Determination of atomic hydrogen density with catalytic probes

Abstract

The determination of atomic hydrogen density with specially designed catalytic probes is described. The probe is a small disc made of a metal with a high recombination coefficient, connected to a pair of thin thermocouple wires. The temperature of a probe placed in a mixture of atomic and molecular hydrogen rises substantially over the temperature of the surrounding gas because of the energy dissipated on the probe surface due to the recombination of hydrogen atoms. The density of atomic hydrogen in the vicinity of the probe is calculated from the measured data on the energy dissipated on the probe surface in a unit time. The density was measured at different total pressures in a vacuum system between 6 × 10 −2 mbar and 2 × 10 1 mbar. The atomic hydrogen source was a low pressure inductively coupled RF hydrogen plasma with the density of the order of 10 16 m −3 and the electron temperature of 6–8 eV. It was found that the atomic hydrogen density increased with increasing pressure reaching the maximum value of 5 × 10 21 m −3 at the total pressure of 8 mbar. At higher pressures, however, the density was found to decrease with increasing total pressure.