Dartmouth Events

Abstract: Retarding potential analyzers (RPAs) have been used to measure plasma ions since the beginning of the space age. However, sheath effects complicate the measurement of low energy (< 1eV) ionospheric ions during sounding rocket missions.  While simplified models of sheaths in stationary, Maxwellian plasmas are well understood, real sheaths can differ significantly.  For example, the small relative plasma flow velocity combined with the focusing of particles traveling through a thick 3D sheath invalidates the planar approximations commonly used during orbital missions. 

Because of the complex sheath physics, ion RPAs have largely fallen out of favor with the sounding rocket community. More costly, higher resource particle imagers and microchannel plate detectors have taken the place of the RPA.  This thesis studies the non-ideal plasma sheath and demonstrates that ion RPAs can be successfully used during sounding rocket missions when modeling of the 3D plasma sheath is included in the data analysis process.

This talk overviews the theory of current-voltage (I-V) curves with an emphasis placed on sheath physics and the impact of the sheath on current collection. We describe the development of an RPA, the Petite Ion Probe (PIP), used in this difficult plasma regime and the new data analysis procedure developed to extract accurate measurements. Data analysis begins by modeling the plasma sheath using the Spacecraft Plasma Interaction System (SPIS), a particle-in-cell code. Test particles are traced through the sheath and detector to determine the detector's I-V response.  A training set is constructed from these simulated curves and support vector regression is used to relate the shape of the I-V curve to the properties of the plasma. 

The first in situ use of the PIPs occurred during the MICA sounding rocket mission which launched from Poker Flat, Alaska in February of 2012.  These data are presented as a case study, providing valuable cross-instrument comparisons to validate the PIP and our new data analysis method.  A heritage top-hat thermal ion electrostatic analyzer, called the HT, and a multi-needle Langmuir probe have been compared. The HT and PIP ion temperature measurements agree with a root-mean-square error of 0.023 eV.  These two instruments also agree on the parallel-to-B plasma flow velocity with a root-mean-square error of 130 m/s.  The PIP with its field of view aligned perpendicular-to-B provided a density measurement with 11% error when compared to the multi-needle Langmuir Probe. Higher error in the other PIP’s density measurement is likely due to simplifications in the SPIS model geometry, which could be improved in future work.  The agreement between the PIP and other instruments on the MICA flight validates the PIP as an accurate and low-resource device for the study of ionospheric plasmas.