A low-pressure microwave plasma source (argon at about ) is sustained by radiation…

PHYSICS · Multi-part · Fermi University Physics · Plasma Physics · Waves in plasmas · Laboratory, industrial and space plasmas

Plasma Physics, Ch Plasma oscillations and light in an unmagnetised plasma

Problem

A low-pressure microwave plasma source (argon at about

10\,\mathrm{Pa}

) is sustained by

2.45\,\mathrm{GHz}

radiation and has an electron density of order

n_e \sim 1.0\times10^{18}\,\mathrm{m^{-3}}

throughout a column of diameter

5.0\,\mathrm{cm}

. At this pressure the electron collision rate is of order

10^{8}\,\mathrm{s^{-1}}

, far below the angular frequency of the drive, so treat the column as a uniform, unmagnetised, collisionless plasma. ((a)) Estimate the electron plasma frequency of the torch and compare it with the drive frequency. ((b)) Estimate the skin depth over which the

2.45\,\mathrm{GHz}

field decays inside the plasma. ((c)) Compare this skin depth with the column diameter and say what this implies about how deeply the microwave field penetrates the plasma.

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