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 $\approx ~50$ cm and length $\approx$ 75 cm). Hydrogen plasma escaping from the CEPS mouth (diameter $\approx$ 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, n $\approx 10^{11}$ cm⁻³, with high $T_{\mathrm {e}}(\approx 8$ eV) and high plasma potentials ( $V_{\mathrm{p}} \approx 50$ V) near the source, at moderate microwave powers ( $\approx 650$ W) and pressures ( $\approx $ few mTorr). At high pressures ( $\approx 8$ mTorr), $T_{\mathrm {e}}$ can become low ( $\approx 1$ eV or lower) away from the source, suggesting favorable conditions for the volume production of H ${}^{-}$ ions. RFEA measurements yield ion energies $\approx 97$ eV at $\approx 1$ mTorr and about $\approx 18$ eV at $p~\approx ~8$ 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 ( $\simeq 0.37~\mu $ s) and the high electric fields ( $\simeq 48.7$ V/cm) that accelerate the ions.