Confusion about floating potential and Langmuir probes in plasmas I am working to understand Langmuir probes and this concept of floating potential keeps popping up that I have some confusion on. I am reading a paper called Understanding Langmuir probe current-voltage characteristics, and at the end of the first paragraph it states:
"The early users of probes naively assumed that the potential of the plasma at the location of the probe known as the plasma potential or space potential and designated as $V_P$ could be determined by measuring the potential on the probe relative to one of the electrodes. However, this procedure determined the floating potential $V_f$ of the probe which is generally not the same as the plasma potential. By definition, a probe that is electrically floating, collects no net current from the plasma, and thus its potential rises and falls to whatever potential is necessary to maintain zero net current"
I understand that the definition of floating potential is the potential at which ion and electron currents cancel each other out, but why does the probe have to stay at zero net current, as the last sentence in the quote above implies? Why does it have to be floating? To obtain an I-V curve with a Langmuir probe, you must go above and below the floating potential, so it is clearly possible to access potentials other than the floating potential. What is so special about the floating potential?
In the wikipedia article for Langmuir probes, under the section "Floating Potential", it makes the remark that the floating potential is the experimentally accessible quantity. Why is this the case? And if it is true, how can you even make a full I-V curve at potentials other than the floating potential?
 A: In a plasma-probe system such as this, a net current implies the accumulation of charge somewhere.  Such an accumulation would result in an electric field.  Electric fields do work to get rid of themselves, thus they would inhibit or enhance currents to get rid of the charge imbalance.  This is part of the reason why probes in plasmas float relative to the plasma, i.e., they accumulate some net charge and generate an associated electric field.  This is sometimes referred to as the plasma sheath around an instrument.  If the instrument is in space and exposed to sunlight, it will have an addition current caused by photoelectrons being ejected from any exposed conducting surface.
In regards to a Langmuir probe, you, the user, typically force the current through the probe.  This is called biasing the probe.  That is, you bias its current positive or negative, which will cause it to float correspondingly.  You can drive the probe to saturation and effectively crush the natural floating potential effect that would otherwise happen when in a plasma (e.g., we sometimes push the instruments to the rails to collapse all surrounding photoelectrons to reduce the normally occurring sheath).
On Parker Solar Probe, they control the biasing currents to the electric field antenna.  The spacecraft is moving upwards of 200 km/s relative to the Sun in parts of its orbit and it's plowing through regions of space with a lot of micron-sized dust (e.g., wrote a more detailed answer about this stuff here https://space.stackexchange.com/a/17646/12508).  As such, it gets bombarded by hypersonic dust, which ablates and ejects material from the spacecraft.  The WISPR instrument can sometimes see this debris leaving the heat shield, which is really neat (and kind of scary for the team).  During every inbound orbit, the FIELDS team changes the biasing current settings to account for the new plasma they expect to pass through.  On one pass they went a little too far and pushed the instrument to the rails for a little bit.  Very interestingly, they saw some of that debris from the heat shield suddenly start to orbit the electric field antenna (i.e., they were acting like a long, current-carrying wire and the debris like huge, charged particles).  It makes for some really neat movies.
