Anomalous electron density of helium, neon, and argon plasmas in high-pressure E-mode inductively coupled plasma

Abstract

Noble gases such as argon, neon, and helium are widely used in low-temperature plasma research and applications. Owing to its lower ionization energy, argon plasma is generally expected to exhibit a higher electron density than neon or helium. Here, we report an anomalous electron density behavior in high-pressure E-mode inductively coupled plasma (ICP), where helium and neon plasmas achieve higher electron densities than argon plasma. At 220 mTorr, the electron density of helium exceeds that of argon by a factor of 2.4. It is found that, under these conditions, argon exhibits a Druyvesteyn electron energy probability function (EEPF) due to the Ramsauer effect, whereas neon and helium maintain Maxwellian distributions. This difference leads to larger collisional energy losses in argon, thereby suppressing its electron density compared to helium and neon. At low pressures or in H-mode operation, this anomalous behavior is absent, and the conventional trend is recovered. These findings highlight the critical role of electron energy distributions in determining plasma density in ICP discharges, and they provide new insight into the use of noble gases for plasma source design under high-pressure conditions.