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EFW for Dummies
1. Hardware
2. Spheres
3. Iv curve
4. Current biasing
5. Modes

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5. Modes of EFW

The instrument can be run in several different scientific modes (as well as a much larger number of diagnostic modes).  The most common mode is the electric field mode experiment.  Two other less frequently used modes are Langmuir mode and sweep mode.

5a. Electric field mode

Electric field mode
In this mode, which is the default mode, all 4 probes are run with current biasing.  Since the current biasing "grounds" each probe to the local plasma potential, the difference between the probe potentials divided by the length between the probes gives the electric field.

5b. Langmuir Mode

Langmuir mode
In this mode, the probes are run with a voltage bias, rather than a current bias.  That is, they are maintained at a fixed voltage with respect to the satellite, and the current is measured.

This mode is most useful in denser plasmas, with the probes biased positive with respect to the plasma.  In this regime, dominated by the plasma electrons, the collected current depends on the plasma density.  This mode therefore gives high time resolution measurements of the plasma density.

5c. Sweep mode

Sweep mode
In this mode, either a bias current or a bias voltage is swept through the range of interest, and the other parameter (voltage or current) is measured.  In this way, the complete I-V curve is determined.  This curve contains information about:
  • the density
  • the ion temperature
  • the electron temperature
  • the maximum photocurrent
In principle, all 4 of these parameters can be found by fitting a curve to the data.  In practice, it is difficult to find the ion temperature.

Okay, that's it for "EFW for dummies".  Now on to how to actually use EFW data.

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last modified on 28-Nov-2002