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Abstract

The reflection of an Ordinary Wave from its cut-off at the electron plasma frequency can be exploited to measure the density profile of laboratory plasmas. We present two codes for the numerical simulation of broad band reflectometry on a Reversed Field Pinch. The RAYRFP code evaluates the h.f. field distribution in the geometric optic approximation. It can rapidly simulate a frequency scan by evaluating the optical length of the "central" ray. It can also be used for example to estimate the angular spread of the reflected wave due to refraction and reflection from a cylindrical convex surface. Because of the large shear characterics of Reversed Field Pinches, coupling between the ordinary and the extraordinary modes in the plasma can be important. Mode mixing is investigated by solving the full wave equation for nearly equatorial waves with a finite element code FELRFP. The results show that indeed a non negligible amount of power can be transferred from the launched O- mode to the unwanted X-mode (or vice-versa). Although stronger in small devices, this effect can also be substantial when the optical path is long, even in plasmas so large that the variation of B-> per wavelength is small. The mixing coefficient is found to be an oscillating function of frequency, indicating that the excitation of standing waves between the plasma edge and the cut-offs also plays a role. Nevertheless, the implementation of reflectometry should still be possible without difficulty in sufficiently large devices.

Additional information

Authors: BRAMBILLA M MAX-PLANCK-INSTITUT FUER PLASMAPHYSIK, GARCHING BEI MUENCHEN (GERMANY) MORESCO M ISTITUTO DI ELETTROTECNICA E DI ELETTRONICA DELL'UNIVERSITA DI PADOVA (ITALY) , MAX-PLANCK-INSTITUT FUER PLASMAPHYSIK, GARCHING BEI MUENCHEN (GERMANY);ISTITUTO DI ELETTROTECNICA E DI ELETTRONICA DELL'UNIVERSITA DI PADOVA (ITALY)
Bibliographic Reference: WRITE TO MAX-PLANCK-INSTITUT FUER PLASMAPHYSIK, 8046 GARCHING BEI MUENCHEN (GERMANY), MENTIONING REPORT IPP 5/11, 1986
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