It is demonstrated experimentally that in the containment of a collisional hydrogen plasma in a PR-6 mirror trap, which is accompanied by strong instability, a quiescent phase occurs as a result of the inflow of cold plasma remaining after the injection is turned off. With the use of the technique of cutting off the inflow it has been shown clearly that the instability begins to develop when the inflow drops to a fairly low value. – It is also shown that in earlier experiments on microwave stabilization of an instability the same inflow brought about the cold-ion filling of the potential well that was formed. By the use of the cut-off it became possible to create an empty well and to fill it with cold ions of a selected gas in a controlled manner. It was found that during filling of the well with hydrogen ions, suppression of the instability apparently takes place at values of cold-ion addition which are substantially lower than 5% of the hot-plasma density. A similar addition of argon ions has no stabilizing effect.