Experimental and theoretical determination of the strongly anisotropic velocity distribution functions of ions in the intrinsic gas plasma in strong fields

The first seven coefficient of expansion of the energy and angular distribution functions in the Legendre polynomials for Hg$^+$ ions in the Hg vapor plasma with parameter $E/P\approx$ 400 V/(cm Torr) are measured for the first time using a planar one-sided probe. The analytic solution to the Boltzmann kinetic equation for ions in the plasma of their parent gas is obtained in the conditions when the resonant charge exchange is the predominant process, and an ion acquires on its mean free path a velocity much higher than the characteristic velocity of thermal motion of atoms. The presence of an ambipolar field of an arbitrary strength is taken into account. It is shown that the ion velocity distribution function is determined by two parameters and differs substantially from the Maxwellian distribution. Comparison of the results of calculation of the drift velocity of He$^+$ ions in He, Ar$^+$ in Ar, and Hg$^+$ in Hg with the available experimental data shows their conformity. The results of calculation of the ion distribution function correctly describe the experimental data on its measurement. Analysis of the result shows that in spite of the presence of the strong field, the ion velocity distribution functions are isotropic for ion velocities lower than the average thermal velocity of atoms. With increasing ion velocity, the distribution becomes more and more extended in the direction of the electric field.