A Prospective Microwave Plasma Source for In Situ Spaceflight Applications

Abstract:

In planetary environments and terrestrial laboratories, in situ chemical analysis of atomic or molecular species is typically carried out using spectroscopy and/or mass spectrometry. Plasma sources atomize and ionize materials, providing data on low analyte concentrations in a wide range of samples. Commercial plasma systems typically require high power (>1 kW) and high gas flow rates (> 10 L/min), making these ion sources restrictive for spaceflight applications. However, recent studies show that plasmas generated with orders of magnitude lower power and gas flow rates can still be used as effective ion sources for spectroscopy and mass spectrometry, supporting the prospect for plasma-based chemical analysis on a future planetary mission.

We analyzed the fundamental properties of a low power (<25 W), low gas flow (<0.2 L/min) microwave plasma, and determined its capacity to atomize and ionize relevant geologic or organic material. Langmuir probe measurements provide electron temperature (T_e) and electron density (N_e) of He and Ar plasmas across a range of forward powers and gas flow rates. Using the Saha equation with inputs of T_e and N_e, we estimate that an Ar plasma can fully ionize atomic or molecular species with high ionization energies, such as sulfur (10.4 eV) or glycine (8.9 eV) , with only 20 W of power and 0.2 L/min of gas flow. In comparison, a He plasma can achieve similar ionization efficiency with as little as 5 W of forward power at the same flow rate. Thermodynamic calculations show that these lower power plasma sources have sufficient heat to atomize ablated geologic particles as large as < 50 nm diameter and sustain a max ablation load of 5x10^-9 g/s of solid sample, comparable to a 50 µm diameter laser beam producing an ablation depth of ~80 nm/pulse at a repetition rate of 10 Hz.