A simple multifoil spectrometer for routine carbon and oxygen measurements

A simple multifoil Ross filter spectrometer for monitoring principal (i.e., n=2−1) soft x‐ray lines of H‐ and He‐like carbon and oxygen has been installed on the Madison symmetric torus reversed‐field pinch. It consists of four silicon surface barrier diodes, each covered by a different multilayer foil filter but viewing a common plasma volume. Each filter is composed of a combination of thin film absorbers with strategically located absorption edges, giving a peak transmission of around 10% at a wavelength of interest (C v 40.30 Å, C vi 33.74 Å, O vii 21.70 Å, O viii 18.90 Å) with good rejection of the others. A linear combination of these four lines is isolated by a given filter and a matrix inversion of the four simultaneous measurements produces the intensities of the individual lines. The time evolution of these lines shows evidence of radial shell structure and gives useful information of the dynamics of the electron temperature during sawtooth activity. This multifoil spectrometer provides a simple and inexpensive alternative to grazing incidence spectrometers as a monitor of C and O influx and radiated power in high temperature tokamaks, and may be especially applicable in a high neutron flux environment.