Experimental Studies of Hydrogen Plasma Produced by Compact ECR Plasma Source

This article presents the capabilities of the highly efficient, permanent magnet-based compact ECR plasma source (CEPS) developed at IIT Delhi for the production of hydrogen plasma. In the experiments, the CEPS was mounted coaxially onto a moderately large expansion chamber (diameter <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\approx ~50$</tex-math> </inline-formula> cm and length <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\approx$</tex-math> </inline-formula> 75 cm). Hydrogen plasma escaping from the CEPS mouth (diameter <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\approx$</tex-math> </inline-formula> 9.1 cm) flows along the diverging field lines of the NdFeB ring magnets of the CEPS into the chamber. Diagnostics include specially designed Langmuir probes (LPs) and an on-axis retarding field energy analyzer (RFEA). Measurements yield typical densities, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\approx 10^{11}$</tex-math> </inline-formula> cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup>, with high <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T_{\mathrm {e}}(\approx 8$</tex-math> </inline-formula> eV) and high plasma potentials (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V_{\mathrm{p}} \approx 50$</tex-math> </inline-formula> V) near the source, at moderate microwave powers (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\approx 650$</tex-math> </inline-formula> W) and pressures (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\approx $</tex-math> </inline-formula> few mTorr). At high pressures (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\approx 8$</tex-math> </inline-formula> mTorr), <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T_{\mathrm {e}}$</tex-math> </inline-formula> can become low (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\approx 1$</tex-math> </inline-formula> eV or lower) away from the source, suggesting favorable conditions for the volume production of H<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${}^{-}$</tex-math> </inline-formula> ions. RFEA measurements yield ion energies <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\approx 97$</tex-math> </inline-formula> eV at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\approx 1$</tex-math> </inline-formula> mTorr and about <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\approx 18$</tex-math> </inline-formula> eV at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$p~\approx ~8$</tex-math> </inline-formula> mTorr in front of the CEPS, indicating that a wide range of ion energies can be attained by controlling the pressure and power. Detailed discussions on the role of the CEPS magnetic field in the confining, heating, and transport of electrons are presented. Scaling laws obeyed by the electrons and ions during their flow are discussed and validated using the experimental results. This article also gives a detailed account of the plasma dynamics inside the source, providing a mechanism for production of the warm electrons observed in the experiments and giving estimates of the charge polarization times (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\simeq 0.37~\mu $</tex-math> </inline-formula>s) and the high electric fields (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\simeq 48.7$</tex-math> </inline-formula> V/cm) that accelerate the ions.