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Typical I-V characteristics of an emissive probe.

Here are some of the basic features of an emissive probe:

1. With increasing heater voltage a large electron emission current is observed when the probe bias is below the plasma potential.
   
   
2. The floating potential, defined as the voltage at zero probe current, shifts toward the plasma potential. The larger the emission current, the closer the emissive probe floats to the plasma potential.
   
   
3. The plasma potential can be approximately obtained from the open-loop voltage of an emissive probe. This method is simpler than finding the knee of a single probe. The plasma potential can be measured instantaneously. Since the emissive probe draws no current it perturbs the plasma much less than the single probe drawing electron saturation current. Disadvantages are the required floating heater supply and the fact that one does not truely measure the exact plasma potential.
   
   
4. The slope of the I-V characteristics is inversely proportional to a differential resistance dV/dI. Considering the probe as a voltage source, a low internal resistance minimizes measurement errors with a voltmeter of finite load resistance. In time-dependent measurements, the probe's internal resistance and the external circuit capacitance determine the time resolution of the probe.
   
   
5. The emissive probe is primarily used for electric field measurements. The electric field in a plasma is defined by the gradient in the plasma potential, E=-gradV_plasma. Gradients in the floating potential of a cold probe are very sensitive to gradients in the temperature and primary electrons, hence too ambiguous for electric field measurements. In order to measure an electric field instantaneously, i.e. without moving the probe, one uses a double emissive probe and measures its open-loop voltage.
   
   

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