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The experiments for the next four weeks will deal with plasma production and losses, plasma diagnostics with probes, measurements of potentials, electric fields and currents inplasmas. For simplicity, we start out with an unmagnetized discharge plasma. The Langmuir probe serves as the main tool to measure density, temperature and plasma potential. Variations include the double probe and emissive probe. In steady state plasmas we will measure the degree of ionization and its dependence on discharge parameters. In a pulsed plasma we will measure the decay time and derive the plasma losses from it. We will study plasma sheaths and the shielding of electric fields by collisionless plasmas. We will see that the resistivity of a collisionless plasma is low. However, the current density is limited and near the limit the plasma properties are changed. Specifically, the following initial experiments are suggested.

1. Langmuir Probes.

Measure the current-voltage characteristics of a Langmuir probe in a steady-state discharge plasma. Evaluate the curve "by hand"so as to obtain the electron temperature, density, floating and plasma potential. Measure the I-V curve in the presence of an electron beam. Identify the presence of primary electrons in the discharge.

2. Properties of Discharge Plasmas.

Using the single Langmuir probe and the Labview data acquisition/evaluation system, measure how the electron density varies with discharge current and neutral gas density. Measure how the discharge current depends on discharge voltage and heater power. Is the emission current temperature or space charge limited?

3. Double Probes.

Measure the current-voltage characteristics of a double probe in a steady-state discharge plasma. This probe consists of two identical Langmuir probes which are biased with respect to each other rather than to ground. Consequently one cannot obtain the plasma potential with respect to ground. But the curve yields the electron temperature and density, the former from the slope at the origin and the latter from the ion saturation current. Measure the I-V curve, evaluate the plasma parameters, and compare the results with those from a single Langmuir probe. Explain the differences and list advantages/disadvantages of each probe.

4. Emissive Probes.

An emissive probe is a Langmuir probe whose electrode consists of a heated wire capable of emitting as many electrons as it collects from the background plasma. Under these conditions the probe "floats" (I=0) at the plasma potential rather than the floating potential. Thus, emissive probes are used to measure the plasma potential, hence electric fields (E = -gradV) in plasmas. Measure the I-V characteristics of an emissive probe. Note that with increasing filament temperature the floating potential approaches the plasma potential. Measure the electric field in the plasma in front of a biased grid. Identify the sheath region and the essentially field-free plasma region. What happens to the plasma potential when the grid bias is positve? Explain.

Read your lab manual and the appropriate chapters in Chen's textbook. A list of questions will follow.

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