C.C. Damm

Attainment of Ultrahigh Vacua, Reduction in Surface Desorption, and the Adsorption of Hydrogen by Evaporated Molybdenum

Deposition of molybdenum by vaporization from simple hairpin filaments has been found to reduce pressures in both unbaked and moderately baked stainless steel vacuum systems to 4×10−10 mm Hg. A deposit of 300 mg from a single filament, 0.050 in. in diameter and 6 in. long, has maintained an unbaked 85‐liter volume at pressures below 10−9 mm Hg for over 40 hr with the aid of a small well‐trapped diffusion pump. For a substrate area of 8×103 cm2, the initial pumping speed of a molybdenum deposit was found to be as high as 105 liters/sec for hydrogen and 8×104 liters/sec for deuterium. The sticking probability for either hydrogen or deuterium on this deposit is estimated at 0.3.A similar system, baked at 200°C for several days while pumped by an oil‐free ion pump, attained 2×10−10 mm Hg with molybdenum evaporation. When the ion pump was valved off, the pressure in this 75‐liter system remained at 2×10−10 mm Hg for two weeks with no external pumping.

Cooperative Effects in a Tenuous Energetic Plasma Contained by a Magnetic Mirror Field

The formation and characteristics of a steady‐state hydrogen plasma contained in a magnetic mirror field are described. The mean ion energy is 20 keV. The plasma is formed by ionizing and trapping a portion of a beam of energetic hydrogen atoms passing through the confining field. The methods of measurement used to determine the plasma properties are described. Measurements of the radial and azimuthal trapped‐ion distributions, the average ion and electron densities, and the plasma potential are compared with the predictions of simple theory, neglecting cooperative plasma effects. The observed deviations from these simple predictions show that the plasma properties are dominated by cooperative phenomena. The plasma density is found to be limited to a low value (∼4 × 107 ions/cm3) by a flute or drift instability. This instability is characterized by a low frequency rotation of the plasma at a frequency typically close to the ▿B precession frequency of a 20‐keV proton in the nonuniform mirror field. The pla...

High Intensity Source of 20‐keV Hydrogen Atoms

A continuous source of energetic hydrogen atoms has been developed for use in a plasma experiment. The mechanical design and some operating characteristics of the ion source are described, as well as the beam focusing and neutralizing system. An output of 65‐mA power equivalent of 20‐keV hydrogen atoms has been measured on a 2.0×6.3 cm target located 350 cm from the ion source.

Enhancement of the excited-state population in a hydrogen-atom beam

A method for enhancing the population of excited states in a hydrogen atom beam is reported which is based on charge exchange of a deuteron or proton beam with lithium vapor rather than water vapor. An experiment was conducted to show the enhancement relative to water vapor; an increase by a factor of --3 was obtained. The excitedstate population varies less than 15% over a range of beam velocities equivalent to 15 to 42 kev. (D.L.C.)

Collisional Processes at Low Densities in Magnetic Mirror Systems

The study of collisional processes in plasmas produced by neutral‐atom injection into magnetic mirror fields is described. The emphasis is on the many collisional processes which occur as the plasma density increases. Experimental and theoretical results are given. The experimental results are discussed first in terms of a simple model which assumes a Maxwellian electron distribution and a monoenergetic ion component of much higher energy. Analytical solutions may be obtained for this model. Also presented is a more complete theory employing two time‐dependent Fokker‐Planck equations to describe the behavior of the electron and ion distribution functions. Both models are in good agreement with measured values of the electron temperature and plasma potential. The equilibrium values of these two quantities are found to vary as the 35 power of the ratio of the plasma density to the background‐gas density.