Kinetic Analyzes of Potential Formation and Transport in Boundary Plasmas

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1. Two formaiton mechanisms of the magnetic presheath are clarified by kinetic analyzes. The magnetic presheath in a cold-ion plasma is formed due to the ion polarization drift, and that in a hot-ion plasma is due to the finite ion-gyroradius effect. An oblique magnetic field has two means of preventing secondary electrons from flowing into a plasma. One is the reflection of secondary electrons by the Lorentz force, which is effective when the Debye length is much larger than the electron gyroradius. Another is enhancement of the space-charge effect in the sheath due to formation of the magnetic presheath. The magnetic presheath acts as a thermal insulator instead of the sheath under conditions of the space-charge limitation.2. Kinetic analyzes have been carried out to study impurity control effects of a spatially expanding magnetic field and effects of neutral-gas collisions on power transmission in the divertor chamber. The spatial variation of magnetic field strength causes an electrostatic potential drop along the magnetic field lines, which decreases with increasing plasma-neutral collision frequency. Cold ions of fuel and ionized impurities are successfully trapped in the divertor by the electrostatic potential if the ionization rate does not exceed an upper limit. Charge exchange replaces energetic ions by cold ions, and amplification of the ion flux by ionization reduces the electrostatic potential drop. Consequently, plasma-neutral collisions markedly reduce the impact energy of ions reaching the divertor plates. Low values of the power transmission factor as low as 2-3 measured in tokamak experiments can mainly be due to reduction of ion energy through plasma-neutral interaction processes in the divertor.