= Solution
Rapid longitudinal <Oscillation>[oscillations] in a weakly collisional <plasma (physics)>[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.
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