Matched dipole probe for magnetized low electron density laboratory plasma diagnostics

Abstract

In this paper, a diagnostic method for magnetized and unmagnetized laboratory plasma is proposed, based on impedance measurements of a short matched dipole. The range of the measured electron densities is limited to low density plasmas (1012–1015 m−3), where other diagnostic methods have strong limitations on the magnetic field strength and topology, plasma dimensions, and boundary conditions. The method is designed for use in both large- and small-dimension plasma (<10 cm) without or with strong non-homogeneous magnetic field, which can be undefined within the probe size. The design of a matched dipole probe allows to suppress the sheath resonance effects and to reach high sensitivity at relatively small probe dimensions. Validation experiments are conducted in both magnetized (B ∼ 170 G) and unmagnetized (B = 0) low density (7 × 1012 m−3–7 × 1013 m−3) low pressure (1 mTorr) 10 cm scale plasmas. The experimentally measured data show very good agreement with an analytical theory both for a non-magnetized and a magnetized case. The electron density measured by the matched dipole and Langmuir probes in the range of 7 × 1012 m−3–7 × 1013 m−3 show less than 30% difference. An experimentally measured tolerance/uncertainty of the dipole probe method is estimated to ±1% for plasma densities above 2 × 1013 m−3. A spatial resolution is estimated from the experiments to be about 3d, where d is the dipole diameter. The diagnostic method is also validated by comparing the measured plasma impedance curves with results of analytical modelling.