MEASUREMENT OF PLASMA-DENSITY USING WAVELET ANALYSIS OF MICROWAVE REFLECTOMETER SIGNAL

A new method of plasma density profile reconstruction in microwave ref lectometry is proposed and implemented on an X-mode broadband reflecto meter of the GAMMA 10 mirror device with an ultrafast sweep rate of 10 -20 mu s. The proposed method makes use of the wavelet transform of th e detected signal. Excellent resolution in the time-frequency domain, inherent to wavelet analysis, allows one to obtain a radial electron d ensity profile for every frequency sweep. The electron density reconst ruction algorithm, besides the wavelet transform of the reflectometer signal, also includes the calibration, profile initialization, and the solution of an integral equation, ultimately yielding the local value s of the electron density. Calibration of the measured signal phase an d profile initialization is performed using the independent results of microwave interferometry. Inversion of the integral equation is imple mented utilizing the gradient method, numerically stable even for plas ma regions with steep density gradients and density profile plateaus. A wavelet-based profile reconstruction algorithm is especially advanta geous for monitoring transient plasma phenomena and fast processes, su ch as in pellet injection, ultrafast swept reflectometry, and short pu lsed reflectometry. (C) 1998 American Institute of Physics.