Influence of Plasma Frequency on the Light Emitted by an Exploding Ionized Gaseous Filament

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Abstract
A very dense and easily reproducible plasma was generated by the explosion of a thin preionized gaseous column. The optical thickness of the discharge was measured by means of a laser and the absorption coefficient was found to be higher than 18 cm−1. The spectral brightness of the discharge is measured from wavelengths 0.2μ to 4.8μ. The instantaneous spectral distribution shows an abrupt drop in the near infrared at 1μ. This is interpreted as being due to the plasma frequency with a corresponding electron density of (1 ± 0.2) × 1021 cm−3. The complex index and the emissivity of the plasma are calculated and give a spectral repartition in good agreement with the measured one. Between 0.5 μsec and 1 μsec after the breakdown, the discharge column radiates as a blackbody at 44 000°K over a large spectral region. The free-free transitions, mainly in the infrared, can explain the radiation observed. The light is found to originate from a shell 2 × 10−3 cm thick, in agreement with the structure found for such sparks.

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