  The tandem mirror GAMMA 10 utilizes an electron cyclotron resonance heating
(ECRH) for forming a confinement potential. Density fluctuation is observed
using microwaves, such as interferometer, reflectometry and Fraunhofer
diffraction (FD) method. An ultrashort-pulse reflectometry has an advantage of
detecting fluctuation locally. The wave number can be obtained by the FD
method. In the edge plasma region electrostatic probes are used. Fluctuation
with coherent mode in several kHz is excited in the hot-ion mode plasma. When
the ECRH is applied, electron density in the central cell increases gradually.
On the contrary, intensity of the density fluctuation decreases and coherent
mode is suppressed. Its frequency also varies with the density increase. In
that time, radial potential distribution, i.e., electric field, changes along
with the formation of plug potential. From this behaviour it is deduced that
the fluctuation deeply relates to the potential formation and improvement of
the confinement.
