Computer Processing of Plasma Data Utilizing a Fast-Fourier-Transform-Generated Spectral Analysis Approach

This paper describes a method of plasma wave diagnostics using digitally computed power spectral density functions. The process involves application of the fast Fourier transform algorithm to digitally converted analog plasma data in order to generate the standard auto-and cross-power density spectra as well as other useful spectral functions. The system employs a 7 channel, 1.5 MHz, analog instrumentation type, tape recorder to record the plasma data signals (e.g. density or potential fluctuations monitored by Langmuir probes). The recorded signals are digitized by an XDS930 computer and the digital output is stored on magnetic tape for subsequent analysis on a CDC6600 computer. The output of the CDC6600 computer analysis consists of autopower density, cross-power density (both magnitude and phase spectra), and coherence functions . The details of the instrumentation and implementation of the system as well as considerations necessary to achieve valid results are described. Examples of the computer generated spectral functions for plasma data are shown. Finally a description of the interpretation of the data analysis and some proposed applications are presented.