Measurements of two-dimensional ion velocity distributions in electron beam generated E x B plasma

Electron beam (e-beam) generated plasmas with applied crossed electric and magnetic (E x B) fields are promising for low-damage processing of materials such as graphene and single-crystal diamond. The minimal damage to these sensitive materials is typically attributed to the low energy of ions incident on the substrate surface, a result of ion confinement by E × B fields. Recently, our team observed the presence of warm ions (~1 eV), which is sufficient to produce a diffusive ion backflux towards the edge, thereby compromising the ion confinement in this system [1]. Given that our system is nearly collisionless, the radial structure of the electric potential is identified as the primary source of ion heating. However, other types of anomalous ion heating mechanisms, such as two-stream instabilities [2] at the plasma periphery, require further investigation.

We have developed a two-dimensional Laser-Induced Fluorescence (LIF) diagnostic technique using Doppler velocimetry [3] to map ion velocity distributions (IVD) across the plane perpendicular to the e-beam generated Argon plasma. A diode laser centered at 668.61 nm is used to excite metastable argon ions, and an ICCD camera is employed to collect the corresponding fluorescence signal. By combining electronic diagnostics such as Langmuir probes and emissive probes with this laser-based diagnostic method, we can extensively study ion dynamics in both the confinement and peripheral regions. This diagnostic system, equipped with an ICCD camera, enables time-efficient measurement of spatially distributed IVDs. In future work, we anticipate that our findings will enhance the understanding of ion heating mechanisms in this system, providing insights into the physics behind warm ion production in E × B plasma devices.

 

[1] Chopra, Nirbhav Singh, Ivan Romadanov, and Yevgeny Raitses. "Production of warm ions in electron beam generated E× B plasma." Applied Physics Letters 124.6 (2024).

[2] Mikellides, Ioannis G., and Alejandro Lopez Ortega. "Growth of the lower hybrid drift instability in the plume of a magnetically shielded Hall thruster." Journal of Applied Physics 129.19 (2021).

[3] Severn, G. D., D. A. Edrich, and R. McWilliams. "Argon ion laser-induced fluorescence with diode lasers." Review of Scientific Instruments 69.1 (1998): 10-15.