Fourier analysis of the wave number of unstable waves in a spiral beam-plasma system

To analyze the wave number of unstable waves in a spiral beam-plasma system, the interferometer output of the propagating wave is Fourier-transformed. By using this method, the ratio between the amplitudes of elementary waves as well as their wave numbers are determined, even when three or more waves are included in the observed wave pattern. The instabilities caused by beam waves of higher order are identified from the comparison of the observed wave number with the theoretical dispersion relation. (author)