Ion-acoustic noise excited by positive probes

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Abstract
Measurements are presented of a low-frequency instability (fpi, where fpi is the ion plasma frequency) in the neighborhood of positively biased disk probes in an unmagnetized multi-dipole plasma. On an isolated probe the spectrum is broad, with relative density perturbation delta n/n<or approximately=1 per cent, but sharpens and increases in amplitude when a second, negatively biased probe is brought into the vicinity. Observed modulations in delta n/n as a function of probe separation are modeled under the assumption that the unstable waves have wave-vectors in the plane of the positive probe, leading to an 'antenna pattern'.