Over the last few years, a method has been developed for obtaining ionization probability curves with essentially monoenergetic electrons. A retarding potential is applied to the electron beam to yield an energy distribution with a sharp low-energy limit. By varying the retarding potential slightly, a new low-energy limit of the distribution can be selected. The difference in the ionization produced in the two cases is ionization by those electrons with a small energy spread selected from the original distribution. By pulsing the electrons and ions, it is possible to eliminate the adverse effect of the ion-drawout field on the electron energy. With this retarding potential difference (RPD) method, a detailed analysis of ionization probability curves is possible. A full description of this method is given in this paper with a discussion of its advantages and limitations. The mass spectrometer used in this series of studies is described, particular attention being given to a description of the ion source. The various electrodes of the electron beam slit system are described in terms of their influence on the electron energy, and on the shapes of ionization probability curves.

The Tokamak Fusion Test Reactor (TFTR) is intended to achieve approximate energy breakeven in D-T plasmas. Construction approval was received in March 1976, and the first plasma was produced in December 1982. Three major experimental run periods, the last ending April 1985, have yielded experimental results on confinement and heating that extend the scaling laws of smaller machines. The plasma parameters in TFTR are now approaching those required for breakeven in unthermalized, two-component regimes. They span the range from high-density operation with ne(0)τE ≈ 4 × 1019 m−3·s to a low-density regime with Ti(0) ≈ (9±2)keV. The maximum product ne(0)τE Ti(0) of about 9 × 1019m−3·s·keV was obtained at a central density, 1.1 × 1020 m−3, by means of pellet injection.

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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.