It has been shown experimentally that plug and thermal barrier potentials are created simultaneously by electron cyclotron resonance heating (ECRH) only in the end-mirror cells of a tandem mirror. The density around the thermal barrier is sustained to be much lower than that in the central cell, which suggests that the ions trapped in the thermal barrier potential escape from there rapidly.
Monte-Carlo simulation was carried out to investigate the electrostatic potential formation in the end-mirror cell of a tandem mirror. It was found that the plug and thermal barrier potentials can be created simultaneously with a non-Maxwellian distribution of electrons heated by ECRH.[1-3] In addition to the electron distributions, the potential formations require the existence of non-Maxwellian ions trapped in the thermal barrier potential, where the non-Maxwellian ions are assumed to be created as a result of a large radial loss there.
The mechanism of radial loss of ions trapped in the axisymmetric end-mirror cells in the present tandem mirror experiment are unknown. It has been measured experimentally that the electrostatic potential is not axisymmetric around the plug region. The orbits of ions trapped in the end-mirror cell were calculated numerically by taking into account the non-uniform electrostatic potential (with many high m azimuthal modes) around the plug. It was found that the trapped ion orbits had a chaotic behavior. The non-axisymmetric electrostatic potential around the plug region, therefore, can be a candidate of a large ion radial loss in the axisymmetric end-mirror cells, which is large enough to explain the present ion radial loss in the tandem mirror experiments.
The analysis of a particle orbit, especially with chaotic behavior, is made by a mapping technique.
The standard map is the most famous one, where is the case that a single m mode perturbation is added to the axi-symmetric electrostatic potential in the end-mirror cell. However, very few ions resonate with a single m mode perturbed electrostatic potential and the amount of the resultant radial transport is not so large. At the conference we will present the relation between a ion radial transport and a non-uniformity of electrostatic potential profile in the axi-symmetric end-mirror cell.

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