Effect of object potentials on the wake of a flowing plasma

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Measurements and calculations have been carried out to determine the structure of electric potential and ion density in the near wake created by the flow of a high-Mach-number plasma past a conducting plate biased with respect to the undisturbed plasma. Results are obtained for a molecular nitrogen plasma with ambient electron densities N of the order of 105 cm−3, ion temperatures of θi≂0.025 eV, electron temperatures of θ∼0.3 eV, and plasma flow velocities of V0≂106 cm/s, corresponding to V0/(θ/M)1/2≂11. For high-Mach-number flow past an unbiased object, the wake structure is very nearly that predicted for the expansion of an initially uniform plasma half-space into vacuum; in this case there is a sharply defined ion front moving under the influence of an electric field produced by charge separation between ions and electrons near the front. With bias potential φp such that qφp/θ is in the range −1 to −10, a well-defined ion front still exists, but its motion is strongly affected by the imposed potential. This effect can be explained in terms of the impulse received by those ions passing through the sheath region near the plate edge. Two-dimensional simulations of the laboratory experiments were performed by using a multiple waterbag technique. The results for both zero and finite ion temperatures are almost noise-free and support the approximate analytical model. The calculated density for 10 θ negative bias on the plate is compared with the measured profile.