LionSAT Hybrid Plasma Probe
 
LionProbe Design
1.0   Introduction
2.0   Background
  2.1  Langmuir Theory
  2.2  PFP Theory
  2.3  Plasma Sheath
3.0   Implementation
  3.1  Microprocessor
  3.2  A/D & D/A Convert
  3.3  Electrometer
  3.4  DDS & Comparator
  3.5  Phase Detection
  3.6  Signal Combination
  3.7  Sensor Heads
  3.8  Boom Selection
  3.9  Other Schematics
4.0   Results
5.0   Project Value
6.0   Cost Analysis
7.0   Conclusion
8.0   Future Work
9.0   References
 

Plasma Sheath Analysis

            It should also be made clear that in order to successfully accomplish the scientific goals of LionSat, it is necessary to characterize the probable length of the plasma sheath surrounding the spacecraft.  In order to make density measurements in the ambient, undisturbed plasma, one or more sensor heads will need to extend into this region via mechanically deployed booms.  Given the data from the International Reference Ionosphere Model, the following plot can be made of the Debye length of the plasma, where the Debye length is defined as:

where                          ne = electron density,
                                    q = electron charge,
                                    Te = electron temperature,
                                    ε0 = permittivity of free space.

This is effectively the length at which the ions and free electrons can impose an effect on each other.  The plasma sheath around the spacecraft should be on the order of a few times the Debye length. [7]

Figure 3: Debye length analysis using IRI data model

It can be seen from this plot that the Debye length is very small in the expected altitude range for LionSat, 300-400 km.  Therefore a suitable boom length for extending the sensor heads into ambient plasma could be on the order of 5 cm, for example.

            In continuing this discussion, it is important to account for the fact that there will be a shock front formed around the spacecraft which causes the ram/wake structure that LionSat wishes to study.  This too has an impact on boom length and the length of boom it will take to reach ambient plasma.  A plot showing the expected shock front structure is shown below in Figure 4.

Figure 4: Shock front simulation of ram/wake effects

In this image, the spacecraft's orbital plane is the plane of the image.  Consequently, the spacecraft spin axis is perpendicular as noted by the rotation arrow.  This image clearly shows the expected density buildup and falloff in front of and behind the spacecraft, respectively.