Langmuir probe

The Langmuir probe, named after Irving Langmuir, is an experimental arrangement used to characterize a plasma. Therethrough electron density, electron temperature and plasma potential can be determined. The method was first described in 1923 and is still the subject of special research in the field of plasma diagnostics.

Construction

In general, a Langmuir probe of a wire of tungsten or molybdenum, which is enclosed in a ceramic. Only the probe tip is not insulated.

Operation

The probe is inserted at a location in the plasma and applied with a voltage. In practice, this is a sawtooth voltage. The resulting current is measured over a measuring resistance.

Thus, a characteristic curve of the plasma are recorded and closed out of this on the potential distribution in the plasma. From the characteristic, the electron and ion density, electron temperature, floating, and the plasma potential and the electron energy distribution can be calculated. However, the probe is a local disturbance of the plasma, so the measured values ​​do not correspond to the exact parameters at this point.

The probe is strongly negatively biased, ions ( because positively charged) are tightened, electrons can not reach the probe. This voltage range is called ion saturation region, in the ideal case, the current is constant here. Here you can determine the ion current.

If the negative voltage of the probe is smaller, can more and more hot electrons ( which are much faster due to their small mass than the heavy and cold ions) reach the probe. The I (U) - characteristic goes into an exponentially growing field over (if the electron gas has a Maxwell - Boltzmann distribution ), the so-called electron - starting range. At a characteristic voltage, the floating potential of so-called ion and electron flow are equal, the total current is therefore zero. This would be the voltage to which the probe tip in the plasma would boost its own. Above this voltage, the ion current is practically zero, the electron current increases exponentially. From the exponents of this increase, the electron temperature can be calculated.

In another characteristic voltage, the exponential increase in the electron current stops. Here, the electron saturation region begins. The characteristic voltage is equal to the plasma potential and the electrical potential of the plasma at this point. Ideally here the power would be off again constant, in reality, the shape of the curve depends strongly on just in the electron saturation region of the probe geometry (spherical probe, cylindrical probe, flat probe ) from.

The plasma parameters can be obtained is by adapting a model to the I (U ) characteristic. There are a number of such models, which are more or less depending on the RF power, the plasma density and gas pressure valid.

Swell

  • M. Kaufmann: plasma physics and fusion research. 1st edition, pp. 237-239, Vieweg Teubner Verlag, 2003, ISBN 978-3519003496.
  • Plasma Physics
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