plasmapy.diagnostics.langmuir.get_ion_density_OML(probe_characteristic: Characteristic, probe_area: Quantity, gas, visualize: bool = False, return_fit: bool = False)[source]

Implement the Orbital Motion Limit (OML) method of obtaining an estimate of the ion density.

  • probe_characteristic (Characteristic) – The swept probe characteristic that is to be analyzed.

  • probe_area (Quantity) – The area of the probe exposed to plasma in units convertible to m2.

  • gas (Quantity) – The (mean) mass of the background gas in atomic mass units.

  • visualize (bool, optional) – If True a plot of the OML fit is shown. Default is False.

  • return_fit (bool, optional) – If True the parameters of the fit will be returned in addition to the ion density. Default is False.


n_i_OML – Estimated ion density in m-3.

Return type:



The method implemented in this function holds for cylindrical probes in a cold ion plasma, i.e. :math:T_i=0` eV. With OML theory an expression is found for the ion current as function of probe bias independent of the electron temperature [mott-smith.langmuir-1926]:

\[I_i \xrightarrow[T_i = 0]{} A_p n_i e \frac{\sqrt{2}}{\pi} \sqrt{\frac{e \left( V_F - V \right)}{m_i}}\]



H. M. Mott-Smith, I. Langmuir, Phys. Rev. 28, 727-763 (Oct. 1926)