The Divertor, a Device for Reducing the Impurity Level in a Stellarator

Abstract

A divertor, designed to reduce the flow of impurities from the wall into a gas discharge, has been used with a small stellarator. In the divertor an outer shell of magnetic flux is bent away from the discharge channel into a large auxiliary chamber. Ions diffusing toward the wall tend to follow the lines of force of this outer shell into the divertor chamber. Impurities produced by wall bombardment in this chamber do not readily return to the discharge. The magnetic design of this device is described, and a phenomenological theory of its performance is outlined. The spectroscopic data with and without the divertor activated indicate that the divertor reduces the influx of impurities by a factor of two to three, while the impurity concentration at the core, or central region, may be diminished by an order of magnitude when the divertor is activated. Kinetic temperatures of positive ions determined from spectroscopic measurements of Doppler broadening increase from 40 ev without the divertor to 60 ev for He⁺ and to 130 ev for O⁺⁺⁺⁺ with use of the divertor.