A PLASMA SOURCE BASED ON THE PENNING IONIZATION OF CESIUM BY PHOTOEXCITED MERCURY

A high-pressure plasma source based on the Penning ionization of caesi um by photoexcited mercury is studied by a kinetic model. Mercury vapo ur is photoexcited by resonant excitation at 253.7 nm to the P-3(1) le vel, then quenched to the metastable P-3(0) level by a small admixture of nitrogen. Helium is added to enhance the electron collision freque ncy, but as a result also enhances the decay of electron density by th ree-body recombination processes. The results show that almost complet e ionization of caesium to densities of order 10(12) ions cm(-3) can b e achieved. Radiation transport in this medium is also studied, and th e effects of pressure broadening and hyperfine splitting are explored. Using excitation sources and plasma sources enriched with different i sotopes of mercury, hyperfine splitting can be exploited to increase t he depth of penetration of the resonant excitation radiation into the plasma source volume and therefore increase the volume over which stro ng ionization can occur.