Plasma production by the trapping of energetic atoms

Abstract

The production of a hot plasma by the injection of streams of energetic atoms into a confining magnetic field is discussed. The experiments described are directed towards injection of 20-keV hydrogen or deuterium atoms into a magnetic mirror field. The results of some numerical calculations of the plasma growth to a steady-state in a constant magnetic field are presented, including the calculation of the spatial distribution of the trapped ions. In these calculations, the primary trapping mechanism is the ionization of beam atoms by trapped ions and electrons. Parametric values are assigned to approximate the experimentally attainable conditions. The indicated equilibrium densities are in the range of 10¹⁴/cm³, at β ≈ 1%, with typical growth times of a few seconds, if the final density is determined by ion-ion scattering into the mirror loss cone. The practical achievement of a hot plasma by this injection method depends upon maximizing the trapping rate, and minimizing the particle loss due to charge-exchange scattering. Severe requirements are therefore placed on the atomic beam intensity and the gas density in the confinement region. Some of the requirements on the build-up conditions imposed by plasma stability considerations are also discussed. Progress toward meeting the technological requirements is described. A highly collimated beam of hydrogen atoms in excess of 5 × 10¹⁷ atoms/sec at 20-keV energy has been produced. The cross-sectional area of the beam is 20 cm² at a distance of 360 cm from the source; the half-angle divergence is less than 10 milliradians. Vacuum techniques have been developed to achieve base pressures in the 10⁻¹⁰ mm Hg range without extensive bakeout procedure. At the same time pumping speeds exceeding 10⁵l/sec for hydrogen are available. A method of trapping the energetic atoms by means of a transient "cold" plasma is also discussed. This procedure greatly increases the initial plasma growth rate. The plasma density attainable depends upon the beam intensity, vacuum, and cold plasma density, the latter two being time-dependent. The generation of a suitable cold plasma is described.