Estimation of Width of Electron Energy Loss Region in Cusp Magnetic Field in Negative Ion Sources

Abstract

The loss width of electron energy in the cusp magnetic field in negative ion sources is investigated analytically. Energy equation and continuity equation for electrons are solved with heat flux and particle flux parallel and perpendicular to the magnetic field. Electron energy and density perpendicular to the magnetic field are assumed to decrease exponentially with each decay length and the cusp loss width is defined by the doubling of the decay length. An expression of the ratio of the cusp loss width of electron energy to that of electron density is obtained. The ratio of the cusp loss width depends on electron collision frequencies across the magnetic field and a heat transmission coefficient defined by the ratio of the heat flux to the particle flux multiplied by the electron temperature along the magnetic field. The heat transmission coefficient is related to the sheath potential on the wall surface. Both parameter dependence and the calculated value of the cusp loss width of electron energy derived from our analysis are in good agreement with Horiike et al.'s experimental result for the magnetic multipole ion source.

Electric Potential in Volume Produced Negative Ion Sources with Magnetic Field Increasing toward a Wall

“…The amount of negative ions produced in the ion source depends on the plasma density related to the particle loss on the wall and the plasma temperature related to the energy loss on the wall. It has been shown that a width of plasma loss region of the electron energy depends on a heat transmission coefficient that is the ratio of the heat flux to the particle flux multiplied by the electron temperature along the magnetic field [1]. Since the heat transmission coefficient is related to a sheath potential formed near the wall in the plasma [2], investigating the electric potential near the wall is important in the production of negative ions in the ion source.…”