Challenging fundamental assumptions on Langmuir probe physics in strongly magnetized fusion plasmas

Langmuir probes are a classic and widespread tool for investigating edge and Scrape-Off Layer physics in magnetically confined fusion plasmas. They are attractive as they are easy to build and allow localized measurements of multiple plasma properties, e.g. T_e, n_e, I_I,sat. The data interpretation, however, is typically subject to simplifying assumptions on the probe-plasma interaction and sheath physics.

In the Wendelstein 7-X stellarator, several fundamental assumptions on Langmuir probe analysis have been experimentally assessed using a reciprocating array of 24 different Langmuir probes, including systematic scans of 1) probe size, 2) probe material, 3) incidence angle to the magnetic field, 4) dielectric shielding of poloidal / radial / toroidal plasma influx.

1) cylindrical probes with diameters between 0.5mm and 4.0mm are employed to study the ion saturation current that is expected to scale linearly with probe diameter. Deviations inform about sheath expansion effects and the role of cross-field currents.

2) identically shaped probes made of Tungsten, Molybdenum, Graphite inform about the effects of different work function regarding electron emission.

3) a scan of magnetic field incidence angle informs about sheath modification at very shallow angles of down to 1°

4) perpendicular (poloidal and radial) particle fluxes are usually considered negligible compared to parallel particle fluxes. A set of selectively shielded probes uncovers perpendicular particle flux contributions.