Role of cold H+ and He+ ions and thermal anisotropy in the electromagnetic proton cyclotron instability

Abstract

A detailed study of the electromagnetic proton cyclotron instability in a plasma containing cold H+ and He+ ions is presented. In the presence of cold He+ ions the unstable spectrum is divided into two regions, LF and HF branches, by a stop band starting at the He+ gyro frequency. By using a semi-cold analytical expression for the convective growth rate we show that in both branches an increase of cold proton concentration shifts the spectrum to lower frequency values. However, an increase in the He+ popUlation shifts the spectrum (1) to lower frequencies in the LF branch and (2) to higher frequencies in the HF branch. This behavior leads to the fact that in the LF branch, the presence of cold H+ and He+ ions slows down the stabilization of the plasma due to decreasing thermal anisotropy; but, in the HF region, this stabilization is strengthened with increasing cold He+ population and is slowed down with increasing cold H+ concentration. Thus, an amplificaqon .of these waves is the result of combining these two effects, first, the shift of the spectrum due to cold ions and, second, the fact that higher frequency waves are the first to be stabilized.