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.

Experimental divertor physics

“…Divertor research goes back to the very beginnings of controlled nuclear fusion experiments. The divertor was first suggested by Spitzer in an early report (Spitzer 1951) and discussed in the open literature in connection with the stellarator concept (Spitzer 1958, Burnett et al 1958. The original purpose of the divertor was primarily related to point (4) in the introduction, that is, to separate the source of impurity production from the main plasma by arranging that the primary plasma-material interaction occurs in a separate chamber.…”

Plasma-material interactions in current tokamaks and their implications for next step fusion reactors

“…As early as 1951, Spitzer [64] recognized the threat to plasma purity by impurities arising from PMIs and proposed a divertor to help alleviate the problem. In the following early years of research in the 1950's on stellarators and pinches [65][66][67][68], relatively primitive vacuum techniques were employed, resulting in severely contaminated plasmas (primarily carbon and oxygen desorbed from the wall). These discharges had such poor confinement and low plasma temperatures that even these low-Z impurities were not fully stripped in the plasma core, resulting in large central radiation losses.…”

Fifty years of Physics of Fluids

“…Already in issue 5 ͑September-October͒ of the first volume ͑1958͒ there were five articles on the stellarator [8][9][10][11][12] out of a total of eleven in the issue. This trend continued, with tokamaks eventually replacing stellarators as the devices of interest.…”