Correction algorithm for cavity ring-down based anion density measurement in a negative ion source having continuously fed cesium vapor

Abstract

Negative ion or atomic anion density measurement is frequently done non-invasively by employing a Cavity Ring-Down Spectroscopy (CRDS) diagnostic system in a negative ion source. The optical cavity in the CRDS system is created by installing two highly reflecting concave mirrors on two collinear opposite ports of the ion source chamber, so that the cavity encloses the plasma as an absorbing medium. In a continuously-fed cesium (Cs)-seeded ion source the CRDS mirror is exposed to Cs vapor environment. As a result, a finite probability of Cs deposition is possible on the mirror surface. In addition, ion sputtering and thermal distortion may degrade the mirror reflectivity and mirror alignment, respectively, during the time of ion source operation. Distorted cavity alignment may affect CRDS functionality. All the above issues increase the mirror loss which can be misinterpreted as absorption losses and lead to an over-estimation of negative ion density for a long ion source operation time. The CRDS sensitivity and accuracy depend on its mirror reflectivity or rather “effective reflectivity”. Since continuous Cs deposition yields a continuous change in mirror reflectivity, the CRDS sensitivity and accuracy also vary in time. A correction factor is needed to take care of the overestimation in negative ion density value if the time difference between the reference instance and measurement instance is significantly large. In this article, an algorithm is presented to find the correction scheme.