Plasma consists of two components in the Gas Dynamic Trap facility: a relatively cold and dense collisional plasma and a population of fast anisotropic ions which oscillate between mirror points. Peaks of fast ion density are made closely to the mirror points. It formes an ambipolar potential difference between these points and the center of the facility. The ambipolar potential restricts a plasma flow through the mirror region, so it influences on the plasma confinement. The ambipolar potential value can be found from the line plasma density in the central facility region. The dispersion interferometer, which is based on a CO2-laser with wavelength = 9.57 m, has been made for this purpose. The minimal line plasma density measurable with the dispersion interferometer is (nel) ~ 1013 cm-2, the time resolution is 100 s. The fast ion line density is 4 times higher than the warm ion line density in the mirror region. The ambipolar potential value is e [approximately equal] 0.7 Te in electron temperature units. Also the flute instability restriction opportunity with gradient of local electric field has been observed. The limiter voltage satisfying the condition U ~ Te is boundary for stabilization of plasma behavior.