A prospective microwave plasma source for in situ spaceflight applications

Abstract

Noble gas plasmas are commonly used as ion and excitation sources in inductively coupled plasma (ICP) optical emission and mass spectrometry for organic and inorganic chemical analysis. However, the high power (∼kW) and voluminous gas flow rate (∼15 L min⁻¹) of commercial plasmas limit their potential deployment in remote terrestrial and planetary environments. Here, using argon and helium gas supplies, we investigate the fundamental characteristics of low power and reduced-pressure microwave plasmas for organic and elemental analysis. These plasmas require a fraction of the power (<25 W) and gas (<0.2 L min⁻¹) compared to conventional ICP systems. Langmuir probe measurements were used to determine electron temperatures and electron densities for both helium and argon plasmas under a range of forward of powers and gas flow rates. Ionization efficiencies for select organic compounds and elemental species were estimated from these results using the Saha equation. Our findings show that at 23 W of forward power and 0.2 L min⁻¹ gas flow rate, an argon plasma can fully ionize (>99%) organic molecules or elements with high ionization potentials (e.g. glycine, 8.9 eV, or S, 10.4 eV); in comparison, helium can achieve similar performance metrics with as little as 5 W of forward power.