Plasma Diagnostics

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Information about Plasma Diagnostics
Science-Technology

Published on September 20, 2008

Author: Abhishekgor90

Source: authorstream.com

Plasma Diagnostics : Plasma Diagnostics 1 Abhishek A. Gor (248) Plasma Diagnostics Plasma diagnostics : Plasma Diagnostics 2 Plasma diagnostics Research about the tools & methods for the research in plasma is the main objective of plasma diagnostics. Various methods & Instruments are developed in plasma physics. Various quantities of plasma : Plasma Diagnostics 3 Various quantities of plasma Nature Number density of particles Speed & Energy of particles Collision frequency Magnitude of Electric &Magnetic Field. Langmuir probe : Plasma Diagnostics 4 Langmuir probe Langmuir probe : Plasma Diagnostics 5 Langmuir probe Langmuir-probe, named after their famous inventor, allow us to infer plasma-density and electron-temperature from simple electrical measurements. Slide 6: Plasma Diagnostics 6 Entire probe: length=66.5cm Probe tip: diameter=0.130cm Construction of probe : Plasma Diagnostics 7 Construction of probe Probe shaft:- Stainless steel body Teflon plug 2 Metal caps Copper wire Glass tubing SMA co-ax connector Probe tip:- Tungsten wire Glass insulation Aluminum covering Ceramic casing (protects from plasma heat) Slide 8: Plasma Diagnostics 8 Slide 9: Plasma Diagnostics 9 Probe : Plasma Diagnostics 10 Probe The cylindrical probe-tips are made from high-melting materials, usually tungsten. The probe area is of the order of a few (mm)2. They are embedded in AlO3-ceramics for electrical insulation. Slide 11: Plasma Diagnostics 11 Langmuir probe : Plasma Diagnostics 12 Langmuir probe When probe is immersed into a plasma, it is exposed to a large flux of ions and electrons. This flux depends upon the density and the temperature (Thermal velocity) of the particles. Langmuir probe : Plasma Diagnostics 13 Since the thermal velocity depends upon the mass of the particles, the electron-flux usually is much higher then the ion-flux. Due to high electron flux we assume the probe as negative potential Uf (thus repelling the electrons) to maintain its charge-balance. Langmuir probe Slide 14: Plasma Diagnostics 14 Slide 15: Plasma Diagnostics 15 Langmuir probe : Plasma Diagnostics 16 Langmuir probe As long as the surface is biased sufficiently negative with respect to the plasma, the electrons will be repelled and one measures a (constant) negative current (the ion-saturation-current) determined by the thermal flux of the ions. From this current we can deduce the the ion-density. Langmuir probe : Plasma Diagnostics 17 Langmuir probe When the voltage approaches the plasma-potential the electron-flux increases according to the boltzman-relation: n/n0=exp(-eU/kBTe) Thus in a logarithmic plot of the characteristic we find a linear slope which yields the electron-temperature. Langmuir probe : Plasma Diagnostics 18 Langmuir probe The equation for the electron temperature & electric current is given. Langmuir probe : Plasma Diagnostics 19 Langmuir probe Similarly the equation of charge density is also given. In case of neutral particle measurement of pressure is enough. i.e. (p=nkt) Langmuir probe : Plasma Diagnostics 20 Langmuir probe All equations are for the Electron temperature, Electric current, Charge density are normalized from I-V trace. Langmuir probe : Plasma Diagnostics 21 Langmuir probe Thus, langmuir probe is one of the important methods in plasma research. Thank You……. : Plasma Diagnostics 22 Thank You……. Abhishek A. Gor (248) Slide 23: Plasma Diagnostics 23

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