Time-resolved spectroscopy on cathode spots of a vacuum discharge

Cathode spots show a very dynamic behaviour. Thanks to great improvements in experimental techniques, our knowledge on vacuum spots could be remarkably enlarged in the last one or two decades, e.g. by the application of fast intensified CCD cameras. Two-dimensional imaging techniques with high time resolution yield a wealth of information on the spot (sub-) structures, their dynamics and lifetimes. Ultra-high time resolution in the sub-ns range with long measuring intervals can be achieved in one dimensional streak imaging. Here, we are interested in the optical emission of the cathode spots. Therefore, we combined a 0.5 m spectrograph with a streak camera enabling a time resolution in the nanosecond range. Limits concerning wavelength and time resolution as well as the emission intensity will be discussed. The investigations were carried out under UHV conditions using a liquid metal cathode of GaIn alloy. We present first results in which spectral lines of the atom and single as well as double charged ions of the cathode spot plasma could be observed simultaneously. At the beginning of the discharge, the emission spectra are dominated by ionic lines. With a delay in the range of hundreds of nanoseconds atomic lines appear. The intensity of atomic lines is much higher than that of the ionic ones and increases the brightness of the spot in this stage.