Editing Langmuir probe (section)

==Practical considerations==

For laboratory and technical plasmas, the electrodes are most commonly [[tungsten]] or [[tantalum]] wires several thousandths of an inch thick, because they have a high melting point but can be made small enough not to perturb the plasma. Although the melting point is somewhat lower, [[molybdenum]] is sometimes used because it is easier to machine and solder than tungsten. For fusion plasmas, [[graphite]] electrodes with dimensions from 1 to 10&nbsp;mm are usually used because they can withstand the highest power loads (also sublimating at high temperatures rather than melting), and result in reduced [[bremsstrahlung]] radiation (with respect to metals) due to the low atomic number of carbon. The electrode surface exposed to the plasma must be defined, e.g. by insulating all but the tip of a wire electrode. If there can be significant deposition of conducting materials (metals or graphite), then the insulator should be separated from the electrode by a {{clarify span|meander|date=October 2016}} to prevent short-circuiting.

In a magnetized plasma, it appears to be best to choose a probe size a few times larger than the ion Larmor radius. A point of contention is whether it is better to use '''proud probes''', where the angle between the magnetic field and the surface is at least 15°, or '''flush-mounted probes''', which are embedded in the plasma-facing components and generally have an angle of 1 to 5 °. Many plasma physicists feel more comfortable with proud probes, which have a longer tradition and possibly are less perturbed by electron saturation effects, although this is disputed. Flush-mounted probes, on the other hand, being part of the wall, are less perturbative. Knowledge of the field angle is necessary with proud probes to determine the fluxes to the wall, whereas it is necessary with flush-mounted probes to determine the density.

In very hot and dense plasmas, as found in fusion research, it is often necessary to limit the thermal load to the probe by limiting the exposure time. A '''reciprocating probe''' is mounted on an arm that is moved into and back out of the plasma, usually in about one second by means of either a pneumatic drive or an electromagnetic drive using the ambient magnetic field. '''Pop-up probes''' are similar, but the electrodes rest behind a shield and are only moved the few millimeters necessary to bring them into the plasma near the wall.

A Langmuir probe can be purchased off the shelf for on the order of 15,000 U.S. dollars, or they can be built by an experienced researcher or technician. When working at frequencies under 100&nbsp;MHz, it is advisable to use blocking filters, and take necessary grounding precautions.

In low temperature plasmas, in which the probe does not get hot, surface contamination may become an issue.  This effect can cause [[hysteresis]] in the I-V curve and may limit the current collected by the probe.<ref name="Amataucci">{{cite journal |author=W. Amatucci |display-authors=etal |date=2001 |title= Contamination-free sounding rocket Langmuir probe|journal=Review of Scientific Instruments |volume=72 |issue= 4|pages=2052–2057 |doi=10.1063/1.1357234 |bibcode = 2001RScI...72.2052A |doi-access=free }}</ref>  A heating mechanism or a glow discharge plasma may be used to clean the probe and prevent misleading results.