Hollow-type plasma absorption probe for measurement of electron density in low pressure plasmas: a sheath thickness–insensitive technique

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Abstract We propose a hollow plasma absorption probe (PAP) designed to improve the accuracy of electron density measurement in low-pressure plasmas regardless of the sheath thickness on the planar probe surface. This improvement is achieved through an artificial hollow space acting as a thick sheath to bring the resonant absorption frequency ( f PAP ) from the hollow PAP close to the surface wave resonant frequency (), insensitive to variation in sheath thickness. Electromagnetic simulations confirmed the efficiency of the hollow design, showing a minimized discrepancy between f PAP and f SWR even with the thin sheath. Optimal configurations with a coaxial core diameter of 5 mm and a hollow depth of 5 mm were found and tested in plasmas ranging from 5 × 10 8 c m − 3 to 5 × 10 10 c m − 3 . Comparative experiments employing a planar cutoff probe (CP) to directly measure f pe demonstrated good agreement between the electron density derived from the hollow PAP and from the planar CP, compared to that from a conventional planar PAP.

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