cell tandem mirror under conditions where the central‐cell plasma density is always larger than the end‐cell density. A factor of 4 increase in the central‐cell density, to a maximum of 1.2×1013 cm−3 with simultaneous plasma beta of 13%, occurs upon application of the end plugging potential. Ion confining potentials of 25 V and 80 V at the two ends of the device, respectively, result in a factor of 2.5 increase in the axial confinement time for Tic =40 eV in agreement with the collisional flow model for ion confinement. The non‐Boltzmann ion confining potential is caused by electron heating in the end cells by rf near the ion‐cyclotron frequency. The initial central‐cell density rise is caused by an increase in the ionization rate that occurs because of an increase in the electron temperature. The density remains high throughout the end‐cell heating pulse as a result of increased particle confinement time. There is no nonambipolar radial ion loss in the core plasma (r≤16 cm) but inward radial transport of ions is observed at a rate consistent with ion–neutral collisions and a radial electric field in the negative radial direction. Steady‐state thermal‐barrier‐like potential dips that are in agreement with the Boltzmann model for potentials are observed in the transitions between the central cell and the end cells.