 Solution

Rapid longitudinal oscillations in a weakly collisional plasma compress the velocity distribution primarily along one direction. There is insufficient time for collisions to redistribute that energy among three directions; the effective one-dimensional adiabatic exponent is $1+2/1=3$. Ordinary monatomic-gas sound with local collisional equilibrium has three translational degrees of freedom, giving $1+2/3=5/3$. Thus $\Gamma =3$ belongs to the stated kinetic/time-scale limit, not to every thermodynamic process involving ionized gas.