We discuss experimental constraints on the free parameter of the nonextensive kinetic theory from measurements of the thermal dispersion relation in a collisionless plasma. For electrostatic plane-wave propagation, we show through a statistical analysis that a good agreement between theory and experiment is possible if the allowed values of the $q$-parameter are restricted by $q=0.77 pm 0.03$ at 95% confidence level (or equivalently, $2-q = 1.23$, in the largely adopted convention for the entropy index $q$). Such a result rules out (by a large statistical margin) the standard Bohm-Gross dispersion relation which is derived assuming that the stationary Maxwellian distribution ($q=1$) is the unperturbed solution.
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