Spatial profile of argon (1s₅) metastables in an electron beam generated plasma

Electron beams with an applied magnetic field generate a secondary cold plasma with a selective chemical composition, featuring low-energy ions and metastable species in the discharge periphery, ideal for low-damage plasma treatment of material substrates. In this work, we studied the plasma generated by an e-beam using a 4 kV voltage in a pure argon gas environment under a magnetic field of 150 G and in the pressure range of 25-90 mTorr. We measured the absolute spatial density profile of argon (1 s₅) metastables in an electron beam generated plasma by laser-induced fluorescence and found it to be of the order of 10¹⁶ m⁻³. The electron temperature and the electron density measured by a Langmuir probe were of the order of 10¹⁶ m⁻³ and less than an eV respectively. Electron-impact quenching was identified as a significant loss mechanism for the Ar(1s₅) state, leading to the saturation of the metastable density at higher pressures. Outside the primary ionization region, the spatial distribution of argon metastables followed a linear diffusion profile, indicating negligible additional production in those regions.