Cavity ring-down spectroscopy of N/sup 2+/ in pulsed and DC atmospheric pressure discharges

Summary form only given. In this paper we present the results of cavity ring-down spectroscopy (CRDS) measurements of N/sub 2//sup +/ in atmospheric pressure discharges. Such discharges are of interest for applications such as biological decontamination, and electromagnetic shielding. N/sub 2//sup +/ is the dominant charged species in nitrogen plasmas and thus enables one of the most direct measurements of electron number density. Because the concentration of N/sub 2//sup +/ (and electrons) is on the order of 1 ppm, a sensitive diagnostic, such as CRDS, is required. Our CRDS setup uses an OPO laser system to probe the rotational structure of the first negative band of N/sub 2//sup +/, in the vicinity of 390 nm. Initially, we study an atmospheric pressure, nitrogen, DC plasma. Spatially resolved measurements are obtained by translating the discharge column relative to the laser beam. An Abel inversion method is used to recover the radial profile of electron number density. We are also interested in performing diagnostics in repetitively pulsed discharges. In such cases, a short (/spl sim/10 ns) high-voltage pulse is used to generate free electrons, which subsequently recombine over about 10 microseconds, after which time another pulse is provided. Conventional CRDS assumes that the absorptive losses are uniform over the duration of the ring-down signal, so that a single exponential decay time characterizes the ring-down, and the absorption. In principle, for a time varying case, one may fit different exponential decays to different temporal windows within the ring-down. Then, each decay time represents the losses in the corresponding temporal interval. We investigate this approach as a means of measuring the peak electron concentration (at the end of the high voltage pulse), as well as the electron concentration during the recombination period between pulses. Data demonstrating the feasibility of the technique are presented.