DETERMINATION OF ATOMIC-HYDROGEN DENSITY WITH CATALYTIC PROBES

The determination of atomic hydrogen density with specially designed c atalytic probes is described. The probe is a small disc made of a meta l with a high recombination coefficient, connected to a pair of thin t hermocouple wires. The temperature of a probe placed in a mixture of a tomic and molecular hydrogen rises substantially over the temperature of the surrounding gas because of the energy dissipated on the probe s urface due to the recombination of hydrogen atoms. The density of atom ic hydrogen in the vicinity of the probe is calculated from the measur ed data on the energy dissipated on the probe surface in a unit time. The density was measured at different total pressures in a vacuum syst em between 6 x 10(-2) mbar and 2 x 10(1) mbar. The atomic hydrogen sou rce 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 e V. It was found that the atomic hydrogen density increased with increasing pressure reaching the maximum value of 5 x 10(21) m(-3) at the total pressure of 8 mbar. At higher pressures, however, the densit y was found to decrease with increasing total pressure.