Surface Morphology of Langmuir Probes

Langmuir probes are instruments used to study space plasma. These instruments are immensely popular owing to the simplicity of their design and hence have a rich heritage of being flown on wide variety of space missions.

Langmuir Probes on-board the Rosetta : Comet Hunter (left) and on-board the International Space Station (right).

In simplest terms, these probes could be described as biased electrodes that current collect from the surrounding plasma. The theory of Langmuir probes involves interpreting these current measurements  into useful physical quantities like density and temperature, which reveal the state of the space plasma. This theory has been developed and advanced for almost a century now, which has enabled these probes to be used in all kinds of plasma. However, these theories are based on the inherent assumption of the probe surface having uniform surface properties. Steps like using inert materials with uniform work function for probe construction, heating the probe through the course of the flight etc. have all been adapted to ensure this requirement.

However, there have been cases where the uniformity of the surface has been disturbed during construction, assembling, in-flight, rendering the measurements to be faulty. Steps to circumvent the change in surface behavior if the change is seen to be uniform across the surface (think of water vapor deposited uniformly across the entire probe surface), has been successfully implemented. But, the case when the change is nonuniform and hence the surface behavior is spatially patchy, has not been resolved so far. With the advent of smaller platform missions through CubeSats, resolving this problem is especially important since elaborate steps to satisfy the requirement of uniform surface properties are not always feasible owing to the cost and space constraints.

My research deals with how to combat this issue. We have developed a model that accounts for this patchy surface behavior through an ensemble of microscopic collectors and the surface variance of this ensemble has been modeled into the resulting current collection. This model has been developed from first principles making it adaptable to all space regimes and we have also constructed models for all probe geometries.

We used this model to study current from a sounding rocket mission called STORMS which had a spatially nonuniform probe surface despite the use of precautionary design measures. Below is a snapshot of the model vs rocket data that shows the success of the model in simulating the current collection under patchy surface conditions. To learn more about the development of the model and the rocket data shown above, head over here.

Here are the places where this work has been presented and reported so far:

  • P. Suresh and C.M. Swenson, Can we make reliable plasma measurements from Langmuir Probes on CubeSats?, Measurement Techniques in Solar & Space Physics, April 2015.
  • P. Suresh and C. M. Swenson , A Novel Method to Analyze Ionospheric Measurements made by a Non-uniformly Contaminated Langmuir Probe On-board a Sounding Rocket.”, 2014 USNC-URSI National Radio Science Meeting.
  • P. Suresh and C.M. Swenson, ITM Study using ISS as a launch platform, CEDAR Poster Session, June 2011.
  • P. Suresh and C. M. Swenson. Implications of contamination and surface area ratios for Langmuir probe diagnostics on CubeSats. EOS Trans. AGU, Abstract SM33C-1573(90(52)): Fall Meet.Suppl., 2009.
  • P. Suresh and C. M. Swenson. Estimation of Langmuir probe currents in the event of surface potential variation. CEDAR, Poster Session, July 2009.