J. A. Ray

Channeltron efficiency for counting of H+ and H− at low energy

The counting efficiency of a channel electron multipier is measured for H+ and H− impact with the input of the Channeltron grounded and for impact energies between 0.3 and 5 keV. The counting efficiency is found to be about 30% greater for H− than for H+ at energies below 1 keV. This difference and an observed difference between H− and H+ in the variation of counting efficiency across the Channeltron opening are attributed to the surface potential of the Channeltron near the opening, which can accelerate or decelerate the incident particles.

A Quantitative Detector for Neutral Particles

A neutral particle detector has been constructed and calibrated for H, He, and N atoms in the energy range 0.5–50 keV. These neutral particles were incident on a Cu or AgMg surface that emitted secondary electrons which were accelerated to 20 keV. These energetic electrons were collected by a silicon barrier detector from which the pulse height spectrum was shown to obey Poisson statistics. From the statistical distribution the detector efficiency was determined. The efficiency of the detector increases from 77% at 0.5 keV to 100% at 2 keV and remains constant at approximately 100% up to 50 keV. Measurements and results are presented for secondary electron and reflection coefficients for electrons and H incident on various surfaces.

Low energy H atom analyzer using a cesium heat pipe

The efficiency of detecting H and D atoms (E<1 keV) escaping a plasma has been increased by use of a cesium vapor cell to form negative ions by electron attachment collisions. A 30 cm long heat pipe confines the cesium to a localized region. The resulting negative ions are energy analyzed with a single‐channel, parabolic parallel‐plate analyzer and detected by a channel multiplier. The H0 conversion efficiency was 10−2 at 100 eV, increasing to 3.8×10−2 at 500 eV. For a H0 energy of 100 eV, the analyzer is 2.5 orders of magnitude more efficient than a comparable N2 stripping cell analyzer. The analyzer has been used to measure ion temperatures as low as 30 eV on the EBT Plasma experiment.

Energy Dispersed Neutral Beams

Energy dispersed neutral hydrogen beams have been obtained by passing H+ or H2+ through an argon gas cell. The gas cell was composed of an 11‐electrode ceramic structure across which a retarding potential was placed. Neutrals formed had an energy characteristic of the equipotential line on which the electron capture collision occurred. Flat energy distributions were obtained with linear retarding potentials.