Excitation of ion acoustic waves with probes

A large cylindrical Langmuir probe (length=30 cm, diameter=1.25 mm) and a probe of similar dimensions consisting of a conductor completely enclosed in an insulator are compared as exciters of ion acoustic waves. When the potential drop across the sheath of the Langmuir probe is increased from −143 to +10 V, the ion wave amplitude increases by a factor of 40. This result can be qualitatively explained with a simple model which relates the wave amplitude to the sheath radius and to the ac sheath impedance. When large negative pulses (≳100 kTe/e) are applied to the probes, the plasma response is similar for the biased and the insulated probe. The advantages of the insulated probe as an exciter of ion waves over ordinary probes are discussed. Ion waves with peak‐to‐peak amplitudes (δn/n) ≃0.44 can be excited with the insulated probe.

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