Determination of density of hydrogen atoms in the ground state

Abstract

An improved electrical probe capable of measuring not only plasma density, electron temperature, and plasma potential, but also the density of neutral hydrogen atoms in the ground state is described. The probe is made of a small disc connected to a pair of thin thermo-couple wires. The disc is made of a material with a high recombination coefficient for the reaction H + H → H 2. When the probe is exposed to hydrogen atoms, extensive recombination takes place on the disc surface, and its temperature rises substantially over the ambient temperature. Probes made of nickel and gold were placed in a side tube of the discharge chamber to prevent direct contact with plasma. The density of neutral hydrogen atoms in the vicinity of a probe was calculated from the time derive of the disc temperature just after the discharge was extinguished, while the H density in plasma was calculated taking into account the H transmission of the tube which was differentially pumped. The probes were used to determine the H density in hydrogen plasma with the density of charged particles of the order of 10 16 m −3 and the electron temperature of 5 eV. The density of atomic hydrogen was found to depend on pressure in the discharge vessel and reached a value of nearly 10 23 m −3 at a pressure of 5 mbar.