Editing Langmuir probe (section)

===Special arrangements===

Arrangements with four ('''tetra probe''') or five ('''penta probe''') have sometimes been used, but the advantage over triple probes has never been entirely convincing. The spacing between probes must be larger than the [[Debye length]] of the plasma to prevent an overlapping [[Debye sheath]].

A '''pin-plate probe''' consists of a small electrode directly in front of a large electrode, the idea being that the voltage sweep of the large probe can perturb the plasma potential at the sheath edge and thereby aggravate the difficulty of interpreting the ''I-V'' characteristic. The floating potential of the small electrode can be used to correct for changes in potential at the sheath edge of the large probe. Experimental results from this arrangement look promising, but experimental complexity and residual difficulties in the interpretation have prevented this configuration from becoming standard.

Various geometries have been proposed for use as '''ion temperature probes''', for example, two cylindrical tips that rotate past each other in a magnetized plasma. Since shadowing effects depend on the ion Larmor radius, the results can be interpreted in terms of ion temperature. The ion temperature is an important quantity that is very difficult to measure. Unfortunately, it is also very difficult to analyze such probes in a fully self-consistent way.

'''Emissive probes''' use an electrode heated either electrically or by the exposure to the plasma. When the electrode is biased more positive than the plasma potential, the emitted electrons are pulled back to the surface so the ''I''-''V'' characteristic is hardly changed. As soon as the electrode is biased negative with respect to the plasma potential, the emitted electrons are repelled and contribute a large negative current. The onset of this current or, more sensitively, the onset of a discrepancy between the characteristics of an unheated and a heated electrode, is a sensitive indicator of the plasma potential.

To measure fluctuations in plasma parameters, '''arrays''' of electrodes are used, usually one{{spaced ndash}}but occasionally two-dimensional. A typical array has a spacing of 1&nbsp;mm and a total of 16 or 32 electrodes. A simpler arrangement to measure fluctuations is a negatively biased electrode flanked by two floating electrodes. The ion-saturation current is taken as a surrogate for the density and the floating potential as a surrogate for the plasma potential. This allows a rough measurement of the turbulent particle flux

<math>
\Phi_{turb} 
= \langle \tilde{n}_e \tilde{v}_{E\times B} \rangle
\propto \langle 
\tilde{I}_i^{max} ( \tilde{V}_{fl,2} - \tilde{V}_{fl,1} ) 
\rangle
</math>