Ambipolar potential effect on a drift-wave mode in a tandem-mirror plasma

The k-\ensuremath{\omega} spectra of low-frequency waves which exist in a tandem-mirror plasma are observed by using the Fraunhofer-diffraction method. The observed dispersion relations are in good agreement with those of drift waves including a Doppler shift due to E\ifmmode\times\else\texttimes\fi{}B rotation velocity. The fluctuation level is observed to depend sensitively on the radial profile of a plasma potential. It has a maximum value when a slightly negative electric field is formed, and decreases with increase in an electric field regardless of its sign.