Stationary afterglow apparatus with CRDS for study of processes in plasmas from 300 K down to 30 K

A cryogenic stationary afterglow apparatus equipped with a near-infrared cavity-ring-down-spectrometer (Cryo-SA-CRDS) for studies of electron-ion recombination processes in the plasma at temperatures 30-300 K has been designed, constructed, tested, and put into operation. The plasma is generated in a sapphire discharge tube that is contained in a microwave cavity. The cavity and the tube are attached to the second stage of the cold head of the cryocooler system, and they are inserted to an UHV chamber with mirrors for CRDS and vacuum windows on both ends of the tube. The temperature of the discharge tube can be made as low as 25 K. In initial test measurements, the discharge was ignited in He/Ar/H₂ or He/H₂ gas mixtures and the density of H₃⁺ ions and their kinetic and rotational temperatures were measured during the discharge and afterglow. From the measured decrease in the ion density, during the afterglow, effective recombination rate coefficients were determined. Plasma relaxation was studied in He/Ar gas mixtures by monitoring the presence of highly excited argon atoms. The spectroscopic measurements demonstrated that the kinetic temperature of the ions is equal to the gas temperature and that it can be varied from 300 K down to 30 K.