https://phys.org/news/2018-05-scientists-ability-electrical-properties-plasma.html

Kraus and principal research physicist Yevgeny Raitses, co-author of the paper and research advisor for Kraus on his first-year graduate project, found that the layer's charge can sometimes be positive, contradicting what scientists have long thought—that the blanket always has a more negative charge than the surrounding plasma. The findings indicate that researchers must determine exactly what kind of charge surrounds the probe to be able to make corrections that will generate an accurate measurement of conditions inside the plasma.

Specifically, research conducted on the Raitses-led Hall Thruster Experiment (HTX) at PPPL, which is typically used to study plasma thrusters for spacecraft and related plasma devices, showed that a heat-emitting diagnostic that is not connected to a grounded wire can sometimes produce the positive charge. The HTX was able to provide a steady, stable plasma that let the scientists detect more precisely what kind of charge was building up next to the probe.