A change in the conductivity of sodium-helium plasma interacting with polarization-modulated laser radiation tuned to sodium $D_1$ line

A change in the conductivity of a sodium-helium plasma interacting with laser radiation tuned to the sodium $3^2S_{1/2}-3^2P_{1/2}$ transition and having sign-changing polarization is experimentally observed. It is shown that this effect is caused by the processes of Penning ionization, spin exchange, and optical orientation of atoms in a gas-discharge plasma.