Electrostatic degrees of freedom in non-Maxwellian plasma

Detailed measurements of the ion velocity distribution function are used to test representations of the electrostatic degrees of freedom of slightly non-Maxwellian plasmas. It is found that fluid theory does not describe the data very well because there exist multiple closely spaced kinetic electrostatic modes. New wave branches appear that theoretically should persist as weakly damped modes even with T_e∼T_i. Both a sum over discrete dispersion relations and the Case-Van Kampen spectral representation can be used to provide working descriptions of the data, but the latter has certain advantages.