Spectroscopic analysis of the blue light emitted from Middleton type cesium sputter negative ion sources

To unravel the mechanisms of ion formation in the Middleton type cesium sputter negative ion source, we have acquired and analyzed the spectrum of the light emitted from the source region during sputtering of three different cathode materials; carbon, aluminum and copper. All cathodes were analyzed under the same source settings but with different total current output. The cathode region was observed from a distance of 3 m and the emitted light was analyzed by a fiber coupled spectrometer with a wavelength range from 350 to 1100 nm. The spectra showed clear differences depending on the cathode material. The emitted light from the carbon cathode consisted almost entirely of lines from neutral cesium and no lines from ionized cesium. Both metallic cathodes additionally displayed several lines originating from positively ionized cesium. A weak, broad feature between 470 and 510 nm was also seen in the carbon spectrum. This feature was not present in spectra from the two metallic cathodes. These results may suggest different mechanisms of negative ion formation in the ion source depending on the cathode material. As a side effect of the measurement it proved relatively easy to determine the source temperature, in the range 1020-1215 K depending on source settings, to an accuracy of ±30 K from the black body radiation.