Langmuir probe diagnostics

Because of the large spread of plasma densities and energies, the experimental techniques to measure its properties constitutes a branch of Plasma Physics by itself. In our experiments with cold, low pressure plasmas we make mostly use of electric probes. A classical technique, denominated Langmuir probes consists in a metallic electrode with a well defined symmetry which is immersed into the plasma. This probe is electrically biased with respect to the plasma and the collected current is monitored.

In the picture  two Langmir probes can be observed at the extreme of a vertical thin rod inmersed into an Argon plasma in our small plasma chamber. The thin cilinder is a collecting probe probe while the bright dot corrrespons to a hot emissive  probe and both are connected to the external measuring circuitry in order tomeasure the properties of the background plasma properties.

Actual measurements are shown in the figure (fig. with blue dots) and it may be appreciated that for low voltages (below -20 volts) only a small negative current is collected. The electric potential imposed to the probe was made negative with respect to the plasma leading positive ions to be collected and the electrons repelled. This small negative current in the figure is denominated ion saturation current. As the probe bias voltage is increased, electrons with energy high enough could reach the probe and the drained current grows. When the bias potential is still increased the current reverts its sign crossing the floating potential where ion and electron current are equals.

The electron current increases further more electrons in the plasma are able to jump the decreasing energy barrier up to the knee located between 0 and 10 volts. The probe bias potential becomes equal to plasma potential at this point and all plasma electrons are able to reach the probe. The electron temperature and density could be deduced from these data as well as plasma potential in some cases. The corresponding current is denominated electron saturation current and permits to determine the electron plasma density Ne. For bias voltages exceeding plasma potential, the probe current increases because of the imperfect electrical shielding and additional physical phenomena (ionizations, ...etc.).

A fast measuring technique was used to obtain the experimental data of these graphs. The measurements for the full curve lasted 1.78 ms and the delay between two successive curves was of 0.34 ms. In the second figure (fig. with red dots) this fast sweeping technique was employed to evidence the superimposed probe current jumps corresponding to jumps caused by groups of ions.

The emissive Langmuir probe are basically inteded for te accurate measurements of the plasma potential. Thery are made of a thin filament heated by an electrical current which emits or collect electrons according  to its electrical polarity with respecct to the plasma. The value of the plasma potential is determined by the abrupt transition beween the electron collection and emission which lies close to the floating potential of the hot probe.

The last figure shows the actual probe current current as a function of the bias voltage for different filament temperatures. As the wire temperature increases the electron emission grows and these electrons are repelled by the probe for voltages below the plasma potential. This fact produce more steeper curves than collecting probes and allows a more precise determination of the plasma potential.

More sophisticated electric probe techniques are emissive Langmuir probes, employed to determine the plasma potential and electrostatic energy analyzers are used to measure energy spectrum of ions among others. Interested readers will find information on these techniques in:

I.M. Hutchinson. "Principles of plasma diagnostics". Cambridge University Press. Chap. 3. (1987)

R.L. Merlino. "Understanding Langmuir probe curent-voltage characteristics". Amer. J. Phys. 75, (12), pp. 1078-1085 (2007).

N. Hershkowithz. "How Langmuir probes work", in Plasma diagnostics. Discharge parameters and chemistry. O. Auciello and J. Flamm (Eds.). London, Academic Press. pp. 113-183 (1989).

J.P. Sheehan and N. Herskowitz. "Emissive probes". Plasma Sources Sci. Technol. 20, pp. 1-22 (2011).